Download Faculty of Medicine

Essay
Ophtalmogy
Submitted for Partial Fulfillment of Master Degree in
By
Hosny Ahmed Zein
C1.
M.B. B.
Under supervision of
Dr. Mohamed Ehab M.Elewa
Assis. Prof of ophthalmology
Faculty of Medicine
Minia University
Shehab
El-Fath
Dr.Azza Abd
Assis. Prof. of Ophthalmology
Faculty of Medicine
University
Mina
Elzemby
A. Ibrahim
Lecturer of
Faculty of Medicine
ophtalmgy
Dr.Hosam
University
Mina
Faculty of Medicine
Mina
University
2005
s
;
CUSTOMIZED CORNEAL ABLATION
CURRENT STATUS AND FUTURISTIC VIEW
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ACKNOWLEDGEMENT
My endless and everlasting thanks to
ALLAH
the source of all
humanity knowledge.
Although no words can express my great gratitude and respect to
Prof.D
Rabie Hassanein Head of Ophthalmology department,
Minia University, I would like to thank him for his outstanding
encouragement, advice, and his sincere endless support throughout
this work.
I feel greatly indebted to Dr. Mohamed Ehab M.Elewa, Assis. Prof.
of Ophthalmology, Minia University, for his great valuable help,
supervision and guidance throughout this work.
I wish to offer considerable thanks to Dr. Azza Shehab Ass. Professor
of Ophthalmology, Minia University, for her supervision and
assistance.
Dr.Hosam
I am also grateful to
A. Ibrahim Elzembely Lecturer of
Ophthalmology, Minia University, for his great care, sincere guidance
and continuous valuable advice throughout this work.
Finally, words will not be enough to express my sincere gratitude and
appreciation to my parents, without their love, blessings and
encouragement this work would never have been accomplished.
ABSTRACT
Although using laser for correction of vision has reached a high
degree of effectivity and safety. It cannot correct all errors of refraction
especially higher order aberrations, which need many diagnostic
techniques to be accurately measured.
Customized corneal ablation using wavefront technology and
corneal topography can treat all refractive errors of the entire eye
including higher order aberration but with some limitations and
disadvantages.
Keywords:
• Customized corneal ablation.
• Wavefront.
• Corneal topography.
• Laser technology.
• Lasers in ophthalmology.
• Laser surgery in vision correction.
• Higher order aberration.
CONTENTS
LIST OF CONTENTS
Topic
Wavefront-guid
List of figures
List of tables
List of abbreviations
INTRODUCTION
REVIEW OF LITERATURE
Ocular aberrations and visual performance
Wavefront analysis
Types of aberrations
Assessment of optical quality
customized ablation
The history and methods of wavefront sensing
Principles of different wavefront sensors
Representation and interpretation of wavefront data
Comparison between methods of wavefront sensing
Diagnostic use of wavefrot in refractive surgery
Effect of wound healing on wavefront analysis
Company and their customized laser platform
Adaptive optics ophthalmoscopy
Clinical experiences with wavefront guided ablation
Topography guided customized ablation
Basic principles
Corneal topographers
Comparison between methods of corneal topography
Interpretation of color-coded topographic maps
Role of topography in refractive surgery
Corneal topographic indices
Corneal topography guided ablations
Prospects for perfect vision
SUMMARY
REFERENCES
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I
II
CONTENTS
LIST OF FIGURES
No.
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2
3
4
5
6
7
9
10
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abenomtry
11
Hartmn-Shck
8
Topic
Losses in the power spectrum of wave-aberration
correction produced by beam spatial filtering for each
aberration order
The effect of tracker latency on higher-order wavefront
corrections with two different scanning laser spot sizes
and ablation depths.
Optical simulation showing the difference in higher-order
aberrations present in the eye after CustomCornea and
traditional LADARVision LASIK
surgery.
Full wavefront and higher-order two-dimensional and
three-dimensional displays for a single wavefront
measurement acquired from an eye with a visually
significant "central island" post-photorefractive
keratectomy (PRK).
Graphic representation of Zernike polynomials.
Visualization of wavefront slope measurements for withthe-rule astigmatism (spherical equivalent = 0).
A Shack-Hartmann wavefront sensor is essentially a
Scheinr
Disk with multiple apertures, each fitted with a
small lens that focuses the beam of light onto a video
sensor.
The
image and its derivative wavefront
form.
Principle of Shack-Hartmann wavefront sensing.
Principles of Tscherning aberrometry with low-energy
laser light as a 13-13 spot grid, passing through 10 mm of
the cornea and defocusing to an aberrated grid of 1 mm
on the retina.
Principles of Tracey Ray Tracing
whereby
an individual laser ray is very rapidly scanned through
the multitude of entry points within the pupil onto the
retina.
Principles of spatially resolved refractometry
Principles of slit skiascopy used in the Nidek OPD scan.
The color-coded map for the wavefront image.
Epithelial hyperplasia may mask features produced by
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CONTENTS
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80
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keratcomy.
LASIK,
22
custom corneal ablation.
LADARWave system wavefront analysis for an eye with
moderate keratoconus.
Optical simulation showing the difference in higher-order
aberrations present in the eye after CustomCornea and
traditional LADARVision LASIK surgery.
Scanning electron micrograph of a portion of the
WASCA lenslet array.
Data measurement and presentation by various corneal
topography systems.
Components of topographic display
Corneal wave aberration contour maps, after surgery,
centered at the pupil center, after realignment and
centered at the corneal reflex, directly from corneal
topography data without realignment.
Total, corneal, and internal wave aberration maps (thirdorder and higher aberrations) before and after
with a particularly good surgical outcome.
PRK difference maps
Hyperopic photorefractive
Topographic map showing central island.
Optical effects of tear film disruption.
Diffraction and aberrations degrade the contrast of a
sinusoidal pattern imaged by the eye.
Photoreceptor sampling.
Schematic view of optical and retinal sampling limits to
visual resolution.
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CONTENTS
LIST OF TABLES
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7
Page
Comparative features of eye tracking in scanning 7
excimer lasers.
Forms of customized ablation.
8
Comparison between Laser Ray Tracing (LRT), the 36
spatially Resolved Refractometer (SRR), and the
Hartmann-Shack sensor (H-S).
Characteristics of three Hartmann-Shack wavefront
sensors
Manufacturer, models, and methods of commercially
used corneal topographers.
System for studying topography display.
Role of topography in refractive surgery
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CONTENTS
List of abbreviations
AA
ACP
ArF
BCVA
BSCVA
CEI
CMS
CSF
Excimer
HOA
H-S
Percent Area Analyzed
Average Corneal Power
Argon Fluorine
Best corrected visual acuity
Best spectacle- corrected visual acuity
Corneal Eccentricity Index
Corneal Modeling System
Contrast sensitivity function
Excited Dimer
Higher-order aberration
Hartmann-Shack
IA
Irregular Astigmatism Index
KCS
Keratoconus suspect
KPI
Keratoconus Prediction Index
LASEK Laser subepithelial keratomileusis
LASIK
Laser insitu keratomilus
LLA
Laser Ray Tracing
PARK
Photoastigmatic refractive keratectomy
PRK
Photorefractive keratectomy
PSF
Point spread function
PTK
Phototherapeutic keratectomy
PVA
Potential visual acuity
RK
Radial keratotomy
RMS
Root mean square
Radius of curvature
ROC
Surface Asymmetry Index
SAI
Simulated Keratometry
SimK
index
SKI
Standard keratomic
Surface. Regularity Index
SRI
SRR
Spatially resolved refractometer
Transverse chromatic aberration
TCA
TOSCA Topography supported customized ablations
Uncorrected visual acuity
UCVA
WASCA wavefront supported customized ablations
INTRODUCTION
CUSTOMIZED CORNEAL ABLATION
Perhaps no field within ophthalmology has changed so dramatically than the
still evolving subspecialty of refractive surgery. The roots of refractive surgery can be
traced back well over 50 years to Jose Barraquer and others. Early work with
keratomileusis laid the groundwork for present-day laser-assisted in situ
(LASIK). The instruments have been markedly improved and are still
keratomilus
being modified (Lazzaro, 2005).
Customized corneal ablation is an exciting frontier in refractive surgery that
incorporates wavefront technology to detect and correct higher order aberrations in
addition to spherocylindrical refractive errors. The goal is to achieve supernormal
vision in terms of acuity and contrast. As the concept of wavefront customized
ablations is still new, there are a number of aspects of its clinical application that
need analysis and understanding. Numerous reports have appeared in the literature
during the past year that address the developments, concerns, and limitations of
wavefront technology and custom ablation (Waheed & Krueger, 2002).
The wavefront analyzers in use today can be used to determine higher order
aberrations that were not possible with older-type videokeratographers. These results
"
can be subsequently translated to wavefront ablations. These custom and now
optimized "custom treatments are being used in an attempt to reduce higher-order
2005).
aberrations and strive for " supervision " (Lazro,
Corneal topography continues to be a critical diagnostic modality for refractive
surgery. Even with the advent of wavefront analysis designed to detect refractive
error and aberrations of the eye, it will be necessary to have detailed corneal
topographic information to understand the contribution the cornea makes to vision so
that custom alteration of that surface can be used to optimize vision (Wilson &
Ambrosio, 2001).
1
I NTRODUCTION
1 ---
ADVANCES IN EXCIMER LASER SYSTEMS
The excimer laser is the starting point for much of what is going on in this field.
There are numerous manufacturers of these lasers, each with different operational
platforms and treatment patterns. There are spot lasers and broader-beam lasers to go
along with wavefront-capable machines (Lazzaro, 2005).
Scanning spot laser delivery
Scanning spot size
Although many of today's commercially available excimer lasers have begun
to offer scanning and flying spot delivery patterns, it is the size of the spot that has
recently been examined with greater scrutiny. This scrutiny has increased with the
advent of wavefront customized corneal ablation, because of the higher-order
aberrations that the laser vision correction platform is now attempting to correct
(Krueger, 2004).
Three independent mathematical analyses have been published
to verify the
importance of the small scanning spot in correcting aberrations and to define the
optimal laser spot-size for adequate correction of certain higher-order aberrations
(Krueger, 2004).
In the first study, by Huang et al, a polynomial analysis reveals that treating up
to fourth-order aberrations (N = 4) with a total optical zone diameter of 6 mm (D = 6)
requires a scanning spot beam diameter of 1.0 mm or less. Correcting further detail,
up to sixth-order aberrations, with a 6-mm zone requires less than or equal to a 0.6mm diameter scanning spot. In an independent analysis by Guirao et al, a scanning
spot beam diameter of 1.0 mm was again determined to be the maximum desirable
spot size for correcting fifth-order aberrations by viewing the power spectrum of the
wave aberration (Huang &
At-if,
1
2002, and Guirao et al., 2003).
p
INTRODUCTION
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Losses in the power spectrum of wave-aberration correction produced by
beam spatial filtering for each aberration order, a 1.0-mm beam is the maximum spot
size for correcting fourth- and fifth-order aberrations (Krueger, 2004).
Finally, a third, unpublished analysis by Bueeler and Miochen shows that not
only a small spot, but also a very fast tracking system and small pulse ablation rate
are needed to optimize aberration correction. For a 1-mm spot, a tracker response
latency of 32 milliseconds may be sufficient to correct simple third-order coma.
However, to correct more detail, a 0.25-mm spot size and tracker response latency of
4 milliseconds are required. More complex higher-order aberrations would require
3
(Buelr
the smaller spot size and shorter tracker latency
& Miochen, 2004).
I NTRODUCTION
32 ms
ms
No eye
tracking
00 2
Fig. (2): The effect of tracker latency on higher-order wavefront corrections with two
different scanning laser spot sizes and ablation depths (Krueger, 2004).
Scanning spot shape
The gaussian beam profile is considered the most desirable scanning spot
shape, because it allows for a very uniform overlap in the creation of the ablation
zone profile. This uniformity is important because small spatial errors in beam
placement could result in spikes and valleys in the ablation profile. This is the case
4
INTRODUCTION
with a top hat beam, whereas the gaussian beam avoids abrupt edges, producing an
overall smooth beam profile (Krueger, 2004).
Scanning spot rate
Although the slower rate is thought to extend the time of treatment, the volume
ablated per shot is also an important factor. The frequency of spot placement is also
important with regard to hydration changes that occur over time, as treatments that
take too long can adversely affect hydration. The scanning spot rate, however, must
not be more rapid than the sampling and response rate that can be adequately
followed by the tracking system (Krueger, 2004).
Finally, the placement of the scanning spot is best when it is nonsequential, or
pointillistic, such that one spot is not directly placed next to the preceding spot. This
placement helps to avoid thermal buildup of energy at any given location, or
improper shielding of the ablation plume during treatment (Krueger, 2004).
Robust eye tracking
Fixation-related eye movements
During patient fixation, frequent saccadic eye movements have been recorded.
They are (1) random, (2) about 5 times per second, and (3) at a rapid rate proportional
to the distance traversed. These characteristics of fixation-related saccadic eye
movements make careful treatment of patients requiring laser vision correction less
effective without the aid of a sophisticated eye tracking system ( Bollen et al., 1993).
Tracking nomenclature
To understand the eye tracking systems, a number of terms must be defined.
These include (1) sampling rate, (2) latency, (3) tracker type, and (4) closed loop
versus open loop.
5
II
INTRODUCTION
Sampling rate
"Sampling rate" describes how often the tracker measures the eye's location.
Tracking frequencies vary from 60 Hz, based on the frame rate of certain video
camera trackers, to 4000 Hz, seen with laserradar tracking.
Latency
"Latency" is the time required to determine the eye's location, calculate the
required response, and compensate, or move the laser tracker mirrors to compensate
for the new location. The latency period is therefore due to both the processing delay
and the mirror readjustment delay. Typical infrared video camera-based tracking
systems have a processing delay due to the time it takes for the image to be integrated
onto the image sensor and for the sensor image to be transferred from the camera to
an image processing unit. Each of these steps requires a period of 16.67 milliseconds
(NTSC) or 20 milliseconds (PAL) with a typical video camera based tracker. This
amounts to a total processing delay of 33 milliseconds (NTSC) or 40 milliseconds
(PAL). Custom high-speed video processing systems are touted as reducing the delay
to 4 8 milliseconds (Huppertz et al., 2001).
-
Tracker type
Typical video camera eye tracking uses infrared light illumination of the iris
against a dark pupil in most refractive surgical systems. This video-based tracking
captures a new image without maintaining a reference of the eye's previous position,
so that no feedback of the eye's current position exists. It simply sees a deviation
from the intended position and moves to respond to that deviation (Krueger, 2004).
INTRODUCTION
Video camera
LADRVison
Autonomous
Method
Transmitted
signal
Detection
frequency (Hz)
Response
time (ins)
Laser radar
905-mm
diode laser
4000
Technolas (120)
Nidek.
(60)
(60)
WaveLight (250)
Zeiss Meditec (250)
CCD/Infrared
none
VisX
Laser system
LaserSiht
3 rise time
(100 Hz
bandwidth)
60, 120, 250
50 rise time
(6 Hz bandwidth)
Table (1): Comparative features of eye tracking in scanning excimer lasers. (Krueger, 2004).
Closed loop versus open loop tracking
In open loop (video) tracking, once a new image is taken, the change from the
previous image location is calculated and an error signal is sent to move the mirrors.
By contrast, closed loop tracking is represented by the laser radar based system
(LADAR), where the rapid sampling rate together with this system's closed loop
servo response feed back information on the eye's new position continually,
maintaining a space-stabilized image and accurate tracking without latency (Krueger,
2004).
Clinical significance of eve tracking
In a presentation at the American Academy of Ophthalmology Meeting in
2001, Hardten et al used the VISX ActiveTrac, which is a 60-Hz video camera
tracker with open loop tracking, to treat a cohort of 202 eyes and compared them with
110 eyes in which the tracker was not engaged. Statistical significance of
i mprovement in vision was noted with the active tracker engaged (mean SCVA
20/19.3) compared with no tracker (mean
20/20.2) (P= 0.020). This study may be compared with that reported by
7
-Er
=
SCVA
=
[spectacle corrected visual activity]
INTRODUCTION
Mrochen et al, where a faster 250-Hz video camera tracker employed by
theWavelight Allegretto laser (Erlangen, Germany) demonstrated a statistically
significant improvement in vision (mean SCVA = 20/17.7) when tracking 20 eyes in
(1)-
comparison with treatment of 20 eyes with no tracker (mean SCVA = 20/20.8)
0.013). Both studies showed clinical improvement with eye tracking, yet the latter
study required a smaller number of eyes to achieve statistical significance (Mrochen
et al., 2001).
It is not possible to perform a similar comparison with the LADARVision eye
tracker system at 4000 Hz, because the laser will not fire with the tracker turned off.
Nevertheless, one can infer that significant clinical improvement would also be
achieved when using the LADARVision tracker. In conclusion, the author
recommends the use of a high sampling rate tracker with minimal or no latency and
closed loop servo response to achieve the maximum benefit (Kruger, 2004).
METHODS OF CUSTOMIZED ABLATION
Vision is a complex process. Customizing corneal laser treatment to create
optimal vision may take several major forms: (1) functional, (2) anatomical, and (3)
optical. An attempt to separate the type of customization into categories may be
helpful. (MacRae, 2000).
Table (2): Forms of customized ablation. (MacRae, 2000).
Functional customization based on patient's need
♦
♦
♦
♦
♦
Age
Presbyopia
Patient's occupational and recreational needs
Refraction
Psychological tolerance
Anatomical customization
♦ Corneal diameter and thickness
♦ Pupil size (also important for optical customization)
8
INTRODUCTION
♦ Anterior chamber depth
♦ Anterior and posterior lens shape
♦ Axial length
Optical customization (measures subtle aberrations)
♦ Customization based on corneal topography
♦ Customization based on wavefront measurements:
o
o
o
o
o
o
•
Hartmann-Shack (Shack-Hartmann) wavefront sensor
Aberrometer (Howland and Howland)
Tschering method
Tracey system
Slit-light bundle
Others
Functional customization
Functional customization is based on the patient's needs and is currently being
done under most circumstances. Functional customization considers factors such as
the patient's age, refraction, occupation, optical requirements (including monovision),
as well as psychological adaptability. A 25 year old -1.25 diopter (D) may require a
different surgical strategy than a 45 year old with the same refraction (MacRae,
2000).
• Anatomical customization
Anatomical customization considers the individual anatomical variation of
each eye to determine the surgical approach. The patient's corneal thickness and
diameter, as well as pupil size in bright and dim light conditions, are anatomical
measurements that are important considerations prior to surgery (pupil size is also
critical for optical customization). In the future, anterior chamber depth, anterior and
posterior lens shape as well as thickness, and axial length may be used to predict the
optimal "optical design" possible for each patient based on his or her anatomy. Each
9
INTRODUCTION
factor deserves further exploration to determine its effect on optimal ablation design
and outcome (MacRae, 2000).
Anatomical customization may require an instrument that can scan the entire
anatomy of the eye to enable the surgeon to deliver the optimal ablation pattern
based on the idiosyncratic anatomical needs of each eye. A -8.00 d myope with thick
corneas, a short anterior chamber depth, and small pupils may require a different
treatment than an individual with the same refractive error who has thin corneas, a
long anterior chamber depth, and large pupils (MacRae, 2000).
• Optical aberration customization
There are 2 forms of subtle optical customization that are particularly exciting
because of their potential to enhance vision. Both systems measure subtle optical
aberration. The first is corneal topography-guided ablation, which measures the
ocular aberrations detected by corneal topography and treats the irregularities as an
integrated part of the laser treatment plan. The second measures wavefront error of
the entire eye and treats based on these measurements. Each of these will have be an
important influence on the future of refractive surgery (MacRae, 2000).
Viewing conditions in which monochromatic aberrations are corrected and
chromatic aberrations are avoided provides an even larger improvement in contrast
sensitivity and visual acuity. These results are in reasonable agreement with the
theoretical improvement calculated from the eye's cortical modulation transfer
function (Yoon & Williams, 2002).
CUSTOMIZED VERSUS CONVENTIONAL LASER REFRACTIVE
SURGERY
The optimal corneal ablation pattern for treatment with the excimer laser has
not been fully characterized. An optimal ablation pattern is one that accounts for all
of the optimal elements of an individual's eye that then would require customization
10
INTRODUCTION
of ablation for that individual eye. One of the critical problems in maximizing the
(Schwegrlin
ablation pattern is the reduction of spherical abberation
et al., 1997).
The spherical abberation increased dramatically after PRK and LASIK.
Another challenging problem is the treatment of irregular astigmatism. There are a
variety of conditions that would benefit from customized ablation, including corneas
with irregular astigmatism. These include contact lens induced corneal warpage,
decentered ablations and irregularities caused by previous corneal foreign bodies,
post-penetrating keratoplasty, scars after infectious keratitis, as well as small ablation
optical zones from previous PRK (MacRae et al., 1999).
The development of videokeratoscopes capable of recording corneal shape in
detail and abberometers that measure the wave abberation of the optics of the entire
eye, have revealed that although standard laser refractive surgery eliminates
conventional refractive errors, higher order errors are typically induced (Oshika et al.,
1999).
The aberrations that are induced by conventional laser vision correction have
the potential to be minimized or eliminated with customized corneal ablation. In
addition, preexisting aberrations may be minimized or eliminated by this same
technology (Krueger, 2004).
The goal is to achieve super normal vision in terms of acuity and contrast. As
the concept of wavefront-customized ablations is still new, there are a number of
aspects of its clinical application that need analysis and understanding (Waheed &
Krueger, 2002).
Optical abberations due to surface irregularity (caused by epithelial irregularity
following PRK, or flap folds following LASIK), inclusion of the excimer ablation
zone edge within the entrance pupil, or alteration of the normal prolate corneal shape,
might all degrade the retinal image, leading to decreased contrast sensitivity function
and visual perturbations (Holiday et al., 1999).
I
Ii
INTRODUCTION
Both the theoretical results and the experimental results under laboratory
conditions point out the potential value of correcting eye's higher-order abberations
with customized corneal ablation (Guirao et al., 2002).
E
EHVDF
HRBPYE
AP
UZFE
F
Pl
to
14
Z
After Customrnea
After Conventional LASIK
LASIK
Fig. (3): Optical simulation showing the difference in higher-order aberrations present in
Customrnea
the eye after
(A) and traditional LADRVison
LASIK surgery (B).
The difference in the amount of total higher-order aberrations remaining between the
two groups 6 months after surgery equates to about 0.2 D of defocus.
12
INTRODUCTION
Zonal Reconstruction
Wavefront
Full
Higher Orders
Only 2D
Higher Orders
Only 3D
and higher-order two-dimensional and three-dimensional displays
for a single
measurement acquired from an eye with a visually significant
wavefront
wavefront
Fig. (4): Full
"central island" post-hrefaciv
keratectomy (PRK).
The plots in the left column are based on a Zernike expansion representation of
the wavefront up to the eighth order, while those in the right column are based on
direct zonal reconstruction from the raw slope data.
13
REVIEW OF LITERATURE
OCULAR ABERRATIONS AND VISUAL PERFORMANCE
For more than a century, there has been awareness of the fact that the eye is not
a perfect optical system and that it suffers from defects or optical aberrations other
than defocus and astigmatism (Marcos, 2001).
Optical aberrations in the human eye impose a major physical limit on spatial
vision. Interest in the study of ocular optics was evident centuries ago. Now the field
is booming because of new optical technology to correct optical aberrations beyond
(Arial
defocus and astigmatism
et al., 2001).
In principle, refractive surgery is designed to optimize the optical performance
of the eye without the aid of glasses or contact lenses. Refractive surgical procedures
have focused on eliminating spherical and cylindrical defocus. However, such an
approach ignores the fact that the eye has significant higher order aberrations. These
naturally occurring higher order aberrations, combined with large increases in the
eye's higher order aberrations induced by refractive surgery can decrease visual
performance despite the elimination of spherocylindrical errors (Applegate and
Howland, 1997).
WAVEFRONT ANALYSIS
In recent years, vision scientists and refractive surgeons have adopted
wavefront-sensing technology to assess the optical aberrations of the eye and to guide
customized corneal ablations. A variety of wavefront-sensing or aberrometry devices
now exist, and they all record in some manner the difference between the aberrant
wavefront of light reaching the retina and the theoretical, ideally focused wavefront.
This difference is called the wavefront error of the eye, and it is typically defined
within the area delimited by the pupil (Smolek & Stephen, 2003).
Wavefront error data have the form of a complex, three-dimensional surface
that can be mathematically decomposed into a canonical set of terms that describe
14
I
REVIEW OF LITERATURE
individual aberration components such as spherical aberration and coma. These terms
are also used for guiding laser ablation during customized refractive surgery.
Currently, the preferred decomposition method uses the Zernike polynomial, which
represents total wavefront error as a series of terms that describe surface shape
components with respect to angular and radially arranged basis functions of different
frequencies and orders (Iskander et al., 2001).
Each Zernike term has a coefficient with a magnitude and sign that indicate the
relative strength and direction of the aberration contributed by that term. The
wavefront error is often expressed as the sum of the root mean square (RMS) error to
avoid sign discrepancies for certain terms, particularly when combining left and right
eyes into a single cohort (Smolek et al., 2002; Smolek
& Stephen, 2003).
The Zernike decomposition process is a reverse-fitting routine. That is, given a
complete set of individual aberration components, the original surface shape can be
theoretically reconstructed. When the process is applied to describing the optics of the
eye, however, there are unresolved questions as to how many terms are sufficient to
describe a given surface, and whether the accuracy of the decomposition process (i.e.,
Zemike fitting) is adequate (Iskander et al., 2002).
The Zernike fitting process is not limited to analysis of wavefront error
surfaces, but can be applied to other ocular surfaces as well, including the front
surface elevation of the cornea. Given the significance of the shape of the front
surface of the cornea to the refraction of the eye and the ability to correct refractive
errors by laser ablation of the front surface of the cornea, detailed wavefront error
analysis of corneal topography data is clinically useful and important. In addition,
corneal topography is a major ocular component that defines the total ocular
wavefront error, and so one can argue that the relevance and accuracy of Zernike
polynomial analysis in general can be studied with corneal data alone. In this type of
analysis, the corneal surface data are used to establish a mean reference wavefront
15
1
REVIEW OF LITERATURE
through a best-fit curve to the elevation data and also to provide the raw elevation
information used to calculate the wavefront error ( Smolek & Stephen, 2003).
It has been recognized that the corneal first surface generally provides the bulk
of the ocular aberrations in the post surgical or pathologic eye. The corneal front
surface in the normal eye contributes approximately half the total aberrations of the
eye, but the number contributed is age dependent, and the contributions increase
substantially with surgery and disease (Artal et al., 2002).
Several thousand topographic data points are necessary for adequate detection
of corneal surface irregularities that can decrease vision; however, the number of data
points measured with wavefront-sensing instruments varies from the low hundreds to
several thousand (Smolek & Stephen, 2003).
TYPES OF ABERRATIONS
The term aberration derives from the Latin word aberration, which means
going off-track or deviating. Aberration is defined as the difference that exists
between the ideal image that we would expect to see when luminous rays are
refracted in the perfect optical system (Snell's law) and what is actually achieved.
These differences are characteristic of each optical system and vary from simple
defocus to highly aberrated wavefronts (Solomon et al., 2004).
Aberrations can be described quantitatively using Zernike polynomials, named
after Frits Zemike, a Dutch mathematician and astronomer who won the Nobel Prize
for his invention of phase contrast microscopy. These mathematical models are
adequate for describing the wavefront measurements of the eye, because they are
defined based on a circular form. The shape of the wavefront is described in the x and
y coordinates; the third dimension, height, is described in the z-axis. The final figure
is obtained from the sum of the Zernike polynomials describing all types of
deformation (Solomon et al., 2004).
16
REVIEW OF LITERATURE
Aberrations can be divided into two groups: chromatic and monochromatic.
Chromatic aberrations are caused by the difference in distribution of incident
polychromatic radiation throughout a medium and depend on the wavelength of the
light that penetrates the eye. They are influenced by variations in the refractive index
of a material in relation to the wavelength of the light that travels through it. This
type of aberration cannot be corrected, because it depends on the composition of the
ocular structures and not their shape. Monochromatic aberrations are related to a
specific wavelength and include spherical refractive error (defocus), cylindrical
refractive errors (astigmatism), and high-order aberrations (1-IA0)
such as spherical
aberrations and coma. Based, on Zernike's polynomials, aberrations are described
numerically and ranked accordingly. First to fourth-order polynomials (Solomon et
al., 2004).
1st
2nd
3rd
4th
Fig. (5): Graphic representation of Zernike polynomials (Solomon et al., 2004).
17
REVIEW OF LITERATURE
Low-order aberrations
Low-order aberrations include the following:
E3
Order-zero (no order). These aberrations are characterized by axial symmetry
and a flat wavefront.
13
First-order. These linear aberrations correspond to tilting around a horizontal
(x) or vertical (y) axis. They describe the tilt or prismatic error of the eye.
Second-order. Spherical defocus and astigmatism describe the spherical error
and astigmatic component and its orientation or axis. These components are
similar to measurements found with basic refraction. (Solomon et al., 2004).
High-orde
aberrations
High-order aberrations are as follows:
E3
Third-order aberrations correspond to horizontal and vertical coma and
triangular astigmatism with the base along the x- or y-axis (trefoil).
Fourth-order aberrations include spherical aberration, tetrafoil, and secondary
astigmatism.
13
Fifth-tenth order aberrations are important only when the pupil is greatly
dilated (Solomon et al., 2004).
Clinically relevant aberrations are as follows:
1. Astigmatism. These low-order aberrations demonstrate different meridian focuses
at different planes and generate a toric wavefront.
2. Astigmatism from oblique beams or astigmatism from oblique incidence. In this
form of extra-axial high-order aberration, where the luminous light source is not
co-axial with the optic axis, the luminous rays generated from it follow a different
pathway to the axis of the system.
18
J
REVIEW OF LITERATURE
3. Defocus (spherical refractive error). This low order aberration is observed in the
where the light rays focus at a
ametropi
presence of myopic and hyperopic
different point than emmetropia.
4. Coma. This high-order aberration of the optic system is produced when light rays
form an angle with the optical axis, or when some peripheral light rays do not
focus on the same retinal plane but are focused at different distances from the
retina. The image of the retinal focal point has a coma like appearance.
5. Spherical aberration. This high-order aberration is produced in a spherical
optical medium where marginal rays focus before the paraxial rays. The
wavefront modifies its curvature as it approaches the pupil edge. It can be
measured by determining the distance between the focus of these two rays. In a
normal eye, it is approximately 0.50 D. All spherical surfaces give rise to
spherical aberrations, which are not generated from planar surfaces where the
dioptric power is equal at all points of the surface (Solomon et al., 2004).
Diffraction
Even in the absence of aberrations, an infinitesimal point be formed on the
retina, and this is because of diffraction. Diffraction is an interaction between light
passing through the eye and the edge of the iris. Without aberrations, the perfect eye
would convert incoming wavefronts into converging spherical waves. However, the
edges of these spherical waves are distorted as they pass through the iris. Thus, the
effect of diffraction is to cause the image of a point to have a definite size. This point
(Schwiegrln,
image resulting from diffraction is called Airy disk
2000).
ASSESSMENT OF OPTICAL QUALITY
The difference between the test wavefront and the reference wavefront as a
function of exit pupil location defines the aberration structure. Typically, the
differences are plotted as a contour map or for more a quantitative representation,
fitted with a polynomial (Applegate et al., 2001).
19
REVIEW OF LITERATURE
Today, optical imperfections of the eye are being re-examined within a
comprehensive theoretical framework that expresses the combined effect of all the
eye's aberrations in a two-dimensional aberration map of the pupil plane. An
aberration map is similar in concept to corneal topographic maps used to describe the
corneal surface. The major difference is that a corneal map describes the curvature of
a physical surface, whereas an aberration amp describes the difference between a
wavefront of light and a reference wavefront. By concentrating our attention on light
p
instead of the refracting surface, we gain an ability to compute image quality on the
retina for simple points of light, for clinical test targets, or for any complex object in
the real world (Thibos and Applegate, 2001).
One systemic method for classifying the shapes of aberrations maps is to
conceive of each map as the weighted sum of fundamental shapes or basis functions.
One popular set of basis functions is the Zernike polynomials. This set of
I
mathematical functions are formed as the product of two other functions, one of
which depends only on the radius r of a point in the pupil plane and the other depends
only on the meridian of a point in the pupil plane. The former is a simple polynomial
of the nth degree and the latter function is a harmonic of a sinusoid or co-sinusoid
(Thibos
and Applegate, 2001).
wavefront
The shape of the
can be analyzed by expanding it into sets of
Zernike polynomials to extract the characteristic components of the wavefront shape.
Polynomials can be expanded up to any arbitrary order if sufficient numbers of
measurements for calculations are made. Spectacles can correct only second order
aberrations, not the third and higher orders that represent irregular astigmatism.
Zernik
Monochromatic aberrations can be evaluated quantitatively using
coefficients of each term (Maeda, 2001).
20
P'R
I
m
rt.
I
REVIEW OF LITERATURE
WAVEFRONT GUIDED CUSTOMIZED ABLATION
Efforts to correct refractive errors have led to the development of spectacles,
contact lenses, and wavefront-guided customized corneal ablation. The possibility of
achieving supernormal vision in terms of acuity and contrast has fueled the
imagination and creativity of vision researchers to pursue the goal of customized
(Yel
wavefront refractive surgery
& Azar, 2004).
Wavefront sensors can be linked to excimer laser systems, making possible
wavefront-guided, customized corneal ablations with two specific targets: (1) the
correction of all measured pre-existing aberrations (sphere, cylinder, higher-order
aberrations), and (2) the noninduction of higher-order aberrations, such as spherical
aberration, that have been disclosed to be induced by any laser treatment (Carones,
2004).
The wavefront analyzers in use today can be used to determine higher order
aberrations that were not possible with older-type videokeratographers. These results
can be subsequently translated to wavefront ablations. These custom and now
"optimized" custom treatments are being used in an attempt to reduce higher-order
aberrations and strive for "super vision." Wavefront custom corneal ablative lasers
likely will be the predominant way to modify the corneal refractive power as LASIK
continues to progress. This method to reduce higher-order errors and make the cornea
prolate will probably be around for some years to come (Lazzaro, 2005).
THE HISTORY AND METHODS OF WAVEFRONT SENSING
Tschernig'
Since
first description of his aberoscp,
several changes have
been incorporated to improve the accuracy of characterizing the aberrations of the
eye. The original aberroscope is a device composed of a +5 diopter (D) lens with a
2]
I
IMP
REVIEW OF LITERATURE
square grid superimposed, through which the patient could see the distorted image of
the grid on his retina and characterize the aberrations of the eye. Since then, a variety
of subjective and objective methods for assaying the wave aberrations of human eyes
have been developed. In 1960, Howland proposed another subjective aberroscope
using a 45-cross-cylinder 5-D lens to investigate the aberrations of camera lenses,
which were applied to clinical ophthalmology in a pioneering study that characterized
the monochromatic aberrations of the human eye. This study not only measured
comatic aberrations for the first time, but also introduced the use of Zernike
polynomials to describe the wavefront aberrations of the human eye (Howland, 2000).
At the end of the twentieth century, having learned from astronomers the
importance of using wavefront analyses to improve the images captured by the
telescopic optic systems, vision researchers started to apply these technologies to the
study of the human optic system. The first borrowed technology was the HartmannShack wavefront sensor, used by astronomers to analyze atmosphere aberrations
above a telescope in real time and adapted by vision scientists to measure higherorder aberrations of human eyes (MacRae & Williams, 2001).
The device focuses a bright spot of light on the retina, and the reflected wavefront is
captured and projected onto a matrix of lenslets that focus spots of light on a CCD
video array. A spot pattern is formed from each emerging wavefront and compared
with the spot pattern of an ideal subject with a perfect wavefront. Aberrations in the
optical system result in an irregularly shaped wavefront and so create an irregular
spot pattern. Displacement of lenslet images from their reference positions are used to
calculate the shape of the wavefront, and this information is converted to a color map
for points over the pupil area (Platt & Shack, 2001 and Maeda, 2001).
Also learning from astronomers, vision scientists applied the adaptive optics
technology in the study of the human eye. In adaptive optics, distorted wavefronts
induced by turbulences in the atmosphere are compensated by deformable mirrors,
REVIEW OF LITERATURE
allowing visualization of telescopic images with minimal aberration. Hartmann—
Shack wavefront
sensing (to measure the aberrations of the eyes) and adaptive optics
deformable mirrors (to correct the detected aberrations) allowed for high-resolution
retinal images and the possibility of providing normal eyes with supernormal optical
quality (Liang et a!., 1997).
keratcomy
The precision of the refractive correction by photorefractive
(PRK)
and laser in situ keratomileusis (LASIK) has replaced the less predictable radial
keratotomy (RK). Wavefront sensing and customization are moving toward
correction of the total ocular aberration by coupling future wavefront sensors with
refined nomograms of ablative lasers, indicating that an ideal correction might be
possible and supernormal vision might be attained. Moreover, the use of wavefront
sensing may improve in the future to capture the alterations of severely irregular
corneas with greater accuracy. Once these improvements are achieved, along with
i mproved laser ablation profiles, the applications of wavefront-guided ablations to
treat severely irregular corneas, which have been unsuccessful so far, may become
& Azar, 2004).
METHODS OF OPHTHALMIC WAVEFRONT SENSING
Wavefront sensors usually use ray-tracing methods to reconstruct the wavefront
and are classified into the following three types:
♦ First, outgoing wavefront
aberomty
is used in the Hartmann-Shack sensor.
♦ Second, ingoing retinal imaging abberometry is used in the cross cylinder
abberoscope, the Tscherning abberoscope (Dresden Wavefront Analyzer (DWA))
and the sequential retinal ray tracing method (Laser Ray Tracing, LLT).
♦ Third, the ingoing feedback
aberomt
is used in the spatially resolved
refractometer (SRR). The optical path difference method (OPD) (Slit retinoscopy
or skiascopy) is a variant of this method. (Maeda, 2001).
'?
3
1
feasible (Yeh
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PRINCIPLES OF DIFFERENT WAVEFRONT SENSORS
There are now a number of different types of wavefront measurement devices
available on the market. Although it is often difficult to adequately categorize new
products in an understandable fashion, there appear to be four different principles by
which wavefront aberration information is collected and measured (Krueger, 2004).
Outgoing reflection aberrometry (Shack-Hartmann)
aberomt
The purpose of an
is to measure the ray aberrations of the eye at
multiple pupil locations. This task is accomplished quickly and objectively with a
Scheiner-Hartmann-Shack wavefront sensor consisting of an array of microlenslets
and a video camera. The lenslet array subdivides the beam of light reflected out of the
eye from a retinal point source. Each lenslet forms a spot image that deviates
horizontally and vertically from the optical axis of the lenslet by an amount that is
directly proportional to the angular ray aberrations of the eye at the corresponding
2004).
24
'
I
'
pupil location (Thibos,
REVIEW OF LITERATURE
Horizontal Slope (mrad)
(mrad)
Vertical Slope
1.5
15
0.5
0.5
0
0
-0.5
-05
-1.5
Radial Slope (mrad)
Ray Aberrations (mrad)
Fig. (6): Visualization of wavefront slope measurements for with the-rule
astigmatism (spherical equivalent = 0).
typically measure the horizontal and vertical components of
Aberomts
wavefront slope at numerous pupil locations (Thibos,
25
2004).
I
REVIEW OF LITERATURE
Scheinr
Fig. (7): A Shack-Hartmann wavefront sensor is essentially a
Disk with
multiple apertures, each fitted with a small lens that focuses the beam of light onto a
video sensor (Tlibos,
2004).
At the turn of the past century, Hartmann first described the principles by
which optical aberrations in lenses could be characterized. These principles were later
modified by Shack and have found practical application in adaptive optics telescopes
to eliminate the aberrations of the earth's atmospheres for the past 20 years. They
were finally introduced into ophthalmology by Liang and Bille in 1994 (Liang et al.,
1994).
The Shack-Hartmann •wavefront sensor was used to measure the wave
aberrations of the human eye objectively. A further application of adaptive optics in
ophthalmology was in using it to view retinal structures with greater detail than ever
before. In 1996, images of the cone photoreceptors were viewed in the living human
eye by adaptive optics defined by a Shack-Hartmann wavefront sensor. This first
attempt at customizing the optics of the eye to increase the resolution of its inner
structures defined the need for the measurement specificity provided by wavefront
technology in achieving better resolution when viewing structures outside the eye
(Miller et al., 1996).
26
REVIEW OF LITERATURE
The typical Shack-Hartmann wavefront sensor uses approximately 100 spots or
more, created by approximately 100 lenslets that focus the aberrated light exiting the
eye onto a CCD detection array. After the light passes through the array of lenslets,
each small segment of the wavefront is focused to a small spot on the detection array,
and the distance of displacement of the focused spot from the ideal accurately defines
the degree of ocular aberration (Gulani et al., 2004).
Constr
Ha
n of Wavefro t Aberration
ann-Shack Image
fieconstrud
Coma
Triangular
Astigmatism
Wavefront
Spherical
Aberration
Fig. (8): The Hartmann-Shack image and its derivative wavefront forms ( Gulani et
al., 2004).
Although the Shack-Hartmann method of wavefront aberrometry is a very
accurate one, the level of accuracy is dependent on the number of spots that are
collected within a 7-mm pupil area. Commercially, Shack-Hartmann wavefront
devices vary from as few as 70 spots (Bausch & Lomb Zywave, Claremont,
California) to as many as 800 spots (Wavefront Sciences COAS, Albuquerque, New
Mexico) (Gulani
et al., 2004).
27
p
REVIEW OF LITERATURE
In general, it appears that >100 spots within a 7-mm pupil area is sufficient for
accurately measuring up to eighth-order aberrations (Durrie & Stahl, 2004). The
potential limitations of this type of wavefront sensing may include multiple scattering
from choroidal structures beneath the fovea, but this is likely to be insignificant in
comparison with the axial length. Also, highly aberrated eyes may find a cross-over
of the focused spots. Although this is potentially a concern with some devices, it has
not been found to be of concern with others, even in capturing the most complex
wavefronts (Pettit et al., 2004 and Reinstein et al., 2004).
Lenslet Array
Samples Pupil
CCD Records
Point Image of Each Lenslet
Fig. (9): Principle of Shack-Hartmann wavefront sensing, where a lower energy laser
light reflects off the retinal fovea, passing through the optical structures of the eye to
create the outgoing wavefront. The wavefront passes through a lenslet array to define
the deviation of focused spots from their ideal (Krueger, 2004).
and ray tracing)
(Tschernig
Retinal imaging aberrometry
Tschernig
The next type of wavefront sensing was characterized by
in 1894,
when he described the monochromatic aberrations of the human eye. However,
Tscherning's description was not supported by the leaders of ophthalmic optics,
Gulstrand,
including
and was not favorably received. It was not until 1977 that
Howland and Howland used Tschernig'
8
2
aberroscope design together with a cros-
REVIEW OF LITERATURE
cylinder lens to measure the monochromatic aberrations of the eye subjectively. This
same idea was more recently modified by Seiler using a spherical lens to project a 1mm grid pattern onto the retina (Mierdel et al., 1997).
This device, together with a para-axial aperture system, could visualize and
photographically record the aberrated pattern of up to 168 spots as a wavefront map.
This 13 13 spot grid of laser light is projected through a 10-mm corneal area and
represents an analysis of approximately 100 spots within a 7-mm pupillary area
(Krueger, 2004).
Fig. (10): Principles of Tscherning aberrometry
The previous Fig. (10) show that with low-energy laser light as a 13-13 spot grid,
passing through 10 mm of the cornea and defocusing to an aberrated grid of 1 mm on
the retina (Krueger, 2004).
The potential limitation of this type of wavefront sensing is the use of an
However, the Gulstrand
(Gulstrand
idealized eye model
Model I) to perform the ray tracing computation.
model, which varies with refractive error, is modified within
the commercial device according to the patient's refractive error to maintain an
accurate assessment of the axial length. An alternative form of retinal imaging has
been introduced over the past several years by a Ukrainian scientist, Vasyl Molebny.
Tracey retinal ray tracing is a slightly different form of retinal imaging in that it uses
29
REVIEW OF LITERATURE
a sequential projection of spots onto the retina, in a "Gatling gun" fashion, that are
captured and traced to find the wavefront pattern (Molebny et a!., 1997).
Fig. (11): Principles of Tracey Ray Tracing aberrometry whereby an individual laser
ray is very rapidly scanned through the multitude of entry points within the pupil onto
the retina. The retinal spots can be traced within 12 milliseconds and converted to a
wavefront pattern on the pupillary plane (Krueger, 2004).
Up to 95 sequential retinal spots can be traced within 12 milliseconds and converted
to a wavefront
pattern on the pupillary plane. The Tracey technology also has the
flexibility to analyze a greater number of spots within a critical area, expanding the
specificity of resolution, especially in highly aberrated eyes (Krueger, 2004).
Ingoing adjustable refractometry (spatially resolved refractometer)
The third method of wavefront sensing is based on the seventeenth-century
Smirnov
principle of Sheiner and was described by
in 1961 as a form of subjectively
adjustable refractometry. Peripheral beams of incoming light are subjectively
redirected toward a central target to cancel the ocular aberrations from that peripheral
point. This method was modified by Webb, Penny and Thompson in 1992 as a
subjective form of wavefront refractometry of the human eye ( Webb et al., 1992).
30
REVIEW OF LITERATURE
Retinal
Corneal Plane, CP:
Plane. RP:
Fig. (12): Principles of spatially resolved refractometry:
In Fig. (12) a peripheral light source projected through a wheel (mask) can be
subjectively redirected by a joystick to overlap a reference point in the center of the
pupil. The movement varies the angle of incidence of the peripheral ray and can be
used to characterize the wavefront error at that point mathematically. Multiple
peripheral rays are tested to construct the wavefront profile (Krueger, 2004).
The "InterWave" (Emory Vision, Atlanta, Georgia) device commercializes
the SRR technology using approximately 37 testing spots that are manually directed
by the observer to overlap the central target in defining the wavefront aberration
pattern. Although this technology is unique and potentially beneficial in its capacity
to verify subjectively the aberration seen by the patient, the limitation of this
technique is the lengthy time required for subjective alignment of the aberrated spots
(Krueger, 2004).
31
AM"
REVIEW OF LITERATURE
aberomty
Double pass
(slit skiacopy)
The final method of wavefront sensing is based on methods of double pass
abeiTomtry
or retinoscopic abeiTomtry
that consider both the passage of light into
the eye and the reflection of light out of the eye in defining the wavefront pattern. Slit
retinoscopy (skiascopy) rapidly scans a slit of light along a specific axis and
orientation. The fundus reflection is then captured to define the wavefront aberration
pattern onto a parallel array of photodetectors, and a pair of perpendicular
photodetectors defines the orientation. A total of four spots can be defined at each
meridian of scanning, with 360 meridia scanned (at each degree of 360) for a total of
1440 data points (MacRae & Fujieda, 2000).
The temporal delay of photo voltage peaks from the photodetector signifies the points
of wavefront information. Although this technique, used by the Nidek OPD scan
(Gamagori, Japan), is also sequential at various axes, the objective capture of the
reflex makes it possible to acquire the information in a rapid sequence. The potential
limitations of this technology include the small amount of information collected
axially within a given meridian (4 spots) and the sequential nature of the capture
(Krueger, 2004; Thibos, 2004).
32
REVIEW OF LITERATURE
Scanning slit light
Reflection light
Myopia
Photo Voltage of Photo Detectors
t
I
Fig. (13): Principles of slit skiascopy used in the Nidek OPD scan.
In the previous Fig. (13) a scanning slit of light enters the eye and is reflected out
with a temporal delay. An array of photodetectors captures the temporal delay, which
defines the refractive or aberration error specific to that meridian. Multiple meridia
characterize the total ocular wavefront profile. (Krueger, 2004).
REPRESENTATION AND INTERPRETATION OF WAVEFRONT DATA
Data from wavefront sensing devices can be displayed in a variety of ways:
The first type is the colorized refractive map, which simply displays the
refractive en or of the eye over the entire entrance pupil. The wavefront device
3
-
♦
REVIEW OF LITERATURE
can also provide a spherocylindrical value for the average refractive correction
that might be required. In this type, wavefront devices show great promise as
autorefractors.
♦ The second type of display is a wavefront error map, which shows the deviation
of light from that a perfect optical system. This is a pseudo-color map that
provides a quantitative representation of the abberations of the measured eye.
Measurements are in actual microns of light, not in microns of tissue.
♦ The third type of display is the Hartmann-Shack pattern of the Hartmann-Shack
device. The clarity of the focus of the grid of spots indicates the quality of the
measurement. Variations in the position, size and shape of the dots provide
additional qualitative information regarding the active prosperities of the eye in a
given region; however, only the spot position needs to be known for wavefront
error calculation (Koch, 2001).
Color-coded map:
Information presented by the maps includes the following:
- Modulation transfer function (MTF). This parameter is the objective equivalent of
the contrast sensitivity and supplies a measurement of the quality of the image.
- Point spread function (PSF). This parameter expresses the effect of the aberrations
on the retinal image and consequently on the quality of the image.
- Phorpte
predicted refraction (PPR). This parameter corresponds to a translation
of the wavefront into a spherocylindrical value similar to the refraction obtained with
the phoropter.
- Quantification of each polynomial. This information is shown by various graphics.
- Root mean square (RMS) or mean sum of the square. This parameter represents
the mean residual square root of the total aberration and permits numerical
34
REVIEW OF LITERATURE
quantification
antification of the error and calculation of to what degree the surface examined
deviates from the perfect reference surface (Solomon et al., 2004).
Ideal Optic
System
Measured Optical
System
Fig. (14): The color coded map for the wave front image (Solomon et al., 2004).
-
COMPARISON BETWEEN METHODS OF WAFEFRONT SENSING
Although a large number of techniques are now available, little work has been
done so far to assess their equivalence or to establish which technique is best suited
for a particular situation. Three different techniques, Laser Ray Tracing (LRT), the
spatially Resolved Refractometer (SRR), and the Hartmann-Shack sensor (H-S), have
been compared (Moreno Barriuso et al., 2001). The comparison between the three
-
types proved that:
♦ LRT and HS, are objectives, but SRR is psychophysical.
♦ LRT and SSR, are Sequential, but HS is Parallel. Thus, The application of LRT
and SSR is limited to real time measurement; however, HS is the fastest
technique (1 to 2 seconds).
♦ All three techniques provide similar information concerning the wave abberation,
when applied to normal human eyes.
♦ H-S works in parallel with a fixed sampling pattern and measures the abberations
in the outgoing (second pass) aerial beam, whereas LRT and SSR are sequential
II
3J
REVIEW OF LITERATURE
with a highly flexible sampling pattern and measure aberrations of the incoming
beam (first pass) at the retinal surface. These two important basic differences
imply that they are going to show each different pros and cons, depending on the
particular application.
presents clear advantages in dynamic measurements, while LRT and SSR
H-S ♦
have advantages in clinical applications where one expect to find high values of
abberations (Moreno Barriuso et al., 2001).
-
Table (3): Comparison between Laser Ray Tracing (LRT), the spatially Resolved
Refractometer (SRR), and the Hartmann-Shack sensor (H-S) (Moreno Barriuso et al.,
-
2001).
LRT
Objective
SRR
HS
/ Objective
Psychophysical
Objective
/ Sequential
Sequential
Parallel
543
543
4
4
psychophysical
Parallel
sequential
543
Wavelength
I
(nm)
of
runs 4
averged for each
3 or 4 min.
—
5s
—
Duration of the
—
subject
2s
measurement
Effective
pupil 6.6
7.2
6.6
First
Second
sampled (mm)
Measurement in First
first / second
pass
36
"71rs'
■1
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DIAGNOSTC USE OF WAVEFRONT IN REFRACTIVE SURGERY
Diagnostic use of wavefront sensing before refractive surgery
The possibility of analyzing refractive errors in a wider more accurate way has
had a primary impact on the field of refractive surgery. Wavefront sensors can be
considered as "advanced autorefractometers" that are able to provide a more precise
measurement of not only aberrations that can be measured with traditional methods
(sphere, cylinder) but also higher-order aberrations that otherwise could not be
measured. The additional information they provide may be useful in selecting the best
procedure when considering surgery to correct refractive errors. Details such as the
amount of pre-existing higher-order aberrations and their relationship with pupil size
may be sensitive factors influencing the choice of a customized ablation instead of a
standard one. ( Carones et al., 2004).
Eyes presenting with preoperative higher-order aberrations may not fully
benefit from the correction of sphere and cylinder, especially when the goal in these
patients is to improve quality of vision or reduce visual symptoms related to higherorder aberrations. In normal virgin eyes, rarely, the amount of preexisting higherorder aberrations is large enough to determine visually significant quality of vision
problems. Such instances may represent specific cases, particularly eyes with a very
large pupil size under scotopic conditions. Eyes presenting with significant
asymmetric astigmatism that cannot be fully corrected with spectacles may have
i mproved best spectacle-corrected visual acuity and reduced night-vision problems
after a customized ablation targeted at correcting the asymmetric astigmatisminduced coma (Carones et aL,
2003).
Unfortunately, for low levels of aberration, the most commonly used index to
quantify higher-order aberrations (root mean square wavefront error or RMS) does
not seem to be a good predictor of visual acuity; therefore, there are no clear
indications to define those eyes that may truly benefit from the reduction of pre37
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REVIEW OF LITERATURE
existing higher-order aberrations. The most reasonable indication is to perform
customized ablation in all candidate eyes, at least to reduce the chance of inducing
new higher-order aberrations (Applegate et al., 2003).
Diagnostic use of wavefront
sensing after refractive surgery
Any refractive surgical procedure may generate higher-order aberrations that
can be detected with the aid of wavefront sensing only. Corneal refractive procedures
are more likely to produce higher-order aberrations as a consequence of the change of
the corneal shape, and their impact on vision is related to several factors, the most
important of which are the type of procedure, the amount of change in corneal shape,
the optical zone size, and the wound healing response. All of these factors are pupil
size - dependent; that is, the larger the pupil, the worse the result. Conventional laser
ablation of corneal tissue generates an increase in spherical aberration for the most
part (fourth-order), whereas flap creation and repositioning are more likely to
increase third-order aberrations, such as coma, and aberrations higher than fourthorder, such as secondary astigmatism, as well as the effects of the wound healing
response. Wavefront-based custom laser ablation not only reduces pre-existing higher
order aberrations but also avoids inducing systematic increases in higher-order
aberrations, mainly spherical aberration. Incisional surgery may generate
biomechanl
unpredictable increases in different higher-order aberration terms owing to the
effects of the incisions and wound healing response (Carones et al.,
2004).
Wavefront analysis after refractive surgical procedures helps the surgeon better
understand the reason for vision complaints by the patient. Visual symptoms as
generated by an increase in higher-order aberrations are closely related to the type of
induced aberration. Spherical aberration is usually reported as a peripheral halo,
particularly when fixating lights at nighttime, and pupil constriction, either lightinduced or pharmacologic, reduces most of this symptom. Coma induces disturbances
38
I
REVIEW OF LITERATURE
referred to as double vision, with multiple fading images; a bright light is referred to
as a comet. Secondary astigmatism induces ghost and double images, which are
particularly difficult to deal with. These latter higher-order aberration terms benefit
less from pupil constriction (Carones et al., 2003).
The diagnostic use of wavefront analysis after refractive surgery becomes
crucial when planning for a second procedure, that is, enhancement or retreatment.
Patients may complain of unsatisfactory results mainly for two reasons: (1) errors in
or overcorrection of sphere
(undercoti
achieving the final targeted refraction
and cylinder), or (2) an increase in higher-order aberrations generating visual
symptoms. These two factors may be combined (which is the most common
situation). Wavefront analysis helps greatly in discriminating between these two
components and understanding the reasons for visual complaints by patients, which
may be related to the spherical or astigmatic offset in addition to other changes.
Based on the wavefront analysis results, the surgeon may choose the most appropriate
procedure to fix the problem, such as a customized ablation for eyes showing higherorder aberration related symptoms (Carones et al., 2004).
EFFECTS OF WOUND HEALING ON WAVEFRONT ANALYSIS
Wavefront analysis is commonly performed preoperatively to map the
aberrations of the eye and guide custom laser treatment. In addition, wavefront
analysis should be performed postoperatively to evaluate changes in the higher-order
aberrations following the laser procedures and correlation with the postoperative
(Chalit
visual symptoms
et al., 2002). Wavefront analysis usually provides
important information regarding the visual results and should be interpreted in
et al., 2003).
(Sehwigrln
conjunction with corneal topography
Wavefront analysis is typically more accurate in normal eyes of refractive
surgery candidates. However, under certain circumstances, the higher order
aberrations acquired to guide the laser treatment may be different from those that are
39
-
REVIEW OF LITERATURE
■•
1
■•
actually present at the time of the ablation. This may happen, for example, when a
flap is created during the LASIK procedure. Flap creation causes unpredictable
changes in the higher-order aberrations, which may vary according to flap diameter,
thickness, and hinge location, but may also be secondary to variations in wound
healing and biomechanical effects. Typically the wavefront changes induced by flap
formation are modest, but they are highly significant in some eyes. (Porter et al.,
2003 and Wang et al., 2003).
of the cornea. It also triggers a
wound healing response due to the epithelial trauma produced by the
microkeat
biomechans
Flap creation certainly affects the
blade and epithelial debris dragged into the lamellar interface. Ideally, all of these
changes secondary to the biomechanical effects and wound healing would be
considered during the wavefront analysis and subsequent treatment. However, it is
impossible to predict the intensity of these highly variable effects in individual eyes.
For example, the level of myofibroblast generation is highly variable, depending on
the type and level of injury, and has the potential to alter corneal shape and hence
wavefront analysis (Netto & Wilson, 2004).
During the first days after LASIK, LASEK, or PRK, wavefront analysis should
be avoided, because it will be affected by transient tissue edema and corneal surface
microirregularities, resulting in inaccurate measurements. Corneal cellularity and
function slowly return toward normalcy after refractive surgical procedures.
Keratocyte apoptosis, necrosis, and proliferation diminish markedly approximately 1
week after surgery. Most immune cells also disappear from the cornea, probably via
apoptosis or necrosis, by 1 week after surgery. However, some components of this
normalization process may take months to years, depending on the type of surgery
and the outcome. Specifically, the alterations secondary to the myofibroblasts'
remodeling, may take many months (Mohan et
aL,
stromal
generation, involving
2003).
40
REVIEW OF LITERATURE
Typically at the third month after surgery most of the corneal changes will
have returned to normal, and all corneal structures will be more stable, providing
more reliable wavefront measurements. Effects of wound healing on customized
treatments. Current refractive surgery procedures, such as PRK, LASIK, and LASEK,
induce different patterns of cellular wound healing response that are likely
determinants of the clinical variations in refractive outcomes (Nett° & Wilson, 2004).
The results after wavefront-guided treatment are directly dependent on these
postoperative corneal biological events. A meticulous corneal sculpture is attempted
with customized treatments, but unexpected patterns of healing may mask these
effects, because custom features are often measured in terms of a few microns, a size
far less than the diameter of a single corneal epithelial cell (Nett° & Wilson, 2004).
custom features
hyperlasi
Fig. (15): Epithelial hyperplasia may mask features produced by custom corneal
ablation.
In this schematic drawing there are exaggerated custom features ablated into the
stroma in the upper panel. The lower panel shows how epithelial hyperplasia may
(Neto
mask these stromal features
& Wilson, 2004).
41
Stromal
REVIEW OF LITERATURE
remodeling and epithelial hyperplasia are thought to be the most
important mechanisms that lead to regression of the refractive effect following laser
surgery and are responsible for the creation of new corneal aberrations. The responses
are also likely to be the major factors that interfere with custom corneal features
(Neto
intended to produce alterations in the aberrations of the eye
& Wilson, 2004).
Laser correction of the lower-order aberrations is also affected by these
biological events, but the higher-order aberrations are most susceptible to the effects
of corneal healing and alterations in corneal biomechanics, due to their subtle nature
& Wilson, 2004).
(Neto
Several different variables can directly influence the pattern of wound healing
following refractive surgery. Individual variability in programmed cellular responses
plays a major role in variations in wound healing, but technical variations between
different procedures also account for significant differences. There are fundamental
differences in the intensity and overall pattern of wound healing following various
keratorefractive procedures. First, the minimal central epithelial trauma that occurs
with normal LASIK is believed to trigger a less intense cellular response (Mohan et
al., 2003).
In addition, the difference in the anatomical elements involved in the two types of
procedure, including the issue of preservation of the central epithelial basement
membrane, may be responsible for the differences between LASIK and PRK or
LASEK. The localization of apoptosis after surface ablation in PRK compared with
LASIK is also different and contributes to the difference in the overall wound healing
response ( Wilson et al., 2002).
In LASIK, keratocyte apoptosis and keratocyte proliferation occur above and
below the lamellar interface. In PRK, keratocyte apoptosis occurs in the superficial
stroma, and keratocyte proliferation occurs in the posterior and peripheral stroma.
Mohan et al reported higher levels of keratocyte apoptosis, keratocyte proliferation,
42
REVIEW OF LITERATURE
and myofibroblast generation following PRK, when compared with the wound
healing response following LASIK for the same level of correction in the rabbit
model (Mohan et al., 2003).
There is much less distance between the epithelium and the healing stroma in
corneas that have undergone PRK than in those that have undergone LASIK.
Consequently, cell interactions leading to myofibroblast generation and haze are
much greater after PRK (Wilson et al., 2002). Creation of the flap during LASIK
maintains a zone of normal cornea between the epithelium and the stromal wound
healing response, diminishing the interactions between the activated stromal cells and
the overlying epithelium (Mohan et al., 2003).
At the flap margins in LASIK there is a direct contact between the activated
stromal cells and epithelium. As a result, the healing response is much greater in the
periphery of LASIK, as can be noted clinically by observing the circumferential scar
at the edge of the flap. Complicated LASIK procedures, such as those with a very
thin or buttonhole flap, often have a wound healing response more akin to high PRK,
stromal
with central haze at the point where there is close interaction between the activated
cells and the overlying epithelium (Netto & Wilson, 2004).
The intensity of corneal response following injury is variable and related to the extent
of ablation. Keratocyte apoptosis, keratocyte proliferation, and myofibroblast
generation are significantly greater following PRK for high myopia than PRK for low
myopia (Wilson et al., 2002; Mohan et a!., 2003). Clinical studies confirm that there
PRK
is a higher incidence of regression after
for high myopia ( > 6 diopters) than
after the procedure for low myopia (Kim et al., 1996; Williams, 1997).
An intriguing question is why the myofibroblasts are generated in high PRK,
but not in low PRK. One possible explanation is that a deeper ablation results in a
quantitatively greater overall healing response, which leads to myofibroblast
g eneration. Another possibility is that deeper ablations affect posterior keratocytes
43
REVIEW OF LITERATURE
A third possibility is
stroma.
that respond differently from keratocytes in the anterior
that higher corrections tend to have greater surface irregularity, which could increase
the surface area of
stromal-epih
interactions. These biological differences are
directly related to the intensity of corneal remodeling after surgery and play a major
(Neto
role in the postoperative outcomes of customized treatments
& Wilson, 2004).
It is unknown whether the corneal wound healing response following LASEK
varies qualitatively or quantitatively from that observed after PRK. In LASEK, a
solution of ethanol is used to promote the separation between the epithelium and
stroma. There is no concentration of ethanol or exposure time that is optimal for
epithla-srom
every eye, because of individual variations in the
adhesion.
Consequently, variability in the exposure time required to prepare the epithelial flap
results in differences in stromal hydration that are likely to affect the accuracy and
precision of laser vision correction. In addition, the ethanol may trigger necrosis of
the anterior keratocytes, especially at higher concentrations or with longer contact
time, and may thus alter the overall wound healing response. A delay in corneal reepithelialization noted in some eyes may result in a prolonged stromal exposure to
epithelium- and tear-derived cytokines, possibly leading to a stronger wound healing
response (Netto & Wilson, 2004).
It is possible that new technologies used to create the corneal flaps, such as the
epi-mcrokat
femtoscecond laser and
(used to create epithelial flaps), will offer
advantages in terms of favorable wound healing responses, contributing to greater
reproducibility and effectiveness of custom corneal ablation. However, further studies
will be needed to establish these advantages (Pallikaris et al., 200 Sand Ratkay-
Traub et al., 2003).
44
REVIEW OF LITERATURE
COMPANY AND THEIR LASER PLATFORM
The wavefront device or aberrometer is the instrument used for the study of
errors and total aberrations of the eye. Aberrations in the human eye are as unique as
a person's fingerprints. The analysis of the emerging wavefront, using currently
available aberomts,
can be based on different principles (Solomon et al., 2004).
Hartmann-Shack system:
This sensor uses a small ray of light that is projected onto the retina and
reflected back through the pupil. The reflected light is focused by a lenslet array, and
a video sensor captures the array of spot images. The location of each spot is captured
by the sensor and compared with its theoretical location in an aberration-free system;
the combined relative offsets of the points provide a wavefront aberration map
et al., 1997; Howland, 2000).
(Liang
Aberrometers based on this principle include the COAS by Wavefront Sciences,
(Albuquerque, New Mexico); the Topcon KR-9000PW Wavefront Analyzer by
Topcon America, (Paramus, New Jersey); the LADARWave by Alcon Laboratories
(Ft. Worth, Texas), the WaveScan by VISX (Santa Clara, California), the Zywave by
Bausch & Lomb Surgical (San Dimas, California), and the ORK Wavefront Analyzer
by Schwind (Kleinostheim, Germany) (Solomon et al., 2004).
Tscherning system:
The Tscherning system is similar to the Hartmann-Shack system; however, the
incident light beam passes through a perforated lens. It uses a frequency-doubled
neodymium: yttrium-aluminum-garnet (Nd:YAG) laser emitting light at 532 nm. A
video recorder registers any deformation of the reflected pattern, analyzes it, and
returns the image to a reticulum. Analyzing the image and comparing the aberrated
45
REVIEW OF LITERATURE
retinal grid pattern with the ideal grid positioning quantifies the optical aberrations at
the level of the entrance of the pupil. The slope of the difference between the actual
i mages compared with the ideal grid represents the wavefront error. Tscheming
aberrometry is used in the Allegretto WaveLight Analyzer distributed by WaveLight
Laser Technologies AG (Erlangen, Germany) ( Liang & Williams, 1997).
Tracey system:
This device measures an ingoing light that passes through the optical system of
the eye and forms an image on the retina. It measures one ray at a time in the entrance
pupil rather than measuring all of the rays at the same time like previously mentioned
devices. This design decreases the chance of crossing the rays in highly aberrated
eyes. The total scanning time is 10 to 40 ms. the instrument processes a complete
refractive map of the eye and a distortion map of the wavefront. Ray tracing
aberrometry is used in the Tracey-VFA distributed by Tracey Technologies (Houston,
Texas) (Mac Rae & Williams, 2001).
Scanning system or optical path difference scans.
This technology resembles the examination in a skiascope. The system scans a
large number of points and, different from other systems, examines the relationship
between the light source and the reflected component. This diagnostic instrument
combines autorefractometry, corneal topography, and analysis of the wavefront to
create a single map of the corneal surface's refractive power. This principle is used in
the Nidek OPD-Scan distributed by Nidek (Fremont, California) (Maeda, 2001;
Doane & Slade, 2003).
Following analysis, all of the systems produce a graphical image with
chromatic scales, the aberrometric maps. These images resemble topographical maps.
They describe the difference in microns between a light wavefront and a reference
wavefront (Solomon et al., 2004).
46
REVIEW OF LITERATURE
Current wavefront sensing devices
used by the authors include
Aberomts
the LADARWave (Alcon Laboratories), WaveScan (VISX), and Zywave (Bausch &
Lomb Surgical), which use the Hartmann-Shack principle (outgoing reflection
aberrometry). Although based on the same optical principles, the devices differ
slightly in their range of capabilities. These differences are important in the ability to
capture wavefronts (Solomon et al., 2004).
Table (4): Characteristics of three Hartmann Shack wavefront sensors (Solomon et
-
al., 2004).
Characteristic
LADARWave
WaveScan
Zywave
I mage acquisition
Hartmann-Shack
Hartmann-
Hartmann-Shack
sensor
Shack sensor
sensor
Line of sight, middle
Line of sight
Line
Reference axis
of natural pupil
of
sight,
middle of natural
pupil
Accommodative
Automatic
Automated
Automatic
6th
5th
fogging
Maximum
order
of 8th
aberrations measured
Pupil size range (mm)
2.5to
8.00
+ 6.00
3.0to
+ 15.00
8.00
5.00
10.0
15.0to
15.0to
Maximum cylinder (D)
+ 15.00
8.0to
Range of sphere (D)
6.0
2.5to 8.0
Wavefront images can be used for diagnostic and therapeutic uses. The information
can also be used to localize aberrations, especially when used in association with
corneal topography systems. Clinically, correction of the spherocylindrical error
alone in some eyes is insufficient to produce an optimal result, because some of the
aberrations present can cause irregular astigmatism. Integration of the aberrometric
finding of the wavefront devices with laser treatment (personalized corneal ablation
or wavefront-guided LASIK) can help correct all of the significant aberrations in the
eye (irrespective of whether they are spherical, cylindrical, or HOA) by increasing
47
REVIEW OF LITERATURE
the resolution of the retinal image and contrast. Correcting HOA in addition to
defocus and astigmatism is advantageous if the entrance pupil is large and in
situations of low light and specific accommodative situations. (Solomon et al., 2004).
LADARVision system
The CustomCornea platform (Alcon, Orlando, Florida) is a wavefront-guided
treatment technology. Although laser vision correction with the LADARVision
system (Alcon, Orlando, Florida) has been commercially available for several years,
the wavefront-based method is relatively novel. The LADARWave device uses the
Shack-Hartmann approach to measure the aberrations in the eye. The ShackHartmann measurement produces a camera image showing a pattern of dots, which
are the individual wavefront pieces divided and focused by the lenslet array. The
device software measures the displacement of each focused dot from its ideal location,
that is, the pattern generated by a perfectly flat plane wave, and uses this information
to calculate the slope of the intact wavefront at each lenslet location. The software
then uses this slope data to generate a mathematical description of the original
wavefront profile. Although this technology is simple in concept, much care must be
aben
taken in building a clinical
-
ometr
that is reliable, easy to operate, and highly
accurate in measuring the wide variety of visual aberrations found among refractive
surgery patients (Liedel & Pettit, 2004).
48
REVIEW OF LITERATURE
analysis for an eye with moderate
wavefront
Fig (16): LADARWave system
keratoconus.
(A) Two-dimensional pseudocolor display of the wavefront profile. The eye is below
the plane of the figure. The color scale is shown on the left side of the panel, with the
units in microns. Red indicates the highest points in the profile, whereas blue shows
the deepest depressions. (B) Instantaneous power map calculated directly from the
wavefront axial curvature. The colors indicate instantaneous refractive power in
diopters, with red indicating the most hyperopic and blue the most myopic regions.
Both maps cover the same 7-mm diameter circle at the cornea and are based on an
eighth-order Zernike polynomial reconstruction (Liedl
49
& Pettit, 2004).
REVIEW OF LITERATURE
VEHRBP
AP
E1.
ENIVDF
V OF
HUZFE
E
D
ti
Ir
After Customern
P
U
CIP1
LASIK
After Conventional LASIK
Fig. (17): Optical simulation showing the difference in higher-order aberrations
present in the eye after Custom Cornea (A) and traditional LADARVision LASIK
surgery (B).
The difference in the amount of total higher-order aberrations remaining between the
ct
two groups 6 months after surgery equates to about 0.2 D of defocus (Liedel
Pettit,
2004).
Aberrometry: WASCA
Shack-Hrtmn
The WASCA aberrometer is based on the
wavefront sensing
principle .Briefly, it operates thus: a lenslet array collects incident light emerging
from the eye. Each lenslet then creates a focal projection onto a CCD camera array .
The position of the spots on the CCD array relative to a reference location (as would
be produced by a flat wavefront) is used to determine the actual wavefront slope of
the incident light onto a particular lenslet .The combination of this array of slopes in a
topographic manner leads to the calculation and digital reconstruction of the incident
wavefront. This wavefront can be either displayed directly from the raw data by"
zonal reconstruction) "as a result of the very high resolution of the WASCA) or
broken down into a collection of shapes of varying amplitude (eg, the Zernike
expansion series). The authors will now delineate some of the specific design features
50
REVIEW OF LITERATURE
of the WASCA that provide for increased accuracy and reproducibility of wavefront
measurement in practice (Reinstein et al., 2004).
lenst
Fig. (18): Scanning electron' micrograph of a portion of the
array.
WASC
In Fig. (18) each lenslet is in fact square and comprises a depression of
approximately 1.6 mm with a diameter of 144 mm. Note the 100% fill (ie, no gap)
between lenslets (Reinstein et al., 2004).
The Carl Zeiss Meditec platform for custom ablation incorporates a suite of
aberomty
wavefront
technology for
(WASCA), corneal surface shape data
(TOSCA), sophisticated excimer laser delivery MEL80),
and surgeon-controlled
(
individualization of treatment protocol (CRS-Master). Together, these components
promise to deliver increasingly higher accuracy and control over corneal sculpting, a
the late Jose Ignacio Barraquer
dream come true for the father of keratomilus,
(Reinstein et al., 2004).
51
1
REVIEW OF LITERATURE
The Zyoptix system
The Bausch & Lomb Zyoptix (Rochester, New York) system includes the
Zyoptix Diagnostic Workstation (combining the slit-scanning technology of the
Orbscan system for true elevation measurement with the Zywave wavefront detector
based on the Hartmann-Shack principle), the Hansatome microkeratome, and the
Technolas 217Z excimer laser. Data in the Zyoptix system are measured at the newly
integrated Dual Head Workstation composed of the Zywave aberrometer and the
Orbscan. These data are then assimilated into a customized ablation using Zylink
software and stored as a treatment file on a disk that is kept with the patient's records.
The treatment data are then uploaded into the Technolas 217Z laser, and Zyoptix
(LASIK) surgery is performed (Gulani et al., 2004).
keratomilus
The Nidek OPD
-
Scan
laser in situ
The Nidek OPD-Scan is a diagnostic instrument that combines dynamic
skiascopy technology and placido disk corneal topography to create unique refractive
power maps that show the total optical system aberrations. It produces diagnostic
image maps to provide high resolution (1440 measurement points for total aberration
and autorefractor measurements and 6480 points or more for corneal topography
measurements), a wide measurement range (-20 to +22 D, with up to 12 D of
cylinder), and a common axis of measurement for optical aberrations and corneal
topography. It also incorporates autorefraction, allowing simultaneous data
acquisition for all measurements in timeline and axis. The Nidek OPD-Scan
EC-50X/
combined with the Nidek
CXII laser form part of the Navex platform
using the Final Fit algorithm. This platform is used outside the United States
(Solomon, 2004).
52
REVIEW OF LITERATURE
The WayeLiOt
Analyzer
The Wave Light Analyzer measures low- and high order aberrations using the
Tscherning principle. A 660-nm diode laser class 2 source collects 800 data points
over a 7.0-mm pupil. The dynamic range of measurement is -12 to +6 D of sphere
and up to -4 D of cylinder, treating up to sixth-order aberrations. Using the software
Allegretto Wave A-Cat, the Wave Light Analyzer can perform wavefront-guided
LASIK with the Allegretto Wave excimer laser. This use has recently been approved
in the United States to treat patients who have nearsightedness up to -12 D and
astigmatism up to -6 D, who are 18 years of age or older, and whose eyesight has
been stable (by 0.5 D) for the year before the preoperative examination (Solomon,
2004).
ADAPTVE OPTICS OPHTHALMOSCOPY
Adaptive optics is the concept of internationally designing the optics of the
observatory system to compensate for the measured abberations of the subject. As a
result one can observe the objects with minimal abberations because of the reduction
of total abberations of the subject and the observatory system. The wavefront sensor
measures the abberations of the subject, and this information is processed and sent to
actuators. The shape of the deformable mirror is controlled by actuators to reshape
the surface of the thin mirror to compensate for the abberation of the subject.
Examples of the possible applications of adaptive optics in ophthalmology are the use
of the fundus camera with adaptive optics, the measurements of visual function
responsible for retinal and central nervous system, and wavefront-guided refractive
surgery (Maeda, 2001).
Adaptive optics was first suggested in 1953 by astronomer Hoarce Babcock to
remove the blurring effects of turbulence in atmosphere on telescopic images of stars.
41
53
1
REVIEW OF LITERATURE
This information would allow vision scientists to apply this technology to better
wilan,
understand the eye's optic and retinal image quality (MacRae &
2001).
Liang et al., used the Shack-Hartmann wavefront sensor to detect the eye's
abberations and then applied an adaptive optics deformable mirror to correct the eye's
lower and higher order abberations. They noted that adaptive optics provide a six fold
increase in contrast sensitivity to high spatial frequencies when the pupil was large.
This study was first to demonstrate that the correction of higher-order abberations can
(Liang
lead to supernormal visual performance in normal eyes
et al., 1997).
Liang et al., used monochromatic light. Normal viewing conditions usually involve
broadband light, and retinal images formed in broadband (white) light are blurred by
chromatic abberations, as well as the monochromatic abberations that adaptive optics
can correct. Yoon and Williams showed that in broadband light that characterizes
normal viewing conditions, adaptive optics still provides a twofold increase in
contrast sensitivity at high spatial frequencies in typical eyes, even when chromatic
abberation is present (MacRae & williams, 2001).
CLINICAL EXPERIENCE WITH WAVEFRONT GUIDED ABLATION
Seiler et al., reported the first application at wavefront-guided LASIk using the
wavelight Allegretto excimer laser (Wavelight Laser Technologies AG, Erlangen,
Germany). The early results were encouraging. In the USA, McDonald performed the
first wavefront-guided LASIK with LADARVision system in a comparative study by
examining the effect of conventional LASIK in one eye and the wavefront-guided
LASIK in the other eye for myopia and hyperopia. Although the wavefront-guided
LASIK produced similar uncorrected visual acuity compared with conventional was
found in some cases. Also, VISX and other laser companies have begun clinical trials
to evaluate wavefront-guided ablations. The safety and efficacy of this procedure
should be reported soon (Maeda, 2001).
54
.
11.
!
REVIEW OF LITERATURE
Nowadays, several studies appear to evaluate the outcome of different
wavefront-guided corneal ablations, and to evaluate the variable wavefront sensing
devices. There are a certain studies aimed to highlight the mathematical, optical,
cellular and clinical basis of wave-front guided corneal ablation.
Of these studies, Bueeler et al., investigate the lateral alignment accuracy needed in
wavefront-guided refractive surgery to improve the ocular optics to a desired level in
a percentage of normally aberrated eyes, the effect of laterally misaligned ablations
on the optical outcome was simulated using measured wavefront aberration patterns
from 130 normal eyes. The calculations were done for 3.0 mm, 5.0 mm, and 7.0 mm
pupils. The optical quality of the simulated correction was rated by means of the rootmean-square residual wavefront error. A lateral alignment accuracy of 0.07 mm or
better was required to achieve the diffraction limit in 95% of the normal eyes with a
7.0 mm pupil. An accuracy of 0.2 mm was sufficient to reach the same goal with a
3.0 mm pupil. Bueeler et al., 2003 concluded that the procedures must be developed
to ensure that the ablation is within a tolerance range based on each eye's original
optical error. Rough centration based on the surgeon's judgment might not be
accurate enough to achieve significantly improved optical quality in a high
(Buelr
percentage of treated eyes
et al., 2003).
To examine the effects of laser in situ keratomileusis (LASIK) flap incision
and healing on the shape of the cornea and the wavefront error of the eye.
Schwiegerling et al., stated that differences in corneal shape and wavefront error
consistent with a mild hyperopic shift were seen as a result of the keratome incision.
Thus, Cutting the flap in LASIK causes subtle changes to corneal shape and the
optics of the eye that may affect customized treatments. Additional work is needed to
quantify these changes so that their effect can be incorporated into future treatments
et al., 2002).
55
(Schwiegrln
REVIEW OF LITERATURE
The clinical experience of Gimbel et al., regarding enhancement (retamn)
guide
of previously performed non-wavefront-guided refractive surgery by wavefrontmultipoint (segmental) custom ablation utilizing the Nidek NA VEX platform,
prove that Topography and wavefront-guided multipoint (segmental) custom ablation
enhancements were safe and effective in improving visual symptoms following
primary refractive surgery. In some eyes, improved visual function without
correspondingly lower RMS of HOA values may be an effect of neutralizing some
chromatic aberrations across the visible light spectrum, thereby improving the
modulation transfer function (Gimbel et al., 2003).
Nikano et al., reported preliminary results of laser in situ keratomilus
(LASIK) using customized ablation and the Nidek OPD-Scan. They report a
prospective, non-comparative interventional case series. After OPD-Scan analysis,
patients underwent LASIK. Eighty-four eyes were included: 44 eyes were treated
with Flex Scan, 22 eyes with customized ablation based on Nidek OPD-Scan analysis,
and 18 eyes were treated with conventional (scanning slit) ablation. Visual outcome
did not differ among groups. No patient experienced a significant decrease (more
than 1 Snellen line) in best spectacle-corrected visual acuity. Nikano et al., concluded
that LASIK with the Nidek OPD-Scan system was safe and effective in this small
group of patients. (Nikano et al., 2003).
Chalita et al., described the surgical outcome of a patient who had a previous
and 3 months later, had
buttonhole after laser in situ keratomileusis (LASIK)
(PRK) with topical mitomycin C
wavefront-guided photorefractive keratcomy
0.02%. One month after wavefront-guided PRK, his uncorrected visual acuity was
20/25 in the right eye, with no symptoms. Best spectacle-corrected visual acuity in
the right eye was 20/15 with +0.25 -0.50 x 110 degrees. No haze or scar was seen on
slit-lamp examination. Wavefront analysis showed a decrease in higher order
(Chalit
aberrations, especially coma and spherical aberration
et al., 2004).
56
"MI
REVIEW OF LITERATURE
LADRVison40
Carones et al., evaluated the clinical results of Alcon
wavefront-guided customized treatment of eyes with myopia and/or astigmatism, and
clinically significant visual symptoms related to the presence of higher order
LADRVison
aberrations. Wavefront-guided custom ablation with Alcon's
4000
was effective in reducing higher order aberrations and related visual symptoms in this
preliminary small series. Longer follow-up on more eyes is necessary to assess the
accuracy of the algorithm in the correction of defocus, which resulted in a slight
aL,
overcorrection in this study (Carones et
2003).
Kinjani et al., designed a clinical study to evaluate the results of wavefrontand topography-guided ablation in myopic eyes using Zyoptix (Bausch & Lomb).
This observational case study comprised 150 eyes with myopia and compound
myopic astigmatism. Preoperatively, the patients had corneal topography with
Orbscan Iz
(Bausch & Lomb) and wavefront analysis with the Zywave aberrometer
keratomilus
(Bausch & Lomb) in addition to the routine workup before laser in situ
(LASIK). The results were assimilated using Zylink software (Bausch
keratomilus
& Lomb), and a customized treatment plan was formulated. Laser in situ
was performed with the Technolas 217 system (Bausch & Lomb). The
patients were followed for at least 6 months. The mean preoperative best corrected
+/-
visual acuity (BCVA) (in decimal equivalent) was 0.83
0.18 (SD) (range 0.33 to
1.00) and the mean postoperative (6 months) BCVA, 1.00 +/- 0.23 (range 0.33 to
1.50). Three eyes (2%) lost 2 or more lines of best spectacle-corrected visual acuity.
The safety index was 1.20. The mean preoperative uncorrected visual acuity (UCVA)
was 0.06 +/-
0.02 (range 0.01 to 0.50) and the mean postoperative UCVA, 0.88 +/-
0.36 (range 0.08 to 1.50). The efficacy index was 14.66. The mean preoperative
spherical equivalent (SE) was -5.25 +/- 1.68 diopters (D) (range -0.87 to -15.00 D)
+/-
and the mean postoperative SE (6 months), -0.36
0.931 D (range -4.25 to +1.25
D). At 6 months, the UCVA was 1.00 or better in 105 eyes (69.93%) and 0.5 or better
in 126 eyes (83.91%). The postoperative aberrations were decreased compared with
57
REVIEW OF LITERATURE
the preoperative aberrations. One eye (0.66%) had a free cap during LASIK with
higer-od
subsequent loss of 2 lines of BCVA and induced
aberrations (HOAs).
Nine patients (11.2%) complained of halos at night. Wavefront- and topographyguided LASIK leads to improve visual performance by decreasing HOAs. Scotopic
visual complaints may be reduced with this method (Kinja
et al., 2004).
Customized laser surgery attempts to correct higher order aberrations, as well
as defocus and astigmatism. The success of such a procedure depends on using a laser
beam that is small enough to produce fine ablation profiles needed to correct higher
order aberrations. In a study by Guirao et al., wave aberrations were obtained from a
population of 109 normal eyes and 4 keratoconic eyes using a Shack-Hartmann
wavefront sensor. Guirao et al., considered a theoretical customized ablation in each
eye, performed with beams of 0.5, 1.0, 1.5, and 2.0 mm in diameter. Then, they
calculated the residual aberrations remaining in the eye for the different beam sizes.
Retinal image quality was estimated by means of the modulation transfer function
(MTF), computed from the residual aberrations. Fourier analysis was used to study
spatial filtering of each beam size. The laser beam acts like a spatial filter, smoothing
the finest features in the ablation profile. The quality of the correction declines
steadily when the beam size increases. A beam of 2 mm is capable of correcting
defocus and astigmatism. Beam diameters of 1 mm or less may effectively correct
aberrations up to fifth order. So, Guirao et al., concluded that large diameter laser
beams decrease the ability to correct higher order aberrations. A top-hat laser beam of
1 mm (Gaussian with FWHM of 0.76 mm) is small enough to produce a customized
ablation for typical human eyes (Guirao et al., 2003).
On the other hand, some optical errors are too localized and random to be detected by
polynomial expression. Mrochen &
Zernik
commercial wavefront devices and
Semchishen, looked beyond aberrations defined by Zernike expression to discuss
i mplications of fine irregularities associated with highly aberrated corneal surfaces
and complex surface roughness that can lead to light scattering. Most fine
58
REVIEW OF LITERATURE
irregularities are related to postoperative surface roughness, complexities of corneal
(LASIK) flap. These can be
keratomilus
ablation, and the laser in situ
characterized mathematically by a random function that includes local surface tilts,
the correlation radius of irregularities (Ic), surface roughness, and other terms. The
Kirchoff method of scatter analysis characterizes fine surface irregularities by
replacing each point on the surface with a tangential plane, allowing it to be governed
by Snellen and Fresnel laws. Mrochen & Semchishen, concluded that the corneal
surface irregularities after laser vision correction may induce significant optical
aberrations and distortions apart from classical wavefront or scattering errors. As
these may not be detected by commercial wavefront devices, and yet contribute to the
degradation of optical performance, alternate techniques should be evaluated to detect
and describe these surface irregularities (Mrochen Semchishen, 2003).
59
REVIEW OF LITRATURE
TOPOGRAPHY GUIDED CUSTOMIZED ABLATION
Corneal topography plays an important role in the management of patients
undergoing refractive surgery. It is therefore vital for any eyecare professionals
involved in this field to have a good understanding of its principles and applications
2000).
(Cobet,
BASIC PRINCIPLES
The goal of computerized videokeratography (CVK), referred to clinically as
corneal topography, is to describe the shape of the cornea accurately in all meridians.
CVK is widely recognized as an important tool in diagnosing, treating, and
(Szcotka,
monitoring the cornea
2003).
The anterior cornea is the major refractive surface of the eye, and is
responsible for over two thirds of its total dioptric power .Therefore, very small
changes in corneal shape resulting from surgery or disease can have a dramatic effect
on the clarity with which an image is brought to a focus on the retina. (Cobett, 2000).
Measurements
There are several ways in which the shape of the cornea can be measured and
represented. Power (in dioptres) is a measure of the refractive effect of the anterior
corneal surface, and is therefore particularly useful in refractive surgery. Radius of
curvature is converted to power using the standard keratometric index (SKI=1.375)
this is an approximate figure derived from assumptions about the thickness and
refractive index of the cornea, and the shape of its posterior surface .Principles
Original methods for assessing corneal topography were all based on the principle of
reflection but more recently, techniques using projection have been developed. In
addition, wave-front analysis can now provide information about the refractive power
of the eye as a whole, rather than just the effect of the anterior corneal surface
(Waring, 1989).
60
REVIEW OF LITRATURE
The selection of a device for purchase, or the development of new devices in
the future, depends upon the precise. Each has its own advantages, and the use of the
most appropriate method for a given clinical situation can enhance the presentation
and interpretation of results (Roberts, 1996).
Projectin-basd
f
Reflction-brd
Topography Systems
cg;
Photke
ratoicpy
V ideokcratspy
interfc
Moir6
Rastercogphy
Laser iretfonly
Kcratenik
cg;
Topography Systems
Radius of Curvature
LOCO P
01bat
.1
nsateo
i
r
Axial r
Sagetr
I
mk-x
Kroac
Stnlarc
(SKI)
l
e
I
ale(ric
rower
Diopterc
Fig. (19): Data measurement and presentation by various corneal topography systems
(Cobett, 2000).
Height:
The fundamental way of describing any surface mathematically is to define the
distance of each of its points from a reference surface for example height above sea
level on a geographical map. Height maps with a flat reference plane show the overall
shape of the cornea. Finer detail can be provided by the use of a spherical reference
(Cobet,
plane, with the height being expressed as the difference from a sphere
61
2000).
REVIEW OF LITRATURE
Height measurements are particularly useful in excimer laser surgery, in which
the refractive effect is dependent upon the depth of tissue removed. Once the true
shape of the corneal surface has been measured in terms of height, then slope,
(Cobet,
curvature and power can be calculated from it
2000).
Radius of curvature (ROC):
Corneas with a steep surface slope have a small radius of curvature and those
which are flatter, have a relatively large radius of curvature. The radius of curvature
is usually expressed in mm, but can be calculated in two ways. Global (axial/sagittal)
radius of curvature measures the distance of points from the optical axis, and
therefore has a spherical bias. Local (instantaneous /tangential) radius of curvature
calculates the curvature at each point with respect to its neighboring points, and is
therefore more accurate for local irregularities and in the peripheral cornea (Roberts,
1996).
Power
Power (in dioptres) is a measure of the refractive effect of the anterior corneal
surface, and is therefore particularly useful in refractive surgery. Radius of curvature
keratomic
is converted to power using the standard
index (SKI =1.3375). This is an
approximate figure derived from assumptions about the thickness and refractive
index of the cornea, and the shape of its posterior surface (Cobett, 2000).
Hardware principles:
Over the last 15 years, a variety of CVK methods have been developed based
on different measuring principles. The topographic tools used routinely in clinical
settings are of two main forms: reflective devices and slit scanning devices. Placidobased CVK systems are reflective devices that use the cornea as a convex mirror. A
series of illuminated annular rings are projected onto the cornea. Using the corneal
tear film as a mirror, the reflected image of the rings is captured by a digital video
REVIEW OF LITRATURE
camera. The captured image is then subjected to an algorithm to detect and identify
(Szcotka,
the position of the rings relative to the videokeratographic axis
2003).
In Placido CVK technology, two types of projected ring systems prevail. Thin
ring projection systems use a peak luminance algorithm wherein the brightest portion
(the center of the ring) is identified as the border. Wide ring projection systems use a
border detection algorithm to detect both edges of each illuminated ring. Once the
borders are detected, a proprietary algorithm (a high-order polynomial equation) is
applied to the digital image, which reconstructs the corneal curvature. From the many
files that are created, a table of curvature values and data point locations are
calculated. These tables are then used to produce a familiar map of curvatures
presented in a color-coded format (Szczotka and Benetz, 2000).
Placido-based CVK devices have been tested frequently and are simple and
reliable methods to measure central and peripheral corneal curvatures. They are the
most popular systems in clinical practice (Wilson et al., 1992). Nevertheless, Placidobased CVK devices have several well-documented inaccuracies. These limitations
include problems with alignment sensitivity, estimation of the corneal apex, skew ray
calculations, and assumptions in formulas (Cairns, 2002).
Although sophisticated software enhancements are being developed to
minimize many of these issues, the inherent errors based on the measurement
technique will never be eliminated. Additionally, Placido-based CVK systems
acquire data from the anterior corneal surface only, leading to further estimations and
assumptions in formulas (as is true in simulated keratometry calculations, which
require knowledge of the posterior surface of the cornea). They also must assume
where the corneal surface lies in space when applying algorithms, which can lead to
inaccuracies that are greatest in the corneal periphery and when converting this data
to elevation maps (Applegate, 1995).
63
REVIEW OF
LITRAUE
Nonetheless, Placido-based technology continues to be a reliable technique,
and it is the most commonly used method to measure the cornea clinically. Slit
scanning methodology for elevation measurement is a significant addition and
advancement in the field of ocular topography. The Orbscan II system (Bausch &
Lomb, Rochester, New York) is an example of this technology. It combines a threedimensional scanning slit beam system for analyzing anterior and posterior surfaces
of the cornea an with a Placido disk attachment to acquire the front surface curvature
data. Orbscan elevation topography uses the principle of triangulation. The system is
calibrated with a flat surface placed at a known distance perpendicular to the
instrument axis (Szczotka, 2003).
When the instrument is subsequently used to measure a curved corneal surface,
the position of each slit is compared with the calibrated plate. ( Cairns, 2002). The
curved slit can be reconstructed proportionally to represent the various curved
elevations of the surface being measured. When combined with the Placido device,
the curvature can be reconstructed with fewer assumptions than when using Placido
technology alone, because it is known where each point of the three-dimensional
surface lies in space. The Orbscan II system can effectively and reproducibly measure
2001, and
Marsich&
(Touzea,
corneal shape and thickness in normal eyes
2000).
Bulitnore,
Software principles:
Software advancements in CVK have come a long way since the late 1980s,
when the axial display was the standard and often only topographic map available.
Today, elevation maps are becoming a common primary display, and multiple
curvature algorithms and their resulting displays are available. Mapping options
range from the selection of various color scales, curvature and height maps, serial
imaging displays, and difference maps to sophisticated refractive surgery simulations
2001).
64
Smolek
and disease detection programs (Maeda N, et al.., 194;and
and Klyce,
REVIEW OF LITRATURE
CORNEAL TOPOGRAPHERS
The corneal modeling system (CMS; Computed Anatomy, NY) was the first of
a growing number of devices for measuring corneal topography and this class of
machine employ in the videocapture of Placido disk images was known as
videokeratoscope. The Placido disk approach has been the most clinically and
commercially successful system (Klyce, 2000).
Table (5): Manufacturer, models, and methods of commercially used corneal
2000).
topographer. (Klyce,
Manufacturer
Model(s)
Method
Alcon Surgical
EyeMap EH-290
Placido
Alliance
Keratron CT; Scout
Placido
Dicon
CT-200
Placido
Euclid Systems
ET-800
Fluorescein
Medical
Mkts
profilometry
EyeSys/Premier
EyeSys 2000; Vista
Placido
EYETEK
CT-2000
Placido
Kera Metrics
CLAS-1000
Phase
modulated
laser topography
Atlas 991, 992
Placido
Medmont
E300
Placido
Oculus
Keratograph
Placido
B & L Surgical
Orbscan II
40 scanned slits &
Humphrey
Insturments
Placid()
B & L Surgical
Orbshot
Placido
PAR Vision Systems
CTS, Accugrid
Fluorescein
profilometry
Sun Contact Lens Co
SK-2000
Placido
C-SCAN
Placido
profilmety
Intraop, CTS
Technomd
Fluorescein
PAR Vision Systems
65
REVIEW OF LITRATURE
Technology
Tomey Technology
AutoTopographer
Placido
Topcon
C M-10 0 0
Placido
America
Corp
COMPARISON BETWEEN METHODS OF CORNEAL TOPOGRAPHY
As reported by Guarnieri and Guarnieri statistical and clinical comparison of
three different corneal topography systems: Placido-based (EyeSys),
rasterstereography (PAR), and slit-scan (Orbscan) was made. Measurements were
obtained from 221 eyes of 119 human subjects. Statistical comparisons of central
curvature, keratometric curvatures, and meridians between the three systems were
made. Good agreement was achieved between the rasterstereography and Placidokeratomic
base systems by measuring the central curvature in 19 eyes and the
curvature and meridians in 48 eyes (Guarnieri and Guarnieri, 2002).
The comparison between Slit-scan and Placido-based systems showed relative
agreement in central curvature in 101 eyes and in keratometric curvature and
meridians in 39 eyes. The results showed similarities in the axial curvature map of the
three systems. In the clinical part of the study, the three systems were able to detect a
clinical keratoconus case with an inferior-superior steepening. Because the Placidobased systems can't detect the central zone, they can't detect small central islands and
decentration after PRK. Axial curvature maps of non-Placido systems detected
pachymetr
central islands. Good agreement was found when comparing Orbscan and ultrasonic
(Guarnieri and Guarnieri, 2002).
The confocal scanning laser tomography of the cornea using a near-infrared
laser beam that is scanned across the anterior segment of the eye to acquire layer-bylayer section images of the cornea. The measurement is non-contact and does not
require a tear film. It thus can be used immediately before and after refractive surgery.
66
REVIEW OF LITRATURE
The instrument allows determination of the anterior and posterior corneal surface
characteristics, thickness and surface roughness of the cornea. Total acquisition time
for each measurement is less than 1 second. The imaging field size for the prototype
system is 3 mm by 3 mm. Such evaluation will serve to highlight any potential
Padnibh,
advantages compared with existing corneal topographic systems (Rao and
2000).
INTERPRETATION OF COLOR CODED TOPOGRAPHIC MAPS
Any topography map can be interpreted if it is approached systematically, and
the basic principles are applied if the topographic display is being studied in
conjunction with a patient, the name, date, eye and other details should first be
confirmed. The scale should then be noted to determine the type of measurement and
the step interval. Only then should the map itself be considered. Further information
can be obtained from the statistical data given with the map. Comparisons can be
made with previous topography examinations of the same eye, or with the other eye,
as normal corneas are often mirror images of each other. The scale of the maps to be
compared must be the same (Corbett, 2000).
Table (6): System for studying topography display (Corbett, 2000).
System for studying
topography displays
Check name, date, eye
(e.g
Scale
- type of measurement
height, curvature, power)
step interval
Map
taiscl
S
-
cursor box, indices)
information (e.g
Compare previous maps of the same eye
(check that the scale is the same)
.
Compare with topography of the other eye
(check that the scale is the same)
67
REVIEW OF LITRATURE
4, Fr
Siate.cl
s
ti-hs
inSf
cid
5 Map
tl'^i+
Ali
mistIc
e*,—■
Inter.
trig
Ii
ktucIio
I.
Fig. (20): Components of topographic display (Corbett, 2000).
Color scale options
Topography maps can be viewed with one of several color scaling options—an
absolute scale (also called a standard scale), a normalized scale (also called a color
map, auto scale, or autosize scale), and an adjustable scale (also called a customized
scale). Although these scaling options have been available for a long time, their
potential benefits and drawbacks are frequently overlooked. An absolute scale always
assigns the same color to a given data value and forces the data to fit within a
(Szcotka
predetermined dioptric range
, 2003).
For example, on curvature maps, the same color is always assigned to the same
dioptric interval of corneal curvature. Absolute scales are instrument specific, and the
range of curvature values may be vastly different from system to system. Some
systems have large ranges from 9 to 100 D and others use ranges typical of manual
keratometers. Because the scales are consistent each time the absolute map is
employed, a direct and rapid comparison can be made between the colors maps of
one eye and another or between the same eyes on two separate occasions. This
68
REVIEW OF LITRATURE
comparison avoids confusion and allows visual familiarity for the user. For the RGP
fitter, the advantage of viewing this map early in the RGP fitting process is that it
allows quick visualization of significant astigmatism and quick insight into the
curvature of any eye. If the RGP fitting approach changes based on the amount of
corneal astigmatism or an average corneal curvature, a mental reference of RGP
design can begin at a glance of the absolute display (Szczotka and Lebow, 1998).
The absolute contact lens fitter will not rely on one map. Using only the
absolute scale would have distinct disadvantages. In systems with a large range of
curvatures, the scales have large intervals, which may mask clinically significant
irregularities. Systems with smaller ranges may not have this disadvantage, but they
may suffer from scale saturation. (The smaller range absolute scales are typically
chosen by the manufacturer to simulate the range of a keratometer.) If a cornea is
unusually steep, such as in keratoconus, the majority of the map may appear as an
"island of red" with no interval definition, because most curvatures are steeper than
the 52-D top interval of its scale (Szczotka , 2003).
A normalized scale automatically adjusts and subdivides the map into multiple
equal dioptric intervals based on the range of dioptric values found for that cornea.
The color intervals may vary in range and dioptric values between different eyes or
occasions for a given eye; therefore, normalized maps should not be compared
visually at a glance without referring to the associated color scale. A normalized
display allows more detail, because the color intervals can be much smaller than the
corresponding absolute maps. In fact, the normalized scale can produce a misleading
map, because it can take a normal cornea and exaggerate its shape to look abnormal
with multiple color changes from one region of the cornea to the next. It also can
provide more definition in maps that have been saturated with the absolute scale
(Szczotka and Lebow, 1998).
Color scaling can alter the appearance of any map, regardless of whether a
curvature, elevation, or thickness map is viewed. Although most practitioners easily
69
I
REVIEW OF LITRATURE
understand curvature, many are not familiar with elevation color legends, which
describe local deviations of the cornea's shape from a chosen spherical surface. It is
more difficult to diagnose abnormal deviations on elevation maps. The Orbscan
II
system has incorporated a scale termed the Normal Band Scale, which is helpful to
depict quickly what data (of any map) lie within a normal range. All normal areas are
shaded green. Data outside the normal range are indicated with hot (red, yellow) or
cool (blue) colors (Szczotka , 2003).
Curvature map options
Curvature map options vary in their application of a mathematical function to
the raw data. The two most informative maps used in contact lens practice are the
tangential radius of curvature representation (also referred to as instantaneous, local,
or true maps) and the axial representation (also referred to as sagittal, color, or default
maps). The axial representation values are defined as the distances along a radial
plane measured perpendicularly from a surface tangent at each point along the plane
to a central (optical) axis. The axial map is based on a spherically biased algorithm
that closely mimics keratometer measurements. Because of the spherical bias, it has
problems mapping abnormal corneas and shows greater error in the corneal periphery.
Additionally, axial maps produce a running average of the analyzed data for a given
peripheral location; therefore, they exclude extreme curvature values (Roberts, 1996).
The tangential representation produces true curvatures based on a standard
definition of the local curvature at a given point along a curve. The surface curvature
is measured along radial planes as in the axial representation; however, the radii of
tangent circles at each point along the plane define the curvatures. Although the
tangential and axial representations offer qualitatively similar views of corneal shape,
tangential maps provide smaller and more centrally located patterns than do the axial
maps. Differences between the two representations become significant for corneal
points further from the reference (optical) axis (or in the mid-to-far corneal
periphery), which results in many clinically significant differences relevant to contact
70
LITRAUE
REVIEW OF
lens fitting. For example, axial maps have larger and more diffuse color patterns than
tangential maps. This difference is most obvious where distinct corneal shape
changes occur in the periphery (Roberts, 1996).
Tangential maps include extreme curvature values; therefore, they offer a more
detailed view of localized corneal curvature. Tangential curvatures of a relatively flat
area of a cornea will appear to be flatter than the respective axial value, and relatively
steep areas will be imaged steeper on tangential maps when compared with their axial
counterparts. Tangential maps depict a more accurate corneal shape better correlated
to slit lamp observations of corneal pathology and RGP fluorescein patterns. They are
useful in RGP optical zone selection if the optical zone is targeted to vaulting a
chosen area of the cornea. Lastly, tangential maps provide more precise definition to
detect irregular astigmatism before RGP lens fitting, to monitor for contact lens—
2003).
(Szeotka,
induced warpage, and to track disease progression
Elevation maps
Elevation maps display the traditional interpretation of a topography height
map. Most elevation maps depict relative height differences from a computer selected
best fit reference sphere. Warmer colors (toward red) identify areas above the
reference sphere, whereas cooler colors (toward blue) denote areas of the cornea that
are lower than the specified reference sphere. Although these colors are identical to
those used in curvature displays, when viewed on elevation maps, they represent
height differences and not curvatures. Green, often the middle color, depicts the best
(Szcotka,
fit between the eye surface and the reference sphere
2003).
Once these elevation maps become familiar to the practitioner, they are useful
in predicting the appearance of an RGP fluorescein pattern on a given cornea without
having to interpret curvature maps. The high (red) areas always displace fluorescein,
and the low (blue) areas always pool fluorescein. In contrast, in curvature maps, both
steep (red) and flat (blue) areas may displace or pool fluorescein depending on the
71
REVIEW OF LITRATURE
corneal location. Steep curvatures will be high when centrally located but will pool
fluorescein when located in the corneal periphery (Szczotka, 2003).
ROLE OF TOPOGRAPHY IN REFRACTIVE SURGERY
In refractive surgery, corneal topography is valuable pre-operatively for
assessment and planning, and post-operatively for monitoring and further
management. In addition, it has roles in education, communication and
documentation (Corbett, 2000).
Table (7): Role of topography in refractive surgery (Corbett, 2000).
Role of topography in refractive surgery
INTRAOP
ERATIV E
• Adjustment of incisions or sutures
• Real time monitoring
•
•
•
•
•
•
Documentation of immediate effects of surgery
Assessment of healing
Investigation of a poor outcome
Planning of augmentation
lens power calculation
Suture manipulation/removal
Intraocul
POST-OPERATIVE
• Screening for ocular disease
keratoconus
- contact lens-induced corneal warpage
• Planning the surgery
- incision location, length, depth
• Intraocul
tens power calculation
.
ATIVE
PR -OPER
THROUGHOUT
• Patient education
• Communication with colleagues
• Documentation for medico legal purposes
Pre operative screening
-
All potential patients presenting for refractive surgery must have topography
performed on both eyes to help exclude conditions associated with an unstable
72
LITRAUF,
REVIEW OF
refraction or an altered wound healing response. In myopic patients presenting for
refractive surgery, as many as 33% have abnormal corneal topography 10. In the
majority of cases, the abnormality is not evident on examination by biomicroscopic
slit lamp or placido disc, which stresses the importance of performing corneal
topography (Corbett, 2000).
Keratocon us
The topography of keratoconus typically shows inferior corneal steepening. It
is classified according to the severity, location and shape of the cone. Sub-clinical
keratoconus has the typical topographic appearance in the absence of other clinical
signs. It may be a normal variant or an early form of the disease. Early results suggest
that the outcome of refractive surgery in sub-clinical keratoconus is similar to that in
normal, but extra care has to be taken in these patients while techniques are still being
aL,
developed (Wilson et
1990; and Corbett, 2000).
Planning of surgery:
Corneal topography is essential before all refractive procedures, to enable the
surgeon to understand the refractive status of an individual eye, and plan the optimum
treatment for it. Astigmatic procedures should treat the refractive astigmatism rather
than the corneal astigmatism. However, topography can help confirm the axis of the
aL,
astigmatism where the two are aligned, and assess its regularity (Levin et
1995)
In highly irregular corneas, such as after corneal transplantation (keratoplasty),
topography is more accurate than refraction or keratometry for determining the semimeridian with greatest astigmatism, and the accurate position of tight sutures (this is
an invaluable guide to the removal of tight sutures that are causing steepening of the
cornea). In these eyes, topography is also valuable for calculating the required power
aL,
of an intraocular lens (Kohen et
1996).
73
REVIEW OF LITRAUF,
topography
Infra-opetiv
Keratoscopy devices (reflected rings) may be used intra-operatively to
incsoal
determine the regularity of the cornea during
surgery and suture adjustment,
e.g. during penetrating keratoplasty. This enables the surgery to be modified to
achieve the minimum astigmatism by the end of the operation using real-time
feedback. However, its value may be limited because the corneal shape achieved at
the end of surgery usually undergoes further changes post-operatively. Work is
currently underway to develop real-time topography devices that can instantaneously
drive the distribution of excimer laser energy intra-operatively during the smoothing
of irregular corneas (Corbett, 2000).
Post-operative topography
All eyes undergoing refractive surgery should have topography soon after the
procedure (ideally within a week) to document the effect achieved by the surgery
itself, before significant changes occur as a result of healing. This can be important in
understanding the mechanisms operating in those who develop complications, and as
a medico legal record. Thereafter, the serial monitoring of routine cases is usually
only beneficial for research. In contrast, in complex cases topography is very
valuable in the investigation of complications, fitting of contact lenses and planning
of further interventions (Corbett, 2000).
Investigation of complications
Visual complaints after refractive surgery may be due to surface anomalies,
which are too subtle to be detected on biomicroscopic examination, but can be
revealed by corneal topography. Reduced Snellen acuity, contrast sensitivity defects,
distortion and halos may be due to surface irregularities or treatment zone
decentration. Following LASIK, a loss of corneal flattening progressing inwards from
74
REVIEW OF LITRATURE
one edge of the treatment zone is characteristic of epithelial in-growth (Corbett,
2000).
TOPOGRAPHY AFTER REFRACTIVE SURGERY
Refractive corneal procedures aim to alter the curvature of the central cornea,
and usually have a minimal effect on the corneal periphery. The area bearing the fullintended correction is called the optical zone. This tends to be surrounded by an
intermediate zone of altered curvature, before normal cornea is reached in the
periphery (Corbett, 2000).
The curvature of the central optical zone is changed more than that of the
periphery, so the asphericity of the cornea is altered. Treatments for hyperopia
steepen the optical zone, so the cornea becomes increasingly prolate. Myopic
treatments flatten the optical zone making the cornea less prolate, or even oblate. This
in which the central flattening is
associated with increased steepening in the mid-periphery
(Bogan
keratomy,
is most marked following radial
et al., 1991).
Changes in corneal topography are shown to best effect by the use of
difference (change/subtraction) maps, in which a later map is subtracted from an
earlier one. The result of the surgery itself is demonstrated by subtraction of the
immediate post-operative map from the pre-operative one. The stability of the change,
and the effect of the ensuing wound healing process, is quantified by the difference
between the map taken soon after surgery and one taken subsequently (Kwitko et al.,
1992).
CORNEAL TOPOGRAPHIC INDICES
Simulated K (SimK): Simulated keratometry measurements characterize
corneal curvatures in the central 3-mm area. The steep simulated K-reading is the
steepest meridian of the cornea, using only the points along the central pupil area
with 3-mm diameter. The flat simulated K-reading is the flattest meridian of the
75
REVIEW OF LITRATURE
cornea and is by definition 90° apart. These readings give an idea about the central
corneal curvature that is frequently visually most significant. The 3-mm diameter was
chosen primarily from historical reasons for the purpose of comparison with standard
keratometry that is used for analysis of 4 central points, 3.2 mm apart (Rabbetts,
1998).
The index of asphericity: The index of asphericity indicates how much the
curvature changes upon movement from the center to the periphery of the cornea. A
normal cornea is prolate (i.e., becomes flatter toward the periphery) and has the
asphericity Q of -0.26. A prolate surface has negative Q values and positive oblate
surface values. Most myopic laser vision corrections change the anterior corneal
surface from prolate to oblate (Rabets,
1998).
Indices characterizing the uniformity and optical quality of the anterior
corneal surface: Currently, best spherocylindrical correction that is used in glasses,
soft contact lenses, and laser vision correction and are reflected in
keratomic
4
indexes do not correct all the optical aberrations of the corneal surface. The best
visual acuity also requires uniform smooth anterior corneal surface (Rabbetts, 1998).
A variety of these indices try to relate the variability of the actual values of the
anterior corneal surface obtained by corneal topography to the optical quality and
potential best visual acuity that would be permitted by the anterior corneal surface.
These indices can be thought of in clinical practice as different mathematical
estimates of the visual disturbance that can be expected to be caused by the amount of
irregularities of the anterior corneal surface. Many indices specific for each
instrument exist. Examples include the following: surface regularity index (SRI),
corneal uniformity index (CUI), predicted corneal acuity (PCA), and point spread
function (PSF). It also is important to realize that patients with normal corneal indices
can have poor vision caused by disturbances in any other part of the optical system of
the eye ( Schuman, 1998).
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REVIEW OF LITRATURE
Limitations of corneal topography:
The error of corneal topography is under optimal conditions in the range of
Dm,
+0.25 D or 2-3
but, in those abnormal corneas seen in clinical practice, it often
is +0.50-1.00 D due to several limitations. The imaging requires an intact epithelial
surface and tear film. Some of the errors of the Placido-based systems are as follows:
focusing errors, alignment and fixation errors with induced astigmatism, difficulty to
calculate the position of the center from the small central rings, increased inaccuracy
toward the periphery because the accuracy of each point depends on the accuracy of
all preceding points, and other errors ( Schuman, 1998).
Different technologies use different measurement methods and algorithms;
thus, the output data are not directly comparable. Also, the technologies undergo
constant modifications, and the results of studies comparing the instruments are
outdated quickly and difficult to interpret for practical clinical purposes. In addition,
normal spherocylindrical surface imaging techniques should be able to characterize a
variety of more complex optical surfaces, as follows: very steep or flat corneas,
keratoconus with local steepening, sharp transition zones after uncomplicated
keratoplsy,
refractive surgeries, diffusely irregular surfaces after penetrating
complex surfaces after complicated refractive surgeries with decentered ablations,
and central islands. These optical surfaces are more difficult to measure. For example,
7 commercial topographers have been tested on nonspherical models, and errors
greater than 4 D may occur ( Schuman, 1998).
Corneal topography in normal corneas
The normal cornea flattens progressively from the center to the periphery by 24 D, with the nasal area flattening more than the temporal area. The topographic
patterns of the 2 corneas of an individual often show mirror-image symmetry, and
small variations in patterns are unique for the individual. The approximate
distribution of keratographic patterns described in normal eyes includes the following:
77
REVIEW OF LITRATURE
round (23%), oval (21%), symmetric bow tie typical for regular astigmatism (18%),
asymmetric bow tie (32%), and irregular (7%) (Liu et al., 1999).
CORNEAL TOPOGRAPHY GUIDED ABLATION
LASIK
Customized
based on topography
Both corneal and total aberrations increased after LASIK surgery for myopia.
The higher the preoperative myopia, the higher the increase. In general, although the
trends are similar when looking at third-order and higher aberrations, we found that
the spherical aberration in the anterior corneal surface was greater than that in the
entire eye. (Bruno et al., 2001
Several studies have shown the impact of refractive surgery for (radial
[PRK])
and photorefractive
keratcomy
[RI(]
keratotomy
on corneal aberrations
those studies computed the corneal aberration pattern by measuring corneal elevation
maps using commercial corneal videokeratoscopes. In these devices, centration is
typically achieved by aligning a set of concentric rings to the corneal reflex of the
fixation light. Corneal aberrations are then typically referred to the corneal reflex
rather than the pupil center. The processing algorithms align the corneal aberration
pattern with the total aberration pattern, which is referred to the pupil center. The
position of the pupil is important for a correct estimation of retinal image quality and
should be taken into account when predicting visual performance from corneal
aberration data. According to Marcos et al., corneal aberration data (third-order and
higher) changed by 10% when the pupil position was taken into account. Although,
as expected, spherical aberration did not change significantly by recentration (3% on
average), third-order aberrations changed by 22% (Marcos et al., 2001).
Total aberrations result from the combination of corneal and internal
aberrations and their inter-relationships. Before surgery, both components contributed
comparable amounts of aberrations. As stated by Marcos et al., comparison of post78
II
REVIEW OF LITRATURE
LASIK corneal and total aberrations revealed an increase in the amount of negative
internal spherical aberration, which tended to slightly attenuate the impact of the
positive spherical aberration induced in the anterior corneal surface. The effect is
larger as the preoperative spherical refractive error increased and did not depend on
aL,
the preoperative internal aberrations. (Baek et
2001).
The correlation coefficient of post-LASIK internal spherical aberration to preLASIK spherical refractive error is 0.73 and of the induced internal spherical
aberration (before minus after surgery) to pre-LASIK spherical refractive error is
0.74. LASIK surgery is not likely to induce changes in the crystalline lens; the
changes therefore seem to occur in the posterior corneal surface. The effect is only
present for spherical aberration, but not for other terms. This finding is consistent
with reports using scanning slit corneal topography. They show posterior corneal
surface changes of curvature after PRK for myopia and LASIK that produce a
forward shift of the posterior corneal surface (Naroo and Charman, 2000). This
suggests that after LASIK and PRK the thinner, ablated cornea may bulge forward
slightly, steepening the posterior corneal curvature. This effect has been thought to
account for the regression toward myopia that is sometimes found after treatment,
Charmn,
particularly in the patients with highest preoperative myopia (Naroo and
2000).
Seitz et al. found that the posterior central corneal power changed significantly
from 26.28 to 26.39 D after LASIK, and the asphericity power changed from 0.98 to
1.14, in a group of eyes with preoperative spherical refractive error similar to those in
our study (range: 21.00 to 215.50, mean, 25.07 to 2.81 D) (Seitz et al., 2001).
1ETI
T
71
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REVIEW OF LITRATURE
Reference: pupil center Reference: corneal reflex
Fig. (21): Corneal wave aberration contour maps, after surgery, centered at the pupil
center, after realignment (left) and centered at the corneal reflex (right), directly from
corneal topography data without realignment (Marcos et al., 2001).
total
corneal
internal
0
Fig. (22): Total (left), corneal (middle), and internal (right) wave aberration maps (thirdorder and higher aberrations) before (top) and after (bottom) LASIK, with a particularly
good surgical outcome (Marcos et al., 2001).
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Customized PRK based on topography
To correct myopia, the excimer laser flattens the central cornea by etching a
negative lens onto its anterior surface. This requires that a greater number of pulses
fall on the centre than the periphery of the treatment zone, resulting in a saucershaped disc of tissue being removed. The correction of hyperopia requires the optical
zone to be steepened. This is achieved by using larger diameter ablations to remove
an annulus of tissue from the mid-periphery of the cornea (Corbett, 2000).
Regular astigmatism is corrected by the differential ablation of tissue in the
steeper meridian, and therefore the treatment zone is usually hemi-cylindrical or oval.
Irregular astigmatism was originally smoothed using fluid masking agents in
phototherapeutic keratectomy (PTK). However, this technique lacks the precision
required for the refractive correction of irregular astigmatism. If topography is to be
used to guide ablation, it is imperative that height maps are used so that the treatment
can be applied to the peaks, rather than the steep sides, of any elevation. The
application of laser energy can be controlled by either passing a broad beam through
a pre-lathed individualised erodible mask complementary to the corneal shape, or by
directing the ablation pattern of a 'flying spot' laser (Hersh et al., 1997).
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REVIEW OF LITRATURE
Fig. (23): PRK difference maps (Corbett, 2000).
The treatment zone is usually easily delineated by the close proximity of
adjacent contours at its edge, especially immediately post-operatively. Following
hyperopic correction, there is steepening of the central cornea, which increases the
corneal asphericity. This is surrounded by a ring of relative flattening at the edge of
the treatment zone, where most corneal tissue has been removed. In astigmatic
treatments, the treatment zone is oval. Decentration can only be identified with
relative certainty by comparison of a map taken in the first week post-operatively,
with a pre-operative map. A similar post-operative appearance can arise from preexisting asymmetric astigmatism, or an asymmetrical healing response. Decentrations
of large diameter (6mm) optical zones are usually only clinically significant if greater
unless patients have particularly large pupils (Corbett, 2000).
Following any
PRK
m,
than l
procedure, the induced flattening or steepening is most
pronounced initially, during the period of overcorrection. It then becomes less
marked with time as new wound healing tissue partially fills in the ablated area.
menw lo
■
miMINFO
mo
d
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REVIEW OF LITRATURE
Postoperatively, there is usually a temporary increase in astigmatism and irregularity,
which then reduces with time (Hersh et al., 1997).
The distribution of the new tissue determines the shape of the post-operative
corneal surface. Eight topographic patterns after PRK have been identified. Patients
with a homogeneous pattern have least astigmatism. Those with regular patterns
(homogeneous or toric) have a better refractive predictability, visual acuity and level
of satisfaction than those with irregular patterns. The irregular patterns include semicircular, keyhole, central islands, focal irregularities and irregularly irregular (Corbett,
2000).
A central island is present when any part of the treatment zone surrounded by
areas of lesser curvature on more that 50% of its boundary. They are classified
according to the power and diameter of the central steep area. The incidence varies
greatly between studies, but always reduces in the first year post-operatively. Several
mechanisms have been proposed to explain their occurrence, but each exerts its
effects through one of three common pathways: reduced central ablation due to
characteristics of the laser and ablation process, reduced central ablation due to
properties of the cornea, or irregularities of healing (Levin et al., 1995).
83
I
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Fig. (24): Hyperopic photorefractive keratectomy.
To correct hyperopia, the excimer laser removes an annulus of tissue from the
mid-periphery to steepen the central cornea. One year after a +3.50D correction, the
central cornea is steepened. In the mid-peripheral cornea, the contours are closer
together than normal, representing the rapid change from central steepening to
flattening over the treated area (Corbett, 2000).
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REVIEW OF LITRATURE
I
Fig. (25): Topographic map showing central island.
One week after a -3.00/-2.00 x 90° PRK, there is a central area of high power
surrounded by an annulus of lesser power. Over the following months, the central
island became less obvious and the statistical indices reverted towards normal
(Corbett, 2000).
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REVIEW OF LITERATURE
THE PROSPECTS FOR PERFECT VISION
The 21st century goal of eliminating all traces of optical blur due to higherorder aberrations of the eye, which clinicians sometimes call "irregular astigmatism"
suggests the prospect of perfect retinal image producing perfect vision. If the goal is
to achieve retinal images of such high quality that they are no longer limited by the
optical imperfections of the eye, then the only remaining optical limitation will be
the unavoidable effects of diffraction. This is the meaning of the phrase "diffractionlimited retinal images." When contemplating the prospects for achieving
diffraction-limited retinal images even under night time viewing conditions when
(Thibos,
pupils are dilated, three questions immediately come to mind
2000).
1) Is this quest for diffraction-limited image quality really possible, or is it just
wishful thinking?
2) What would be the potential benefits of perfect retinal images, if they can be
obtained?
3) What are the potential penalties? In other words, is there a down side to having
perfect retinal images? (Thibos, 2000).
Are perfect retinal images really possible?
Many studies make the optimistic assumption that refractive surgery going
to become absolutely perfect. In other words, they assume that modern technology
is going to make it possible for a clinician to diagnose all of the optical
imperfections of a patient's eye, and that these aberrations can be corrected
(Thibos
exactly, without error, by a laser surgeon
2000).
Despite this optimistic assumption, there are at least two good reasons to be
skeptical that perfect retinal images and perfect vision will follow. 1) Aberrations are
86
REVIEW OF LITERATURE
a moving target because of variability that may occur over time scales ranging from
seconds to years. For example, the magnitude of ocular aberrations varies with the
state of accommodation of the eye and micro-fluctuations around a fixed state of
accommodation causes instability in the aberration structure of eyes on a time scale
of seconds, or less (Thibos, 2000).
During accommodation, aberrations of the eye are changing. These changes
are related to the crystalline lens and its performance over time during the aging
process. The dynamic variable in accommodation is the lens, since the cornea
curvature theoretically does not change. Nature has made our optical system in such
a way that the lens and cornea complement each other, at least in the peripheral part
of the optics. All abberrations and of most importance, higher order aberrations,
(Palikrs
change continuously both during the aging process and everyday vision
et al., 2001).
In a recent study six eyes of six subjects were examined in an accommodative
and non-accommodative state. Patients were asked to concentrate their attention on
the accommodation target. One observer, using the steps specified in the user
manual, performed all measurements with the Tracey Wavefront Aberrometer
(Palikrs
(tracey technologies Inc., Houston, TX)
et al., 2001).
For case no. 1 (35-year-old subject), accommodation was measured under three
accommodative conditions corresponding to the accommodative stimulus that
ranged in 0 to -6.00 diopters (D) in steps of 3.00D. For cases no. 2, 3, 4, 5, and 6,
accommodation was measured under two accommodative conditions, 0 and -6.00 D.
For case no. 6, the accommodation process was measured with and without
mydriatics (Pallikaris et al., 2001).
For evaluation of the differences between the two states for each eye, refractive
maps for change in refraction as well as the root mean square (RMS) values for
higher order aberration change during accommodation were compared. The RMS
87
REVIEW OF LITERATURE
wavefront error provides a general estimate of the variation of the wavefront from
the ideal. The higher the RMS wavefront error, the larger the wavefront aberration
and the worse the image quality. In cases no 2, 3 and 4, the higher order RMS
decreased with accommodation; in case no. 1 it increased, perhaps because the
subject was asked to accommodate slowly through three steps (not two); in case no.
6, the RMS value did not change due to the age of the subject, although the RMS
value was different depending on whether cycloplegia was used. The RMS for case
no. 5 was much decreased for the accommodating stage because me pupil was
smaller. In this case we see that the higher order aberrations are very dependent on
pupil size (Pallikaris et al., 2001).
The conclusion, based on this limited analysis, is that higher order aberrations
seem to increase with rapid accommodation in younger patients and cycloplegia and
show no change with older patients. Slow, stepwise accommodation seems to show
decrease in higher order aberrations. However, an analysis of a larger number of
eyes is required in order to draw more conclusions and substantiate these findings
(Pallikaris et al., 2001).
In an earlier study the wavefront aberration of the human eye was measured
for eight subjects using a spatially resolved refractometer. The eyes were undilated
and presented with accommodative stimuli varying from 0 to -6 diopters. The overall
results shows that Monochromatic wavefront aberrations tend to increase with
increasing levels of accommodation, although there are substantial individual
variations in the actual change in the wavefront aberration. While spherical
aberration always decreased with increasing accommodation, it did not change from
positive to negative for every observer. The direction and amount of change in fourth
order aberrations varied between observers. Aberrations with orders higher than
fourth are at a minimum near the resting state of accommodation. The
accommodation-induced change in wavefront aberration was not strongly related to
the total amount of aberration in the eight eyes studied (He et al., 2000).
88
REVIEW OF LITERATURE
2). Additional instability may arise from evaporation of the corneal tear film,
which has been shown to be capable of producing huge optical aberrations
(Thibos, 2000).
I
.
.
*
*
*,
•
elk
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Fig. (26): Optical effects of tear film disruption(Thibos and Hong, 1999).
This figure(26) shows that the upper row of images was captured immediately
after a blink; the bottom row of images were obtained after the subject had held his
lids open for about 40 seconds. Left column contains images obtained by retroillumination of the pupil; middle column shows the data images captured by the SH
aberrometer; right column shows contour maps of the aberrated wavefront emerging
from the eye computed from the SH data image. Contour intervals in the
reconstructed wavefront are 1 mm and the wavefront phase at pupil center has been
set to zero. Pupil coordinates are in mm (Thibos and Hong, 1999).
(Fig.26)
The previous figure
compares the optical aberrations of a normal eye
immediately after a blink (when the tear film is intact) and again 40 seconds later,
after the tear film was disrupted by drying. This particular subject had relatively
unstable tears that began to break up just a few seconds after each blink. Other
89
I
REVIEW OF LITERATURE
subjects retained an intact tear film even after holding the lids open for several
minutes, a fact which emphasizes the large individual differences in tear film
integrity in otherwise healthy eyes. The upper row of images in previous figure is the
control results obtained immediately after a blink. On the left is the retroillumination view of the pupil, which is clear and free of any signs of tear film
disruption. In the middle is the SH data image, which shows a regular array of
clearly imaged dots indicating low levels of optical aberration or scatter. The phase
map calculated from the SH data image is shown at the right. As a control for
accommodative fluctuations, the defocus term of the wavefront aberration was set to
zero before computing the phase map. The widely spaced contours indicate an eye
that is relatively free of optical aberrations. The bottom row of images in that figure
was obtained after the subject had held his lids open for about 40 seconds (Thibos
and Hong, 1999). Large amounts of intra-subject variability, and also inter-subject
variability, encountered in these experiments prevents easy generalizations about the
topography of tear breakup or its latency after a blink. Nevertheless, it is clear that
the drying of the tear film had a major impact on the quality of the optical system of
this individual's eye. Tear film drying will include an evaluation of the relative
contributions of light scatter and optical aberrations. These optical effects appear to
have a significant visual impact according to a recent study by Tutt who documented
a significant loss of retinal image quality and visual contrast sensitivity associated
with drying of the tear film. Future studies which monitor optical image quality with
the SH aberrometer. While simultaneously monitoring visual performance and
retinal image quality, should enable testing of the hypothesis that reduced optical
quality of the eye is the root cause of blurry vision associated with dry eye syndrome
and tear film disruption (Thibos and Hong, 1999).
Is it really possible to completely correct higher order wavefront aberrations
with a scanning spot laser system and, if so. What laser parameters are needed to do
that?
REVIEW OF LITERATURE
Theoretical study was done to find answers to these important questions. This
theoretical study showed that. all of the scanning spot systems or the market cannot
completely correct higher order optical aberrations. Best results for corrections of
l
m
optical aberrations up to the 6th order were obtained with
Gaussian beam even
these systems will produce an error in the order of 10%. The only way to increase
the accuracy of the corrections is to reduce the ablation volume per pulse. This
requires excimer laser systems with higher repetition rates (500Hz to 1000Hz) to
reduce surgery duration. The future will show if such 2 systems can be adapted to
and
2002).
Mrochen,
(Kaemr
the human eye
What are the potential benefits of perfect retinal images for vision?
The consequence of having diffraction and aberrations in the eye is that the
contrast in the image formed on the retina is reduced. Fig. (27) shows a high contrast
sinusoidal pattern being imaged by the eye. As this pattern is affected by diffraction
and imaged through an eye with aberrations, the contrast of the image will be
degraded. On the retina in Fig. (27), the degraded image of the sinusoidal target
caused by diffraction and aberrations is formed. Note that the degraded image is still
sinusoidal, but that its contrast (i.e., the difference between the dark and light
portions of the pattern) has been reduced. Aberrations and diffraction degrade
contrast in the image formed on the retina. However, the degree of degradation is
dependent upon the spatial frequency of the pattern. Spatial frequency is related to
the angular subtense between the black lines of the sinusoidal target. Higher spatial
frequencies correspond to smaller' angular subtense of the black lines. Each
aberration type affects various ranges of spatial frequencies differently. In general
though, loss of contrast tends to be more severe for high spatial frequencies, and the
higher the degree of aberrations, meaning the less the converging wavefront looks
like a sphere, the worse the contrast degradation (Schwlegerting 2000).
91
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REVIEW OF LITERATURE
Lens
Fig. (27): Diffraction and aberrations degrade the contrast of a sinusoidal pattern
imaged by the eye (Schwiegerling 2000).
In general terms, vision should improve for any visual task for which human
performance is limited by the amount of contrast in the retinal image, or by
distortions of spatial phase induced by optical aberrations, but not for those tasks for
which performance is limited by the spatial grain of the retina. For example, the
detection of visual objects by their luminance contrast is a visual task that is
inherently contrast-limited, it follows that any improvement in retinal contrast
achieved by reducing the eye's aberrations will improve contrast sensitivity for
object detection in direct proportion to the improvement in the eye's modulation
transfer function (Thibos, 2000). This expectation has been confirmed
experimentally for monochromatic light by using adaptive optics technology to
(Liangetl,
reduce the eye's aberrations
1997). More recently, William's group has
shown modest improvements in polychromatic letter acuity as well (Williams,
1999).
If the optical aberrations of an eye could be reduced enough to eliminate
insufficient contrast as the limiting factor for visual performance then some other
factor, such as the sampling density of retinal neurons, will surface as the limit to
visual performance (Thibos, 2000).
Photoreceptor sampling ultimately limits the resolution limit of the eye. The
size of each photoreceptor and the spacing between adjacent photoreceptors
determines how small a feature can be processed and how high a spatial frequency
92
REVIEW OF LITERATURE
can be resolved. Fig. (28) demonstrates the effects of finite sampling size and
spacing. A sinusoidal pattern falling on a row of photoreceptors can be detected if
the neural signals from adjacent photoreceptors are significantly different. In
Fig.(28) (a), there is one photoreceptor for each dark portion of the sinusoid and one
photoreceptor for each bright portion. Each photoreceptor averages the intensity of
the light falling on its aperture, so that the neural signal coming out of the receptors
takes on a square-wave pattern. As long as the difference in the peaks and valleys of
the neural signal is large enough, the sinusoidal pattern will be detected. In
Fig.(28)(b), the spatial frequency of the target has been doubled. In this case, a peak
and a valley of the sinusoidal pattern fall on an individual photoreceptor. The
receptor can only average the intensity falling on it, so the neural signal coming out
of the receptors is uniform. Therefore, the pattern is not detected in the example
shown in Fig. (28) (Schwiegerling 2000).
Input
rah
Photoreceptors
— — Output
Input
Photrecps
\
Amiv
(b)
Output
Fig. (28): Photoreceptor sampling.
93
•
REVIEW OF LITERATURE
The highest frequency that a neural sampling array can resolve is called the
Nyquist frequency. Stimuli beyond the Nyquist frequency are undersampled and
therefore are misrepresented by the neural visual system. Thus, visual resolution is set
by the lesser of the two parameters: optical bandwidth of the retinal image and
sampling density of retinal neurons. Under normal circumstances the optical
bandwidth of our aberrated eyes determines the resolution limit of foveal vision
because optical band-width is typically less than the Nyquist frequency of the
photoreceptor array. However, if aberration correction in the future by surgery or
ophthalmic lenses succeeds in increasing the optical bandwidth beyond the Nyquist
frequency, then neural sampling will replace optical filtering as the mechanism which
limits visual resolution. When this happens, vision of the objects that exceed the
Nyquist frequency will become possible, but unfortunately such vision will be nonundersamplg
veridical and therefore unreliable this is because neural
misrepresents
fine spatial details as coarse details, a phenomenon known as "Aliasing ". Aliasing
produces a kind of misperception in which objects appear to have a different spatial
scale, orientation, form, or direction of motion compared to the physical stimulus
2000)
94
0
(Thibos,
I
REVIEW OF LITERATURE
Fig. (29): Schematic view of optical and retinal sampling limits to visual' resolution.
Fig. (29) shows that the broken and solid curves show contrast sensitivity
functions for normal vision and for supernormal vision associated with improved
optical quality, respectively. The highest spatial frequency that is above detection
threshold for maximum stimulus contrast is a measure of the visible optical bandwidth
(Thibos,
of the eye, which is typically below the Nyquist frequency of the neural retina
2000).
However, this condition is common place in peripheral vision where the eye's
natural optical bandwidth typically exceeds the neural sampling density of the
peripheral retina This suggests that studies of spatial aliasing and motion aliasing in the
periphery may help predict the potential consequences of perfect foveal images
achieved by future clinical treatment. Those prior studies predict there will be an
improvement in contrast sensitivity for the task of visual detection, but not for the task
of spatial resolution because of the ambiguity introduced by aliasing (Thibos 2000).
To estimate the theoretical limit on visual acuity, it will be assumed that the
wavefront-sensing technique accurately measures the monochromatic aberrations of
the eye at a given wavelength and that these monochromatic aberrations do not vary
with wavelength. Furthermore, it is assumed that the perfect compensating optic,
which accounts for aberrations, healing, and biomechanical changes, is delivered
accurately to the cornea. The resulting eye would have its visual performance limited
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REVIEW OF LITERATURE
only by diffraction at a single wavelength and by chromatic aberration over the visible
spectrum. Exact ray-tracing of a schematic eye model is used to determine the
modulation transfer function, which accounts for diffraction, photopic response,
chromatic aberration, the Stiles-Crawford effect, and pupil size (Schwiegrln,
2000).
By analyzing an eye model that is free of monochromatic aberration and that
contains natural strategies for reducing the effects of diffraction and chromatic
aberration, such as the photopic response and the Stiles-Crawford effect, the theoretical
limits on foveal vision can be predicted. These limits were shown to be between 20/12
and 20/5 for common pupil sizes. These acuity levels are optimistic because of the
assumptions of the model. Furthermore, the acuity levels represent a grating acuity,
and letter performance tests would be expected to have slightly worse acuities. The
results of this study, however, are encouraging. They suggest the possibility that
emerging refractive surgery technology will be able to markedly improve the level of
vision of patients who undergo the procedures (Schwiegerling, 2000).
The potential penalties of perfect retinal images
The use of high technology to increase optical bandwidth of eyes may clash with
those high tech industries that depend on the eye's imperfections to make their product
took good. For example, television and computer monitors use rasters which produce
pixel patterns that are about equal to the resolution limit of the eye for viewing at arm's
length. Similarly, the printing industry uses half-tone images, which may become
objectionable if the individual dots of the image are clearly visible to a person with
"supernormal vision" (Th ibos, 2000).
Finally, we should consider possible public healthand safety issues associated
with perfect retinal images. Current safety standards for lasers and other light hazards
are based on the assumption that eyes are aberrated, so light from a bright point source
is spread out across the retina, which helps dissipate the damaging heat. If we improve
i mage quality significantly, we may be putting the retina at risk of accidental exposure
96
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REVIEW OF LITERATURE
to bright point sources of light. We should also consider costs and benefits to the
national and global communities of refractive surgery aimed at reducing the eye's high
order aberrations (H.O.A) (Thibos, 2000).
There are many reasons for lack of ability of laser technology alone making it
possible to achieve visual acuity that is better than conventional eyeglasses, contact
lenses and traditional refractive surgery. One has to reminder that when we try to
correct H.O.A even very minor changes count. Sometimes, it is enough for only one or
two shots delivered in a slightly wrong position or timing and all the expected results
will not be achieved. So, we have to take into account that we need nothing but
absolutely perfect results in order to achieve what we want. These reasons are divided
into 4 main groups:
Inherent physiological differences and changes of the optical and biornechal
properties of different corneas.
Uncontrolled optical changes during the healing process of the cornea.
Technological limitations of the surgical equipment.
(Lipshtz
Uncontrollable surgeon dependent variables
I
and Gad, 2002).
Inherent physiological differences and changes of the optical and biomechanl
properties of different corneas (Ocular problems):
1- Changes in wavefront with age. So, even if we can achieve supervision for a
certain eye, new aberrations will appear sooner or later with age (McLellan et al.,
2001).
2- Changes of wavefront during accommodation (dynamic vision factor-DVF). So,
even if we achieve supervision for distance vision, the near vision will have
induced H.O.A and may even be worse compared to the situation before correcting
the distance H.O.A (Arta!, 2002).
3- Effect of pupil size on ocular H.O.A change of the pupil size dramatically affects
vision. Constricting the pupil avoids the mid-peripheral H.O.A., increases the depth
97
REVIEW OF LITERATURE
of focus and corrects even lower order aberrations. In a 3 mm pupil, one needs to
correct only the 4` order Zernike coefficient in order to achieve supervision,
h
th
Zernik
whereas for a 7 mm pupil, we will need to correct up to the 8 order
coefficient in order to achieve supervision and that is very difficult (Lipshitzet al.,
2002).
4- Biomechanical changes between different corneas. Corneas differ considerably in
their biomechanical properties. These differences depend on various factors such as
corneal thickness and collagen properties (Keratoconus), thus the effect of laser
shots on a specific cornea is not precisely predictable (Moller Pedersen et al.,
-
2000).
5- Flap biomechanics. Once a flap is created the biomechanical properties of the
cornea change depending on the depth of the incision, diameter of the flap, size of
the hinge, location of the hinge. The thickness of the flap differs from the center to
the periphery and from areas close to the hinge to the peripheral cornea (Roberts
and Dupps, 2001).
6- Changes of the tear film optical properties of different eyes (dry eye, blepharitis,
etc.). Dramatic change curvature after LASIK changes the tear film distribution
and dissecting of the corneal nerve fibers by the microkeratome decreases tears and
2000).
(Thibos,
causes dry eye that affect the vision
7- Change of abberation after creating the flap. The H.O.A. that are measured before
creating the flap which are used for calculating the laser treatment after the flap is
created and even returned to its place without ablation there is a change in the
wavefront measurement and the calculations for laser application are incorrect
(Lipshitz and Gad, 2002).
8- Changes of wavefront during cycloplegia. Measurement of wavefront has to be
taken through dilated pupils after cycloplegia (most of H.O.A. correspond to the
corneal mid-periphery) but when the pupil constricts back, the wavefront
measurements change significantly, so the preoperative measurements are not valid
for performing surgery (Fankhauser et al., 2000).
98
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REVIEW OF LITERATURE
9- Variation of ablation rate in different depth of the cornea. The water content of the
cornea is changing according to the depth inside the cornea, higher content of
water causes less ablation, so the ablation rate per pulse changes all the times
(Lipshitz and Gad, 2002).
10-
Change of corneal thickness in different meridians. Usually the cornea is
thinnest in the inferior or the infero-temporal side because these parts are dryer as
they get less tears, so the ablation rate is higher at these locations compared to
others and this creates an effect of cornea on the refraction (Lipshitz and Gad,
2002).
11- Thin corneas ablate more than thick corneas per shot. Thin corneas (as for
instance prolonged soft contact lens wearers) have lower content of water so they
process of the cornea
(Healing
,
the
healir
changes
Uncontrolled optical
during
ablate more for each laser shot (Lipshitz and Gad, 2002).
problems):
12- Corneal epithelium wound healing (Epithelial remodeling). After changing the
corneal curvature with the laser, and as a result of epithelial injury by the
microkeratome or sponge drying, the epithelial cells regenerate and invade the
teared area, thus changing the refractive power. The amount of hyperplasia is
different from eye to eye. It is enough for one layer of epithelial cells to
dramatically change the H.O.A. of the eye (Roberts and Dupps, 2001).
13-
Corneal collagen wound healing. Collagen healing is different in various people.
It depends on the depth of ablation, incision profile and the new biomechanics of
the cornea (Zieske, 2001).
14-
Changes of tear film after surgery. Type of microkeratome incision and hinge
location, as well as depth of ablation, have an effect on tear film production ad
distribution after LASIK (Arils
et al., 2000).
15- Effect of corneal biomechanics on healing process (corneas have different
biomechanics before surgery and it changes even more after surgery). The corneal
REVIEW OF LITERATURE
biomechanl
properties of the entire cornea change considerably after LASIK.
The collagen healing process differs with the depth of ablation, from center to
periphery (Moller-Pedersen et al., 2000).
Technological limitations of the surgical equipment (Technological problems):
16- Eye positioning during laser treatment. Axis alignment, refraction, corneal
topography, and wavefront testing are taken when the patient is sitting. The exact
head position is not fixed when refraction, corneal topography, or wavefront are
measured. When a patient lies down, we can never be sure that the head position is
exactly the same as when it was when taking the measurements (Mrochen, 2002).
17- Accuracy of laser ablation. The exact laser ablation of corneal tissue changes
continuously, among other things it depends on humidity, temperature, laser
mirrors, cavity output that affect the tissue laser interaction of every single shot.
Usually output of the laser is measured as an average of many laser applications,
while when treating H.O.A. we depend on very few shots (Lipshitz and Gad,
2002).
18- Microkeratome accuracy and profile. Many studies have shown that
microkeratome don't cut exactly as specified. The incision depends on many
variables starting from diameter of microkeratome incision, blade quality, corneal
curvature, surgical technique and microkeratome performance (Lipshitz and Gad,
2002).
19- Cornea-microkeratome relationship. Corneal K-reading, corneal diameter,
corneal biomechanical properties, corneal hydration and environmental parameters
all affect the end results of corneal flaps which cause changes in the ablations
clinical results (Lipshitz and Gad, 2002).
20- Tracking problems. We can measure 4000 times/second, but after getting the
measurement the mechanics and optics of the laser have to adjust them selves
exactly and perform the exact delivery of the laser energy to the predetermined
topographical location of the cornea. The optical alignment and mechanical
100
REVIEW OF LITERATURE
adjustment take time and there will always be a critical delay between
measurement alignment and delivery (for a 10 m sec. of latency, the normal eye
movement are 11-19 microns/sec.) (Mrochen, 2002).
21-
Decentration. Laser alignment can be done on the optical axis, the geometric
axis or according to the pupil. All these minor changes affect the H.O.A. specially
the coma aberration (Mrochen, 2002).
22-
Accuracy of wavefront sensors. There are several ways to measure wavefront.
Each method work differently and a different resolution capability (number of
points measured per cornea). Recently, at one of the meetings, a patient suffering
from keratoconus was measured by 6 different wavefront analyzers and only one
was able to detect such a serious problem as keratoconus (Lipshitz and Gad,
2002).
Wavefront-laser inter-phase. The results of wavefront sensing as well as the
23-
other data, such as refraction, corneal diameter, flap quality, topography, etc.. Have
to be calculated into the laser. The ability of the laser to integrate all these
enormous amount of data and to give to each piece of data the exact value by the
laser is questionable. For different patients any information has a different
influence on the end refraction (for instance, in a patient with high astigmatism, the
wavefront measurement is not as important as the topography) (Lipshitz and Gad,
2002).
24-
Accuracy of laser machines. The laser machines not only changes continuously
in their output when they are used but also even differ between the same machine
and the same company (2 NIDEK 5000 lasers work differently and different
nomograms are used for each machine) (Lipshitz and Gad, 2002).
25-
The exact laser output changes all the time and over a relatively short period.
When we finish a certain procedure, the laser works slightly different compared to
before starting the procedure (Mrochen, 2002).
26-
Chromatic abberrations are not detected by the wavefront sensors. All the
wavefront analyzers are using a monochromatic light which is thrown into the eye
101
REVIEW OF LITERATURE
and detected when it leaves the eye and from this they build a colored picture, but
real live is polychromatic and this can not be measured by current wavefront
machine, so how can we correct all the spectrum of lights by using measurements
from only one wave length (Lipshitz and Gad, 2002).
27-
Place of measurements of the wavefront machines. The wavefront machines
take their measurement at the exit of light from the pupil at the pupillary plane, but
the correction by the laser is done on corneal surface that is far away. The
wavefront is detected as a plan of light but laser works on a curve (cornea)
(Lipshitz and Gad, 2002).
sur,zeon
28-
dependent variables
(Sumeon
Uncontrollable
problems):
Dryness of the ocular surface. It is crucial for precise ablation, but we still don't
have any method to exactly measure the degree of moisture of the cornea before
ablation and the degree and rate of change of corneal moisture during ablation (this
depends not only on the corneal properties which cannot be measured but also by
the number of laser shots, frequency of their delivery etc.) (Lipshitz and Gad,
2002).
Retinal problems of
abertion
29-
free optics. We still don't know if directing all
the rays of light directly on the Fovea (which is what we are going to do if we want
to create super vision) will not cause thermal or toxic damage to the macular (may
(Thibos,
be that's why God created optical abberration in our eyes)
30-
2000).
Environmental issues. We can control humidity or temperature in our operating
theatres but we cannot control them at the cornea-laser interaction. The
temperature there changes constantly due to the heat that is generated by the laser
on the surface of the corneal bed. This is also associated with corneal moisture
(Lipshitz and Gad, 2002).
31- Positioning of the flap after the ablation. Even though we are usually marking
the cornea before the microkeratome incision, we can be sure that the exact
position of the flap on the cornea is achieved. It is enough that only a few microns
102
REVIEW OF LITERATURE
of change of the flap position compared to the original to destroy all what we want
to achieve (Lipshitz and Gad, 2002).
32- LASIK or laser surgery is an irreversible procedure, so that if only one or two of
all the above-mentioned problems occur, it may jeopardize the super vision results
(Lipshtz
and this can't be reversed
and Gad, 2002).
Many unanswered questions remain. How will normal visual mechanisms
respond to non-physiological levels of contrast never before encountered by the retina
and brain?. Will we discover that we are all amblyopic to objects which exceed the
optical bandwidth of normal eyes because we have been deprived of these stimuli all
of our lives? . Will motion aliasing and spatial aliasing become a significant problem
when the optical quality of our eyes exceeds the neural sampling limits of the retina?
Many such interesting and important issues will need to be addressed by future
researchers before we will have an inkling of what vision will be like when retinal
images become perfect (Thibos, 2000).
Wavefront analysis will provide the means necessary to measure and minimize
aberrations. Topography will help us measure and predict the biomechanical corneal
response in ways that have not been elucidated. The future of customized laser
refractive surgery is bright, but we must step into the future with a better
understanding of our current procedures rather than leap without knowing where we
will land (Roberts, 2000).
103
PE
1
0
.
01•-
SUMMARY
SUMMARY AND CONCLUSION
Refractive surgery is an increasingly popular procedure to decrease spectacle
or contact lens dependency. The risks of refractive surgery are low on an individual
basis, but the impact on the population must be carefully evaluated by the medical
community. Photorefractive keratectomy (PRK) and laser in situ keratomileusis
(LASIK) are two refractive procedures currently leading the field.
Customized corneal ablation is an exciting frontier in refractive surgery that
incorporates wavefront technology to detect and correct higher order aberrations in
addition to spherocylindrical refractive errors. The goal is to achieve supernormal
vision in terms of acuity and contrast. As the concept of wavefront-customized
ablations is still new, there are a number of aspects of its clinical application that
need analysis and understanding. Numerous reports have appeared in the literature
during the past year that address the developments, concerns, and limitations of
wavefront technology and custom ablation.
Corneal topography instruments used in clinical practice most often are based
on Placido reflective image analysis. This method of imaging of the anterior corneal
surface uses the analysis of reflected images of multiple concentric rings projected on
the cornea. Multiple light concentric rings are projected on the cornea. The reflected
image is captured on charge-coupled device (CCD) camera. Computer software
analyzes the data and displays the results in a variety of formats.
Every map has a color scale that assigns particular color to certain keratometric
dioptric range. Never base an interpretation on color alone. The value in keratometric
dioptric range is crucial in the clinical interpretation of the map and has to be looked
at with the interpretation of every map.
104
SUMMARY
Corneal topography will continue to be a critical diagnostic modality for
refractive surgery. Even with the advent of wavefront analysis designed to detect
refractive error and aberrations of the eye, it will be necessary to have detailed
corneal topographic information to understand the contribution the cornea makes to
vision so that custom alteration of that surface can be used to optimize vision. This
will be true of the normal eye, but it will be of special importance in eyes with
abnormalities that were induced by corneal surgery.
Wavefront technology provides a fast noninvasive method to measure the
sphere, cylinder, and axis of a patient and HOA such as coma and spherical
aberration. The aberrations measured are not just corneal but aberrations of the entire
optical system. Studies have demonstrated that the correction of HOA with adaptive
optics systems can lead to supernormal visual performance in normal eyes. The
success of the technique encourages implementation of high order correction in
everyday vision through customized laser refractive surgery, contact lenses, or
intraocular lenses.
The possibility of achieving supernormal vision in terms of acuity and contrast
has fueled the imagination and creativity of vision researchers to pursue the goal of
customized wavefront refractive surgery. This goal is achieved by generating an
optimal ablation pattern based on individual anatomical and functional characteristics
of the treated eye. However, increasing concerns regarding the clinical applicability
of customized wavefront correction have emerged, and the possibility of achieving
supernormal vision in all patients has been challenged. It is by understanding current
optical, physiologic, and technological challenges faced by treating physicians and
basic researchers that we can develop clear objectives for the future development of
customized refractive surgery.
At present, several different technology platforms exist. A more thorough
understanding of these platforms will help in evaluating which system is best at
eliminating aberrations. The following technology is required to provide what is
105
SUMARY
considered optimal within the current state of knowledge in this field: (1) scanning
spot laser delivery, (2) robust eye tracking, (3) an accurate wavefront device, and (4)
the wavefront-laser interface.
It is concluded that:
Optical aberrations inherent in the eye adversely affect the visual acuity and
the optical performance of the eye. With wavefront guided ablation it is now possible
to reduce these abberrations, and the initial results reported are extremely
encouraging. This procedure may not enhance our vision much above normal levels
but it will certainly make us refine our definition of normal vision.
106
air
mip,
I
imp
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