Download Outcomes of Corneal Spherical Aberration

ORIGINAL ARTICLE
Outcomes of Corneal Spherical Aberrationguided Cataract Surgery Measured by the
OPD-Scan
Jonathan D. Solomon, MD
ABSTRACT
PURPOSE: To determine based on preoperative corneal
spherical aberration, the practicality of targeting zero
total ocular postoperative spherical aberration when selecting an aspheric intaocular lens (IOL).
METHODS: Consecutive cataract patients were selected to receive an aspheric IOL based on corneal spherical
aberration. A target of zero postoperative total spherical aberration Z(4,0) was calculated. One of three IOLs
was chosen, based on the corneal spherical aberration
Z(4,0) measurement at the 6-mm optical zone. The IOL
was selected based on the summation of the corneal
spherical aberration and the aspheric value of the prolate optic. The intention was an absolute value of zero
total spherical aberration. Statistical analysis of the postoperative total ocular wavefront profile was performed to
assess the accuracy of aspheric IOL selection.
RESULTS: Forty eyes of 40 patients were available
for postoperative assessment. The Tecnis Z9003
(Abbott Medical Optics) was implanted in 25 eyes
with a preoperative corneal spherical aberration of
⫹0.311⫾0.054 µm, the AcrySof IQ (Alcon Laboratories Inc) in 13 eyes (⫹0.188⫾0.034 µm), and the
SofPort-Advanced Optic with Violet Shield (Bausch &
Lomb) was implanted in 2 eyes (⫹0.0915 µm). Total postoperative ocular spherical aberration for the
entire group measured ⫹0.019⫾0.051 µm (Tecnis:
⫹0.024⫾0.058 µm; AcrySof IQ: ⫹0.010⫾0.035 µm; and
SofPort AOV: ⫹0.037 µm). Mean absolute predictive error,
for the entire group, measured ⫹0.025⫾0.020 µm.
CONCLUSIONS: Skiascopy-derived total wavefront measurement of spherical aberration is a reproducible method
of aspheric IOL selection and permits more precise control of total ocular spherical aberration. [J Refract Surg.
2010;xx:xxx-xxx.]
doi:10.3928/1081597X-
O
nce a procedure solely devoted to the safe removal
of a cataract, contemporary microincisional phacoemulsification and intraocular lens (IOL) implantation has developed into lens-based refractive surgery for
those who have developed visually significant cataracts. As
a result, increasing visual demands require the surgeon to
achieve greater control of refractive outcomes and ultimately
more precise visual enhancement.
Numerous studies have demonstrated the change in optical
aberration in the aging eye.1-6 The development of wavefront
sensors has made it possible to identify otherwise unquantifiable refractive measures. The study of wavefront optics has
led to the creation of aspheric IOLs, specifically designed to
mitigate the naturally occurring positive spherical aberration
of the cornea. Clinical studies comparing aspheric implants
with spheric implants have found that modified prolate optics significantly improve contrast sensitivity under mesopic
conditions.7-9
This increased knowledge has led to the management of
spherical aberration in the cornea and is the basis for the
US Food and Drug Administration (FDA) designating New
Technology IOL (NTIOL) status on four aspheric IOLs. Despite
disagreement as to the absolute value, studies suggest that
reduction of total ocular spherical aberration is optimal when
approaching emmetropia.10-13 Measuring 696 eyes, Beiko
et al14 reported a normal Gaussian distribution of corneal
spherical aberration. The mean spherical aberration across
a 6.0-mm pupil was ⫹0.274⫾0.089 µm and is consistent
with previously reported values in a study by Holladay et al.6
Therefore, the Tecnis Z9003 IOL (Abbott Medical Optics,
Santa Ana, Calif), the first aspheric lens granted NTIOL status, induces negative spherical aberration of 0.27 µm through
From Solomon Eye Physicians & Surgeons, Bowie, Md.
Dr Solomon is a consultant to Marco Ophthalmic, Tustin, Calif.
Correspondence: Jonathan D. Solomon, MD, 14999 Health Center Dr, Ste 108,
Bowie, MD 20716. Tel: 301.464.1886; Fax: 301.464.5455; E-mail: jdsolomon@
hotmail.com
Received: June 2, 2009; Accepted: November 11, 2009
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Aspheric IOL Selection Based on Corneal Spherical Aberration/Solomon
a modified anterior prolate surface, whereas the
AcrySof SN60WF IOL (Alcon Laboratories Inc, Ft
Worth, Tex) provides 0.20 µm of negative spherical aberration with a blue-light filtering chromophore and a
posterior prolate surface. Consequently, with decreasing dioptic power from center to edge, centration of
aspheric optics becomes critically important. Subsequent studies have shown an exaggeration of both lower and higher order aberration with as little as 0.5 to 1.0
mm of displacement from the visual axis.15,16 Bausch
& Lomb, therefore, created an IOL with zero spherical
aberration. The SofPort Advanced Optic with Violet
Shield IOL (LI61AOV; Bausch & Lomb, Rochester, NY)
combines an ultraviolet and violet-light filtering chromophore to a biprolate, silicone optic that is aspherically neutral, and therefore will not add or subtract
from pre-existing higher order aberrations. The SofPort
lens has a uniform thickness and refractive power resisting the refractive effect of decentration, even when
displaced as much as 1.0 mm.16
A growing clinical interest in wavefront profiles creates a strong commercial interest in making aberrometry
a part of daily clinical practice. Currently, instruments
used for the determination of ocular wavefront aberration are designed to measure focal shift—the principle
that an aberrated lens refracts an incident beam of light
in front or behind the expected focal point, given a perfect optical system, resulting in a shift of the refracted
beam with respect to the focal plane.
Practical comparison between the available aberrometers is difficult due to the variety of principles
used, such as Hartmann-Shack, Tscherning, ray tracing, and automatic retinoscopy. The ARK-10000 (OPDScan; NIDEK Co Ltd, Gamagori, Japan) uses the principle of dynamic skiascopy and is the only manufacturer
of this type of wavefront sensor for clinical purposes.
As a serial, double-pass aberrometer, an infrared light
slit and photodetectors are placed on a rotating wheel
along the same rotational position across the pupil.
Moving the incident beam along a specific pupillary
meridian results in a reflected beam that will go in the
same or opposite direction. As the wheel is rotated, the
instrument then compares the time it takes for light to
peak at each photodiode after passing a beam splitter.
The device calculates the optical pathway difference
and derives the wavefront error by comparing the results with the theoretical reference time and generates
a refractive map and wavefront profile. This procedure
yields 1440 data points within 0.4 seconds.17-19
The intention of the current study is to determine
the feasibility of targeting a total ocular spherical aberration of zero, utilizing the current cache of aspheric
IOLs, based on the calculated preoperative corneal
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spherical aberration and measured total ocular spherical aberration with the OPD-Scan.
PATIENTS AND METHODS
This prospective study comprised 40 eyes of 40
consecutive patients (23 men and 17 women) with
age-related visually significant cataracts, unsatisfactorily corrected with spectacles, who desired visual
improvement. Patients were excluded if any of the
following conditions were present: existing pathology
that would limit visual acuity or potentially compromise centration of the implant (ie, amblyopia, diabetic
retinopathy, macular degeneration, glaucoma, history
of uveitis, pseudoexfoliation syndrome), previous keratorefractive surgery, pupil dilation of ⬍6.0 mm (with
or without pharmacologic mydriasis), ⬎1.00 diopters
(D) of corneal astigmatism that would warrant correction, and potential visual acuity worse than 20/25.
Preoperatively, each patient was evaluated with corrected distance visual acuity (CDVA) and detailed slitlamp microscopic examination with applanation tonometry, which followed manual keratometry. Biometry,
anterior chamber depth, and corneal white-to-white
measurements were obtained with the IOLMaster (Carl
Zeiss Meditec Inc, Dublin, Calif) along with lens power calculations performed with the SRK-T formula.
Preoperative corneal topography was acquired through
an integrated Placido image and the corneal spherical aberration Z(4,0) is derived from the sixth order
Zernike expansion generated from the refractive map.
Near simultaneous measurement of total ocular spherical aberration Z(4,0) was reconstructed at the 6.0-mm
optical zone transposed to the corneal vertex. All measurements were performed with interphase software
analysis on the OPD-Scan version 2.11.04.
With the intent of achieving total ocular spherical
aberration of zero, the lens selection was based on preoperative corneal spherical aberration. Consequently,
the Tecnis Z9003 was the lens of choice for corneal
spherical aberration of ⬎⫹0.235 µm, whereas the
LI61AOV was selected for corneal spherical aberration
⬍⫹0.10 µm. For eyes with corneal spherical aberration
⬎⫹0.10 µm and ⬍⫹0.235 µm, the AcrySof IQ SN60WF
was selected.
All surgical procedures were performed by the
same surgeon (J.D.S.) using the Infinity (Alcon Laboratories Inc) phacoemulsification machine. Perioperative treatment included diclofenac (Voltaren; Novartis, E Hanover, NJ) and moxifloxacin (Vigamox, Alcon
Laboratories Inc). After topical anesthesia, surgery was
performed through a 2.75-mm, self-sealing, on-axis,
clear corneal incision and 1.0-mm paracentesis at 90°
from the main incision. Following anterior chamber
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Journal of Refractive Surgery • Vol. xx, No. x, 2010
Silicone
6
Three-piece
UV & violet blocking
0
Bausch & Lomb
Acyrlic
6
One-piece
Blue-light filter
⫺0.20
Alcon Laboratories Inc
Acrylic
⬍⫹0.10
⬍⫹0.235
⭓⫹0.10
⭓⫹0.235
IOL
Selection
*Total ocular spherical aberration was measured with a 6-mm pupil at 4 weeks postoperatively.
Material
Optic size (mm)
Design
Chromophore
Spherical aberration (µm)
Manufacturer
LI61AOV
Material
Optic size (mm)
Design
Chromophore
Spherical aberration (µm)
Manufacturer
AcrySof IQ
Material
6
Three-piece
Design
Optic size (mm)
UV blocking
⫺0.27
Advanced Medical Optics
Chromophore
Spherical aberration (µm)
Manufacturer
Tecnis
Aspheric IOL
71.5
74.5⫾4.8
(68 to 81)
78.5⫾4.8
(67 to 82)
Age (y)
2 (0:2)
13 (4:9)
25 (13:12)
No. Eyes
(F:M)
⫹0.0915
⫹0.188⫾0.034
(0.129 to 0.228)
⫹0.311⫾0.054
(0.244 to 0.408)
Preop Corneal
Z(4,0) µm
Aspheric IOLs Implanted in 40 Eyes After Cataract Surgery
TABLE
⫹0.037
⫹0.01⫾0.035
(⫺0.054 to 0.064)
⫹0.024⫾0.058
(⫺0.076 to 0.121)
Postop Ocular*
Z(4,0) µm
Aspheric IOL Selection Based on Corneal Spherical Aberration/Solomon
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Aspheric IOL Selection Based on Corneal Spherical Aberration/Solomon
Figure 1. Distribution of eyes based on the preoperative corneal Z(4,0)
spherical aberration in the study population.
Figure 2. Postoperative total ocular Z(4,0) spherical aberration compared to preoperative corneal Z(4,0) spherical aberration in the study
population.
inflation of an ophthalmic viscosurgical device, a capsulorrhexis of 5.5 mm in diameter was created. Hydrodisection and hydrodelineation allowed for uneventful phacoemulsification and irrigation/aspiration. The
IOL was implanted and centered in the bag followed
by ophthalmic viscosurgical device removal and gentle hydration of the corneal incisions.
Four weeks postoperatively, a thorough ophthalmic
examination was performed, which included CDVA and
repeat OPD-Scan with pupillometry, topography, and a
dilated measurement of corneal spherical aberration and
total ocular spherical aberration for a 6.0-mm pupil.
Predicted spherical aberration based on the summation of the preoperative corneal spherical aberration
and the labeled spherical aberration of the implanted
IOL, was compared to the measured total ocular spherical aberration at a 6.0-mm pupil.
measured for a 6.0-mm pupil. The two eyes with the
LI61AOV IOL measured 0.033 µm and 0.041 µm, respectively, for total ocular spherical aberration postoperatively. Eyes implanted with the Tecnis IOL measured
0.024⫾0.058 µm for postoperative total ocular spherical aberration, and the SN60WF measured 0.01⫾0.35 µm.
No statistically significant differences were found in total ocular spherical aberration between the two groups
(P=.32); because of the small sample size, comparisons
were not made with the L161AOV IOL.
When the predicted total ocular spherical aberration was compared to the measured postoperative total
ocular spherical aberration, the mean absolute error
was 0.025⫾0.020 µm. Subgroup analysis, assuming
equal variance, demonstrated no statistical significance between the mean absolute errors of the Tecnis
and SN60WF eyes (P=.76). Additionally, measurements of corneal spherical aberration postoperatively
were compared to preoperative values, which revealed
no statistical difference (P=.48), irrespective of lens selection. All but three eyes achieved a spherical aberration value within 0.1 µm of the intended zero. Each
of the three eyes measured ⬎0.37 µm for preoperative
corneal spherical aberration and all eyes, regardless of
preoperative measurements, were within 0.15 µm of
zero (Fig 2).
RESULTS
Of the 40 consecutive patients, mean preoperative
spherical equivalent refraction was ⫺1.19⫾2.44 D, and
the preoperative corneal spherical aberration measured
at a 6.0-mm pupil was 0.260⫾0.084 µm for the entire
study group (Table, Fig 1). As a result, 25 eyes were
selected to receive the Tecnis IOL and therefore had
corneal spherical aberration of ⬎0.235 µm with a mean
spherical aberration of 0.311⫾0.054 µm, whereas 13
eyes had Z(4,0) between 0.1 and 0.235 µm and received
the AcrySof SN60WF with a mean of 0.188⫾0.034 µm.
Two study patients had corneal spherical aberration
⬍0.1 µm and were implanted with the neutral aspheric L161AOV. The first patient’s right eye measured
⫹0.098 µm of corneal spherical aberration, and the
second patient, also a right eye, measured ⫹0.085 µm.
Postoperative mean spherical equivalent refraction for the entire group was ⫺0.13⫾0.33 D, with total
ocular spherical aberration of 0.019⫾0.051 µm when
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DISCUSSION
Precise postoperative outcomes are no longer determined by spherocylindrical refraction outcomes alone.
The appreciation of higher order aberrations and their
effect on visual performance is well documented.6-11
Combination of aberrations from both the anterior
cornea and the internal optics is a delicate process,
and aspheric IOLs are designed to mimic the refractive properties of the youthful crystalline lens by compensating for the natural prolate surface of the cornea.
Copyright © SLACK Incorporated
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Aspheric IOL Selection Based on Corneal Spherical Aberration/Solomon
This study was intended to determine the capability of
managing spherical aberration by coupling preoperative corneal spherical aberration with the appropriate
aspheric IOL to achieve a reproducible reduction in
total ocular spherical aberration. In the present study,
when measured by the OPD-Scan, the average total ocular spherical aberration following corneal spherical
aberration-guided cataract surgery is 0.019⫾0.051 µm.
To adequately investigate the control of postoperative ocular spherical aberration, precise preoperative corneal spherical aberration measurements are
required. It is important to remember that the refractive surface that provides the majority of the refractive
power of the eye is the anterior cornea surface. Unfortunately, a correlation between corneal radius and
keratometry and spherical aberration and Q-value has
demonstrated a poor correlation.14 A significant repeatability of corneal higher order aberration measurement with the Placido-disk–based videokeratoscope
has been reported previously,20,21 as well as the effect
small-incision cataract surgery has on corneal spherical aberration. Tong et al,22 when calculated with the
Humphrey Atlas topographer (Carl Zeiss Meditec) and
customized MatLab ray-tracing program (The MathWorks Inc, Natick, Mass), found a 3.0-mm clear corneal
incision merely reduced corneal spherical aberration
by an average 0.016 µm. In the current study, a similar
degree of consistency was found with pre- and postoperative corneal spherical aberration (preoperative
corneal spherical aberration 0.260⫾0.084 µm/postoperative corneal spherical aberration 0.247⫾0.089 µm
[P=.48]).
Postoperative ocular spherical aberration has been reported previously in eyes implanted with aspheric IOLs
and compared to eyes implanted with spheric IOLs.
Awwad et al9 found that when measured at a 6.0-mm
pupil with the LADARWave aberrometer (Alcon Laboratories Inc), 15 eyes implanted with an AcrySof SN60WF
IOL significantly reduced ocular spherical aberration
0.09⫾0.04 µm when compared to 13 eyes implanted
with an AcrySof SN60AT IOL (Z[4,0] = 0.43⫾0.12 µm)
(P⬍.0001). Caporossi et al23 and Kasper et al24 both measured total ocular spherical aberration with the Zywave
aberrometer (Bausch & Lomb). The former study calculated postoperative spherical aberration as 0.05⫾0.06 µm
at a 5.0-mm pupil for the Tecnis Z9000, 0.11⫾0.1 µm
for the AcrySof SN60WF, and 0.19⫾0.08 µm for the
Sofport L161AO,23 whereas Kasper et al24 implanted 20
eyes with the Tecnis Z9000 IOL and calculated a mean
postoperative ocular spherical aberration of ⫹0.17 µm
and a mesopic pupil diameter of 3.84 mm.
The LADARWave and the Zywave are both Hartmann-Shack aberrometers, which are objective, parJournal of Refractive Surgery • Vol. xx, No. x, 2010
JRSonlineSOLOMON_Jan15.indd 5
allel, double-pass methods and therefore limited in
their spatial resolution by the number and size of the
microlens.25 In addition, neither the LADARWave
nor the Zywave are capable of measuring the corneal
spherical aberration alone. As a result, in the study by
Caporossi et al,23 the corneal spherical aberration was
derived from analysis of the corneal elevation data of
the Zernike coefficients using a separate corneal topographer (Eye-Top; Costruttori Strumenti Oftalmici,
Florence, Italy). Kasper et al24 was forced to generate
measurements of corneal spherical aberration with
the Orbscan IIz (Bausch & Lomb). The use of two instruments, which require a patient change position to
calculate the corneal spherical aberration and the ocular spherical aberration has the potential to introduce
error, such as cyclotorsion. Unique to the OPD-Scan
is the integrated corneal topographer, which enables
corneal wavefront and total ocular wavefront data to
be generated within 0.2 seconds of each other aided by
a built-in eye tracker to ensure direct correlation and
minimize the chance for movement.
Furthermore, the aforementioned Hartmann-Shack
aberrometers calculate ocular spherical aberration relative to the line of sight,26 or the axis joining the pupillary center to the fovea, whereas Placido-based topographers use the vertex normal as the derivative for
corneal spherical aberration. Salmon et al27 reported on
subtle underestimates of fourth order aberrations owing
to the lack of a common axis. In the current study, the
OPD-Scan calculates both total and corneal aberrations
with regard to the vertex normal, without the need for
realignment, thereby avoiding the ambiguity of locating
the pupil center that is known to change with pupil size.
In fact, the power of the study is best appreciated when
we compare the postoperative total ocular spherical aberration to the predicted total ocular spherical aberration. For all eyes in our study, the mean absolute error
is 0.025⫾0.020 µm. Previously, Packer et al28 using the
iTrace (Tracey Technologies, Houston, Tex) to calculate the corneal spherical aberration, and then measuring postoperative ocular spherical aberration with the
WASCA aberrometer (AMO Wavefront Sciences LLC,
Albuquerque, NM) achieved a similar predictive value
when selecting an IOL based on corneal spherical aberration. Both aberrometers in their study measured ocular wavefront slope from the line of sight. Despite the
discrepancy in vertex normal and line of sight, previously published data have demonstrated reproducible
correlation between total aberration measured by the
iTrace and OPD-Scan.29 Furthermore, should alignment
be a significant concern, the anticipated effect would be
elevated coma, which was not evaluated in either study
and remains a source for future research.
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Aspheric IOL Selection Based on Corneal Spherical Aberration/Solomon
Aside from line of sight misalignment, when interpreting these data we must also remain cognizant of
the aspheric IOL’s sensitivity to positioning and acknowledge the possible contribution that tilt, decentration, and defocus may have in neutralizing total ocular
higher order aberrations, as well as spherical aberration. Although a more thorough optical analysis of IOL
centration represents a deficiency in the study, it was
presumed that the presence of a continuous curvilinear capsulorrhexis with 360° anterior capsular contact
minimized any effective of tilt or decentration.
As a feasibility study, we were concerned about the
potential for over- and under-estimation of both the
measurement and calculation of lower and higher order aberrations. He et al30 suggested that there are some
compensatory factors between both eyes that in part
are responsible for the aberrations in the cornea and
the whole eye. Therefore, in the design of this study,
only one eye of each patient was included to eliminate
the effective exaggeration of statistical tests as aberrations in right and left eyes exhibit some degree of correlation.14,31
Consistent with reports suggesting super vision is
found with greater prevalence in individuals with
slightly positive ocular spherical aberration,10,11 Beiko32
found improved contrast sensitivity under photopic
and mesopic conditions with a targeted residual ocular Z(4,0) of ⫹0.10um. This differs from the study by
Piers et al,13 which demonstrated in-focus contrast performance peaks with 0.0 µm of Z(4,0) using an adaptive optics vision simulator with five eyes of five patients. Further experimentation with adaptive optics
by Wang et al12 showed that for a 6.0-mm pupil and
zero defocus, the optimal ocular Z(4,0) was slightly
negative (⫺0.05 µm), shifting toward ⫹0.20 µm with
myopia of ⫺0.50 D, and back toward ⫺0.30 µm with
hyperopia of ⫹0.50 D. Image quality appears to be
maximized through an inverse relationship between
defocus and spherical aberration, although this remains a topic for further research.
Considering the variability in the literature combined with the propensity to err on the side of low levels of myopia when calculating lens power, it was our
decision to aim for near total elimination of spherical
aberration. Although ⫺0.11⫾0.33 D is the measured
postoperative mean spherical equivalent refraction for
all eyes, ⫺0.01⫾0.35 D, ⫺0.12⫾0.35 D, and ⫺0.31 D
are the postoperative mean spherical equivalent refraction for the Tecnis, SN60WF, and L161AOV IOLs, respectively. In turn, postoperative total ocular spherical
aberration for all eyes measured 0.02⫾0.051 µm and
0.024⫾0.58 µm for the Tecnis group, 0.01⫾0.35 µm for
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the SN60WF group, and 0.04 µm for the two eyes implanted with the L161AOV. This relationship between
lower order and higher order aberrations is important
to acknowledge as it appears to correlate most strongly
with visual performance.
Although limited in scope, this study demonstrates
the feasibility of analyzing preoperative corneal spherical aberration and calculating ocular spherical aberration following aspheric IOL selection. Furthermore,
dynamic skiascopy appears to be a reproducible method for calculating higher order aberration, specifically
spherical aberration in the pseudophakic eye. Further
clinical study is desirable to address the effective quality of vision.
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