Download Optical Considerations for IOLs

IN THE NAME OF GOD
Optical Considerations for IOLs
A number of factors must be taken into consideration
when selecting IOLs,including :
1.Image magnification
2. Power selection
3.Piggy back IOls
4.Power after cornal refractive surgery
Image Magnification
Theoretically , replacing a clear or cataractous
crystalline lense with an artificial lens is optimal
form of aphakic correction.The majority of
aberrations and distortions produced by aphakic
spectacles derive from their placement anterior to
pupillary plane.These include:
Image magnification,Ring scotoma,Peripheral
distortion,jack-in-the box phenomen,decrease
useful peripheral field.
Image magnification – as much as 20% - 35% - is the
major disadvantage of aphakic spectacles.Contact lens
correction of aphakia magnifies the image 7%-12%
,IOLs magnify by 4% or less.Naturally ,a lens located in
the
posterior
magnification
chamber
than
a
produces
lens
in
less
the
image
anterior
chamber.Unilateral pseudophakic patients have better
stereoacuity and less aniseikonia than aphakic patients
whose vision is corrected with contact lens or
spectacle.
IOL Power Selection
Nontoric IOLs provide spherical correction of the
refractive error without any cylindrical component
to this spherical correction should be calculated
for each eye as accurately as possible.The optimal
postoperative refraction depends on the situation
and visual needs of the patient.
For example, if a unilateral cataract is present and
the fellow eye is more than 1.5 – 2 D hyperopic ,
the surgeon can consider making the operated eye
slightly hyperopic as well.This strategy avoid
inducing aniseikonia and anisophoria.In most
settings , however it is desirable to produce
emmetropia or slight myopia.
Biometric Assumption In IOL Selection
1.Axial length :
a) A scan ultrasonography
b) Partial coherence interferometry
2.Kerotometry or corneal topography
3.Anterior chamber depth or optical chamber
depth(OCD)
Ascan do not actually measure axial length.They
measure the time required for a sound pulse to
travel through the ocular media , reflect from the
retina , and return through the media.Sound moves
faster through the crystalline lens than through the
cornea , aqueous and vitreous.Even with in the lens
itself , the speed of sound can varry according to
the hardness of the cataract.
Ascan tends to understimate the axial lens of short eyes
and overstimate long eyes. 1 mm error in the
measurement of axial length result in a refractive error
of approximately 2.5 – 3 D . The two primary
Ascan
techniques – applanation (contact) and immersion – give
different readings.Applanation method may give a
shorter axial length measurement perhaps due to
corneal indentation.
Partial coherence interferometry measures the time
required for infrared light to travel to the retina . This
technique does not require contact with the cornea , so
corneal compression artifacts are eliminated . In
addition the patient must fixate a target thus , the
length measured is the path the light takes to the
fovea , the physiological axial length .
However , the media must be clear enough to
allow fixation and light transmission , in dense
cataract , ultrasound axial length measurements
may still be necessary . Partial coherence
interferometry
technique
is
probably
more
accurate and reproducible than ultrasound method
, although some cases will still require ultrasound
biometry .
Keratometry
Keratometry or corneal topography does not
measure corneal power directly , keratometry
measures only a small portion of central
cornea , viewing the cornea as a convex mirror.
Both
front
and
back
corneal
surfaces
contribute to corneal power and a keratometer
measures only the front surface.
Anterior Chamber Depth
Formulas
based
on
geometrical
optics
generally require a third parameter ,
anterior chamber depth or optical chamber
depth (OCD).
Power Prediction Formulas
The first type of IOL formula based on geometrical optics:
nvit
K
PIOL =
AL - acd
1 – [ K(acd) / naq ]
PIOL = Power of the IOL
K
= Dioptric
AL = Axial length
acd = anterior chamber depth
nvit = index of refraction of the vitreaus
naq = index of refraction of the aqueous
In the 1980 , Sanders , Retzlaff and Kraff took a
different approach.formula turned out to be a simple
linear equation , which was introduced as the SRK
formula.
P = A – (2.5 * axial length in mm) – ( 0.9 * average keratometry in diopter )
A constant , which is provided by manufacturers for their
lens ,is specific to each lens type .
Factors related to A constant: 1.lense position in the eye
2.haptic angulation
3.lens shape
This formula was less accurate for long or short eyes.
The power was too low in short eyes and too high in
longer eyes . the formula was later modified as the
SRKⅡ formula :
Axial length (AL)
Modified A constant
20 > AL
A=A+3
21 > AL ≥ 20
A=A+2
22 > AL ≥ 21
A=A+1
24.5> AL ≥ 22
AL ≥ 24.5
A = A (No change)
A = A − 0.5
Second generation formulas added modifications
based on AL for short or long.
Third and fourth generation formulas added
modifications for other factors such as corneal
curvature , ACD , and so on .
These formulas are too complex for convenient
hand calculation.
In the normal range of axial length (22 – 24.5 mm)
almost all formulas function does not have
discrepancies.
– But in medium long eye (24.5 – 26 mm) ,the
Holladay 1 formula is the most accurate.
– And is very long eyes ( > 26 mm) , the SRK/T is
more accurate.
– In short eyes ( < 22 mm ) the Hoffer Q formula is
more accurate.
– Holladay 2 formula equql the Hoffer Q in short
eyes but is not as accurate as the Holladay 1 or
Hoffer Q in average and medium long eyes.