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Warren Hill, M.D.
IOL Intraocular Lens Power Calculations
After LASIK and PRK
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The following information contains an outline of several popular central corneal
power calculation methods that can be used after the various ablative forms
of keratorefractive surgery for myopia, such as LASIK and PRK, for IOL power
calculations.
As a general rule, IOL power calculations following all forms of
keratorefractive surgery should not be run using an uncorrected
2-variable, third generation formula. Instead, the "double K" feature
of the Holladay 2 formula (the small box labeled "Prior RK, LASIK...")
should be employed in this setting. If your office does not have the
Holladay IOL Consultant software package, a trial version can be
downloaded from the Internet at:
Most 2-variable formulas such as SRK/T, assume that the anterior and
posterior segments of the eye are more or less proportional and use
axial length, and keratometric corneal power, to estimate the
postoperative location of the IOL, known as the effective lens
position (ELPo). Unless a special correction is made, the artifact
of flat Ks following keratorefractive surgery will cause these formulas assume
a falsely shallow post-operative ELPo.5
The end result is that 2-variable formulas following RK, PRK and LASIK
will typically recommend less IOL power than required. This is a second and
little recognized source of unanticipated post-operative hyperopia.
This potential pitfall can be avoided by using the double K feature of
the Holladay 2 formula, or applying Aramberri's
"double K method" correction to the Holladay 1, Hoffer Q or SRK/T
formulas. (see references)
For the sake of illustration, we will run through the corneal power estimation
and IOL power calculation for an actual patient from our office.
Clinical history method
3,4
It is important to have accurate pre-LASIK refractive data for this calculation. The clinical history method for corneal power estimation was first described by Holladay, and later by Hoffer as:
Kp + Rp - Ra = Ka
Where...
Kp = the average keratometry power before keratorefractive surgery, and...
Rp = the spherical equivalent before keratorefractive surgery, and...
Ra = the stable spherical equivalent after keratorefractive surgery, then...
Ka = the estimate of the central corneal power after keratorefractive surgery.
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Central corneal power by keratometry will be referred to in diopters (D), even though it is better termed keratometric diopters. This is due to the fact that the cornea has a different index of refraction than manual keratometers or corneal topographers (1.3333 vs. 1.3375). 7
Corrected for an estimated vertex distance of 13 mm, the historical estimation
of the central corneal power of our patient for the right eye following LASIK
would be carried out as follows:
If the Ks before LASIK were 44.25 D / 45.50 D, and...
The refraction before LASIK was -7.50 +1.50 x 123, and...
The stable refraction after LASIK was -0.50 +0.50 x 150, then...
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Kp
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+ |
Rp
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- |
Ra
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= |
Ka
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(44.9)
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+ |
(-6.21)
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- |
(-0.25)
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= |
38.92 D
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Corrected for an estimated vertex distance of 13 mm, the historical estimation of the central corneal power of our hypothetical patient for the left eye following LASIK would be carried out as follows:
If the Ks before LASIK were 44.00 D / 45.25 D,
The refraction before LASIK was -8.75 +0.75 x 064, and...
The stable refraction after LASIK was -1.25 +1.00 x 060, then...
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Kp
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+ |
Rp
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- |
Ra
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= |
Ka
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(44.6)
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+ |
(-7.55)
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- |
(-0.74)
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= |
37.82 D
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Using this technique for estimating central corneal power, the following IOL powers are recommended by the Holladay 2 formula:
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SA60AT
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Target Refraction
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Right
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+21.50 D
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-0.50 D
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Left
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+23.00 D
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-0.50 D
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Feiz and Mannis IOL power adjustment method
1
Another method that is helpful to use when good historical data is available
is the IOL power adjustment method of Feiz and Mannis. With a target refraction
set for -0.50D, this is the method least likely to result in a post-operative
hyperopic surprise. As you will see at the end of this exercise, we will be able to use this
method to set an upper limit of possible IOL powers.
Using this technique, first the IOL power is calculated using the pre-LASIK corneal power as though the patient had not undergone keratorefractive surgery. This pre-LASIK IOL power is then increased by the amount of refractive change at the spectacle plane divided by 0.7. The calculation is for a -0.50 D result. This approach is outlined as follows:
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IOLpre
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+ |
(ΔD / 0.7)
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= |
IOLpost
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Where...
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IOLpre = the power of the IOL as if no LASIK had been performed, and...
ΔD = the refractive change after LASIK at the spectacle plane, then...
IOLpost = the estimated power of the IOL to be implanted following LASIK.
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The Feiz and Mannis IOL power adjustment method for the right eye following LASIK would be carried out as follows:
If the Holladay 2 calculated IOL power before LASIK is +12.50 D, and...
The change in refractive power at the spectacle plane is +6.50 D, then...
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IOLpre
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+ |
(ΔD / 0.7)
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= |
IOLpost
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+12.50 D
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+ |
(+6.50 / 0.7)
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= |
+21.78 D
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The Feiz and Mannis IOL power adjustment method for the left eye following LASIK would be carried out as follows:
If the Holladay 2 calculated IOL power before LASIK is +13.00 D, and...
The change in refractive power at the spectacle plane is +7.63 D, then...
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IOLpre
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+ |
(ΔD / 0.7)
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= |
IOLpost
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+13.00 D
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+ |
(+7.63 / 0.7)
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= |
+23.90 D
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Using this technique, the following IOL powers are recommended by the Holladay 2 formula:
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SA60AT
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Target Refraction
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Right
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+22.00 D
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-0.50 D
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Left
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+24.00 D
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-0.50 D
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Modified Maloney method
2,9
Another very useful method of post-LASIK corneal power estimation is one that
was originally described by the well-known refractive surgeon Robert Maloney
and subsequently modified by Doug Koch and Li Wang.
The advantage of this method is that it requires no historical data and has a
very low variance when used with either the Holliday 2 formula or the 2-variable
formula "double K method" correction nomogram published by Koch and Wang.2
Using this technique, the central corneal power is obtained by placing the cursor at the exact center of the Axial Map of the Zeiss Humphrey Atlas topographer. This value is then converted back to the anterior corneal power by multiplying the Axial Map central topographic corneal power by 376.0/337.5, which is the same as 1.114. An assumed posterior corneal power of 6.1 D is then subtracted from this product.
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(CCP x 1.114) - 6.1 D = Post-LASIK adjusted corneal power
Where...
CCP
= the corneal power with the cursor in the center of the topographic map.
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The Modified Maloney method for the right eye following LASIK is carried out as follows:
If the axial map central corneal power is 40.00 D, then...
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(CCP x 1.114)
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- |
6.1 D
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= |
Post-LASIK
adjusted corneal power
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(40.00 D x 1.114)
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- |
6.1 D
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= |
38.46 D
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The Modified Maloney method for the left eye following LASIK is carried out as follows:
If the axial map central corneal power is 39.90 D, then...
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(CCP x 1.114)
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- |
6.1 D
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= |
Post-LASIK
adjusted corneal power
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(39.90 D x 1.114)
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- |
6.1 D
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= |
38.35 D
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Using this technique for estimating central corneal power, the following IOL powers are recommended by the Holladay 2 formula:
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SA60AT
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Target Refraction
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Right
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+22.00 D
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-0.50 D
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Left
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+22.50 D
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-0.50 D
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Postoperative regression method
8
Yet another method of post-LASIK corneal power estimation is the postoperative regression method that has been shared with us by Doug Koch and Li Wang.
Using this technique, the 0 mm, 1 mm and 2 mm annular rings of the Numerical
View of the Zeiss Humphrey Atlas topographer are averaged together. This topographic-based
averaged central corneal power is then multiplied by 1.23 and then 10.41 is then
subtracted from this product. Like the Modified Maloney method, the advantage
of this method is that it requires no historical data.
(CCP
x 1.23) - 10.41 D = Post-LASIK adjusted corneal power
Where...
CCP
= the averaged values of the 0 mm, 1 mm and 2 mm annular rings.
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The postoperative regression method for the right eye following LASIK would be carried out as follows:
If the averaged topographic central corneal power is 40.07 D, then...
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(CCP x 1.23)
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- |
10.41 D
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= |
Post-LASIK
adjusted corneal power
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(40.07 D x 1.23)
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- |
10.41 D
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= |
38.88 D
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The postoperative regression method for the left eye following LASIK would be carried out as follows:
If the averaged topographic central corneal power is 39.40 D, then...
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(CCP x 1.23)
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- |
10.41 D
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= |
Post-LASIK
adjusted corneal power
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(39.40 D x 1.23)
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- |
10.41 D
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= |
38.05 D
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Using this technique for estimating central corneal power, the following IOL powers are recommended by the Holladay 2 formula:
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SA60AT
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Target Refraction
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Right
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+21.50 D
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-0.50 D
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Left
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+23.00 D
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-0.50 D
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Topographic central corneal power adjustment method
2,9
The last method of post-LASIK corneal power estimation was originated by Doug
Koch and Li Wang and is based on determining the central power of the cornea
using either the Zeiss Humphrey Atlas topographer, or the adjusted effective
refractive power (EffRPadj) of the Holladay Diagnostic Summary of
the EyeSys Corneal Analysis System.
Using this technique, the 1 mm and 2 mm annular power rings of the Numerical
View of the Zeiss Humphrey Atlas topographer are averaged together, or the adjusted
effective refractive power (EffRPadj) is determined, and that figure is reduced
by 19% for every diopter of myopia corrected by LASIK.
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CCP
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-
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(ΔD x 0.19) = Post-LASIK adjusted
corneal power
Where...
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CCP
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= |
the EffRPadj, or the averaged Zeiss Atlas central
corneal power, and...
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ΔD
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= |
the refractive change after LASIK at the spectacle plane.
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The topographic central corneal power adjustment method for the right eye would be:
If the averaged topographic central corneal power is 40.06 D, and...
The change in refractive power at the spectacle plane is +6.50 D, then...
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CCP
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- |
(ΔD x 0.19)
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= Post-LASIK adjusted corneal power
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40.06 D
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- |
(+6.50 x 0.19)
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= 38.83 D
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The topographic central corneal power adjustment method for the left eye would be:
If the averaged topographic central corneal power is 39.36 D, and...
The change in refractive power at the spectacle plane is +7.63 D, then...
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CCP
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- |
(ΔD x 0.19)
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= Post-LASIK adjusted corneal power
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39.36 D
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- |
(+7.63 x 0.19)
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= 37.91 D
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Using this technique for estimating central corneal power, the following IOL powers are recommended by the Holladay 2 formula:
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SA60AT
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Target Refraction
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Right
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+21.50 D
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-0.50 D
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Left
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+22.50 D
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-0.50 D
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Hard contact lens method
3,6
Following all forms of ablative keratorefractive surgery (LASIK, PRK, etc.) the literature now suggests that the hard contact lens method may be less accurate than originally thought. For this reason it is not recommended in this clinical setting.
Higher order optical aberrations following keratorefractive surgery
The higher order optical aberrations that often accompany the various forms of keratorefractive surgery, and the multifocal nature of some of these corneas, will remain unchanged following cataract surgery. Understandably, some patients mistakenly expect that cataract surgery will alleviate these symptoms. Unfortunately, this is not the case. It is important to discuss this fact with these patients prior to surgery so that their expectations will be realistic.
For example, the third and fourth order higher order aberrations produced by radial keratotomy can be as much as 35 times normal values. Significantly elevated third and fourth order aberrations are also seen following some of the early forms of LASIK that employed a large spot size and a non-Gaussian distribution strategy.
Also, the change from a prolate (steep central cornea) to an oblate (flattened
central cornea) ocular system produced by lowering the central corneal power
may result in decreased discrimination at higher spacial frequencies. This will
not change after cataract surgery.
Accuracy of intraocular lens power calculations following LASIK
It is important to explain to your patient in no uncertain terms that intraocular lens power calculations following all forms of keratorefractive surgery are, at best, problematic. You should also discuss the fact that in spite of our very best efforts, the final refractive result may end up more hyperopic, or more myopic than expected. The fact that five different methods are in use is eloquent testimony to how far we still have to go in this area.
It is a disappointment to all of us that given available technology, there is
simply no single reliable method to accurately determine the net central power
of these unusual eyes. Intraocular lens exchange or secondary piggyback implantation
after all forms of refractive surgery, are important parts of informed consent
prior to cataract surgery. Given the limitations of available technology, this
fact must be clearly understood by every patient as a well-recognized consequence
of prior keratorefractive surgery.
Below is a summary of IOL powers, generated by several forms for central corneal
power estimation. Some have certain characteristics, which we can use to better
understand what the correct IOL power may be. By effectively bracketing, it is
possible to modestly improve the accuracy of an inherently inaccurate exercise.
However, when refractive surgery results in a highly multifocal cornea, or there
is unaccounted for lenticular myopia, this system can show variable and unexpected
results.
When this system of bracketing breaks down, one or more pieces of the mathematical puzzle are either missing, masked, or inaccurate. Often, the calculations may be influenced our inability to determine the true post-LASIK refractive state (without the influence of lens-induced myopia).
It is generally accepted that IOL power calculations following keratorefractive surgery are always placed on the myopic side and are typically for -0.50 D. This helps to prevent unexpected post-operative hyperopia.
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IOL Power |
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IOL Power |
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Calculation Method
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OD
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OS
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Feiz and Mannis
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+22.00 D |
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+24.00 D |
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Often higher limit |
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correct IOL power |
| - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - |
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Clinical history
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+21.50 D |
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+23.00 D |
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Correct IOL |
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Modified Maloney
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+22.00 D |
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+22.50 D |
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power is often |
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in this area |
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Topo central power adjust
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+21.50 D |
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+22.50 D |
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between upper |
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and lower limits |
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Postoperative regression
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+21.50 D |
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+23.00 D |
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| - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - |
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Flattest measured K
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+20.50 D |
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+22.00 D |
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Often lower limit |
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correct IOL power |
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Sim Ks by topography
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+18.50 D |
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+20.00 D |
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Usually below |
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correct IOL power |
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Measured Ks
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+20.00 D |
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+21.00 D |
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Usually below |
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correct IOL power |
| - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - |
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SA60AT
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SA60AT
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Right Eye |
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Left Eye |
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Best overall |
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Recommended power
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+21.50 D |
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+22.50 D |
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estimate of |
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Target refraction
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-0.50 D |
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-0.50 D |
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IOL power |
| - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - |
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MTA4UO
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MTA4UO
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Right Eye |
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Left Eye |
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Recommended power
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+18.50 D |
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+19.50 D |
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Target refraction
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-0.50 D |
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-0.50 D |
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References
1. Feiz V., Mannis M.J. Garcia-Ferrer F. Intraocular lens power calculation after laser in situ keratomileusis for myopia and hyperopia a standardized approach.
Cornea 2001; 20:792-797.
2. Koch, D., Wang I. Calculating IOL power in eyes that have had refractive surgery.
J Cataract Refract Surg 2003 29(11) 2039-2042.
3. Holladay JT. Consultations in refractive surgery.
Refract Corneal Surg 1989; 5:203
4. Hoffer KJ. Intraocular lens power calculation for eyes after refractive keratotomy.
J Refract Surg 1995; 11:490-493
5. Aramberri J. Intraocular lens power calculation after corneal refractive surgery:
Double K method. J Cataract Refract Surg 2003; 29: 2063-2068.
6. Haigis W. Corneal power after refractive surgery for myopia: contact lens method.
J Cataract Refract Surg 2003 29 (7) 1397-1411.
7. Seitz B. Intraocular lens power calculation in eyes after corneal refractive surgery.
J Refract Surg 2000; 16:349-361
8. Personal communication, April, 2004. Douglas D. Koch, MD, Cullen Eye Institute, Baylor College of Medicine, Houston, Texas.
9. Wang L, Booth MA, Koch DD. Comparison of intraocular lens power calculations
methods in eyes that have undergone LASIK. Ophthalmology 2004 111(10) 1825-1831.
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iol power calculation after lasik
2008-09-13
Cataract Removal after lasik Cataract Removal after lasik Cataracts 5 years post-op Cataracts 5 years post-op IOL calculation error - IOL Calculations After RefractIOL calulations After Refract cataracts post-after lasik Cataract Removal after lasik I had After lasik 5 years ago. I had cataracts in both eyes. Now 5 years later the cataracts in both eyes have progressed to where my vision is down to 20/40 some days and 20/50 other days. I am considering cataract removal and need to know if there will be any problems because I've already had After lasik, such as determining the correct power of the IOL. After lasik Calculating the implant after lasik is very challenging. It is critical that your surgeon have the following information Your preop corneal curvature (called K's) Your preoperative refraction Your final refraction anywhere from 1 month to a year or so after the procedure (before the cataract progressed). With this information, a calculation of the proper intraocular lens can be made for the cataract surgery. You should be aware that the chance of getting right on target is much lower than for a patient that has never had AFTER LASIK. Cataracts 5 years post-op I will have my 5 year after lasik "anniversary" 2008-09-13T14:17:04-07:00. I think I may have had cataracts prior to after lasik, but my doc didn't pick up on that. They were diagnosed 17 months after lasik by a well-known opthalmologist 1,000 miles away. Fast forward to October 2004. My vision had been getting progressively worse. At first, my surgeon did not think it was the cataract causing all the diminishing vision. I had cataract surgery with my after lasik surgeon. He had all my stats from pre-op after lasik and everything since. Let me say that I was a high myope at -9.5. I came out of cataract surgery at -4.0 in that eye. Apparently my surgeon had problems calculating for my iol. Cataracts 5 years post-op What other options were offered to you to correct your IOL calculation error? IOL calculation error What were the options offered to you to correct your IOL calculation error? IOL Calculations After Refractive Surgery What does the Surgeon usually offer to correct IOL calculation error to refractive surgery patients? Is a lens exchange a reasonable option or enhancement with AFTER LASIK? IOL Calculations After Refractive Surgery What does the Surgeon usually offer to correct IOL calculation error to refractive surgery patients? Is a lens exchange a reasonable option or enhancement with AFTER LASIK? cataracts post-after lasik I have anisometria resulting from the miscalculations for my iop. I'm -4.0 in my post-cataract eye. My surgeon suggests that he can piggyback another iol over the first iol and make up for the over-minus. I've had that comment from other doctors I've consulted. I've been burned by after lasik resulting in dry eyes, corneal dystrophy, diplopa, irregular astigmatism, I don't feel having another surgery right now is an option. I reacted badly to after lasik, plus my outcome on the cataract surgery was less than good. My track record is not good with eye surgery. I've had some advice saying the piggy-backing iol's is not a good idea in my case. While I don't understand the dynamics of it all, it had something to do with pressure within the eye on an already compromised cornea. All the surgeon has to do is put in another iol. I also have capsular opacification already. This will require a procedure of laser by the YAG laser. Several docs have told me it's a 10 second procedure that can be done at the office. It seems that one thing just leads to another. The reason I'm posting this is for others who may not anticipate any problems post-after lasik. They can, and do happen. I, and others, are just finding out what some of the long-term complications post-after lasik can be. Docs sometimes recommend the piggy-backing of another iol because an iol exchange is more difficult. Or so I've heard. Personally, I would never consider having after lasik again and starting my whole stuff all over. I've actually had a new consult dr suggest that to me. He wrote on the insurance slip, wavefront over after lasik.
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