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IOL Calculations
Determining Corneal Power
Following LASIK and PRK
The following information contains an outline of several popular IOL power calculation
methods that can be used following the various ablative forms of keratorefractive surgery
for myopia, such as LASIK and PRK.
IOL power calculations following keratorefractive surgery should not be carried out using
standard keratometry combined with any one of several popular 2-variable third generation
theoretic formulas, such as SRK/T without a special correction. Instead, formulas with
special adaptations, such the Holladay 2 formula (contained in the Holladay IOL
Consultant), more modern regression techniques, or the myopic surgery specific Haigis-L
formula (contained within the newest software release of the IOLMaster) should be used in
this setting. To date, our office has had the best overall success with the combination of
the Holladay Equivalent K feature of the Pentacam and the Holladay 2 formula contained
within the Holladay IOL Consultant.
Basic information about the Holladay Equivalent K feature of the Oculus Pentacam can be
found at:
http://www.oculususa.com/prd_comp.php
If your office does not have the Holladay IOL Consultant software package, a trial version
can be downloaded from the Internet at:
http://www.docholladay.com/iolprogram.html
A major shortcoming of most 3rd generation, 2-variable formulas, such as SRK/T, is that
they often assume that the anterior and posterior segments of the eye are mostly
proportional and use only the axial length and keratometric corneal power to estimate the
postoperative location of the IOL, known as the effective lens position (ELPo).
Unless a specific correction is made for this situation, the artifact of centrally flattened Ks
following keratorefractive surgery will have these formulas assume a falsely shallow postoperative ELPo.5 The end result is that without a special correction, following LASIK
these formulas will typically recommend less IOL power than is actually required. This is a
second, and little recognized, source of unanticipated post-operative hyperopia following
keratorefractive surgery for myopia.
The SRK/T, Hoffer Q and Holladay 1 formulas can be used with caution, but it must be in
conjunction with what has been termed an Aramberri double K method correction.5 You
can review a post-keratorefractive surgery “Aramberri double K method” IOL power
correction table based on recent literature at the following Internet location:
www.doctor-hill.com/iol-main/prior-keratorefractive.htm
Using the Holladay 2 formula, or a special “Aramberri double K method” correction for the
SRK/T, Hoffer Q, or Holladay 1 formulas, you and your staff should now be able to do
these calculations.
For the sake of illustration, we will run through several methods of central corneal power
estimation and IOL power calculation using the data from a patient recently seen in our
office.
If you are not yet experienced in doing this type of calculation, it is recommended that you
read through this entire document before beginning.
Clinical history method 3,4
The clinical history method for corneal power estimation was first described by Holladay
and later by Hoffer as:
Kp + Rp - Ra = Ka
Kp
Rp
Ra
Ka
=
=
=
=
Where …
the average keratometry power before keratorefractive surgery, and …
the spherical equivalent before keratorefractive surgery, and …
the stable spherical equivalent after keratorefractive surgery, then …
the estimate of the central corneal power after keratorefractive surgery.
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 keratometers, or corneal topographers (1.3333 vs. 1.3375).7
Corrected for an estimated vertex distance of 12 mm, the historical estimation of the
central corneal power of the right eye following LASIK would be carried out as follows:
If the Ks before LASIK were 45.37 D / 46.00 D, and …
The refraction before LASIK was -6.00 +0.75 x 135, and …
The stable refraction after LASIK was -0.25 sphere, then …
Kp
+
Rp
-
Ra
=
Ka
(45.69) + (-5.27) - (-0.25) = 40.67 D
Corrected for an estimated vertex distance of 12 mm, the historical estimation of the
central corneal power of the left eye following LASIK would be carried out as follows:
If the Ks before LASIK were 45.25 D / 46.12 D, and …
The refraction before LASIK was -5.50 +0.75 x 060, and …
The stable refraction after LASIK was -1.25 +0.25 x 070, then …
Kp
+
Rp
-
Ra
=
Ka
(45.69) + (-4.83) - (-1.11) = 41.97 D
If you are unfamiliar with how to correct for vertex distance, click here to download an
Excel spreadsheet that you can use to do this calculation. Be sure to maintain the correct
sign (+ or -) when entering the refractive data, or the result will not be accurate.
Using this technique for estimating central corneal power, the following IOL powers are
recommended by the Holladay 2 formula:
Right
Left
SN60WF
+17.00 D
+17.50 D
Target Refraction
Plano
-0.38 D
+16.00 D
+16.50 D
Plano
-0.38 D
Feiz and Mannis IOL power adjustment method 1
Another method that I like to use as an aid in determining the overall refractive picture
when good historical data is available is the IOL power adjustment method of Feiz and
Mannis. This is the method that is least likely to result in a post-operative hyperopic
surprise. As you will see on the last page, we can use this method to set the upper limit of
possible IOL powers as the Feiz and Mannis method will frequently produce a myopic
over-correction.
Using this technique, first the IOL power is calculated using the pre-LASIK keratometry as
though the patient had never undergone keratorefractive surgery. There is no special
formula correction that needs to be employed for the Feiz and Mannis method. This
calculated pre-LASIK IOL power is then increased by the amount of refractive change at
the spectacle plane divided by 0.7. This approach is outlined as follows:
IOLpre - (∆D 0.7) = IOLpost
Where …
IOLpre = the power of the IOL using pre-LASIK keratometry, and …
∆D = the stable refractive change after LASIK at the spectacle plane, then …
IOLpost = the estimated power of the IOL to be implanted following LASIK.
The Feiz and Mannis IOL power adjustment method for the right eye following LASIK
would be carried out as follows:
If the calculated IOL power before LASIK is +9.18 D, and …
The change in refractive power at the spectacle plane is -5.38 D, then …
IOLpre
+9.18 D
-
(∆D / 0.7)
= IOLpost
- (-5.38 / 0.7) = +16.87 D
The Feiz and Mannis IOL power adjustment method for the left eye following LASIK
would be carried out as follows:
If the calculated IOL power before LASIK is +10.27 D, and …
The change in refractive power at the spectacle plane is -4.00 D, then …
IOLpre
-
(∆D / 0.7)
= IOLpost
+10.27 D - (-4.00 / 0.7) = +15.98 D
Using this technique, the following IOL powers are recommended:
Right
Left
SN60WF
+17.00 D
+17.50 D
Target Refraction
Plano
-0.38 D
+16.00 D
+16.50 D
Plano
-0.38 D
Modified Maloney method 2, 9
Another 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 been reported to
have a low variance when used with either the Holliday 2 formula or a 2-variable formula
combined with an Aramberri double K method correction nomogram published by Koch
and Wang.2
Using this technique, the central corneal power is estimated 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.
(CCP x 1.114) - 6.1 = Post-LASIK adjusted corneal power
Where …
CCP = the corneal power with the cursor in the center of the Axial Map
of the Zeiss Humphrey Atlas topographer.
The Modified Maloney method for the right eye following LASIK is carried out as follows:
If the Axial Map central corneal power is 40.91 D, then …
(CCP x 1.114) - 6.1 = Post-LASIK adjusted corneal power
(40.91 D x 1.114) - 6.1 = 39.47 D
The Modified Maloney method for the left eye following LASIK is carried out as follows:
If the Axial Map central corneal power is 41.36 D, then …
(CCP x 1.114) - 6.1 = Post-LASIK adjusted corneal power
(41.36 D x 1.114) - 6.1 = 39.98 D
Using this technique for estimating central corneal power, the following IOL powers are
recommended by the Holladay 2 formula:
Right
Left
SN60WF
+16.50 D
+17.00 D
Target Refraction
Plano
-0.38 D
+17.00 D
+17.50 D
Plano
-0.38 D
Masket method 10
Another useful method of post-LASIK corneal power estimation is a postoperative
regression method developed by Samuel Masket and recently published in the Journal of
Cataract and Refractive Surgery. This technique takes advantage of the fact that there
appears to be a linear relationship between the spherical equivalent of the total amount of
the stable laser vision correction (LSE) and the over-estimation of central corneal power by
simulated keratometry. This works following both myopic and hyperopic LASIK.
By this method, the IOL power is calculated using the Holladay 1 formula for axial lengths
greater than 23.0 mm and the Hoffer Q formula for axial lengths less than 23.0 mm. The
SRK/T formula is generally not recommended here as the artifact of very flat Ks may
sometimes result in an under-correction. The IOL power is calculated without a double K
method correction. Ks are provided by simulated keratometry. The calculated IOL power is
then adjusted by the vertex distance corrected laser vision correction spherical equivalent
determined at four to six months after the procedure, multiplied by -0.326 with 0.101 added
to this product. Click here for an Excel spreadsheet to correct for vertex distance.
(LSE x -0.326) + 0.101 = Post-LASIK IOL power adjustment
Where …
LSE = The vertex distance corrected laser vision correction spherical equivalent.
The Masket method for the right eye following LASIK would be carried out as follows:
If the calculated IOL power is +15.26 D for a -0.25 D result, and …
The stable laser vision correction spherical equivalent is -5.38 D then …
(LSE x -0.326) + 0.101 = Post-LASIK IOL power adjustment
(-5.05 D x -0.326) + 0.101 = +1.75 D = IOL power adjustment
+1.75 D + 15.26 D = 17.01 D = Final adjusted IOL power
The Masket method for the left eye following LASIK would be carried out as follows:
If the calculated IOL power is +15.09 D for a -0.25 D result, and …
The stable laser vision correction spherical equivalent is -4.00 D then …
(LSE x -0.326) + 0.101 = Post-LASIK IOL power adjustment
(-3.82 D x -0.326) + 0.101 = +1.35 D = IOL power adjustment
+1.35 D + 15.09 D = 16.44 D = Final adjusted IOL power
Using this technique for estimating central corneal power, the following IOL powers are
recommended:
Right
SN60WF
+17.00 D
+17.50 D
Target Refraction
Plano
-0.38 D
Left
+16.50 D
+17.00 D
Plano
-0.38 D
Modified Masket method 10, 12
During the process of validating the original Masket method, our data produced a slightly
different regression formula. As with the Masket method, this technique takes advantage of
the fact that there appears to be a linear relationship between the spherical equivalent of the
total amount of the stable laser vision correction (LSE) and the over-estimation of central
corneal power by simulated keratometry. This works for myopic and hyperopic LASIK.
By this method, the IOL power is calculated using the Holladay 1 formula for axial lengths
greater than 23.0 mm and the Hoffer Q formula for axial lengths less than 23.0 mm. The
SRK/T formula is generally not recommended as the artifact of very flat Ks may sometimes
result in an under-correction. Ks are provided by simulated keratometry. The IOL power is
calculated without a double K method correction. Ks are provided by simulated
keratometry. The calculated IOL power is then adjusted by the vertex distance corrected
laser vision correction spherical equivalent determined at four to six months after the
procedure. It is then multiplied by -0.4385 and 0.0295 added to this product. Click here for
an Excel spreadsheet that you can use to correct for vertex distance.
(LSE x -0.4385) + 0.0295 = Post-LASIK IOL power adjustment
Where …
LSE = The vertex distance corrected laser vision correction spherical equivalent.
The modified Masket method for the right eye would be carried out as follows:
If the calculated IOL power is +15.26 D for a -0.25 D result, and …
The stable laser vision correction spherical equivalent is -5.38 D then …
(LSE x -0.4385) + 0.0295 = Post-LASIK IOL power adjustment
(-5.05 D x -0.4385) + 0.0295 = +2.25 D = IOL power adjustment
+2.25 D + 15.30 D = 17.55 D = Final adjusted IOL power
The modified Masket method for the left eye would be carried out as follows:
If the calculated IOL power is +15.09 D for a -0.25 D result, and …
The stable laser vision correction spherical equivalent is -4.00 D then …
(LSE x -0.4385) + 0.0295 = Post-LASIK IOL power adjustment
(-3.82 D x -0.4385) + 0.0295 = +1.71 D = IOL power adjustment
+1.71 D + 15.09 D = 16.80 D = Final adjusted IOL power
Using this technique for estimating central corneal power, the following IOL powers are
recommended:
Right
SN60WF
+17.50 D
+18.00 D
Target Refraction
Plano
-0.38 D
Left
+17.00 D
+17.50 D
Plano
-0.38 D
Topographic central corneal power adjustment method 2, 9, 11
Another 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, 2, 3 and 4 mm power values of the Numerical View of the
Zeiss Humphrey Atlas topographer are averaged together and used as the central corneal
power value (Ccp). As an alternative, the adjusted effective refractive power (EffRPadj) of
the Holladay Diagnostic Summary of the EyeSys Corneal Analysis System can be used.
This value is then reduced by 19% for every diopter of myopia corrected by LASIK. We
have added the 3 mm and the 4 mm power values from the Numerical View of the Zeiss
Humphrey Atlas topographer to this calculation based on the fact that Holladay has found
that sampling the 4.0 mm central cornea for the post-LASIK eye has returned very good
results with the Oculus Pentacam.11 Our own clinical results have also shown an increase
in accuracy by doing this.
CCP - (∆D x 0.19) = Post-LASIK adjusted corneal power
Where …
CCP = the EffRPadj, or the averaged Zeiss Atlas central corneal power, and …
∆D = the refractive change after LASIK at the spectacle plane.
The topographic central corneal power adjustment method for the right eye would be:
If the averaged topographic central corneal power is 41.97 D, and …
The change in refractive power at the spectacle plane is -5.38 D, then …
CCP + (∆D x 0.19) = Post-LASIK adjusted corneal power
41.97 D + (-5.38 D x 0.19) = 41.95 D
The topographic central corneal power adjustment method for the left eye would be:
If the averaged topographic central corneal power is 42.61 D, and …
The change in refractive power at the spectacle plane is -4.00 D, then …
CCP + (∆D x 0.19) = Post-LASIK adjusted corneal power
42.61 D + (-4.00 D x 0.19) = 41.85 D
Using this technique for estimating central corneal power, the following IOL powers are
recommended by the Holladay 2 formula:
Right
Left
SN60WF
+16.50 D
+17.00 D
Target Refraction
Plano
-0.38 D
+16.00 D
+16.50 D
Plano
-0.38 D
Corneal bypass method
14
Another method of post-LASIK corneal power estimation is one that was recently described
by Walter et al in the March, 2006 issue of the Journal of Cataract and Refractive
Surgery. The advantage of this method is that it is done without having to calculate the postLASIK corneal power.
Using this technique, the IOL power is calculated using the post-LASIK axial length and the
pre-LASIK keratometry. The target refraction is set for the pre-LASIK spherical equivalent.
This “bypasses” the post-LASIK corneal power.
The corneal bypass method for the right eye would be carried out as follows using the postLASIK axial length:
If the pre-LASIK keratometry was 45.37 D / 46.00 D, and...
The spherical equivalent before LASIK was -5.62 D, and ...
The post-LASIK IOLMaster axial length is 26.32 mm, then ...
The IOL power for a -5.62 D refractive result would be +17.27 D
The corneal bypass method for the left eye would be carried out as follows using the postLASIK axial length:
If the pre-LASIK keratometry was 45.25 D / 46.12 D, and...
The spherical equivalent before LASIK was -5.12 D, and ...
The post-LASIK IOLMaster axial length is 25.95 mm, then ...
The IOL power for a -5.12 D refractive result would be +18.23 D
Using this technique for estimating central corneal power, the following IOL powers are
recommended by the Holladay 2 formula:
Right
Left
SN60WF
+17.50 D
+18.00 D
Target Refraction
Plano
-0.38 D
+18.50 D
+19.00 D
Plano
-0.38 D
Oculus Pentacam 12, 13
The final and most useful method uses the Oculus Pentacam which images the anterior
segment of the eye by a rotating Scheimpflug camera for 360°, generating an anterior
segment picture in three dimensions. 25,000 corneal elevation points are measured and the
center of the cornea is measured precisely as a result of this rotational imaging process. All
keratometers and most corneal topographers do not see the central cornea, which gives the
Pentacam a significant advantage over these other methods in terms of accuracy.
The “Holladay Report” feature of the Pentacam uses information from direct
measurements of both the anterior and the posterior cornea within a 4.0 mm area to
generate a central corneal power value in keratometric diopters. This value can then be
used with the Holladay 2 formula for IOL power calculations, or with a 3rd generation, 2variable formula and an Aramberri double K method correction.
We have found the Pentacam to be one of the single most accurate measurement
techniques to date for the eye with prior myopic LASIK.
Pentacam 4.0 mm Holladay Equivalent K value for the right eye: 40.30 D
Pentacam 4.0 mm Holladay Equivalent K value for the left eye: 41.00 D
Using this technique for estimating central corneal power, the following IOL powers are
recommended by the Holladay 2 formula:
Right
Left
SN60WF
+17.50 D
+18.00 D
Target Refraction
Plano
-0.38 D
+17.50 D
+18.00 D
Plano
-0.38 D
Hard contact lens method 3, 6
Following all forms of ablative keratorefractive surgery (LASIK, PRK, etc.) a review of
the literature now suggests that the hard contact lens method may be less accurate than
originally thought. For this reason it is no longer 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, such as an increase in positive spherical aberration (Z 4,0) and
the multifocal nature of some of these corneas, will remain 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. 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 may not significantly improve after cataract surgery.
One helpful addition to our surgical armamentarium is the use of special lenses, such as
the AMO Tecnis, or the Alcon IQ lens which help to reduce the addition of positive
spherical aberration resulting from a flattened central cornea.
Accuracy of intraocular lens power calculations following LASIK
It is important that our patients understand that intraocular lens power calculations
following all forms of keratorefractive surgery are, at best, problematic and represent only
an estimate. 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
multiple methods are currently in use in this regard is eloquent testimony to how far we
still have to go in developing a meaningful system of intraocular lens power calculations
for the post-keratorefractive eye.
The possibility of an intraocular lens exchange, or a secondary piggyback implantation
after all forms of refractive surgery, are important parts of informed consent prior to
cataract surgery in this clinical setting. Given the limitations of available technology, this
fact must be clearly understood by every patient as a well-recognized consequence of
prior keratorefractive surgery.
Regarding Monovision
Many patients opt for monovision as part of their laser vision correction strategy for
LASIK or PRK. This works well in the setting of laser vision correction, but for IOL
power calculations following LASIK or PRK, the level of accuracy required to achieve
this may not be possible. For monovision to work in a satisfactory manner, the IOL power
calculation accuracy typically needs to be carried out to an accuracy of within ±0.25 D,
which is far beyond the resolution of this exercise. If your patient would like to be more
myopic than the values obtained by normal calculation methods, you can do so by simply
increasing the power of the IOL by approximately 1.4 times per diopter . However, the
farther away from emmetropia, the less accurate the calculation becomes.
IOL Power Determination
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 bracketing between what is most likely an over-correction and what is most certainly
an under-correction, 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 approach 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). This is why for any of the historical
methods that the post-LASIK refractive error is usually determined at four to six months
after LASIK. This is long enough to be stable, but close enough to the procedure that
lens-induced myopia does not become a factor.
It is generally accepted that IOL power calculations following keratorefractive surgery are
typically placed on the myopic side of plano anywhere from -0.25 D to -0.75 D. This helps
to lessen the possibility of unexpected post-operative hyperopia.
Below is the bracketing method for the patient whose calculations were carried out above.
Calculation method
Feiz and Mannis
IOL Power OD
+17.50 D
IOL Power OS
+16.50 D
Sometimes will over-correct.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Clinical history
+17.50 D
+16.50 D
Modified Maloney
+17.00 D
+17.50 D
Masket method
+17.50 D
+17.00 D
Correct IOL power is often in this
Modified Masket
+18.00 D
+17.50 D
area between upper and lower limits
Corneal power adjustment
+17.00 D
+16.50 D
Corneal bypass method
+18.00 D
+19.00 D
Pentacam 4.0 HEK
+17.50 D
+18.00 D
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Measured Ks
Recommended power
SN60WF (IQ lens)
Target refraction
+16.00 D
Right Eye
+15.50 D
Always below correct IOL power
Left Eye
+17.50 D
+17.50 D
-0.38 D
-0.38 D
Best overall estimate of IOL power
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) 18251831.
10.
Masket S, Masket SE. Simple regression formula for intraocular lens power
adjustment in eyes requiring cataract surgery after excimer laser photoablation.
J Cataract Refract Surg 2006 32 (3) 430 - 434.
11.
Personal communication, April, 2006. Jack Holladay, MD, Houston, Texas.
12.
Hill WE. IOL power calculations following keratorefractive surgery. Presented at
Cornea Day of the Annual Meeting of the American Society of Cataract and
Refractive Surgery, San Francisco, California, March 17, 2006
13.
Holladay JT. Measuring corneal power after corneal refractive surgery. Insert to
Cataract and Refractive Surgery Today. January, 2006. 4–6.
14.
Walter KA, Gagnon MR, Hoopes PC, Dickenson PJ. Accurate intraocular lens power
calculation after myoic laser in sitiu keratomileusis bypassing corneal power. J
Cataract Refract Surg 2006 32 (3) 425 - 429.