Myopic LASIK and PRK.
Determining Corneal Power after Myopic LASIK.
This section contains outlines 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.
If your office does not have the Holladay IOL Consultant software
package, a Free Trial Version can be downloaded
from the Internet at the following link:
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 post-operative ELPo.1 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.1 Follow this link to learn
how to perform a post-keratorefractive
"Double K" Method IOL power correction based on recent
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.
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
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
Accuracy of intraocular lens power calculations —
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
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 -0.50 D. This helps to lessen the possibility of unexpected post-operative
Below is the bracketing method for the patient whose calculations were carried
||correct IOL power
|SN60WF (IQ lens)
| Target refraction
1. Aramberri J. Intraocular lens power calculation after corneal refractive
surgery: Double K method. J Cataract Refract Surg 2003; 29: 2063-2068.