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Post Myopic LASIK and PRK
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Myopic LASIK and PRK.

Determining Corneal Power after Myopic LASIK.

  prk

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:

Holladay IOL Consultant

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 surgery Aramberri "Double K" Method IOL power correction based on recent literature.

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.

read more about iol power calculations 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 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 -0.50 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.


  IOL Power   IOL Power    
Calculation Method   OD     OS    
           
Clinical History Method +17.50 D   +16.50 D    
           
Corneal Bypass Method +18.00 D   +18.50 D    
           
Feiz-Mannis Method +17.50 D   +16.50 D   Sometimes will
          over-correct
Latkany Method +17.50 D   +17.50 D    
           
Masket Method +17.50 D   +17.00 D   Correct IOL
          power is often
Wang Koch Maloney Method +17.00 D   +17.50 D   in the area
          between upper
Modified Masket Method +18.00 D   +17.50 D   and lower
          limits.
Topographic Central Power +17.00 D   +16.50 D    
           

Standard Keratometry +16.00 D   +15.50 D   Always below
          correct IOL power

Recommended Power   OD     OS    
           
SN60WF (IQ lens) +17.50 D   +17.50 D   Best overall
          estimate of
Target refraction -0.38 D   -0.38 D   IOL power


References

1. Aramberri J. Intraocular lens power calculation after corneal refractive surgery: Double K method. J Cataract Refract Surg 2003; 29: 2063-2068.

 

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