2-variable IOL power calculation formulas
The Question:
Assume Mrs. Smith had LASIK several years ago for -8.00 D of myopia and is now scheduled to undergo cataract surgery…
You have carefully gathered all the necessary historical information and measured the corneal power at its exact center using one of the newer, highly sophisticated topographers. The measured and historical numbers all line up, and you are confident that your estimation of central corneal power is as accurate as possible. Using the SRK/T formula, you calculate an IOL power that seems appropriate.
Her surgery goes well, but to your great surprise, the first post-operative week, Mrs. Smith has a spherical equivalent of +2.00 D. You go back and re-calculate everything, confirm the axial length, and still arrive at the same IOL power… What happened?
The Answer:
Variations within the IOL power calculation formulas
Until recently, our attention to the post-keratorefractive eye has mainly been focused on accurately determining the central corneal power. It is now becoming more widely understood that a flattened central cornea not only renders keratometry inaccurate but also causes problems with many IOL power calculation formulas that were previously trusted.
Third-generation, two-variable formulas, such as SRK/T, assume that the anterior and posterior segments of the eye are mostly proportional and use a combination of axial length and keratometric corneal power to estimate the postoperative location of the IOL, known as the effective lens position (ELPo). If the central corneal power is very low, as we see following keratorefractive surgery, the formula assumes that the anterior chamber is shallow.
Why is this important? It turns out that in a two-lens system, such as the eye (cornea and lens), the power of the intraocular lens can be thought of as something of a relative figure. For example, if a +21.00 D IOL placed within the capsular bag produces emmetropia, with only a 0.50 mm posterior displacement, that same lens has an effective power of +20.00 D. If that same lens is moved anterior by only 0.50 mm, it then would have an effective power of +22.00 D. Try holding your fingers 0.50 mm apart and the it’s easy to see why even a small miscalculation can lead to problems.
With a low central corneal power, the formula assumes that the IOL following cataract surgery will end up sitting closer to the cornea than normal and requires less power. And the flatter the cornea, the bigger a problem this becomes. Where the IOL power calculation formula thinks the lens will sit inside the eye can have a profound effect on the power that it recommends.
Unless a correction is made for this situation, the artifact of centrally flattened Ks following keratorefractive surgery will cause these formulas to assume a falsely shallow post-operative ELPo incorrectly. The result is that without a special correction, 2-variable formulas following LASIK, PRK, and RK will 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.
So, what do we do?
IOL power calculations following any form of keratorefractive surgery are best carried out using the Holladay 2 formula (contained within the Holladay IOL Consultant). If your office does not have this software package, a trial version can be downloaded from:
“Double K” correction method
The following articles are excellent and well worth the time it takes to go through them.
Lacking the Holladay 2 formula, instead you can use the “Double K” correction method in conjunction with the SRK/T, Hoffer Q, or Holladay 1 formulas as described in:
| Aramberri J. Intraocular lens power calculation after corneal refractive surgery: Double K method. J Cataract Refract Surg 2003; 29(11): 2063-2068. |
View this article here:
The companion editorial piece, cited below, has a very nice “Double K” method nomogram for the Hoffer Q, SRK/T, and Holladay 1 formulas:
| Koch, D., Wang, I. Calculating IOL power in eyes that have had refractive surgery. J Cataract Refract Surg 2003 29(11) 2039-2042. |
This article and its Hoffer Q, SRK/T, and Holladay 1 formula correction tables can be viewed at:
Basic correction tables for the Aramberri “Double K” method are available for each of the following formulas, as outlined in the article by Drs. Koch & Wang:
By making this additional correction, the accuracy of your refractive outcomes in the setting of cataract surgery following keratorefractive surgery should be significantly improved.
