Laser vision correction was introduced in the 1980s.23–25
By now, many of the early patients have reached an age when cataracts are common,26–28
and some have already had cataract surgery.2,13,29–31
With millions of laser vision correction procedures performed each year, the number of cataract patients with previous laser vision correction will eventually increase. Therefore, a method to select IOL power that is not biased by previous laser vision correction is essential. The main challenge in calculating IOL power for cataract patients with previous laser vision correction is measuring the true posterior corneal power because the assumption about the fixed relationship between anterior and posterior curvatures no longer applies. This also leads to biased IOL depth prediction, which further compounds the problem.
Many methods have been proposed to improve the accuracy of IOL power selection for eyes that have undergone laser vision correction. These methods fall into two major categories. The first includes procedures that estimate corneal power.30,32–37
Some use historic information such as preoperative keratometry and laser vision correction–induced refractive change to correct the bias in conventional keratometry.32
Some use rigid gas permeable (RGP) contact lens over-refraction to estimate corneal power and bypass conventional keratometry.33,34
Others alter the calculation of corneal power from conventional keratometry by assuming a fixed posterior corneal curvature rather than a fixed keratometric index. This ensures that surgical alteration of the anterior curvature does not get propagated into an error in the estimation of posterior curvature (Koch method).30
The second category of methods that improve the accuracy of IOL power selection make direct adjustment to the IOL power.31,38–40
These techniques adjust the recommended IOL power using historical information such as laser vision correction–induced refractive change and include the methods proposed by Masket,38
and Latkany et al.31
These approaches are partially successful in reducing the unexpected refractive error after cataract surgery, but they are not always applicable or successful. Historical data are often missing or incomplete. Because the RGP contact lens over-refraction method relies on clinical refraction, it is not reliable in cases of denser cataracts when the vision is poor and the visual endpoint for clinical refraction is vague.41
Because of these limitations, poor cataract surgery outcome in postoperative laser vision correction patients is still a growing problem waiting for a solution after years of active research.26
Optical coherence tomography corneal mapping provides a reproducible estimate of the posterior corneal power. We reported the reproducibility to be within 0.10 D.20
The new OCT-based IOL formula combines OCT corneal mapping, OCT biometry, and IOLMaster measurements to accurately predict postoperative cataract surgery refraction. The early results indicate that the new formula performs as well as the standard theoretic IOL formula in eyes without previous laser vision correction. The OCT formula actually has smaller mean error in the predicted refractive outcome. This is probably because we based our IOL depth formula on the same experimental data, whereas the theoretic formulae used A-constants based on previous data. The standard deviation for the OCT formula predictive error is similar to those of the theoretic formulae. Thus, the OCT formula can be as predictive and precise as the current best practice.
Optical coherence tomography may also provide more accurate biometry inputs for theoretical ray-tracing formulae for IOL power calculations.42,43
In those methods, the surface data are necessary to trace rays from object to retina or vice versa based on Snell’s law (in paraxial limit, vergence tracing is equivalent to ray tracing). Optical coherence tomography is able to map both surfaces of the cornea, crystalline lens, and IOL with high resolution. The system used in this study is not suitable for this purpose because of motion artifact. Nevertheless, with the increasing scanning speed of Fourier domain OCT technology,44
motion artifacts can be reduced and accurate, reproducible ocular surface data may be obtained by OCT.
Although the sample size in the current study is relatively small, the results validate the accuracy and predictability of the new IOL formula in cataract patients without prior laser vision correction. The new OCT-based IOL formula might be a promising solution for cataract patients with prior laser vision correction because OCT measures the actual posterior corneal curvature instead of assuming a fixed value or a fixed relationship with the anterior curvature. It does not require the use of clinical history, which can be easily lost and forgotten in the intervening decades between laser vision correction and cataract surgery. It does not rely on subjective measurements such as RGP contact lens over-refraction. In the next step of development, we are conducting a prospective clinical study of the OCT-based IOL formula on cataract surgery patients who underwent laser vision correction. The IOL depth formula may be subject to revision after more cases are collected because currently it is based on a small sample. Future studies that compare the OCT-based formula with other tomography-based formulae (Orbscan [Bausch & Lomb, Rochester, NY], Pentacam [Oculus Optikgeräte GmbH, Wetzlar, Germany], and others) are necessary to ensure the most accurate procedure.