feature
Wavefront-Driven Ablations:
Which patients benefit?
BY STEVEN C. SCHALLHORN, M.D.

Minimizing the induction of higher-order aberrations (HOAs) after LASIK is important to achieve the best quality of vision. It is well known that conventional LASIK induces HOAs, the most prominent being spherical. We also know that, with most techniques, wavefront-guided LASIK still induces HOAs in most patients. However, it can also reduce HOAs, especially in patients with high amounts of preoperative HOAs. Wavefront-optimized LASIK is designed to be spherical-aberration neutral, meaning it does not increase spherical aberration. However, the treatment basis for wavefront optimized LASIK is still sphere and cylinder, as with conventional LASIK.

So the question then arises: Do patients with low preop HOAs benefit more from wavefront-guided procedures?

My colleagues and I have recently developed a model based on a simulation of collected wavefront data from a large sample size of patients, prior to and 1 month after surgery, for both conventional and wavefront-guided LASIK. Using an observed HOA distribution of normal eyes, 10,000 random preop wavefront maps were generated (simulated eyes). Postoperative HOAs were randomly assigned to be within one standard deviation of the induced terms dependent on both the type of surgery and the level of preop HOAs. This method ensured that the observed variability in outcome was matched in the model.

It was assumed that wavefront-optimized LASIK would produce no change in spherical aberration compared with conventional. It was also assumed that wavefront-optimized would increase or decrease other HOAs to the same amount as conventional surgery. I believe this is reasonable because while wavefront-optimized was designed to be spherical-aberration neutral, the treatment basis is still sphere and cylinder and does not reduce the induction of other types of HOAs.

The simulated eyes were grouped by the amount of preop HOAs: low (less than 0.3 μm), negative-spherical aberrations and highly aberrated. The basic outcome of the model revealed that conventional LASIK induces more HOAs than wavefront-optimized, and wavefront-optimized LASIK induces more HOAs than wavefront-guided.

What is the Clinical Relevance?

To discuss clinical relevance, it is important to define a significant increase in HOAs. After studying the effect of HOAs on vision as well as the precision of aberrometers to measure them, we defined an increase of 0.1 μm HOA or greater as significant.

Using an induction of >0.1 μm HOA as a clinical benchmark, we can re-analyze the model outcome to predict clinical relevance. In simulated eyes with less than
0.3 μm HOA preop, conventional is predicted to be twice as likely to induce a significant increase in HOAs compared to wavefront-optimized. Conventional is over five times more likely than wavefront-guided to induce a significant amount of HOAs. Wavefront-optimized is twice as likely as wavefront-guided to induce significant HOAs. Therein lies the clinical relevance.

This model predicts that the likelihood of inducing a significant amount of HOAs is twice as high with wavefront-optimized than with wavefront-guided in eyes that have very few HOAs preop.

In conclusion, based on these simulations, optimized is an improvement over conventional and is less likely to induce significant HOAs. However, the same advantage that optimized has over conventional, wavefront-guided has over optimized. Thus, in my opinion, patients with low preop HOAs are predicted to benefit more with a wavefront-guided vs. wavefront-optimized procedure.

Steven C. Schallhorn, MD, is the director of Cornea and Refractive Surgery at the Naval Medical Center, San Diego, Calif.