Correlational Analysis of the Effective Optical Zone with Myopia, Myopic Astigmatism, and Spherical Equivalent in LASIK, PRK, and SMILE.

Purpose: We assess the relationship between preoperative myopic sphere, astigmatism, and spherical equivalent and effective optical zone (EOZ) size, shape, and decentration within individual populations of post-LASIK, PRK, and SMILE patients.

Patients And Methods: A retrospective chart review was conducted with 118 LASIK, 144 PRK, and 41 SMILE eyes from 179 total patients that underwent compound myopic ablation. One-year postoperative Pentacam tangential difference maps were used for EOZ data measurements. Correlational analysis between compound myopic measures [sphere, cylinder, manifest refractive spherical equivalent (MRSE)] and EOZ parameters was performed, and differences between groups of myopic sphere and cylinder within each surgery type were assessed.

Results: An increase in absolute myopic sphere (and subsequent MRSE) is associated with a smaller EOZ area in SMILE (r=0.454, p=0.003) and a more circular EOZ shape in LASIK (r=0.396, p<0.001) and PRK (r=0.563, p<0.001). An increase in absolute myopic cylinder is associated with an increased EOZ area in all three surgery types [LASIK (r=-0.459, p<0.001), PRK (r=-0.716, p<0.001), SMILE (r=-0.429, p=0.005)] and a more elliptical EOZ in LASIK (r=-0.491, p<0.001) and PRK (r=-0.538, p<0.001).

Conclusion: While astigmatism may be correlated to EOZ size within all three refractive surgery types, myopic sphere alone is insufficient to estimate EOZ size differences for procedures with a large blend zone of ablation like LASIK or PRK. Shape is just as important a factor as size to consider when examining corneal EOZ differences; reported correlative findings likely result from inherent differences in surgical technique and abruptness of planned surgical ablation borders.