Abstract
Purpose: :
Clinical observations indicate that wavefront guided myopic laser refractive surgery causes hyperopic over–correction. The goal of this study was to investigate the effect of correcting spherical aberration on refractive outcome after customized ablation.
Methods: :
The wavefront aberration and manifest refraction on 340 myopic eyes treated with Zyoptix customized LASIK (Bausch & Lomb) were measured up to six months post surgery. Mean preoperative sphere and cyl were –3.32 ± 1.54 D and –0.68 ± 0.59 D, respectively. Theoretical retinal image quality was assessed by calculating the optical modulation transfer function to study interactions between lower and higher order aberrations.
Results: :
Seventy four eyes (22% of our population) attained postoperative hyperopia (> 0.5D). On average, positive spherical aberration at 6 months post surgery was increased (0.17 ± 0.15 um for a 6 mm pupil) relative to before the treatment. Eyes with larger amounts of preoperative positive spherical aberration had a smaller increase in positive spherical aberration but attained larger hyperopic over–correction. This smaller induced spherical aberration had a statistically significant correlation with the degree of postoperative hyperopia (p < 0.001). This is because positive spherical aberration causes a myopic shift (–0.23D per 0.1 micron spherical aberration for a 6 mm pupil) in refraction. Conventional ablation algorithms tend to over–correct myopia to empirically compensate for this myopic shift. However, in a customized algorithm that reduces some of the positive spherical aberration, this interaction becomes smaller, resulting in postoperative hyperopia.
Conclusions: :
Hyperopic over–correction in wavefront guided myopic refractive surgery can be explained by the interaction between positive spherical aberration and defocus. Empirically adjusted conventional ablation algorithms need to be refined for wavefront guided ablation in a way that correction of spherical aberration is taken into account. Incorporating preoperative higher order aberrations into a wavefront guided ablation algorithm could improve the predictability of refractive outcome in customized laser refractive surgery.
Keywords: refractive surgery: optical quality • refractive surgery: LASIK • refractive surgery: other technologies