Kuang-Mon Tuan

Corneal asphericity and visual function after wavefront-guided LASIK.

Purpose. In recent years, a theory has been advanced that corneal asphericity in and of itself determines visual function; that the natural, optimal shape of the cornea is prolate; and that changing the cornea from prolate to oblate negatively impacts visual results. This article presents an analysis of the impact of corneal asphericity on wavefront-guided LASIK. Method. A retrospective analysis was conducted of 160 myopic eyes that had undergone wavefront-guided LASIK. Surgical procedures and data collection were conducted at six clinical sites. Corneal topography, visual acuity, and contrast sensitivity data were collected before and 6 months after surgery. The topographically measured corneal surface of each eye was fitted to a conic, and a Q-value was computed for a 5.5-mm pupil. Multivariate regression analysis was performed to evaluate the correlation between Q-value and visual function. The relationship of changes in the corneal surfaces to visual performance was also investigated. Results. Preoperative corneas exhibited negative (prolate) conic shape constants. Postoperative corneas were about equally divided between positive (oblate) and negative conics. There was no statistically significant correlation between corneal shape and visual acuity or contrast sensitivity function. Changes in corneal asphericity after surgery had no significant correlation with changes in visual acuity or contrast sensitivity. Conclusion. Visual acuity and contrast sensitivity after wavefront-guided LASIK are not dependent on corneal asphericity. Neither preserving nor inducing asphericity ensures better visual outcome. Better visual outcomes are more likely to result from the application of a customized shape than a standard conic shape.

Adaptation to the prismatic effects of refractive lenses

Adaptation to the simple visual displacement of prisms was compared to that for refractive lenses, which have a varied prismatic effect. Subjects were made myopic using contact lenses, then corrected using spectacle lenses. The effect on the perceived direction of a randomly located target was assessed from pointing behavior. Prism adaptation showed a negative directional aftereffect but lacked intermanual transfer. Lens adaptation lacked a negative aftereffect but exhibited intermanual transfer. The results suggest that lens adaptation involves a recalibration of extraretinal eye movement information and multiple sets of lens adaptation can be retained for short periods.