Corneal ablation profiles for correction of hyperopia with the excimer laser.

PURPOSE

To calculate smooth corneal ablation profiles for the correction of hyperopia by means of excimer laser photorefractive keratectomy and to quantify the typical topographical changes, especially in the transition zone, in terms of axial distance and instantaneous radius of curvature.

METHODS

Conditions were determined to yield a smooth transition surrounding a hyperopic photorefractive keratectomy. Functions for the ablation depth in the transition zone were calculated. Theoretical optical zones of 4-mm diameter with 1 or 2-mm transition zones were required. The variation in axial and instantaneous power along one semimeridian was subsequently calculated. The power profile of a +10 diopters (D) correction on a spherical surface (47 D) was presented as an example for both ablation profiles.

RESULTS

In photorefractive keratectomy for hyperopia, the transition zone is necessarily flatter than the original cornea. The more abrupt the transition, the more pronounced the flattening. A central steepening of +10 D entailed a mid-peripheral flattening of -7.00 D or -15.00 D in transition zones of 2 and 1 mm width, respectively. In the transition zone, the mean rate of change of axial power was 15 and 45 D/mm, respectively.

CONCLUSION

Making the central cornea steeper by means of photorefractive keratectomy implies that the periphery should be flattened (axial power). The amount of flattening is directly proportional to the degree of steepening, and is relatively larger if the transition is more abrupt.