Assessment of ablation zone centration after photorefractive keratectomy using a vector center of mass formula.

Abstract

Adequate centration of keratorefractive surgical procedures is essential for a successful outcome. An accurate technique to mathematically describe the centration and topography of the ablation zone after photorefractive keratectomy (PRK) would be valuable in assessing the effects of these variables on subsequent visual results. A vector center of mass formula and computerized videokeratography were used to study the postoperative treatment zone centration and topography of 17 consecutive highly myopic patients (-6.00 to 12.00 diopters [D]). Each had undergone PRK using either a single 6.0 mm (n = 11) or three-stepped ablation zone (n = 6), with good visual results. Calculations disclosed mean ablation zone decentration relative to the pupil center for all patients to be 0.20 +/- 0.16 mm using the vector center of mass formula. Areas of uniform central corneal dioptric power (mean diameter 3.4 +/- 0.8 mm) and surrounding transition zones of declining dioptric power (mean slope 1.61 +/- 0.44 D/mm) were also determined. A new vector center of dioptric power distribution that analyzes centration and transition zone topography offers a rigorous but straightforward means to assess the effects of refractive corneal surgery procedures on central corneal topography.