Photorefractive keratectomy for the correction of myopia and astigmatism

Photorefractive keratectomy corrects myopia with or without astigmatism by decreasing the refractive power of the central cornea. The Colliac matrix formula determines the anterior corneal radius of curvature expected to achieve emmetropia. We used the formula as an algorithm for the computer-controlled delivery of the laser beam. The formula was evaluated by using theoretical and clinical cases. We examined the relationship between the correction induced by photorefractive keratectomy, the diameter of ablation zone, and the thickness of the ablated corneal lenticule on the optical axis. Comparison between attempted and achieved keratometric readings showed the accuracy of the formula to be in the order of +/- 0.75 diopter.