Evaluation of apparent ectasia of the posterior surface of the cornea after keratorefractive surgery

PURPOSE: To calculate the apparent posterior corneal changes after keratorefractive surgery and reevaluate corneal ectasia displayed by Orbscan (Orbtek). SETTING: Department of Ophthalmology, Nara Medical University, Nara, Japan. METHODS: Postoperative:preoperative magnification ratio of the posterior surface of the cornea was calculated in a theoretical eye model. RESULTS: Assuming the preoperative corneal thickness is 600.00 microm, the preoperative refractive power of the anterior corneal surface is 48.0 diopters (D), the refractive power of the cornea is 1.376, the ablation diameter is 6.0 mm, the postoperative corneal thickness is 480.00 microm, the postoperative refractive power of the anterior corneal surface is 38.0 D, and the posterior surface of the cornea does not change postoperatively, the apparent image of the posterior surface of the cornea becomes 0.778% smaller postoperatively. If the posterior radius of curvature of the cornea is 6.2 mm, it becomes smaller by 48.24 microm. If this change directly affects the difference map, the posterior surface of the cornea moves forward by 48.24 microm. CONCLUSION: The results correspond to the amount of ectasia in previous reports. This artifact may explain the apparent ectasia detected by Orbscan.