Evaluation of the Apparent Change in Posterior Corneal Power in Eyes With LASIK Using Orbscan II With Magnification Compensation

PURPOSE. To develop a mathematical model for the assessment of the apparent posterior corneal curvature changes after myopic keratorefractive surgery with Orbscan (Bausch & Lomb) and to assess its accuracy with clinical data. METHODS, Part 1: Postoperative and preoperative magnification. ratio of the posterior corneal surface was calculated from a theoretical eye model, Changes of radius and power of the posterior corneal surface were estimated over a wide range of refractive corrections and preoperative anterior corneal power. Part 2: The measured radius of the posterior cornea by Orbscan II 3 months postoperatively was converted to a new value based on the mathematical model. Both the measured radius and corrected radius were compared with preoperative values using paired sample t test. RESULTS: Myopic correction is associated with apparent steepening of the posterior curvature. The higher the amount of correction, the more the apparent difference. A statistically significant difference was noted between preoperative and measured postoperative radius of the posterior cornea (0.35 +/- 0.16 mm, P<.01). However, no significant difference was seen between preoperative and corrected postoperative radius of the posterior cornea (0.008 +/- 0.11 mm, P=.44). CONCLUSIONS: The change of corneal magnification induced by refractive correction partly explains the apparent steepening of the posterior corneal curvature. The mathematical model effectively compensates for the error in Orbscan II in the assessment of posterior corneal curvature after LASIK.[J Refract Surg. 2009;25:221-228.]