Minor influence of myopic laser in situ keratomileusis on the posterior corneal surface.

PURPOSE To check whether myopic LASIK induces changes on the posterior corneal surface. METHODS A Scheimpflug system (Pentacam; Oculus, GmbH, Wetzlar, Germany) was used to measure preoperative and postoperative posterior corneal topography in 27 eyes (of 14 subjects) that had undergone standard myopic LASIK surgery (attempted corrections between -1.25 and -8.50 D) and on 18 nonoperated eyes (9 subjects). A hybrid porcine-plastic eye model was developed to validate the measurement technique. Longitudinal displacement of the posterior corneal apex and changes of the apical radius of curvature and asphericity were computed. RESULTS Measurements on a hybrid model eye of known posterior corneal geometry showed that the measured posterior corneal radius of curvature was minimally affected by the geometry of the anterior surface. The measurements on patients showed that, on average, the only relevant (though clinically unimportant) change in radius of curvature and asphericity occurred the first day after surgery (DeltaR = -28 +/- 34 microm and DeltaQ = -0.06 +/- 0.06). No statistically significant change was observed afterward. The change in radius was more pronounced in the vertical direction than in the horizontal direction. On average, there was no significant displacement of the posterior corneal apex. Individual changes over time did not show a systematic trend across patients, and control subjects experienced changes of the same order of magnitude. CONCLUSIONS The Pentacam Scheimpflug system can be used reliably to assess changes in the posterior corneal radius of curvature after LASIK. There is no evidence of surgically induced changes in the corneal posterior surface beyond 1 week after surgery.