Posterior corneal surface changes after hyperopic laser in situ keratomileusis

Purpose: To evaluate posterior corneal surface topographic changes after hyperopic laser in situ keratomileusis (H-LASIK) using Orbscan I (Orbtek, Inc.). Setting: Department of Ophthalmology, Nara Medical University, Nara, Japan. Methods: In 25 eyes of 15 patients who had H-LASIK, the posterior corneal surface was measured with slit-scanning corneal topography (Orbscan I) preoperatively and 1 year postoperatively. The center as a fit zone and calculated posterior corneal surface changes were taken at 4 points: nasal, temporal, superior, and inferior sides in the 5.0 mm diameter. The posterior corneal topographic changes were analyzed using an analysis of variance. The postoperative:preoperative magnification ratio of the posterior corneal surface was calculated in a theoretical eye model. Results: When a "+" reading was defined as the forward displacement and "-" was defined as the backward displacement, the mean posterior corneal topographic changes were -2.8 mu m +/- 27.9 (SD) at the nasal side, -4.5 +/- 27.8 mu m at the temporal side, -3.9 +/- 20.1 mu m at the superior side, and -2.3 +/- 20.1 mu m at the inferior side. The posterior corneal surface between any 2 examined points showed no significant difference after H-LASIK In addition, the hypothetical change in the posterior cornea was -8.3 mu m after +3.0 diopter H-LASIK, which was approximately closer to the study results. In each side, the amount of the attempted correction was significantly correlated with the posterior corneal topographic change. Conclusions: Clinical measurement of the posterior corneal displacement after H-LASIK with Orbscan revealed a backward shift. This change corresponded to the hypothetical artifactual changes with Orbscan; that is, changes in the magnification ratio.