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 μm ± 27.9 (SD) at the nasal side, −4.5 ± 27.8 μm at the temporal side, −3.9 ± 20.1 μm at the superior side, and −2.3 ± 20.1 μm 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 μm 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.