Geometric ray tracing analysis of visual acuity after laser in site keratomileusis

PURPOSE: Using a geometric ray tracing model, we explain the increase in vi sual acuity observed in myopic patients after laser in situ keratomileusis (LASIK). METHODS: This study included 37 eyes of 23 patients who underwent LASIK. Al l patients had myopia and a spectacle-corrected visual acuity of 0.95 or wo rse. Clinical tests included biometry, corneal topography, pachymetry, and refraction (with and without cycloplegia). Calculations were made by tracin g rays through all the refractive surfaces of the eye based on a Le Grand-t ype theoretical model of the whole eye. RESULTS: Comparison of spectacle-corrected visual acuity of the eye before surgery, the size of the blur circle calculated by ray tracing, and the mag nification for the ocular system facilitated a numerical criterion to asses s visual acuity by geometric calculation. This criterion was applied to myo pic eyes that underwent LASIK, and the maximum increase in spectacle-correc ted visual acuity was predicted. An actual increase in visual acuity of app roximately 40% of the predicted maximum was observed in patients. CONCLUSIONS: With geometric ray tracing, it was possible not only to obtain an estimate of the visual acuity before LASIK but also to assess the value of the maximum and probable increases in visual acuity after LASIK.