Influence of corneal shape on limbal light focusing.

PURPOSE Light incident at the temporal cornea is focused by the peripheral anterior eye to the nasal limbus, the usual site of pterygium formation. Parameters that may contribute to observed individual variations in the degree of limbal light focusing were assessed. METHODS Computer-assisted optical ray tracing techniques were applied to a human anterior segment model. The angle of incident light (theta, 95 degrees to 108 degrees posterior to the sagittal plane), corneal central radius of curvature (ro, 7.2 to 8.4 mm), and shape factor (p) were varied, and the effect on distal limbal intensity (I) was calculated. RESULTS The magnitude of intensity peaks (Ipeak) is dependent on theta and ro. Steeper corneas have higher intensity peaks (Ipeak approximately 21.5X at ro = 7.2 mm, p = 0.75), and flatter corneas have lower intensity peaks (Ipeak approximately 8X at ro = 8.4 mm, p = 0.75) (cf Ipeak approximately 14X for a standard cornea, ro = 7.8 mm, p = 0.75). Anteroposterior location of intensity peaks is dependent on theta and ro. Steeper corneas have intensity peaks situated more anteriorly, whereas flatter corneas have more posteriorly placed peaks. Distal light distribution profiles demonstrate that intensity peaks are not always centrally located. At lower angles of incidence (theta = 100 degrees, ro = 7.8 mm, p = 0.75), peak intensity is located approximately 1 mm above and below the horizontal plane. The overall distribution (envelope) of light at the distal limbus is apparently independent of corneal shape. CONCLUSIONS Differences in corneal topography can account for the clinical observation of individual variation in the degree of limbal light focusing. Whether individuals with corneas capable of developing intense limbal foci may be more predisposed to developing pterygium requires further study.