Imaging of birefringent properties of keratoconus corneas by polarization-sensitive optical coherence tomography

PURPOSE: To investigate and map the polarizing properties of keratoconus corneas in vitro and to compare the results with those obtained in normal corneas. METHODS: Corneal buttons of five keratoconus corneas were investigated by polarization-sensitive optical coherence tomography (PS-OCT). The instrument measures backscattered intensity (conventional OCT), retardation, and (cumulative) slow axis distribution simultaneously. Three-dimensional (3-D) data sets of the polarizing parameters are recorded, and two-dimensional (2-D) cross-sectional images as well as en face images of the distribution of these parameters at the posterior corneal surface are derived. The results are compared to similar maps obtained from normal corneas. RESULTS: Compared with normal corneas, the retardation and slow axis orientation patterns are heavily distorted in keratoconus corneas. Larger areas of increased and decreased retardation can be found in keratoconus corneas, markedly increased retardation (up to >50 degrees ) can especially be found near the rim of corneal thinning. Contrary to normal corneas, regions where the slow axis markedly changes with depth (by up to 50 degrees -90 degrees ) are observed in keratoconus. CONCLUSIONS: The observed changes in the cornea's birefringence properties indicate a change in the arrangement of collagen fibrils in the corneal stroma associated with keratoconus. PS-OCT may be a useful tool for the study and diagnosis of corneal disease.