Topographic and tomographic properties of forme fruste keratoconus corneas

To investigate the efficacy of topography and tomography indices combined in discriminant functions to detect mild ectatic corneas. The authors retrospectively reviewed the data of 143 eyes separated into three groups by the Corneal Navigator OPD scanning system (Nidek, Gamagori, Japan): normal (N; LASIK surgery with a 2-year follow-up; n = 72), forme fruste keratoconus (N topography with contralateral KC; FFKC; n = 40), and KC (n = 31). Topography and tomography indices, corneal thickness spatial profile (CTSP), and anterior and posterior curvature spatial profiles were obtained with the Orbscan IIz (Bausch & Lomb Surgical, Rochester, NY). The percentage of thickness increase (PTI) from the thinnest point to the periphery, the percentage of variation of anterior (PVAK), and posterior curvature were calculated and compared by Kruskal-Wallis test. The usefulness of these data to discriminate among the three groups was assessed by receiver operating characteristic (ROC) curve analysis. Posterior elevation of the thinnest point (TP), all positions of CTSP, PTI for all distances from the TP, and PVAK from a 5- to 7-mm distance from the TP were significantly different in the FFKC compared with the N group. The discriminant functions between the FFKC and the N groups and between the KC and the N groups reached an area under the ROC curve of 0.98 and 0.99, respectively. PTI indices and maximum posterior central elevation were the most important contributors to the discriminant function. Indices generated from corneal thickness and curvature measurements over the entire cornea centered on the TP can identify very mild forms of ectasia undetected by a Placido-based neural network program.