Astigmatic Vector Analysis of Posterior Corneal Surface: A Comparison Among Healthy, Forme Fruste, and Overt Keratoconic Corneas.

PURPOSE: To determine novel diagnostic parameters for keratoconus, and to assess the correlation between anterior and posterior corneal surfaces based on vectorial astigmatism analyses.

DESIGN: Retrospective case-control study.

METHODS: Six hundred and ninety-eight eyes of 698 patients were enrolled in the study. Healthy corneas, or controls (C, n = 264), were compared to keratoconic corneas, further categorized as forme fruste (FFKc, n = 212) and overt keratoconus (Kc, n = 222). Corneal measurements were obtained from a Scheimpflug-based tomographer. Vectorial analyses were conducted in accordance with the method proposed by Thibos.

RESULTS: Posterior corneal astigmatic power vector (APV) >0.23 diopter (D) yielded a test for overt Kc with sensitivity and specificity rates of, respectively, 81% and 77%, indicating a positive likelihood ratio (LR+) of 3.5 and a negative likelihood ratio (LR-) of 0.25. Posterior corneal overall blur vector (Blur) >6.45 D yielded a test slightly less sensitive and specific, with rates of 75% and 72%, respectively, associated to LR+ of 2.7 and LR- of 0.35. The highest (Spearman ρ) correlation coefficients between anterior and posterior corneal astigmatisms were associated with Blur, being 0.93 for Kc, 0.87 for C, and 0.81 for FFKc. The astigmatism vectors along the 45-degree (J45) and 0-dregree meridians (J0) and APV most often presented higher coefficient values for Kc and FFKc than for C (P = .01).

CONCLUSIONS: Posterior corneal vectors APV and Blur constitute objective supplemental parameters for the diagnosis of Kc. Anterior and posterior corneal surfaces correlate in all groups, although it was not possible to accurately predict posterior astigmatism from anterior astigmatism.