Characteristics of Corneal Astigmatism of Anterior and Posterior Surface in a Normal Control Group and Patients With Keratoconus

Purpose: To evaluate and compare power and axis orientation of anterior and posterior astigmatism in eyes with keratoconus with healthy eyes. Methods: In this retrospective cohort study, we examined 861 eyes of 494 patients diagnosed with keratoconus at the Department of Ophthalmology, University Hospital Frankfurt, and 256 eyes of 256 healthy individuals. Using a Scheimpflug device (Pentacam HR), we measured the magnitude and axis orientation of anterior and posterior corneal astigmatism, corneal thickness, and conus location. The results were compared between different stages of the disease according to the Amsler-Krumeich classification and the control group. Results: Magnitude of corneal astigmatism was 3.47 +/- 2.10 diopters (D) on the anterior surface and 0.69 +/- 0.40 D on the posterior surface in eyes across all keratoconus stages. We found a significant increase of anterior and posterior corneal astigmatism with progression of disease (P < 0.01, 1-way analysis of variance) and a significant correlation between anterior and posterior corneal astigmatism (r = 0.77, P < 0.01). In contrast to eyes of healthy individuals, in which posterior corneal axis alignment is vertical in most cases independent of anterior alignment, we found in eyes with keratoconus a match between anterior and posterior alignment when alignment was vertical in 97% of eyes, 46% when oblique and 61% when horizontal (Cohen kappa coefficient kappa = 0.55, P < 0.01). With progression of disease, alignment of anterior and posterior corneal astigmatism became increasingly vertical. Conclusions: In eyes with keratoconus, posterior axis alignment of corneal astigmatism is in line with alignment of the anterior surface in the majority of cases. Posterior astigmatism axis alignment could potentially be used in algorithms to support diagnosis and staging of keratoconus.