Clinical Interest of Posterior Corneal Astigmatism Measured With the Orbscan to Predict Subjective Astigmatism

Abstract

Purpose: : To evaluate the interest of posterior corneal astigmatism measured with Orbscan to predict subjective astigmatism in various clinical situations.

Methods: : We prospectively recorded subjective astigmatism, anterior and posterior corneal astigmatism (Orbscan) in three groups of 100 eyes with congenital astigmatism, keratoconus or penetrating keratoplasty. Total corneal astigmatism (anterior + posterior) were calculated using rectangular coordinates (x,y) of regular astigmatism. Anterior, posterior and total corneal astigmatism were compared and correlated to the subjective astigmatism. The difference between two successive measurements was recorded to evaluate the reproducibility of Orbscan.

Results: : Reproducibility was worse for posterior astigmatism than for anterior corneal astigmatism: 9.9° versus 5.2° p<0.001 for the axis and 26.0% versus 10.9% p<0.001 for the cylinder. According to the subjective axis, the axis error was significantly higher for posterior (14.7°) than for the anterior corneal astigmatism (9.2°) or total corneal astigmatism (10.5°) (p<0.001). The cylinder error (absolute value) was not significantly different for the total corneal astigmatism (0.93D) and for the anterior corneal astigmatism (0.95). The anterior corneal cylinder shows higher correlation with the subjective cylinder (r=0.89 p<0.001) than the total cylinder (r=0.83 p<0.001) or the posterior cylinder (r=0.64 p<0.001).

Conclusions: : Posterior corneal astigmatism data obtained with the Orbscan appears useless for the prediction of subjective astigmatism.

Keywords: astigmatism • topography • refraction