The effect of posterior corneal flat meridian and astigmatism amount on the total corneal astigmatism estimated from anterior corneal measurements

Abstract

To evaluate the effects of posterior corneal astigmatism and the absolute flat meridian difference between anterior and posterior corneal surfaces (AMD(Ant-Post)) on the estimation of total corneal astigmatism using anterior corneal measurements (simulated keratometry [K]). Ninety-nine eyes of 99 healthy participants were enrolled. Anterior, posterior, and total mean corneal power, cylinder power, flat meridian, and vector components J(0), and J(45) measured by a dual Scheimpflug camera were analyzed. The correlation between the posterior corneal cylinder power, AMD(Ant-Post), and the difference in the cylinder power between simulated K and total cornea (cylinder power difference(SimK-Tot)) were evaluated. The cylinder power difference(SimK-Tot) was positively correlated with the posterior corneal cylinder power (rho = 0.704 and P < 0.001) and negatively correlated with AMD(Ant-Post) (rho = -0.717 and P < 0.001). In the multivariate linear regression analysis, anterior corneal J(0) was strongly associated with the posterior corneal cylinder power and the AMD(Ant-Post). When corneal J(0) had a positive value, the cylinder power of simulated K tended to be larger than the total corneal cylinder power. In comparison, the opposite trend was presented in eyes with negative anterior corneal J(0). When anterior corneal J(0) was larger than 1.0 or smaller than -0.9, the errors from estimating the total corneal cylinder power using anterior corneal measurements tended to be larger than 0.25 D. Posterior corneal astigmatism should be considered for more accurate corneal astigmatism predictions, especially in eyes with anterior corneal astigmatism greater than 2.0 D of with-the-rule astigmatism or greater than 1.8 D of against-the-rule astigmatism.