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

Background: 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]).

Methods: Ninety-nine eyes of 99 healthy participants were enrolled. Anterior, posterior, and total mean corneal power, cylinder power, flat meridian, and vector components J₀, and J₄₅ 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.

Results: 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 J₀ was strongly associated with the posterior corneal cylinder power and the AMD_Ant-Post. When corneal J₀ 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 J₀. When anterior corneal J₀ 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.

Conclusion: 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.