Posterior corneal surface: new insights in curvature and astigmatism

Abstract

PurposeIn normal eyes, a fixed relationship between anterior and posterior corneal astigmatism is assumed, especially for lens power calculations. This study assessed the association between anterior and posterior corneal astigmatism and thickness.MethodsNormal corneas were examined by slit‐lamp microscopy and Scheimpflug imaging. The differences between vertical and horizontal thicknesses at 4‐mm and 6‐mm‐diameters were calculated. Anterior and posterior astigmatism were calculated as the difference between steep and flat powers based on the corresponding radii of curvature and refractive indices. Comparisons and significances of correlations were assessed by using generalized estimating equation models.ResultsFifty‐four subjects (101 corneas) aged 18–80 years were included. Peripheral corneas were thicker vertically than horizontally (p < 0.001) at 4 mm by 13.4 ± 9.4 µm (mean ± standard deviation) and at 6 mm by 8.6 ± 4.8 µm. Anterior corneal astigmatism was 1.08 ± 0.57 D and was aligned vertically (73%), horizontally (6%), or obliquely (21%). Posterior corneal astigmatism was −0.31 ± 0.13 D and was aligned vertically (94%) or obliquely (6%). Anterior and posterior astigmatism were correlated in eyes with vertical anterior astigmatism (r = 0.65; p < 0.001), but not in eyes with oblique or horizontal anterior astigmatism. The difference between vertical and horizontal corneal thickness correlated with posterior astigmatism at 4 mm (r = −0.58; p < 0.001) and at 6 mm (r = −0.61; p < 0.001).ConclusionsThe posterior cornea is normally ellipsoid in shape with a steep vertical meridian that corresponds to the cornea being thicker in the vertical paracentral region. Understanding the normal relationship between anterior and posterior corneal surfaces is important when assessing the same in corneal diseases.