Evaluation of the Shape Symmetry of Bilateral Normal Corneas in a Chinese Population

Purpose: To investigate the bilateral symmetry of the global corneal topography in normal corneas with a wide range of curvature, astigmatism and thickness values Design: Cross-Sectional Study Methods: Topography images were recorded for the anterior and posterior surfaces of 342 participants using a Pentacam. Elevation data were fitted to a general quadratic model that considered both translational and rotational displacements. Comparisons between fellow corneas of estimates of corneal shape parameters (elevation, radius in two main directions, R-x and R-y, and corresponding shape factors, Q(x) and Q(y)) and corneal position parameters (translational displacements: x(0), y(0) and z(0), and rotational displacements: alpha, beta and gamma) were statistically analyzed. Results: The general quadratic model provided average RMS of fit errors with the topography data of 1.7 +/- 0.6 mu m and 5.7 +/- 1.3 mm in anterior and posterior corneal surfaces. The comparisons showed highly significant bilateral correlations with the differences between fellow corneas in R-x, R-y, Q(x) and Q(y) of anterior and posterior surfaces remaining insignificantly different from zero. Bilateral differences in elevation measurements at randomly-selected points in both corneal central and peripheral areas indicated strong mirror symmetry between fellow corneas. The mean geometric center (x(0), y(0), z(0)) of both right and left corneas was located on the temporal side and inferior-temporal side of the apex in anterior and posterior topography map, respectively. Rotational displacement angle alpha along X axis had similar distributions in bilateral corneas, while rotation angle beta along Upsilon axis showed both eyes tilting towards the nasal side. Further, rotation angle gamma along Z axis, which is related to corneal astigmatism, showed clear mirror symmetry. Conclusions: Analysis of corneal topography demonstrated strong and statistically-significant mirror symmetry between bilateral corneas. This characteristic could help in detection of pathological abnormalities, disease diagnosis, measurement validation and surgery planning.