Research on distribution characteristics of corneal asphericity of myopic eyes

Abstract

Objective To evaluate the distribution characteristics of the asphericity (Q-value) of the corneal surfaces in a Chinese population who were candidates for excimer laser surgery; to provide some theoretical bases for clinical diagnosis and treatment. Methods This was a perspective study. Using the Pentacam HR measuring and analyzing system based on Scheimpflug photography, the right eyes of 75 Chinese myopes were evaluated for excimer laser surgery. Patients were divided into a low corneal astigmatism (<+1.00 D) group and a medium-high corneal astigmatism (≥+1.00 D) group according to the anterior corneal surface astigmatism. The analysis was based on the distribution characteristics of the mean Q-values of both the anterior and posterior corneal surfaces and the distribution characteristics of the Q-value at different angles (20°, 25° and 30°) and along different semi-meridians (superior, inferior, nasal and temporal). Single factor analysis of variance was used to determine if there was a difference among Q-values within a particular group. The differences between the two groups were analyzed by an independent samplest-test. Results The mean Q-values of the 30° angle in Chinese myopes evaluated for excimer laser surgery were-0.17±0.09 and-0.02±0.16 on the anterior and posterior corneal surfaces, respectively. The mean Q-values of the anterior corneal surface at different angles were negative and there was no statistically significant difference between the low corneal astigmatism group and medium-high corneal astigmatism group. There was a significant difference between the Q-values of the 2 groups at different angles (F=6.340, 9.963,P 0.05). In the medium-high corneal astigmatism group, QT30 was significantly bigger than QN30(P QN30 and QI30>QS30, but there was no statistically significant difference between QT30 and QS30. Conclusion The curvature along the anterior corneal surface becomes flatter from the center to the periphery, while it becomes steeper from the center to the periphery at the posterior corneal surface. The rate of the flattening of the anterior corneal surface increased as the range of the angle increased, while the rate decreased as the posterior corneal surface decreased. All the shapes on the anterior surface along different semi-meridians were long ovals. Corneal astigmatism does not affect corneal shape. The shapes of the posterior surface along the superior and inferior semi-meridians were short ovals, but were long ovals along the nasal and temporal semi-meridians. Key words: Cornea; Asphericity; Scheimpflug Photography