Abstract
Abstract: :
Purpose: To obtain the shape of the posterior corneal surface in a healthy population. Accurate data on the shape of the back surface of the cornea is scarce because imaging of this surface must always be performed through the anterior surface of the cornea. This will distort the perceived shape of the posterior surface of the cornea due to the magnifying effect of the anterior surface of the cornea. Methods: Scheimpflug images were made in six meridians of 30 subjects (mean age 28 ± 6.5 years). Using ray tracing, each image was corrected for distortion due to the geometry of the Scheimpflug camera and the magnifying effect of the front surface. The radius at the vertex, the astigmatism and the asphericity were determined at an aperture of 7.5mm. The asphericity (k) describes the flattening (or steepening) of the periphery (k=0: parabola; 0<k<1: ellipse; k=1: circle). Results:The average radius (±s.d.) of the anterior and posterior corneal surface was 7.8±0.22 mm and 6.55±0.22 mm, respectively. The orientation of the axis of astigmatism of both corneal surfaces were practically the same. The difference (±s.d.) between the radius of the steepest and flattest meridian of the anterior and posterior surface was 0.174±0.002mm and 0.308±0.007mm, respectively.The mean k of the anterior and posterior surface did not significantly differ and was 0.87±0.1 and 0.85±0.1, respectively. This indicates that both surfaces are described by a flattened ellipse. The asphericity of both the anterior and posterior surface was independent of radius of curvature at the vertex or the refractive error. Conclusions: Astigmatism of back surface (0.29 D) does partly (30%) compensate the astigmatism of the front surface (1.05 D). In agreement with the anterior surface, the asphericity of the posterior surface of the cornea does not vary substantially between meridians and is independent of the radius of curvature.
Keywords: cornea: basic science • astigmatism • topography