Location of the corneal apex and its influence on the stability of the central corneal curvature. A photokeratoscopy study.

A new photokeratoscopic method is presented for the determination of the corneal apex. Any asymmetry of photokeratoscopic data is interpreted as an apex displacement xa, defined as the chord distance between the visual axis and the corneal apex. I find the normal corneal apex to be displaced about 0.5 mm temporally and upward. In the area around the visual axis the corneal shape as well as xa influence the constancy of the radius of the corneal curvature. Quantitatively this curvature stability is depicted by an apex factor (AF) in a given meridian defined as the change in the corneal refractive power per millimeter chord distance in the corneal intersection point of the visual axis. The magnitude of AF is found to be about 0.3 D/mm in both the horizontal and the vertical meridians. The term central corneal cap (CCC) is defined as an area, symmetric around the visual axis, in which the change in the corneal refractive power is less than 0.5 D in a given meridian. CCC is calculated to have an area of 2.4 mm2.