Abstract
Abstract: :
Purpose: We have developed a computer program (Ocularea©) which calculates the exposed wetted area of the eye from photographs, at selected values of corneal and scleral curvature (Adv Exp Med Biol 1998;438:433–439). Curvature of the highly reflective cornea is readily measured, but no such simple method is available for the rougher sclera, which nevertheless covers 40–60% of the exposed area. We have used the Scheimpflug technique, as developed for lens studies, to examine ocular surface curvature. Methods: Photographs were taken with the Marcher Scheimpflug camera of human right eyes, viewing both along the principal axis and with the eye rotated nasally to fixate a series of markers giving defined lateral vergence. The digital images were printed at standard magnification and the curvature of the visible arc of cornea or sclera was measured by overlaying a template of a standard set of curves and finding the best fit. The curves were calibrated from clinical keratometry data of corneal curvature for each subject. Results: Six normal adult subjects were studied (4M, 2F). The radii of curvature of their scleras close to the limbus ranged from 13.3 to 14.3 mm (mean ± SD 13.8 ± 0.53 mm). As vergence increased, curvature remained essentially constant at the limbal value; hence, although radius must ultimately fall towards the mean horizontal equatorial value of 12.5 mm, a single larger value was derived for the exposed temporal sclera of each subject. Conclusions: These results indicate that the values of curvature of cornea and sclera used in the Ocularea© program must ideally be separately determined for each individual subject. Although the error introduced by using a mean value for cornea (7.8 or 7.9 mm) is small, errors are greater over the sclera if the equatorial value of 12.5 mm is used instead of a larger value (mean error of 9.2% in this study). The error can be reduced to less than 2%, without the necessity for individual curvature measurements, if the mean scleral radius of 13.8 mm is used in all cases. These results will aid evaporation studies on the eye by providing more reliable data of the wetted area.
Keywords: sclera • imaging/image analysis: non–clinical • anatomy