A new method for determining the shape of the sclera

Abstract

Purpose. The aim of the present theoretical modelling study is to improve the fitting of scleral lenses by means of a novel determination of the scleral shape of the eye. Material and Methods. . The movement of scleral lenses and soft lenses is affected by scleral shape around the limbus. Based on this, it is important to take a closer look at the edge of the sclera. An area of 4 mm in diameter around the limbus is considered as a suitable area for shape fitting, which is shown by 44 profile photographs. As a first step, a straight line (y = m • x + b) was fitted on scleral data of 44 perpendic- ular cuts. Then, these measuring points were rotated around the first “scleral measuring point after the limbus” (angle of rotation = − arctan (m)) so that all measuring points were horizontal. A parabola was then fitted as best as possible (y = a • x2 + b • x + c) to the rotated measuring points. The parabola coefficient a or “pca value” plays a central role as a shape criterion of the corneo-scleral profile (CSP). A pca value a ≥ 0.1 means the CSP is a concave parabola, while a ≤ −0.1 implies that the CSP is a convex one. If the pca value is between −0.1 < a < 0.1, the CSP is a straight line. Results. Based on the 44 profile section photos, 85.8 % of the CSPs were straight, 11.1 % were convex and 3.1 % were concave parabolic shapes. The probability of determining a parabolic CSP section is about 18.5 % (normal distribution, the normal distribution test for the calculated pca values shows p < 0.01). Conclusion. A specification of the corneo-scleral-profile (CSP) with the corresponding scleral angle “alpha”, gives significantly more information about the scleral shape than if only one of the two parameters is specified. Furthermore, this simplifies the determination of the geometry of contact lenses with large diameters, especially scleral lenses. Keywords Soft contact lenses, scleral lenses, corneo-scleral profile, scleral shape, sclera