Correlation of corneal behavior under the action of orthokeratological contact lenses with tension states in thin curved plates

Abstract

Contact lenses are currently a practical and efficient solution for the correction of refractive errors but also for the application of ortho-keratology procedures in cases of keratoconus pathologies or high myopia. The component of the cornea is represented by a stratified cell structure determining its central dimension (on the optical axis) of the order of 0.5 mm and the marginal one of approximately 0.7-1.0 mm. The radii of curvature of the cornea are different in the two main meridians (vertical and horizontal) causing the radii of curvature of the concave surface of the contact lens to be made accordingly. Because in ortho-keratology the contact lenses used are rigid and are made so as to have effects of deformation of the corneal surface, in the latter are induced a series of residual and inertial stress states through which changes in refractive power are obtained. In this paper are presented some aspects related to the correlation and assimilation of corneal behavior, under the action of ortho-keratology contact lenses, with the states of tension developed in thin curved plates. The first part of the paper analyzes the structures of the cornea in terms of the properties of the component materials (anisotropic elastic structure) and then identifies the geometric dimensions that can be used in modeling stress states. In the second part of the paper are proposed the simplifying hypotheses of working with thin isotropic curved surfaces and with spherical or toric radii of curvature, which are the basis for writing the equations for determining stress states. In the final part of the paper are presented the observations and conclusions of the use of these fundamental principles of construction of procedures for modeling the behavior of the cornea under the action of ortho-keratological contact lenses.