Biomechanically coupled curvature transfer in normal and keratoconus corneal collagen

To examine the hypotheses that a "pushed" biomechanically coupled intrafibrillar transfer of curvature may be involved in corneal warpage (pseudo cone) development, which is observed with superior cornea-bearing rigid contact lens fittings, and that a "pulled" biomechanically coupled intrafibrillar transfer of curvature may be involved in keratoconus cone development.

The mechanical properties of corneal stromal fibrils have been reviewed to examine the histologic basis for fibril inextensibility and the associated potential for them to demonstrate biomechanical coupling.

The hypothesis that apical clearance rigid contact lenses may promote pushed biomechanical responses that aid cone development in some normal or keratoconic corneas is examined.

Stromal fibrils that run limbus to limbus and have circumferential stability at the limbus, appear to be relatively inextensible: a finding that is consistent with the need for stable refraction. The property of inextensibility could also support intrafibrillar biomechanically coupled curvature transfer. For example, in some normal corneas, definite apical clearance fittings may be associated with biomechanically coupled push forces generated by midperipheral bearing that squeeze the central cornea to greater curvature. A disease-related biomechanically coupled pull mechanism for cone formation may be accelerated by an apical clearance fitting push mechanism, with additional synergy generated by subatmospheric pressure in the postlens tear pool. These fitting-related mechanisms appear likely to increase with the degree of apical clearance. Cone formation in keratoconus and models of external forces acting on the cornea (tonometry and contact lens-related) are mathematically analyzed for an idealized one-dimensional elastic model of a corneal stromal fibril. The classic theory of "elastic rods" is adopted, and despite the nonlinear nature of the differential equations involved, a method of dealing with biomechanically coupled curvature transfer in an "exact" manner is described and used to model the related clinical phenomena.