Ocular rigidity and decay curves analyzed by two nonlinear systems.

A mathematical approximation technique has been used for the analysis of pressure-volume and decay curves to see which of two concepts of the mechanical properties of the coats of the eye is the more valid. One concept—that the outer coats are nonlinear elastic—stems from the early work of Friedenwald* and is currently in use. The other concept—that the outer coats are nonlinear viscoelastic—stems from more recent work. When either of these concepts is used to analyze the behavior of an eye with inflow and outflow, the system becomes nonlinear viscoelastic (either simple or complex). Solution of such a system can as yet be obtained only by a mathematical approximation. With such an approach we can analyze equally well by both systems the pressure-volume curves, but not the decay curves. From a single eye two decay curves may be obtained, their positions depending on whether the eye is kept at an elevated pressure for a long or a short time. These decay curves cannot be analyzed or matched without conceding that the outer coats of the eye are viscoelastic. By means of pressure-volume and decay curves, it is thus possible to obtain quantitative data on the fundamental nonlinear viscoelastic properties of the enucleated human eye.