Interactions between geometry and mechanical properties on the optic nerve head.

PURPOSE To determine the relative strength, independently and in interaction, of the influence of factors representing the geometry and mechanical properties on the IOP-induced stresses and strains within the optic nerve head (ONH). METHODS A computational model of the eye was developed such that 21 factors could be varied independently or simultaneously. A fractional factorial screening analysis was used to identify the factors and interactions with the largest influences on the lamina cribrosa (LC) and prelaminar neural tissue (PLNT). RESULTS Nine factors and their interactions accounted for the majority of the variance in the responses (between 95% and 99.8%). These factors were: the properties of the sclera (modulus, eye radius, and shell thickness), LC (modulus and radius), PLNT (modulus and compressibility), and optic nerve (modulus), and IOP. The interactions were stronger on the PLNT than on the LC (up to 16.4% and 9.0% of the response variances, respectively). No factor was the most influential on all the responses or sufficient to ensure high or low levels of strain or stress. Although the modulus of the sclera was among the most influential factors, its effects could be outweighed by other factors. CONCLUSIONS There were strong interactions between and within the geometry and mechanical properties of the tissues of the ONH. This suggests that to ascertain individual susceptibility to IOP it may be necessary to determine several properties of the eye, as well as their interactions. The influential factors and their covariances should be better characterized.