ANTERIOR-POSTERIOR STRAIN VARIATION IN NORMALLY HYDRATED AND SWOLLEN RABBIT CORNEA

PURPOSE. To investigate the variation in anterior and posterior strain ing under intraocular pressure changes for the central cornea of norma lly hydrated and swollen rabbit eyes. METHODS. A new method of measuri ng regional corneal strains, by imaging a specific tissue location at various intraocular pressures, was developed. Sixteen freshly enucleat ed, New Zealand White rabbit eyes were investigated. either in their n ormal hydration state or after swelling of the deepithelialized cornea . The eyes were mounted on a specially designed eye fixture, and laser -scanning confocal microscopic images of a selected region in the ante rior stroma or endothelium were taken at intraocular pressures of 5, 1 2.5, 20, 35, and 65 mm Hg. The positions of individual keratocytes or endothelial cells were used to calculate the nonhomogeneous two-dimens ional strain field over the image. Corneal thickness was measured at t he lowest and highest intraocular pressures (5 mm Hg and 65 mm Hg). RE SULTS. All pressure strain curves were highly nonlinear for intraocula r pressures between 5 mm Hg and 65 mm Hg; the maximal posterior strain s (normally hydrated, 2.1 +/- 0.1%; swollen, 4.8 +/- 0.8%) were larger than the maximal anterior strains (normally hydrated, 1.8 +/- 0.1%; s wollen, 1.5 +/- 0.2%). Swelling significantly decreased the anterior s train response but increased the posterior one. The corneal thickness decreased 7.4 +/- 0.4% for the normally hydrated and 6.3 +/- 0.5% for the swollen corneas for an intraocular pressure step from 5 mm Hg to 6 5 mm Hg. CONCLUSIONS. Bending was found to play a significant role in central corneal deformation of swollen eyes but not in the normal hydr ation state. Microscopic strain measurements of the cornea, using a la ser-scanning confocal microscope, are a valuable tool for the assessme nt of regional nonhomogeneous strains in various depths and locations of the cornea.