Anterior-posterior strain variation in normally hydrated and swollen rabbit cornea

To investigate the variation in anterior and posterior straining under intraocular pressure changes for the central cornea of normally hydrated and swollen rabbit eyes.

A new method of measuring regional corneal strains, by imaging a specific tissue location at various intraocular pressures, was developed. Sixteen freshly enucleated, New Zealand White rabbit eyes were investigated either in their normal 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 anterior stroma or endothelium were taken at intraocular pressures of 5, 12.5, 20, 35, and 65 mm Hg. The positions of individual keratocytes or endothelial cells were used to calculate the nonhomogeneous two-dimensional strain field over the image. Corneal thickness was measured at the lowest and highest intraocular pressures (5 mm Hg and 65 mm Hg).

All pressure strain curves were highly nonlinear for intraocular pressures between 5 mm Hg and 65 mm Hg; the maximal posterior strains (normally hydrated, 2.1 +/- 0.1%; swollen, 4.8 +/- 0.8%) were larger than the maximal anterior strains (normally hydrated, 1.8 +/- 0.1%; swollen, 1.5 +/- 0.2%). Swelling significantly decreased the anterior strain 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 65 mm Hg.

Bending was found to play a significant role in central corneal deformation of swollen eyes but not in the normal hydration state. Microscopic strain measurements of the cornea, using a laser-scanning confocal microscope, are a valuable tool for the assessment of regional nonhomogeneous strains in various depths and locations of the cornea.