In vitro measurement of corneal strain, thickness, and curvature using digital image processing

A video camera, a microscope, and a PC-based digital image processing board were assembled for in vitro studies of corneal biomechanics. Central tangential corneal strain was determined by placing pairs of tiny mercury droplets on the epithelial surface and on the endothelial surface of the cornea. A distance of 3.2 mm could be determined with a standard deviation of 1.2 micron. Central corneal radius of curvature was measured from digital traces of the corneal surface contour. The standard deviation on estimating the radius of an 8 mm steel sphere was 15 microns. Corneal thickness was measured by digital optical pachometry. The standard deviation on measuring a thickness of 1 mm was 4.6 microns. The corneal extensibility was investigated in a total of 10 enucleated human eyes with increased corneal hydration. Tension was applied by varying the intraocular pressure from 2 to 100 mmHg with a column of isotonic saline. The epithelial side corneal strain and the increase in corneal curvature were approximately 1% for a change in intraocular pressure from 2 to 100 mmHg. When a high intraocular pressure was kept constant for 2 h the corneal thickness, radius of curvature, and the epithelial side strain gradually decreased, whereas the endothelial side strain increased. The elastic and visco-elastic behaviour of the human cornea was found to be closely related to changes in corneal hydration. We found digital image processing useful for in vitro biomechanical studies of the cornea.