In vivo determination of corneal swelling pressure

A method to determine the swelling pressure of the corneal stroma in vivo was developed. Discs of transparent poly(glyceryl methacrylate) hydrogel were implanted into rabbit cornea and observed for months. The thickness of the hydrogel and the surrounding cornea was measured with a pachometer. The thickness of the hydrogel decreased to an equilibrium with the stroma at which point the swelling pressure of the hydrogel and the stroma were presumed equal. The thickness-swelling pressure relationships of the hydrogel discs were measured in vitro using two independent methods. Using these data, the disc thickness in vivo was found to correspond to about 50 mmHg of stromal swelling pressure. This value approximates that obtained earlier in vitro. The existence of a definite stromal swelling pressure in vivo is a strong argument for the existence of an outward pump acting across the endothelium or epithelium.