Biomechanical behavior of the cornea and its response to radial keratotomy.

BACKGROUND: Radial keratotomy reduces myopia by flattening the central cornea, but the mechanism remains a matter of controversy. In this article, we studied the biomechanical behavior of the cornea and its response to radial keratotomy. METHODS: A human cadaver eye model without corneal epithelium was used in this study. We studied the effects which varying intraocular pressure (IOP) and corneal hydration would have on the keratometric power of unoperated eyes and eyes following radial keratotomy. For nonoperated eyes, first, normal corneal hydration was maintained while the IOP was varied. Second, the IOP was maintained at a constant level of 20 mm Hg while the corneal hydration was changed. The effects of separately varying the IOP and corneal hydration of postoperative eyes following an eight-incision radial keratotomy were studied in a similar fashion. RESULTS: In the nonoperated eye, a very high IOP was associated with a general reduction of corneal astigmatism without significantly affecting the overall keratometric spherical equivalent refraction. A steepening change of less than 0.50 diopters (D) was obtained in all eyes when dehydrating the cornea from 700 +/- 50 microns (centrally) and 830 +/- 70 microns (peripherally), to 495 +/- 25 microns (centrally) and 655 +/- 45 microns (peripherally). Following radial keratotomy, changes in IOP within the physiological range were found to have minimal influence (< 0.50 D) on the radial keratotomy keratometric power. However, after hydrating the cornea with balanced salt solution for 30 minutes, we obtained a mean flattening of 10.00 D. When dehydrating these corneas with topical hyperosmotic solution over a period of 3.5 hours, the flattening reversed to near preoperative values. The change in keratometric power resulting from radial keratotomy was significantly modulated by varying the hydration state of the deepithelialized cornea: the greater the hydration, the flatter the central cornea; therefore, the unpredictable surgical outcomes and diurnal fluctuations observed after radial keratotomy may be affected by applying topical hyperosmotic agents. CONCLUSIONS: We hypothesize that the corneal stroma is an inelastic, anisotropic, layered collagen structure that distributes tensile stress unequally throughout its thickness as a function of the amount of hydration. IOP, within physiological levels, did not have a significant effect on corneal flattening.