Effects of intrastromal corneal ring size and thickness on corneal flattening in human eyes.

Intrastromal corneal rings can flatten the human cornea, providing a potential new method of keratorefractive surgery. We investigated the effect of implanting various ring sizes of a given thickness in 8.50-mm intrastromal channels dissected in human eye bank corneas. A new intraoperative corneal topography device was used to obtain serial data. Smaller rings with no expansion pressure on the channels were found to induce corneal flattening presumably due to the ring thickness alone. Rings of increasing diameters produced high degrees of corneal flattening at progressively declining rates; this suggests that shear stresses may have expanded the channel due to mechanical stresses placed by our particular experimental technique. Implantation techniques that minimize stress on the outer channel lamellae appear advisable. New ring designs should take ring thickness into consideration along with outer edge configurations that minimize shear stresses on the lamellar channels.