The importance of the corneal epithelium in excimer-laser photorefractive keratectomy.

To evaluate the importance of the epithelium for postoperative regression we undertook excimer-laser photoreactive keratectomy (PRK) in six rabbits. After the epithelium had been removed mechanically, two rabbits underwent a -3.0 D procedure; two rabbits, a -6.0 D procedure; and two rabbits, a -9.0 D procedure. Refraction was monitored pre- and postoperatively using streak retinoscopy. Rabbits were killed at 3 weeks after PRK and the thickness of the corneal epithelium in the centre of the ablation zone and in an untreated area just outside the treatment zone was measured using light microscopy. At the time of killing, no stromal haze was visible on slit-lamp biomicroscopy. In all rabbits the thickness of the corneal epithelium over the untreated area was between 31.4 and 36.7 microns. In contrast, the thickness of the epithelium in the centre of the ablation zone in rabbits treated with a -3.0 D procedure was only slightly increased, and retinoscopy showed a change in refraction of around 3 D. All other rabbits showed a significant increase in epithelial thickness, whereas the amount of thickness correlated with the amount of myopic regression. One rabbit in the -6.0 D group showed an increase in epithelial thickness of 18.4 microns and a myopic regression of 1.5 D. In both rabbits in the -9.0 D group the amount of corneal tissue removed by the laser was nearly completely substituted by epithelial hyperplasia. Postoperative retinoscopy showed nearly the same refraction as had been obtained preoperatively. In summary, the postoperative control of epithelial thickness seems to be an important factor for the success of excimer-laser PRK and, in particular, for high degrees of myopia.