Histology of rabbit corneas after 10-diopter photorefractive keratectomy for hyperopia

To measure histological changes in the optical and transition zones of rabbit corneas treated with excimer laser photorefractive keratectomy (PRK) to correct hyperopia and to estimate the optical effect of these changes on the induced power profile.

Corrections of +10.00 D were ablated by means of a Questek 2620 excimer laser and rotational masks on 8 rabbit eyes. On frozen sections of 4 stained corneas and historesine sections of 4 other corneas, stromal, new stromal, epithelial, and total corneal thickness were measured on four significant locations. The optical effect of each parameter on the axial power profile was estimated in a computer simulation.

Mean stromal regrowth was 50% of the ablated tissue. Deposition was lenticular and could account for -5.00 D of regression. Stromal thickening without regrowth occurred in the optical zone and over the shoulder, causing augmentation instead of regression. The epithelium thickened 20% in the mid-transition zone and was thinner over the shoulder, accounting for 1.00 D of regression and an increase in asphericity in the optical zone.

In these hyperopic rabbit PRK corneas, stromal regrowth and epithelial hyperplasia were lenticular and contributed to regression of the optical effect. Stromal swelling around interrupted collagen lamellae seemed to occur, augmenting the optical effect. The epithelium filters high spatial frequency stromal irregularities.