Effects of phototherapeutic keratectomy on peripheral corneal thickness

Purpose. To test the hypothesis that excimer laser phototherapeutic keratectomy causes a sustained increase in peripheral corneal thickness in vivo. Method. A total of 12 pigmented rabbits were divided equally into groups A and B. Phototherapeutic keratectomy (PTK) using an excimer laser (VISX 20/20, Santa Clara, CA) was performed on one eye of each rabbit. Group A received PTK with an ablation diameter of 6mm and total depth of 150 microns (trans epitheliat treatment) while the total ablation depth of group B was 100 microns. Prior to treatment, profile slit lamp images of the cornea were obtained using a regular slit lamp. Photographs were taken one hour post PTK and one week post PTK to allow for complete reepilhelialization. The profile pictures were then digitally analyzed, and the thickness at the periphery of the ablation zone was measured. Results. The preoperative peripheral thickness was 402±5 microns (n=12 eyes). Following PTK, in group A, the one hour post PTK average peripheral thickness increased by 47+11 microns (n=5, p=0.012) compared with preop thickness. A regression in the peripheral thickening to 18±4 microns (n=5, p=0.011) was observed one week post PTK. Similarly, in group B, we noted an average increase of 53±5 microns (n=5, p<0.001) one hour post PTK, and an average regression in peripheral thickening to 27±3 microns (n=5, p<0.001) one week post PTK. Conclusions. These measurements suggest that the biomechamcs of PTK involve an increase in peripheral corneal thickness secondary to the relaxation of anterior lamellar tension.