Ablation rates and surface ultrastructure of 193 nm excimer laser keratectomies

Purpose. To determine whether photorefractive keratectomy can be performed at lower energies than are currently employed in clinical trials. Methods. Fresh pig corneas were ablated using a clinical excimer laser to study the effects of various energy densities (100-200 mJ/cm²) and beam diameters on ablation rates and on the surface ultrastructure of the ablated cornea. Results. A 20-mJ increase in energy density was associated with a 0.03 μm per pulse increase in the ablation rate. A nearly linear increase in the pseudomembrane thickness occurred with increasing energy densities (r² = 0.83) or decreasing ablation area diameter (r² = 0.86). Conclusions. Our findings suggest that fluences less than those currently used in clinical trials (160-180 mJ) are capable of ablating tissue while producing thinner electron-dense pseudomembranes on the corneal surface. The relationship between pseudomembrane thickness and clinical factors such as reepithelialization and postoperative haze remains to be determined. Operating at lower fluences does have the advantages of allowing larger diameter ablations, reducing possible shockwave damage, and reducing the maintenance requirements for the laser.