ABLATION RATES AND SURFACE ULTRASTRUCTURE OF 193 NM EXCIMER-LASER KERATECTOMIES

Purpose. To determine whether photorefractive keratectomy can be perfo rmed at lower energies than are currently employed in clinical trials. Methods. Fresh pig corneas were ablated using a clinical excimer lase r to study the effects of various energy densities (100-200 mJ/cm2) an d beam diameters on ablation rates and on the surface ultrastructure o f the ablated cornea. Results. A 20-mJ increase in energy density was associated with a 0.03 mum per pulse increase in the ablation rate. A nearly linear increase in the pseudomembrane thickness occurred with i ncreasing energy densities (r2 = 0.83) or decreasing ablation area dia meter (r2 = 0.86). Conclusions. Our findings suggest that fluences les s than those currently used in clinical trials (160-180 mJ) are capabl e of ablating tissue while producing thinner electron-dense pseudo-mem branes on the corneal surface. The relationship between pseudomembrane thickness and clinical factors such as reepithelialization and postop erative haze remains to be determined. Operating at lower fluences doe s have the advantages of allowing larger diameter ablations, reducing possible shockwave damage, and reducing the maintenance requirements f or the laser.