Theory concerning the ablation of corneal tissue with large-area, 193-nm excimer laser beams

A study was conducted to investigate why 193 nm excimer laser beams of uniform fluence produce corneal ablations that are deeper at the edge than at the center. Enucleated porcine eyes were ablated and measured with an optical profilometer. A dehydrated cornea was also ablated. Enucleated porcine eyes and PMMA were ablated with and without narrow central masks, and ablation plumes were photographed. Characteristic patterns of central underablation were present in the porcine corneas. Ablation craters ranging in diameter from 2.0 to 6.5 mm exhibited increasingly shallower central ablation and nearly constant depth at the edges. There was no significant depth variation in the dehydrated cornea. Masks did not change the depth or shape of craters near the edges of the zone; but depth adjacent to the images of the masks was more than twice that with no mask. The depth adjacent to the mask image was nearly the same as at the edge of the zone, and the rate of change in depth with position was nearly equal in both areas. The area of maximum plume density centered over the entire ablation with or without the mask. Redeposition of plume particles is not the major cause of central underablation; rather, propagating transverse energy from the absorption of photons by the peptide bonds increases pressure on the excited components within the irradiated area, increasing recombination, which in turn raises the ablation threshold.