A new procedure for evaluating smoothness of corneal surface following 193-nanometer excimer laser ablation

The routine technique for evaluating the smoothness of an excimer laser keratectomy has been scanning electron microscopy (SEM). However, this method suffers from tissue shrinkage and surface artifacts, and evaluates the surface only in a qualitative manner. In our study, we used a Zygo microscope to quantitatively assess the smoothness of the excimer laser ablated corneas, without complicated tissue processing.

Five rabbit eyes and five PMMA blocks underwent 193-nanometer excimer laser ablations (5.00-millimeter diameter and 80-micrometer depth). The ablation zones of the corneas were trephined and fixated prior to the measurement. The surface smoothness was measured under the Zygo and characterized by two surface parameters (Ra and RMS). After measurement, the corneal and PMMA samples were processed for SEM.

The Ra and RMS (mean +/- SD) for the ablated corneas are 183.33 +/- 20.6 and 240.06 +/- 23.10 nm, respectively; for the PMMA blocks, 79.49 +/- 23.02 and 96.45 +/- 27.16 nm, respectively. There are significant differences of Ra and RMS between the ablated corneas and PMMA blocks (p < .01), indicating the ablated corneal surfaces are rougher than the ablated PMMA surfaces. Qualitatively, SEM correlated well with the results of the Zygo measurements.

We found this technique to be a simple, accurate, and reproducible means of objectively assessing the surface smoothness following excimer laser ablation. Combined use of the Zygo and SEM enables more complete evaluation of the ablated corneal surface.