Silicon cast method for quantification of photoablation.

BACKGROUND

Topometry and measurement of photoablation patterns are key questions for keratorefractive photoablation. Ablation rates have been determined previously by either tissue perforation or by micrometry performed on histologic sections.

METHODS

A three-dimensional cast of cornea after irradiation was made by using a two-component silicon gel that polymerizes within minutes, thus preserving the corneal topography immediately after photoablation. Polymerization is athermal and nontoxic. The resulting silicon blocks were cut perpendicularly to the anterior surface and measured by calibrated light microscopy.

RESULTS

The silicon surface is extremely smooth and the accuracy of the cast is better than 0.25 micron. Reproducibility and long-term stability were demonstrated for casts of photoablated polymethylmethacrylate. Thus, ablation rates and profile, volumetry, and topometry can be determined following laser ablation. The method has been applied for 193-nanometer excimer laser in vitro irradiation of the human cornea. Ablation rates in Bowman's layer and stroma for various radiant energies and distinct pulse numbers were found to be in agreement with published data, and an incubation effect for the first laser pulses could be demonstrated.

CONCLUSIONS

The method is nondestructive, accurate, inexpensive, practical, and reduces requirements for laboratory animals.