Thermal load from a CO₂ laser radiant energy source induces changes in corneal surface asphericity, roughness, and transverse contraction

PURPOSE. We examined corneal surface response to an isolated thermal load. METHODS. Cadaveric porcine eyes were pressurized and stabilized for processing and imaging. A carbon dioxide (CO₂) laser (1.75 W) delivered a uniform disk of continuous wave thermal radiant energy to the exposed corneal stromal surface without ablation. Thermal load was determined by measuring corneal surface temperature during CO₂ laser irradiation. Corneal profilometry was measured with broadband optical interferometry before and after CO₂ laser irradiation. Photomicrographs of the stromal surface were taken before and after irradiation, and the images were superimposed to examine changes in positional marks, examining mechanical alterations in the stromal surface. RESULTS. Thermal load from uniform laser irradiation without ablation produces central corneal steepening and paracentral flattening in the central 3-mm diameter. Q values, measuring asphericity in the central 2 mm of the cornea increased significantly and it was correlated with the temperature rise (R²=0.767). Surface roughness increased significantly and also was correlated with temperature rise (R²=0.851). The central stromal surface contracted and underwent characteristic morphologic changes with the applied thermal load, which correlated well with the temperature rise (R² = 0.818). CONCLUSIONS. The thermal load created by CO₂ laser irradiation creates a characteristic spectrum of morphologic changes on the porcine corneal stromal surface that correlates to the temperature rise and is not seen with inorganic, isotropic material. The surface changes demonstrated with the CO₂ laser likely are indicative of temperature-induced transverse collagen fibril contraction and stress redistribution. Refractive procedures that produce significant thermal load should be cognizant of these morphologic changes.