Suitability of Filofocon A and PMMA for experimental models in excimer laser ablation refractive surgery

Abstract

Experimental corneal models in plastic (in PMMA, and more recently in Filofocon A, a contact lens material) have been proposed recently to overcome some of the limitations of the theoretical approaches aiming at improving the predictability of corneal reshaping by laser ablation. These models have also been proposed for accurate assessment of corneal laser ablation patterns. In this study Filofocon A and PMMA optical and ablation properties were studied using an experimental excimer laser set-up. The effective absorption coefficient and the ablation thresholds of these materials were obtained as a function of the number of pulses. Both materials follow a Beer-Lambert law in the range of fluences used in refractive surgery, and the number of incubation pulses is less than 4 (PMMA) and 2 (Filofocon A) above 140 mJ/cm2. We found that above 40 pulses for Filofocon A and 70 pulses for PMMA, ablation threshold and effective absorption coefficients can be considered constant (Fth = 90 mJ/cm2 and α(eff) = 36000 cm-1, for Filofocon A, and Fth = 67 mJ/cm2 and α(eff) = 52000 cm-1 for PMMA, respectively). The absence of ablation artifacts (central islands), a lower number of incubation pulses, a lower pulse-number dependence of the ablation threshold, and a good correspondence between α(eff) and the absorption coefficient α estimated from spectroscopic measurements make Filofocon A a more appropriate material than PMMA for experimental models in refractive surgery and for calibration of clinical lasers.