Comparative shock wave analysis during corneal ablation with an excimer laser, picosecond laser, and femtosecond laser

Purpose: With the event of topographic steep central islands following excimer laser surgery and the potential damage to the corneal endothelium, shock waves are playing an increasingly important role in laser refractive surgery. With this in mind, we performed a comparative shock wave analysis in corneal tissue using an excimer laser, picosecond laser, and femtosecond laser. Methods: We used a Lamda Physik excimer laser at 308 nm wavelength, a Nd:YLF picosecond laser at 1053 nm wavelength and a synchronously pumped linear cavity femtosecond laser at 630 nm wavelength. The pulse widths of the corresponding lasers were 8 ns, 18 ps, 150 fs, respectively. The energy density of irradiation was 2.5 to 8 times the threshold level being 2 J/cm₂ (excimer laser), 86 J/cm₂ (picosecond laser) and 10.3 J/cm₂ (femtosecond laser). Shock wave dynamics were analyzed using time-resolved photography on a nanosecond time scale using the picosecond laser in corneal tissue, water and air. Shock wave dynamics using the femtosecond laser were studied in water only while the excimer laser induced shock wave during corneal ablation was studied in air only. Results: We found the dynamics of shock waves to be similar in water and corneal tissue indicating that water is a good model to investigate shock wave effects in the cornea. The magnitude of the shock wave velocity and pressure decays over time to that of a sound wave. The distance over which it decays is 3 mm in air with the excimer laser and 600-700 μm in air with the picosecond laser. In water, the picosecond laser shock wave decays over a distance of 150 nm compared to the femtosecond laser shock wave which decays over a distance of 30 μm. Overall the excimer laser shock wave propagates 5 times further than that of the picosecond laser and the picosecond laser shock wave propagates 5 times further than that of the femtosecond laser. Conclusion: In this preliminary comparison, the time and distance for shock wave decay appears to be directly related to the laser pulse duration. The decay distance of the excimer laser shock waves appear to be 5 times longer than that of the picosecond laser in air while the picosecond laser shock wave distance appears to be 5 times longer than that of the femtosecond laser in water. The shorter shock wave distance of the picosecond laser is advantageous for corneal surgery by minimizing potential acoustic damage to the stromal and endothelial cells. Femtosecond lasers further minimize acoustic damage and their use should be considered in future corneal laser procedures.