Investigation of corneal ablation efficiency using ultraviolet 213-nm solid state laser pulses

PURPOSE. TO determine the threshold and efficiency of corneal ablation for various values of laser fluence at the ultraviolet wavelength of 213 nm. METHODS. A commercial Q-switched Nd:YAG laser was used to produce the fifth harmonic wavelength of 213 nm. Ablation trials were carried out on porcine corneas. Slit ablations of dimensions 0.5 x 2.5 mm were performed using se ven values of laser fluence to obtain the most efficient fluence for ablati on. The morphology of each ablation was obtained using a computer-automated confocal profiling system. These profiles were then analyzed to determine the ablation depth for the range of fluence values used. RESULTS. A fluence in the region of 200 mJ/cm(2) was found to be the most e fficient for ablation. The efficiency ill this region was approximately 0.3 5 mm(3)/J, and the ablation rate was found to be 0.6 mu m/pulse. The ablati on threshold was found to occur at a fluence of 50 mJ/cm(2). In the region of highest efficiency, the peak varied slightly in the fluence range betwee n 150 and 250 mJ/cm(2) CONCLUSIONS. This study confirms that the corneal ablation properties at 21 3 nm are comparable with those at the 193-nm excimer laser wavelength. Incr eased pulse energy mas obtained for the fifth harmonic of Nd:YAG lasers at 213 nm through the use of new nonlinear optical crystals to perform the fre quency conversion. A solid state laser is feasible to replace the excimer g as laser for performing refractive surgery procedures. For the first time, the increased energy at 213 nm allows large-beam ablations to be performed at this wavelength.