Comparison of tissue ablation with pulsed holmium and thulium lasers

The ablation rates and tissue effects produced by a pulsed holmium laser (wavelength 2.12 μm, pulse duration 250 μs) and a pulsed thulium laser (wavelength 2.01 μm, pulse duration 250 μs) were compared. Because the absorption coefficient of water is almost three times greater at the shorter wavelength (65 versus 24 cm^-1), the thulium laser should have a significantly lower threshold of ablation and produce significantly less residual thermal injury. These hypotheses were tested in vitro. Ablation rates were measured using fresh liver and a mass loss technique were found to increase linearly with delivered radiant exposure. The threshold radiant exposure for ablation was derived from the mass loss measurements and found to be 36 J/cm² for the holmium laser and 29 J/cm² for the thulium laser. The corresponding heats of ablation were 10 kJ/cm³ for the holmium laser and 9.7 kJ/cm³ for the thulium laser