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-mu-m, pulse duration 250-mu-s) and a pulsed thulium laser (wavelength 2.01-mu-m, pulse duration 250-mu-s) 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 and 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/cm3 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. These values were not statistically different. Ablation craters were made in freshly excised guinea pig skin and examined microscopically. The thulium laser produced slightly smaller zones of thermal residual injury but more than predicted by theory. When used for tissue ablation, the holmium and thulium lasers are comparable.