Tissue damage by laser radiation: an in vitro comparison between Tm:YAG and Ho:YAG laser on a porcine kidney model

The understanding of tissue damage by laser radiation is very important for the safety in the application of surgical lasers. The objective of this study is to evaluate cutting, vaporization and coagulation properties of the 2 µm Tm:YAG laser (LISA Laser Products OHG, GER) in comparison to the 2.1 µm Ho:YAG laser (Coherent Medical Group, USA) at different laser power settings in an in vitro model of freshly harvested porcine kidneys. Laser radiation of both laser generators was delivered by using a laser fiber with an optical core diameter of 550 µm (RigiFib, LISA Laser GER). Freshly harvested porcine kidneys were used as tissue model. Experiments were either performed in ambient air or in aqueous saline. The Tm:YAG laser was adjusted to 5 W for low and 120 W for the high power setting. The Ho:YAG laser was adjusted to 0.5 J and 10 Hz (5 W average power) for low power setting and to 2.0 J and 40 Hz (80 W average power) for high power setting, accordingly. The specimens of the cutting experiments were fixed in 4 % formalin, embedded in paraffin and stained with Toluidin blue. The laser damage zone was measured under microscope as the main evaluation criteria. Laser damage zone consists of an outer coagulation zone plus a further necrotic zone. In the ambient air experiments the laser damage zone for the low power setting was 745 ± 119 µm for the Tm:YAG and 614 ± 187 µm for the Ho:YAG laser. On the high power setting, the damage zone was 760 ± 167 µm for Tm:YAG and 715 ± 142 µm for Ho:YAG. The incision depth in ambient air on the low power setting was 346 ± 199 µm for Tm:YAG, 118 ± 119 µm for Ho:YAG. On the high power setting incision depth was 5083 ± 144 µm (Tm:YAG) and 1126 ± 383 µm (Ho:YAG) respectively. In the saline solution experiments, the laser damage zone was 550 ± 137 µm (Tm:YAG) versus 447 ± 65 µm (Ho:YAG), on the low power setting and 653 ± 137 µm (Tm:YAG) versus 677 ± 134 µm (Ho:YAG) on the high power setting. Incision depth was 1214 ± 888 µm for Ho:YAG whereas Tm:YAG did not cut tissue at 5 W in saline solution. On the high power setting, the incision depth was 4050 ± 1058 µm for Tm:YAG and 4083 ± 520 µm for Ho:YAG. Both lasers create similar laser damage zones of <1 mm in ambient air and in saline solution. These in vitro experiments correspond well with in vivo experiments. Thereby, Tm:YAG offers a cutting performance, coagulation and safety profile similar to the standard Ho:YAG lasers in urological surgery.