Fast and effective skin ablation with an Er:YAG laser: determination of ablation rates and thermal damage zones.

Abstract

Er:YAG lasers are known to superficially ablate skin and other tissues with minimal thermal coagulation zones. The ablation efficacy and thus the clinical applicability of these lasers, however, was limited due to small beam diameters and repetition rates. Aim of this study was to determine the ablation efficacy and the amount of thermal damage with a new high-power high-repetition-rate Er:YAG laser and to find optimal treatment parameters for skin ablation. In vitro and some in vivo ablation trials on human skin were performed with the Er:YAG laser (MCL 29, Aesculap-Meditec, Heroldsberg, Germany, 2.94 microns, max. 500 mJ per pulse, 250 microseconds pulse length, 3 or 4 mm spot size, repetition rate 7-10 Hz) and evaluated microscopically. The ablation threshold was around 1.6 J/cm2. The ablation rates increased linearly with the fluence, and the above-threshold ablation efficacy was around 2.5 microns per pulse per J/cm2, leading to ablation velocities of 70-100 microns per second and higher. With increasing pulse numbers applied to one tissue spot, the ablation per pulse decreased significantly. The amount of thermal damage was clearly dependent on the number of pulses applied (around 25 microns with < 10 imp., up to 100 microns with 40 imp.), whereas higher fluences increased the coagulation zones only minimally. The in vivo trials confirmed these results: overlapping pulses in the 4 J/cm2-range, applied in a sweeping motion, proved optimal for an efficient skin ablation with a smooth resulting surface and a thermal damage zone not exceeding 50 microns. The high power and the high repetition frequency make this laser a fast and effective tool for skin ablation without increasing the thermal damage, but the ablation remains limited to the superficial dermis, since hemostasis cannot be achieved due to the absence of coagulation.