Comparison of the maximum coagulation depth in human skin for different types of medical lasers.

The values for the maximum coagulation depth (MCD) of various types of lasers which are specified in the literature are not comparable, because often different irradiation times were used. MCD depends not only on the wavelength of laser light, but also on the time of action of the laser beam, because of heat transfer. In excised human skin MCD was determined histologically for irradiation with the argon laser, Nd:YAG laser, and CO2 laser. Extending the irradiation time from 0.2 s to 10 s results in a 4-6-fold increase in MCD. Coagulation experiments performed with a soldering iron have shown results similar to those obtained with the CO2 laser. Enlargement of the laser-beam diameter from 1 to 2 mm leads to a 50% increase in MCD. Additional chilling of the skin with water during laser irradiation protects the skin from evaporation, and the values for MCD increase to 3.5 mm for the argon laser and 5.5 mm for Nd:YAG laser. Detailed knowledge of the MCD is necessary to obtain the desired therapeutic effect, and also to avoid unwanted effects.