Principles of lasers and biophotonic effects

In this review, we discuss how, due to a variety of different interactions between laser radiation and biological tissue, the laser has become an established instrument in most medical fields. Depending on the interaction time and the effective power density, three types of laser tissue interaction can be distinguished: photochemical effects, photothermal effects, and photomechanical and photoionizing effects. After a description of the physical mechanisms, the typical parameters, and the medical applications of these effects, a review of the laser types used in medicine is given. For percutaneous laser disc decompression (PLDD), lasers in the near-infrared region (Nd:YAG, Ho:YAG, and diode lasers) and with visible green radiation (frequency doubled Nd:YAG, called "KTP laser") were reported to be effective.