Comparison of thermal tissue effects induced by contact application of fiber guided laser systems

Laser light of various wavelengths is being used for surgical procedures in otolaryngology. Apart from well-known fiber guided laser systems such as Nd:YAG- and Ho:YAG-lasers, newly developed diode-laser systems of different wavelengths have recently become popular in surgery. In order to compare the effects of fiber guided laser light with respect to their induced tissue effects, these laser systems have been studied and compared under reproducible test conditions.

The laser fibers of four common medical laser systems (Ho:YAG- (lambda = 2,080 nm), Nd:YAG- (lambda = 1,064 nm), and diode-laser (lambda = 830 and 940 nm)) were fixed to a computer controlled stepper motor. The laser light was applied in contact mode onto ex vivo muscle tissue, using identical power settings and a reproducible application procedure (application velocity, application angle) under constant conditions (temperature of tissue and volume). The size of the thermal effects on the tissue (e.g., coagulation, ablation, and carbonization zones) were measured and photographed via optical microscopy.

Depending on the laser wavelength used, the experimental results proved different degrees of tissue responses. Nd:YAG- and diode-lasers provided for only low coagulation effects in the depth of the tissue, but produced severe carbonization at the surface. Ho:YAG-laser light revealed the highest ablation capabilities of the lasers investigated in addition to large coagulation zones which were of larger extent than those produced by Nd:YAG- and diode-laser light.

Contact treatment by Ho:YAG-laser light might provide for a precise and effective tissue reduction in a bloodless manner because of its high ablation and coagulation capabilities, especially if large volumes are treated and structures beneath are non-critical. In comparison, Nd:YAG- and diode-laser treatment in contact application showed low thermal tissue effects (i.e., coagulation) in the depth, resulting from a high power loss caused by the development of large carbonization zones at the surface of the tissue. Therefore, the degree of blood-perfusion and the capability of vessel-closure induced by these lasers should be taken into account. The presented investigation also revealed that in contact mode, the tested laser systems produced tissue effects, which were highly different from those already described for applications in non-contact mode. Physicians who are performing laser treatments in close boundaries must be aware that changing from non-contact to contact mode in laser application greatly influences the resulting tissue effects.