TEMPERATURE-CONTROLLED LASER PHOTOCOAGULATION OF SOFT-TISSUE - IN-VIVO EVALUATION USING A TISSUE WELDING MODEL

Background and Objective: Laser surgical procedures involving photocoa gulation of soft tissue have relied on subjective visual endpoints. Th e thermal damage to the denatured tissue in these procedures is highly dependent on the tissue temperatures achieved during laser irradiatio n. Therefore, a system capable of real time temperature monitoring and closed loop feedback was used to provide temperature controlled photo coagulation (TCPC). Study Design/Materials and Methods: The TCPC syste m consisted of a 1.32 mu m Nd:YAG laser, an infrared thermometer, and a microprocessor for data acquisition and feedback control. A porcine skin model was used. Tissue welds were completed to evaluate the photo coagulation effects at different predetermined temperatures. A quantit ative measurement of tissue photocoagulation was obtained by tensile s trength measurements of the laser repairs. Histology of the irradiated tissue was used to determine the extent of thermal injury associated with different photocoagulation temperatures. Results: The TCPC system was capable of maintaining a relatively constant temperature (+/-4 de grees C) during laser irradiation. The tensile strengths of acute repa irs increased with temperature over the range studied (65-95 degrees C ). Tensile measurements made after several days of healing showed that higher temperature (95 degrees C) welds had lower strengths than repa irs completed at lower (65 degrees C or 75 degrees C) temperatures and were significantly lower at 3 days. Acute histology showed that the a mount thermal damage was strongly dependent on the tissue temperature and increased both in tissue depth and lateral to the repair with temp erature. The histologic results suggest that the increase in the acute repair tensile strength as the weld temperature increased was due to an increase in the depth of tissue photocoagulation. The increase in t he lateral tissue injury measured histologically for higher temperatur e welds Likely resulted in the decreased chronic tensile strengths, as a healing response to excessive thermal damage. Conclusion: Tissue te mperatures can be controlled during laser photocoagulation of skin. Th e degree of acute and chronic tissue damage is highly dependent on the temperature during welding. By controlling the tissue temperature dur ing laser procedures, the surgical outcome can be more reliably predic ted and reproduced, as compared to the conventional open loop methods. In addition, the use of a TCPC system should significantly reduce the learning curve for photothermal surgical procedures. (C) 1996 Wiley-L iss, Inc.