Fundamental Studies on Tissue Reaction after Low Power Laser Irradiation

It is generally recognized that the lower power Nd: YAG laser provides some therapeutic effects on tissue without irreversible changes. On the other hand, high power laser causes irreversible changes. The criterion about low and high power laser is, however, not established. The aim of this study is to verify the effect of lower and high power laser on wound healing of rat skin by measuring blood flow and histomorphologic findings with HE and SEM. Each laser were irradiated to the center of 6mm round full-thickness skin defect at the back of the rat at either (1) 300mW×3min, (2) 500mW×3min, (3) 700mW×3min, (4) 300mW×20min, (5) 500mw×20min, (6) 700mW×20min.The results obtained are as follows;1) The histological findings showed that irradiation time caused more tissue damage in the deeper layer, whereas irradiation power caused more damage to the surrounding tissue.2) Small craters were starred on the bottom of the damaged tissue by laser irradiation at 700 mW x 20min. The formation of these craters was related to the absorption of laser irradiation.3) On the irradiated area, blood volume decreased significantly as irradiation power increased. However, both blood flow and tissue damage increased as irradiation time increased. As for the surrounding tissue, the findings showed an increase in the blood flow and tissue damage as irra diation power increased.4) High power laser at 700mW caused tissue damage regardless of irradiation time. On the other hand, 500mW irradiation power caused minimum damage to the tissue. Therefore, the range between 300 to 500mW and the irraditation time over 3min to less than 20min are classified as a lower power laser.