[Fracture toughness of human enamel irradiated by CO2 laser]

The fracture toughness (KIC) of human enamel irradiated by CO2 laser was examined by means of indentation microfracture method (IM method). The irradiation of laser from 1.5 watt (16 J/cm2) to 12.3 watt (133 J/cm2) was applied to enamel for 0.2 s, and the diamond cone of micro vickers hardness tester was pressed down by the 2.45 N (250 gf) under ordinary circumstances. At the irradiated spot, KIC was 0.24-0.86 MPam1/2 and Hv was 3.6 x 10(3)-4.3 x 10(3)MPa (370-440). At the non-irradiated spot, KIC was 0.83-0.9 MPam1/2 and Hv was 3.9 x 10(3)-4.3 x 10(3)MPa (400-440). A decreasing tendency of KIC was seen in all irradiated spots. The marked decrease of KIC started at the irradiation of 4.4 watt (47 J/cm2) and the rate of decrease was 22%, and 54% when exposed to the irradiation of 5.7 watt (61 J/cm2). The rate of the lowering of the KIC of 0.05 seconds' irradiation x 4 times was less than that of 0.2 s x 1 time, while cumulative total irradiation energy was equal. The measurement of KIC by the low force of 0.98 N (100 gf) seemed to be possible when KIC was 0.41 MPam1/2 and less.