[Fracture toughness of human enamel].

The fracture toughness (KIC) of human enamel was examined by means of Indentation Microfracture Method (IM method). The IM method was a simple, efficient and economical means to obtain KIC of human enamel. Judging from the relationship between load and crack size, a median crack occurred in the range of 2.45-5.88 N, and KIC was hardly affected by the difference of surface roughness. KIC of a 58-year-old man's teeth was 0.61 MPam1/2 in 1 and 0.77 MPam1/2 in 7 and the difference was significant (P less than 0.01). KIC differed much from the direction of the indenter pressed to enamel rods and the KIC of the direction of enamel rods axis I was markedly higher than that of others. In the 58-year-old man's 1, KIC was 0.61 MPam1/2 in the direction of enamel rods axis V, 0.66 MPam1/2 in the direction of H and 0.84 MPam1/2 in the direction of I, and the difference was significant (P less than 0.01). The values for a 10-year-old man's permanent teeth was slightly higher but showed a similar inclination to the 58-yer-old man's teeth.