[Fracture toughness of cortical bone in tension, shear, and tear--a comparison of longitudinal and transverse fracture].

The fracture toughness at crack initiation was determined for bovine cortical bone under tension (mode I), shear (mode II), and tear (mode III). A total of 130 compact tension specimens, compact shear specimens and triple pantleg specimens were used for the measurement of fracture toughness under tension, shear, and tear, respectively. Multiple-sample compliance method was utilized to measure the critical strain energy release rate (Gc) at the a/W = 0.55 (crack length, a, to specimen width, W, ratio). The critical stress intensity factor (Kc) was also calculated from the critical loading (PQ) of the specimens at the a/W = 0.55. The effect of the anisotropy of bone on its resistance to crack initiation under shear and tear loading was investigated as well. The fracture toughness of bone with precrack orientations parallel(designed as longitudinal fracture) to and that with precrack orientations normal (designed as transverse fracture) to the longitudinal axis of bone were compared. In longitudinal fracture, the critical strain energy release rates(Gc) of cortical bone under tension, shear, and tear were 644 +/- 102, 2430 +/- 836, and 1723 +/- 486 N/m, respectively. In transverse fracture, the critical strain energy release rates(Gc) of cortical bone under tesion, shear, and tear were 1374 +/- 183, 4710 +/- 1284, and 4016 +/- 948 N/m, respectively. An analysis of variance demonstrated that the crack initiation fracture toughness of bone under shear and tear loading is significantly greater than that under tensile loading in both longitudinal and transverse fracture. Our results also suggest that cortical bone has been "designed" to prevent crack initiation in transverse fracture under tension, shear, and tesar.