The fracture mechanics parameter of the critical stress intensity factor (Kc) was determined by a modified compact tension test method, for the fracture of bovine tibia cortical bone at orientations of 0°, 15°, 30°, 45°, 75° and 90° to the bone axis. It was established that, for a given loading rate, a variation in orientation from 0–90° produced average increases in Kc from 3.2 to 6.5 MN m−3/2.