Measurement Method of Fracture Surface Orientations of Cortical Bones Under Multidirectional External Loads

The unclear and unpredictable fracture surface orientations of cortical bones have restricted the deep understanding of fracture mechanisms. The multidirectional loads aggravate the complexity and difficulty in the measurement of the fracture surface orientations. This article focused on novel testing instrument and measurement method of the fracture surface orientations of cortical bones. The controllable external loads were realized by a self-developed in situ tester under approximate in vivo environment and various sampling orientations of bone specimens. A normal vector measurement method was proposed to obtain the normal vector of fracture surfaces under various sampling angles. Quantitative variation of fracture surface orientations along with the direction of external loads was obtained. As the sampling angle gradually increased in a range from 0° to 60°, the normal vector angles exhibited a “slow first then fast later” monotonous increasing tendency in a range from 69.18° ± 9.40° to 84.48° ± 9.87°. On the basis of proposed complete fitting method and eliminating abnormal point method, the measurement error of normal vector angle exhibited a slight fluctuation in a range from 0.22% to 1.83%, dependent on the straightness error of intersection line and flatness error of fracture surface. The approximatively invariable propagation distance along the osteon/interstitial lamellae interface and the “bending ductility effect” induced longer propagation distance inside the osteons resulted in the increased normal vector angles of the fracture surfaces when the external loads gradually deviated from the long axis of bones.