In situ observation of fracture behavior of canine cortical bone under bending

Cortical bone provides many important body functions and maintains the rigidness and elasticity of bone. A common failure mode for bone structure is fracture under a bending force. In the current study, the fracture behavior of canine cortical bone under three-point bending was observed in situ using an atomic force microscope (AFM), a scanning electron microscope (SEM), and an optical microscope to examine the fracture process in detail. Nanoindentation was carried out to determine the elastic modulus and hardness of different building blocks of the canine cortical bone. The results have shown that the special structure of Haversian systems has significant effects on directing crack propagation. Although Haversian systems contain previously believed weak points, and micro-cracks initiate within Haversian systems, our findings have demonstrated that macro-cracks typically form around the boundaries of Haversian systems, i.e. the cement lines. Micro-cracks that developed inside Haversian systems have the functions of absorbing and dissipating energy and slow down on expanding when interstitial tissue cannot hold any more pressure, then plastic deformation and fracture occur. •Macro- and micro-cracks occur in unique patterns in the bone fracturing process under a bending force.•Early developed micro-cracks inside Haversian systems absorb and dissipate energy in order to delay fracture initiation.•The mechanical properties of Haverisan systems and its surrounding structures influence the developments of macro- and micro-crack formation.•Previously believed weak spots in the bone matrix are not necessarily the origins of fracture development.