Micromechanical properties of epiphyseal trabecular bone and primary spongiosa around the physis: an in situ nanoindentation study.

The elastic modulus and hardness of the mineralized bone around the growth plate was measured to determine its regional micromechanical properties. Multiple nanoindentation tests, >10 sessions, with depths ranging from 100 to 1,000 nm at loading rates of 12.5 and 750 microN/s, were performed on the trabecular bone in the epiphysis, trabecular bone at the junction of the physis and epiphysis, primary spongiosa in the metaphysis, and surrounding cortical bone of the distal femur of 300-gm Sprague-Dawley rats. The indentation load-displacement data obtained in these tests were analyzed to determine the elastic modulus and hardness of the tissues. The nanoindentation results highlighted the regional variations in the material properties of the mineralized tissues around the growth plate. The primary spongiosa had a lower elastic modulus and hardness than both epiphyseal trabecular and cortical bone (p < 0.01). A relatively well-defined thick trabecular band at the physeal-epiphyseal junction had modulus and hardness values comparable to those of cortical bone (p > 0.05). These findings support the hypothesis that the primary spongiosa has micromechanical properties that are significantly lower than the epiphyseal trabecular bone. On this basis, it is speculated that the fracture patterns commonly seen in patients with physeal injuries are influenced by the micromechanical properties of these tissues, as well as by the nature and direction of the applied force.