Brittle Bone Fracture Risk with Transverse Isotropy

Osteogenesis imperfecta (OI) is a heritable bone fragility disorder characterized by skeletal deformity and “brittle” bones. This fragility is believed to stem from a combination of bone mass deficiency and compromised bone material properties [1-3]. Poor bone quality poses major orthopaedic and rehabilitation challenges. Risk of fracture is a major consideration when prescribing activity restrictions and physical therapy. Quantifying bone fracture risk would be an invaluable clinical treatment tool. Finite element (FE) models have the potential to provide patientspecific feedback on the effects of fracture risk factors in long bones. As more information on OI becomes available, the model is evolving to include the latest data. The latest modification is focused on material properties. It is clear that OI bone does not behave like normal pediatric bone [1-5]. However, quantification of these differences has only recently been explored. Accurate material properties are essential for a computational fracture risk assessment (FE) model. The previous femoral fracture risk assessment model for an OI patient implemented material properties obtained during nanoindentation testing of OI bone specimens [5]. At the time, this was the only data available on OI bone properties. Based on these results, the femur was modeled as an isotropic material. Recent mechanical testing by our group has shed new light on the flexural properties of OI bone. These tests have shown that OI bone is, in fact, not isotropic and has a much lower Young’s modulus (E) than what was calculated via nanoindentation testing. Transverse isotropic properties have been implemented into the OI femur model to examine effects on the maximum principal stress experienced during mid-stance of the gait cycle (highest load phase). The goal of study is to compare the isotropic and transverse isotropic models.