Biomechanical assessment of fracture risk in young children with healthy bone and osteogenesis imperfecta.

Osteogenesis imperfecta (OI) is a group of genetic bone disorders that can range from a mild increase in fractures in one’s lifetime to perinatally lethal. Biomechanical assessment via finite element analysis (FEA) can be used to provide subject specific evaluation of fracture risk and thus aid in furthering understanding of likelihood of fracture due to loading associated with common daily activities. Thus, the purpose of this dissertation research was to characterize the risk of femur fracture in young children with OI compared to that of healthy children when exposed to loading associated with common activities, specifically gait. To accomplish this, development of tools and methods to generate the 3D femur using 2D radiographs was necessary. First, healthy femur morphology was characterized by developing a statistical shape model (SSM) using 79 computed tomography scans of femurs from children (aged 0-3 years old). Methods to generate 3D models from 2D radiographs were evaluated using geometric and biomechanical analysis; the resulting method used a combination of the developed pediatric SSM along with morphing to provide a 3D outer femur surface which conforms vi to the femur shape in the 2D radiographs. This method was then applied to radiographs of OI femurs where 3D femur models were generated using the 2D radiographs from a 19-month-old child diagnosed with OI Type I. A parametric FEA was conducted varying the cortical widths of the OI femur model and material properties (elastic moduli) using values documented in the literature. A healthy femur model control representative of a child with the same age, height and weight was generated using the SSM for comparison. For the FEA, simulated loading conditions were experimentally determined from a motion analysis study of 5 newly walking children (13-22 months old) from which regressions predicting the hip joint reaction loads onto the femurs during peak resultant force in gait were developed. While the FE findings suggest an increased risk of fracture in the one OI Type I bone evaluated compared to the healthy femur, loading associated with walking was found to have a low likelihood of fracture in the evaluated subject with OI Type I.