New QCT analysis approach shows the importance of fall orientation on femoral neck strength

Introduction: Osteoporosis is associated with decreased BMD and increased hip fracture risk, but it is unclear whether specific osteoporotic changes in the proximal femur lead to a more vulnerable overall structure. Nonhomogeneous beam theory, which is used to determine the mechanical response of composite structures to applied loads, can be used along with QCT to estimate the resistance of the femoral neck to axial forces and bending moments. Materials and Methods: The bending moment [M-y(theta)] sufficient to induce yielding within femoral neck sections was estimated for a range of bending orientations (theta) using in vivo QCT images of 37 male (mean age, 73 years; range, 65-87 years) femora. Volumetric BMD, axial stiffness, average moment at yield (M-y,M-avg), maximum and minimum moment at yield (M-y,M-max and M-y,M-min), bone strength index (BSI), stress-strain index (SSI), and density-weighted moments of resistance (R-x and R-Y) were also computed. Differences among the proximal, mid-, and distal neck regions were detected using ANOVA. Results: M-y(theta) was found to vary by as much as a factor of 2.8 for different bending directions. Axial stiffness, M-y,M-avg, M-y,M-max, M-y,M-min, BSI, and R-x differed significantly between all femoral neck regions, with an overall trend of increasing axial stiffness and bending strength when moving from the proximal neck to the distal neck. Mean axial stiffness increased 62% between the proximal and distal neck, and mean M-y,M-avg increased 53% between the proximal and distal neck. Conclusions: The results of this study show that femoral neck strength strongly depends on both fall orientation and location along the neck axis. Compressive yielding in the superior portion of the femoral neck is expected to initiate fracture in a fall to the side.