Predicting regional variations in trabecular bone mechanical properties within the human proximal tibia using MR imaging.

Trabecular bone density changes throughout the proximal tibia are indicative of several musculoskeletal disorders of the knee joint. Many of these disorders involve not only changes in the amount of bone, but also in the surrounding soft tissue. Osteoarthritis, for instance, involves bone density changes below the subchondral bone and throughout the proximal tibia, along with degradation evident in the articular cartilage. Osteoporosis, characterized by low bone density may also involve changes in bone size, structure or microarchitecture, each of which may contribute to fracture risk. Recent studies have shown that magnetic resonance (MR) imaging, most frequently applied for soft tissue imaging, also allows non-invasive 3-dimensional characterization of bone microstructure. The purpose of the current study is to use whole joint MR images to acquire regional apparent bone volume fraction (appBVF) throughout the proximal tibia and correlate with mechanical properties measured on the corresponding ex vivo specimens. To compare our method to a high-resolution imaging modality, micro-CT analysis was performed in a subset of specimens. Using linear mixed-effects models, significant correlations (p<0.05) were determined between MR appBVF and Young's modulus (r2=0.58, MPSE=3633 MPa2), yield stress (r2=0.73, MPSE=1.53 MPa2) and ultimate stress (r2=0.72, MPSE=2.29 MPa2). Comparable significant correlations (p<0.05) were also determined between micro-CT BVF and Young's modulus (r2=0.47, MPSE=5179 MPa2), yield stress (r2=0.80, MPSE=1.23 MPa2) and ultimate stress (r2=0.83, MPSE=1.76 MPa2). The current study demonstrates that MR imaging may be used as an in vivo imaging tool to determine differences in bone strength between subjects and regional variations within a single tibia.