Characterization of bone and bone marrow lesions in the osteoarthritic hip
Bone marrow lesions (BMLs) are common in osteoarthritis (OA) of the hip and knee and present as cysts in computed tomography scans. BMLs in knee OA are associated with pain, cartilage loss, and attrition of subchondral bone, suggesting that they play a key role in progression of OA. However, the etiology of BMLs remains unclear. The goal of this study was to better understand the changes that occur in bone in OA, through the characterization of BMLs, bone microarchitecture, and bone stiffness. Femoral heads obtained from patients undergoing total hip arthroplasty because of end-stage OA were imaged using micro-computed tomography (µCT) to identify the cysts. The bone volume fraction (BV/TV), bone mineral density (BMD), and tissue mineral density (TMD) were evaluated for two volumes of interest: an area immediately surrounding the cysts and a cylindrical core from the primary load-bearing region of the femoral head. Further, the cylindrical core was modeled using finite element analysis in order to evaluate the stiffness of this mechanically critical region. After imaging, the heads were sectioned and stained for histological analysis. Overall, the specimens exhibited wide variation in the number of cysts and cyst volume normalized by total volume of the femoral head (CV/TV). The cysts were found primarily in the subchondral bone underlying regions of damaged cartilage. The µCT images and histological sections revealed the presence of sclerotic bone around the cysts. vi The lesions themselves contained fibrous, fatty, osseous, and cartilaginous tissues. Lesions were absent from the cylindrical cores, and no correlations were found between core stiffness and any cyst properties. The cores were also found to have a higher bone volume fraction compared to values from published studies on cadaveric samples obtained from a pool of donors not specifically limited to those with end-stage OA. The cores also exhibited a modestly different dependence of apparent modulus on volume fraction, as compared to those published data. A pilot study was next carried out on the femoral necks from four of the patients. This study consisted of using nanoindentation to measure the modulus of cortical, trabecular, and periosteal bone. These preliminary results suggested that the moduli varied substantially among patients, and that the modulus of cortical tissue was in some locations for some patients, lower than that of trabecular tissue, despite published reports to the contrary in non-OA bone. The results of this project demonstrate that cysts associated with severe OA involve extensive perturbations in local bone morphology and cellular activity, and yet comparatively minimal disruption to the primary load-bearing region of the femoral head. These findings suggest that despite the association of cysts with symptoms of OA in the knee, cysts in hip OA are not strongly associated with a global loss of function of the primary bony structure of the joint. Further study of these cysts is necessary to identify their mechanistic relationship with the progression of hip OA.