Characterization of bone and bone marrow lesions in the osteoarthritic hip
OA Version
Citation
Abstract
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.
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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.