Bone mineral density varies as a function of the rate of joint space narrowing in the hip.

OBJECTIVE: To determine whether patients with a rapid rate of joint space narrowing (JSN) in the hip have higher initial bone mineral density (BMD) in the proximal femur and/or lumbar spine than corresponding patients with a slow rate of JSN.
METHODS: Twenty-eight patients undergoing unilateral total hip replacement (THR) for osteoarthritis (OA), but whose contralateral hips were asymptomatic and had minimal or no radiographic OA, were evaluated. The contralateral proximal femur (i.e., non-operated hip) and lumbar spine were scanned by dual energy x-ray absorptiometry at baseline (prior to THR) and at 2 years. The rate of JSN was determined by serial longitudinal quantification of the joint spaces over the 2 year period following THR from conventional radiographs, and the patients were divided into a group with a slow rate of JSN (< or = 0.2 mm/yr, n = 20) and a group with a rapid rate of JSN (> 0.2 mm/yr, n = 8).
RESULTS: The baseline BMD z and t scores at the femoral neck, Ward's triangle, and lumbar spine of the patients with subsequent rapid rates of JSN were significantly higher than those of patients with slower rates (p < 0.05). There was no difference between the rapid and slow narrowers at the greater trochanter (p > 0.2). Age, sex, weight, height, body mass index, Kellgren- Lawrence scores, and initial joint space width were not significantly different between the 2 groups.
CONCLUSION: Patients with a rapid rate of JSN of the asymptomatic hip after unilateral THR are characterized by elevated local and remote BMD. The local elevation in BMD implies that increased local bone density may contribute to or serve as a marker for increased risk of development of OA (assuming that JSN can be used as a predictive marker). The presence of elevated BMD in the spine suggests that there are systemic as well as local aspects of OA pathogenesis, at least in patients with one THR and rapid JSN in the contralateral hip.