Hip osteoarthritis (OA) is a major public health concern. The determinants of hip OA, however, are not as well understood as those of other OA sites, such as the knee. In recent years, the role of subchondral bone in the pathogenesis of OA has been emphasized but data are lacking for hip OA. Therefore, we aimed to determine which bone characteristics were associated to hip OA.

We made a cross-sectional analysis of 1537 postmenopausal women included in the QUALYOR prospective cohort. At baseline, we measured areal BMD by DXA at the lumbar spine and the hip, volumetric BMD and geometry by hip quantitative computerized tomography (QCT) using the Bone Investigational Toolkit (BIT) software, as well as microarchitecture at the distal radius and tibia by high-resolution peripheral quantitative tomography (HR-pQCT). We built a hip OA score (CT OA score) with images from the hip QCT, based on the depiction of the four major signs of osteoarthritis: subchondral bone sclerosis, joint space narrowing, osteophytes and subchondral cysts. The severity of each of these four signs was graded as absent, mild, moderate or severe (semi-quantitative score ranging from 0 to 3 for each sign). The absence of hip OA was defined as CT score equal to 0, mild hip OA as CT score between 1 and 4 and moderate to severe hip OA as CT score>4. Women with and without hip OA were compared using t tests and multivariable modeling.

The mean age was 65.9 (±6.7) years and the mean body mass index was 24.6 (±3.6) kg/m². Among these 1537 women, 601 had an OA score of 0, 756 between 1 and 4 (mild OA) and 180 greater than 4 (severe OA). Women with hip osteoarthritis had lower trabecular total hip vBMD (125 vs. 129mg/cm³, P<0.01). Cortical hip vBMD did not differ between women with and without hip OA (966.5 vs. 963.5mg/cm³, P n.s). Patients with hip OA also had larger femoral neck volume (11.55 vs. 11.27mm³, P<0.001). The BIT analysis showed greater bone resistance to bending (cross-sectional moment of inertia [CSMI] min with 6.03 vs. 5.6cm⁴ and section modulus [Z] polar with 7.98 vs. 7.59cm³, P<0.05) at the femoral neck in patients with mild hip OA and even greater in women with severe hip OA. Patients with hip OA had significantly higher trabecular area measured at the radius by HR-pQCT (205.11 vs. 192.61mm², between group difference 12.50 95% CI [8.15–16.86] P<0.01), lower trabecular number at the tibia (1.57/μm vs. 1.63/μm, between group difference −0.06 95% CI [−0.09 to −0.03] P<0.001) and higher trabecular spacing at the tibia (0.58 vs. 0.56μm, between group difference 0.02 95% CI [0.011–0.038] P<0.005). Cortical parameters were not different in patients with hip OA compared to controls.

Women with hip OA have larger femoral neck and lower trabecular bone parameters, suggesting a sizeable role of hip bone geometry and remodeling in the pathophysiology of hip OA.