Objective. Previous case-control studies have shown various degrees of inverse relationship between osteoarthritis (OA) and osteoporosis (OP). The aim of this study was to examine the relationship between radiographic hip OA and bone mineral density (BMD) at the affected and contralateral hips, as well as at more distal sites. We also explored the possibility that this association might be confounded by genetic factors. Methods. Using the discordant twin model to reduce selection bias and adjust for genetic factors, plain pelvic radiographs of white female twins aged >40 years, from the St. Thomas' UK Adult Twin Register, were assessed for radiographic features of hip OA. Overall OA was classified using a 6-point global grading system (Croft). Osteophytes (OPH) and joint space narrowing (JSN) were also examined separately. BMD was measured by dual x-ray absorptiometry at the left hip, lumbar spine, and total body. The association of OA with BMD was assessed using conditional logistic regression. Adjustments were made for body mass index, lifetime physical activity, menopausal status, use of estrogen, and smoking. Results. The analysis included a total of 1,148 women comprising 160 monozygotic and 414 dizygotic twin pairs. The median age of the twins was 53 years (range 40-70). The crude and adjusted odds ratios and 95% confidence intervals for having radiographic features of hip OA were 1.63 (1.06, 2.50) and 1.80 (1.05, 3.12), respectively, per unit difference in standardized BMD of the ipsilateral femoral neck. The presence of OPH, but not JSN, was associated with higher BMD. Twins with hip OPH had 3.5% higher femoral neck BMD than their unaffected cotwins. No clear association was found between hip OA and BMD at the contralateral site, lumbar spine, or total body. Conclusion. This twin study confirms the existence of an inverse relationship between OA and OP at the hip. However, the relationship was localized to the OA-affected hip. The generalized and greater increase in BMD in osteoarthritic subjects seen in previous studies of unrelated populations is therefore likely to be due, in part, to genetic factors shared by hip OA and high bone mass. It also suggests that local changes in bone density may be a component of the disease process in hip OA.

Two methods for diagnosing radiological osteopenia in thoracic (TS) and lumbar (LS) spine radiographs were assessed: a subjective conventional method (A) and a semiquantitative method (B), by comparing them with bone mineral density (BMD) measured by dual energy X-ray absorptiometry (DEXA), in a population of "normal" women aged 45-70 years (n = 818). For both methods there was good intraobserver and interobserver reproducibility. BMDs were significantly lower with increasing radiological osteopenia grades (p < 0.001), and remained lower after adjustment for age and body mass index (p < 0.01). The proportion of subjects with DEXA-defined osteoporosis rose with increasing radiological osteopenia grades for both methods. The worst osteopenia categories identified 29.7-55.3% of women with DEXA-defined osteoporosis, compared with 6.1-11.7% in the "normal" categories. Both methods, however, showed a large degree of overlap of BMDs between the various radiological osteopenia grades. The sensitivity and specificity of method A in diagnosing osteoporosis were 45.3% and 78.4%, respectively, for the TS and 19.0% and 94.3%, respectively, for the LS. For method B the sensitivities and specificities were 8.8% and 96.1%, respectively (TS), and 10.2% and 95.6%, respectively (LS). Although both methods have poor sensitivities, "definite" or "high" grade osteopenia should be an indication for bone densitometry. The high specificities suggest that a "normal" (no osteopenia) X-ray is unlikely to have a significantly low BMD.