The effects of position on the radiographic joint space in osteoarthritis of the hip.

The aim of the study was to assess whether radiographic hip joint space thickness was changed by weight-bearing (WB) compared with non weight-bearing (NWB) position, and to evaluate whether radiographs centered on the hip were more sensitive than pelvic X-rays to detect such a change. Anteroposterior radiographs of the pelvis were made in 30 patients with hip osteoarthritis OA (46 OA and 11 normal hips). Osteoarthritic, as well as contralateral normal hips were analyzed. Radiographs centered on OA hip were performed in 28 other patients. X-rays were made in WB and NWB positions using a standardized radiological procedure. Measurements of mean joint space width (MeanJSW) maximum joint space narrowing (MaxJSN) and joint space surface area (JSA), were made using a computerized image analysis system. The joint space width was unaffected by WB in normal joints but decreased with WB in OA joints. The decrease was significant only when considering MaxJSN in patients with a joint space thickness smaller than 2.5mm. The difference between WB and NWB was larger in radiographs centered on the hip than on pelvic X-rays. MeanJSW and JSA were found to be less sensitive than MaxJSN. The decrease of joint space width was inversely correlated with joint space size in WB. These results suggest that WB radiographs of the hip should be used in preference to NWB in studies of hip OA.