Knee pain reduces joint space width in conventional standing anteroposterior radiographs of osteoarthritic knees

Objective. A suspected, but heretofore undemonstrated, limitation of the conventional weight-bearing anteroposterior (AP) knee radiograph, in which the joint is imaged in extension, for studies of progression of osteoarthritis (OA) is that changes in knee pain may affect extension, thereby altering the apparent thickness of the articular cartilage. The present study was undertaken to examine the effect of changes in knee pain of varying magnitudes on radiographic joint space width (JSW) in the weight-bearing extended and the semi-flexed A-P views, in which radioanatomic positioning of the knee was carefully standardized by fluoroscopy. Methods. Fifteen patients with knee OA underwent a washout of their analgesic/nonsteroidal antiinflammatory drug (NSAID) agents (duration 5 half-lives), after which standing AP and semiflexed AP knee radiographs of both knees were obtained. Examinations were repeated 1-12 weeks later (median 4.5 weeks, mean 6.0 weeks), after resumption of analgesic/NSAID therapy. Knee pain was measured with the pain subscale of the Western Ontario and McMaster Universities Osteoarthritis (WOMAC) Index (Likert scale). JSW was measured with a pair of calipers and a magnifying lens. Mixed model analyses of variance were used to test the significance of changes in pain and JSW within and between 2 groups of knees with mild-to-moderate radiographic severity of OA: (a) "flaring knees," in which the patient rated standing knee pain as severe or extreme after the washout and in which pain decreased to any degree after resumption of analgesics and/or NSAIDs (n = 12) and (b) "nonflaring knees," in which standing knee pain was absent, mild, or moderate after the washout or did not decrease after resumption of treatment (n = 15). Results. After reinstitution of treatment, WOMAC pain scores decreased significantly in both flaring and nonflaring knees (-44%; P < 0.0001 and -18%; P < 0.01, respectively). After adjustment for the within-subject correlation between knees, mean JSW (+/-SEM) in the extended view of the flaring OA knee increased significantly from the first to second examination (0.20 +/- 0.06 mm; P = 0.005). In contrast, the change in adjusted mean JSW in the extended view of the nonflaring OA knee was negligible (-0.04 +/- 0.04 mm) and significantly smaller than that observed in flaring knees (P < 0.01). Mean JSW in the semiflexed AP view was unaffected by the severity or responsiveness of standing knee pain in flaring and nonflaring OA knees. Conclusion. JSW in weight-bearing extended-view radiographs of highly symptomatic OA knees can be altered significantly by changes in joint pain. In clinical trials and in epidemiologic studies of OA progression that use this radiographic technique, longitudinal variations in pain may confound changes in the apparent thickness of the articular cartilage.