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Objective. To determine a cut-off point above which a change in joint space width (JSW) could be considered as relevant in patients with hip osteoarthritis (OA) on the basis of predicted need for subsequent total hip arthroplasty (THA). Methods. A multicentre, prospective, longitudinal, 5-yr follow-up study was performed. A pelvic radiograph was obtained at entry and after 1 and 2 yr. For each film, the narrowest JSW was measured using a 0.1 mm graduated magnifying glass. The absolute and relative differences between baseline and 1 and 2 yr of follow-up were calculated. We determined the cut-off points above which an absolute or relative decrease in JSW between baseline and 1 and 2 yr of follow-up could be considered relevant on the basis of the predicted need for THA during the remaining years of the study. The need for THA was categorized as ‘yes’ or ‘no’. Thereafter, for each observed change in JSW (0.1 per 0.1 mm or 1% per 1%), the sensitivity and specificity for subsequent THA were calculated. The choice of cut-off was based on maximal sensitivity and specificity, using the graphic representation of correct classification probabilities. In this way it was possible to obtain the best measured JSW threshold with maximal true positive and minimal false positive results. Results. A total of 423 and 385 patients met the criteria for analysis using the decrease in JSW between baseline and 1 and 2 yr respectively. The best cut-off points were absolute decreases in JSW of 0.2 and 0.4 mm and relative decreases in JSW of 15 and 20% after 1 and 2 yr respectively, with corresponding ranges of sensitivity and specificity of 68‐75 and 67‐78%. Conclusion. This work determined the cut-off above which a change in JSW could be considered clinically relevant in patients with hip OA, on the basis of predicted subsequent need for THA. For validation, similar studies should be conducted in other countries with different health-care systems.

Aim. To determine the minimum clinically important difference (MCID) in joint space width (JSW) progression in patients with hip osteoarthritis (OA), based upon evaluation by a panel of clinical experts as a gold standard. Methods. A sample of 298 patients with hip OA was selected from a multicentre, prospective, longitudinal, 3-yr follow-up study. A pelvic radiograph was obtained at entry and after 3 yr. For each film, the narrowest JSW was measured using a 0.1-mm graduated magnifying glass. The difference between baseline and 3-yr follow-up JSW was calculated. Two senior rheumatologists, who were experts in osteoarthritis, evaluated each pair of films and noted whether a clinically relevant deterioration in osteoarthritis stage occurred at 3 yr compared with baseline. Interobserver reliabilities were evaluated using the k coefficient and proportions of agreements. Then, for each measured difference in JSW (0.1 mm per 0.1 mm), the sensitivity and specificity for MCID, defined as the assessment of expert 1, expert 2 or a combination of both, were calculated. This allowed us to obtain, from graphic representations of the correct classification probabilities, the best measured JSW threshold, with the maximal true positive and the minimal false positive results. Results. The mean measured change in JSW was 2 0.63" 0.74 mm. Experts 1 and 2 considered the decrease in JSW to be clinically relevant in 122 (40.9%) and 100 pairs (33.6%) respectively. The proportion of agreements between the experts was 79.9%, with a k coefficient of 0.572. The best measured JSW threshold was 2 0.4 mm for expert 1, expert 2 and the combination of both; sensitivity and specificity were 0.75 and 0.8, 0.71 and 0.72, and 0.75 and 0.7 respectively. Conclusion. This study suggests that a change of at least 0.4 mm in the radiological JSW could be considered clinically relevant. Other studies using other sets of patients and other methods are needed for validation.