CONCISE REPORTS Variability in knee radiographing: implication for definition of radiological progression in medial knee osteoarthritis

Objectives—(1) To assess reproducibility of medial knee joint space width (JSW) measurement in healthy subjects and osteoarthritic (OA) patients. (2) To define minimal relevant radiological change in knee JSW based on the reproducibility of its measurement. Patients and methods—(1) Healthy volunteers: in the first part of the study, 20 knees of healthy adult volunteers were radiographed in the weightbearing, anteroposterior extended view, twice, two weeks apart, using three diVerent radiographic procedures: (a) without guidelines, (b) with guidelines and without fluoroscopy, (c) with guidelines and fluroroscopy. (2) Knee OA patients: in the second part of the study, 36 knees of OA patients were radiographed twice with guidelines and without fluoroscopy. JSW was measured blindly using a graduated magnifying glass. Based on the Bland and Altman graphic approach, cut oV points defining minimal relevant radiological change are proposed. Results—Standard deviation (SD) of differences in JSW measurement between two sets of knee radiographs in healthy subjects were 0.66 mm for radiography performed without guidelines, 0.37 mm for radiography performed with guidelines and without fluoroscopy, and 0.31 mm for radiography with guidelines and fluoroscopy. SD of diVerences in JSW measurement in OA patients were 0.32 mm for radiography performed with guidelines and without fluoroscopy. A minimal relevant change in JSW between two radiographs performed in healthy subjects can be defined by a change of at least 1.29 or 0.59 mm when radiographs are taken without guidelines, and with guidelines and fluoroscopy, respectively. When radiographs are taken with guidelines and without fluoroscopy, the change must be at least 0.73 mm. A similar figure, 0.64 mm was observed in knee OA patients. Conclusion—Definition of radiological progression varies greatly according to the radiographic procedure chosen. Use of guidelines reduces the threshold of progression required to consider that change between two measures is relevant. (Ann Rheum Dis 1998;57:624–629) At the present time, conventional radiographs are considered the most relevant outcome measure to assess the progression of osteoarthritis (OA) in clinical trials and epidemiological studies. Joint space width (JSW) measurement seems to be the most sensitive radiological criteria to detect changes over time, compared with other radiological variables such as bone response (for example, subchondral sclerosis, cysts, and osteophytes). JSW can be evaluated on diVerent types of views: extended view, flexed view, semi-flexed view. Whatever the degree of flexion many sources of error or variability in JSW measurement have been identified. 5 The influence of the measurement process (reader, measuring instrument) has been widely investigated. 7 Several authors have also drawn attention to the influence of patient positioning and of the radiographic procedure in modifying the radiographic image and therefore JSW measurement. 5 Most of the variability in JSW measurement can be attributed to variations in the radiographic procedure or joint positioning. Reproducibility of JSW measurement on multiple plain radiographs from the same person largely depends on the care taken in performing the radiography. The interest of precise guidelines has been emphasised to obtain comparable joint images on successive radiograhic visits. 4 7 Some of these guidelines propose the use of fluoroscopy to facilitate capture of the joint space and determination of optimal foot rotation. 7 However, in numerous centres fluoroscopy is not available and the real benefit obtained from using fluoroscopy remains to be evaluated. When radiological abnormalities in OA are measured using a continuous variable such as JSW measurement (expressed in millimetres), we need to know whether an observed diVerence constitutes a relevant change. To be considered relevant—that is, related to a real Ann Rheum Dis 1998;57:624–629 624 Department of Rheumatology and Department of Radiology, Cochin Hospital and René Descartes University, Paris, France P Ravaud G-R Auleley M Dougados J-L Drape Department of Biostatistics and Medical Computing, Saint-Louis Hospital, Paris, France B Giraudeau C Chastang Department of Radiology, Ambroise Paré Hospital, Boulogne, France B Rousselin Cassenne Laboratories, Puteaux, France L Paolozzi