Measurement of joint space width in hip osteoarthritis: influence of joint positioning and radiographic procedure

Objectives. We assessed the influence of patient positioning and radiographic procedure, and defined a smallest detectable difference (SDD) in hip osteoarthritis (OA). Methods. OA hip patients each had a standardized pelvic radiograph and, 5 min later. a modified pelvic radiograph with the feet internally rotated 5 degrees (part 1 of the study), the X-ray beam centred on the umbilicus (part 2), or another standardized pelvic radiograph (part 3). Results. Corresponding mean differences in joint space width (JSW) measurements (limits of agreement) between views were + 0.03 (- 0.53 to + 0.59), - 0.31 (- 1.15 to + 0.53) and - 0.02 (- 0.48 to + 0.44) mm. The two views differed significantly in mean JSW in part 2 of the study (P = 1.6 x 10(-4)). but not in part I (P = 0.375) and part 3 (P = 0.580). The SDD estimate was 0.46 mm. Conclusions. Modifying the X-ray beam and foot rotation increases variability in JSW measurements. Use of urograms to evaluate radiological progression should be avoided. A change greater than 0.46 mm could define radiological hip OA progression.