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

Objectives. We assessed the influence of patient positioning and radiograph ic procedure, and defined a smallest detectable difference (SDD) in hip ost eoarthritis (OA). Methods. OA hip patients each had a standardized pelvic radiograph and, 5 m in later. a modified pelvic radiograph with the feet internally rotated 5 d egrees (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) measurem ents (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 dif fered 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 variabili ty in JSW measurements. Use of urograms to evaluate radiological progressio n should be avoided. A change greater than 0.46 mm could define radiologica l hip OA progression.