We assessed the influence of patient positioning and radiographic procedure, and defined a smallest detectable difference (SDD) in hip osteoarthritis (OA).
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).
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=1.6x10(-4)), but not in part 1 (P=0.375) and part 3 (P=0.580). The SDD estimate was 0.46 mm.
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.