title: Validation of the rotation ratios method.
abstract: BACKGROUND
  The rotation ratios method describes rotations between pairs of sequential pelvic radiographs. The method seems promising but has not been validated.
  
  
  OBJECTIVE
  To validate the accuracy of the rotation ratios method.
  
  
  METHODS
  Known pelvic rotations between 165 radiographs obtained from five skeletal pelvises in an experimental material were compared with the corresponding calculated rotations to describe the accuracy of the method. The results from a clinical material of 262 pelvic radiographs from 46 patients defined the ranges of rotational differences compared. Repeated analyses, both on the experimental and the clinical material, were performed using the selected reference points to describe the robustness and the repeatability of the method.
  
  
  RESULTS
  The reference points were easy to identify and barely influenced by pelvic rotations. The mean differences between calculated and real pelvic rotations were 0.0 degrees (SD 0.6) for vertical rotations and 0.1 degrees (SD 0.7) for transversal rotations in the experimental material. The intra- and interobserver repeatability of the method was good.
  
  
  CONCLUSIONS
  The accuracy of the method was reasonably high, and the method may prove to be clinically useful.
authors.affiliation: Norwegian Orthopedic Implant Research Unit and Department of Radiology, St. Olavs Hospital, Trondheim University Hospital, Trondheim, Norway. olav.foss@ntnu.no
authors.forename: O A
authors.initials: OA
authors.lastname: Foss
authors.forename: J
authors.initials: J
authors.lastname: Klaksvik
authors.forename: P
authors.initials: P
authors.lastname: Benum
authors.forename: S
authors.initials: S
authors.lastname: Anda
mesh_terms: D000328:Adult; D000368:Aged; D005260:Female; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D008297:Male; D008875:Middle Aged; D015588:Observer Variation; D010384:Pelvic Bones / Q000000981:diagnostic imaging*; D011859:Radiography; D015203:Reproducibility of Results; D012399:Rotation