title: Central X-ray beam correction of radiographic acetabular cup measurement after THA: an experimental study.
abstract: OBJECTIVE
  Accurate assessment of cup orientation on postoperative radiographs is essential for evaluating outcome after THA. However, accuracy is impeded by the deviation of the central X-ray beam in relation to the cup and the impossibility of measuring retroversion on standard pelvic radiographs.
  
  
  METHODS
  In an experimental trial, we built an artificial cup holder enabling the setting of different angles of anatomical anteversion and inclination. Twelve different cup orientations were investigated by three examiners. After comparing the two methods for radiographic measurement of the cup position developed by Lewinnek and Widmer, we showed how to differentiate between anteversion and retroversion in each cup position by using a second plane. To show the effect of the central beam offset on the cup, we X-rayed a defined cup position using a multidirectional central beam offset. According to Murray's definition of anteversion and inclination, we created a novel corrective procedure to balance measurement errors caused by deviation of the central beam.
  
  
  RESULTS
  Measurement of the 12 different cup positions with the Lewinnek's method yielded a mean deviation of [Formula: see text] (95 % CI 1.3-2.3) from the original cup anteversion. The respective deviation with the Widmer/Liaw's method was [Formula: see text] (95 % CI 2.4-4.0). In each case, retroversion could be differentiated from anteversion with a second radiograph. Because of the multidirectional central beam offset ([Formula: see text] cm) from the acetabular cup in the cup holder ([Formula: see text] anteversion and [Formula: see text] inclination), the mean absolute difference for anteversion was [Formula: see text] (range [Formula: see text] to [Formula: see text] and [Formula: see text] (range [Formula: see text] to [Formula: see text] for inclination. The application of our novel mathematical correction of the central beam offset reduced deviation to a mean difference of [Formula: see text] for anteversion and [Formula: see text] for inclination.
  
  
  CONCLUSIONS
  This novel calculation for central beam offset correction enables highly accurate measurement of the cup position.
authors.affiliation: Department of Orthopedic Surgery, Regensburg University Medical Center, Kaiser-Karl-V.-Allee 3, 93077, Bad Abbach, Germany. Timo_Schwarz@hotmail.com.
authors.forename: T
authors.initials: T
authors.lastname: Schwarz
authors.affiliation: Department of Orthopedic Surgery, Regensburg University Medical Center, Kaiser-Karl-V.-Allee 3, 93077, Bad Abbach, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Weber
authors.affiliation: Department of Orthopedic Surgery, Regensburg University Medical Center, Kaiser-Karl-V.-Allee 3, 93077, Bad Abbach, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Wörner
authors.affiliation: Department of Orthopedic Surgery, Regensburg University Medical Center, Kaiser-Karl-V.-Allee 3, 93077, Bad Abbach, Germany.
authors.forename: T
authors.initials: T
authors.lastname: Renkawitz
authors.affiliation: Department of Orthopedic Surgery, Regensburg University Medical Center, Kaiser-Karl-V.-Allee 3, 93077, Bad Abbach, Germany.
authors.forename: J
authors.initials: J
authors.lastname: Grifka
authors.affiliation: Department of Orthopedic Surgery, Regensburg University Medical Center, Kaiser-Karl-V.-Allee 3, 93077, Bad Abbach, Germany.
authors.forename: B
authors.initials: B
authors.lastname: Craiovan
mesh_terms: D000077:Acetabulum / Q000000981:diagnostic imaging* / Q000601:surgery; D019644:Arthroplasty, Replacement, Hip; D005260:Female; D006622:Hip Prosthesis; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D008962:Models, Theoretical; D010388:Pelvis / Q000000981:diagnostic imaging; D011183:Postoperative Complications; D011859:Radiography; D015203:Reproducibility of Results; D016896:Treatment Outcome; D014965:X-Rays
keywords: Anteversion; Central beam offset; Cup position; Inclination; Retroversion; THA