title: A new CT method for measuring cup orientation after total hip arthroplasty: a study of 10 patients.
abstract: BACKGROUND
  It is difficult to assess the orientation of the acetabular component on routine radiographs. We present a method for determining the spatial orientation of the acetabular component after total hip arthroplasty (THA) using computed tomography.
  
  
  METHODS
  Two CT-scans, 10 min apart, were obtained from each of 10 patients after THA. Using locally developed software, two independent examiners measured the orientation of the acetabular component in relation to the pelvis. The measurements were repeated after one week. To be independent of the patient position during scanning, the method involved two steps. Firstly, a 3D volumetric image of the pelvis was brought into a standard pelvic orientation, then the orientation of the acetabular component was measured. The orientation of the acetabular component was expressed as operative anteversion and inclination relative to an internal pelvic reference coordinate system. To evaluate precision, we compared measurements across pairs of CT volumes between observers and trials.
  
  
  RESULTS
  Mean absolute interobserver angle error was 2.3 degrees for anteversion (range 0-6.6 degrees), and 1.1 degrees for inclination (range 0-4.6 degrees). For interobserver measurements, the precision, defined as one standard deviation, was 2.9 degrees for anteversion, and 1.5 degrees for inclination. A Student's t-test showed that the overall differences between the examiners, trials, and cases were not significant. Data were normally distributed and were not dependent on examiner or trial.
  
  
  CONCLUSIONS
  We conclude that the implant angles of the acetabular component in relation to the pelvis could be detected repeatedly using CT, independently of patient positioning.
authors.affiliation: Department of Hand Surgery, Södersjukhuset, SE-118 83 Stockholm, Sweden. henrik.olivecrona@sos.sll.se
authors.forename: Henrik
authors.initials: H
authors.lastname: Olivecrona
authors.forename: Lars
authors.initials: L
authors.lastname: Weidenhielm
authors.forename: Lotta
authors.initials: L
authors.lastname: Olivecrona
authors.forename: Mats O
authors.initials: MO
authors.lastname: Beckman
authors.forename: André
authors.initials: A
authors.lastname: Stark
authors.forename: Marilyn E
authors.initials: ME
authors.lastname: Noz
authors.forename: Gerald Q
authors.initials: GQ
authors.lastname: Maguire
authors.forename: Michael P
authors.initials: MP
authors.lastname: Zeleznik
authors.forename: Lars
authors.initials: L
authors.lastname: Svensson
authors.forename: Torbjörn
authors.initials: T
authors.lastname: Jonson
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D005260:Female; D006621:Hip Joint / Q000000981:diagnostic imaging* / Q000601:surgery*; D006622:Hip Prosthesis; D006801:Humans; D008297:Male; D008875:Middle Aged; D015203:Reproducibility of Results; D014057:Tomography, X-Ray Computed