title: Accuracy of acetabular cup positioning using imageless navigation.
abstract: BACKGROUND
  Correct placement of the acetabular cup is a crucial step in total hip replacement to achieve a satisfactory result and remains a challenge with free-hand techniques. Imageless navigation may provide a viable alternative to free-hand technique and improve placement significantly. The purpose of this project was to assess and validate intra-operative placement values for both inclination and anteversion as displayed by an imageless navigation system to post-operative measurement of cup position using high resolution CT scans.
  
  
  METHODS
  Thirty-two subjects who underwent primary hip joint arthroplasty using imageless navigation were included. The average age was 66.5 years (range 32-87). 23 non-cemented and 9 cemented acetabular cups were implanted. The desired position for the cup was 45 degrees of inversion and 15 degrees of anteversion. A pelvic CT scan using a multi-slice CT was used to assess the position of the cup radiographically.
  
  
  RESULTS
  Two subjects were excluded because of dislodgement of the tracking pin. Pearson correlation revealed a strong and significant correlation (r = 0.68; p < 0.006) for cup inclination and a moderate non-significant correlation (r = 0.53; p = 0.45) between intra-operative readings and cup placement for anteversion.
  
  
  CONCLUSIONS
  These findings can be explained with the possible introduction of systematic error. Even though the acquisition of anatomic landmarks is simple, they must be acquired with great precision. An error of 1 cm can result in a mean anteversion error of 6 degrees and inclination error of 2.5 degrees. Whilst computer assisted surgery results in highly accurate cup placements for inclination, anteversion of the cup cannot be determined accurately.
authors.affiliation: Musculoskeletal Research Unit, CQ University, Yaamba Road, Rockhampton 4700, Australia. ehohmann@optusnet.com.au
authors.forename: Erik
authors.initials: E
authors.lastname: Hohmann
authors.forename: Adam
authors.initials: A
authors.lastname: Bryant
authors.forename: Kevin
authors.initials: K
authors.lastname: Tetsworth
mesh_terms: D000077:Acetabulum; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D019644:Arthroplasty, Replacement, Hip / Q000295:instrumentation / Q000379:methods*; D060750:Bone Anteversion / Q000000981:diagnostic imaging / Q000517:prevention & control*; D001843:Bone Cements; D001863:Bone Screws; D005260:Female; D006621:Hip Joint / Q000000981:diagnostic imaging / Q000601:surgery*; D006622:Hip Prosthesis; D006801:Humans; D008297:Male; D019300:Medical Errors / Q000517:prevention & control; D008875:Middle Aged; D012189:Retrospective Studies; D025321:Surgery, Computer-Assisted / Q000379:methods*; D036542:Tomography, Spiral Computed; D016896:Treatment Outcome
chemical_list: D001843:Bone Cements