title: Initial Results of an Acetabular Center Axis Registration Technique in Navigated Hip Arthroplasty with Deformed Acetabular Rims.
abstract: BACKGROUND
  In cementless total hip arthroplasty, imageless computer-assisted navigation is usually used to register the anterior pelvic plane (APP). The accuracy of this method is influenced by the subcutaneous tissues overlying the registration landmarks. On the other hand, the acetabular center axis (ACA) is determined from the acetabular rim. Precise registration of the ACA is possible because of direct palpation using a pointer. Imageless navigation using the ACA usually targets patients with normal acetabular morphology. The aim of this study was to investigate the accuracy of imageless navigation using the ACA instead of the APP in patients with normal or deformed acetabular rims.
  
  
  METHODS
  The intraoperative cup position was compared with that obtained from the postoperative computed tomography (CT) images in 18 cases.
  
  
  RESULTS
  The inclination angle derived from the navigation system was 3.4 ± 5.3 degrees smaller and the anteversion angle was 1.4 ± 3.1 degrees larger than those derived from the CT images.
  
  
  CONCLUSIONS
  The inclination cup angle of the navigation system was significantly inferior to the true value, particularly in cases with large anterior osteophytes.
authors.affiliation: Department of Orthopaedic Surgery, Graduate School of Comprehensive Human Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8575, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Wada
authors.affiliation: Department of Orthopaedic Surgery, Institute of Clinical Medicine, and University Hospital, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8575, Japan.
authors.forename: Hajime
authors.initials: H
authors.lastname: Mishima
authors.affiliation: Department of Orthopaedic Surgery, Graduate School of Comprehensive Human Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8575, Japan.
authors.forename: Tomohiro
authors.initials: T
authors.lastname: Yoshizawa
authors.affiliation: Division of Regenerative Medicine for Musculoskeletal System, Department of Orthopaedic Surgery, Faculty of Medicine, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8575, Japan.
authors.forename: Hisashi
authors.initials: H
authors.lastname: Sugaya
authors.affiliation: Department of Orthopaedic Surgery, Institute of Clinical Medicine, and University Hospital, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8575, Japan.
authors.forename: Tomofumi
authors.initials: T
authors.lastname: Nishino
authors.affiliation: Department of Orthopaedic Surgery, Institute of Clinical Medicine, and University Hospital, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8575, Japan.
authors.forename: Masashi
authors.initials: M
authors.lastname: Yamazaki
keywords: Acetabular center axis; cementless total hip arthroplasty; computed tomography; developmental dysplasia of the hip; imageless navigation; osteophyte.