title: Understanding why metal-on-metal hip arthroplasties fail: a comparison between patients with well-functioning and revised birmingham hip resurfacing arthroplasties. AAOS exhibit selection.
abstract: BACKGROUND
  A large proportion of metal-on-metal hip arthroplasty failures are due to unexplained pain. The mechanism of failure has been thought to be associated with factors that increase material loss, including specific design features and surgical positioning of components. However, recent evidence suggests that there is not a simple dose-response relationship. An analysis of failed metal-on-metal hip arthroplasties involving a single design was performed in an attempt to help resolve this issue. Our aim was to identify the clinical and component variables associated with failure of metal-on-metal hip arthroplasties, particularly in patients undergoing revision arthroplasty because of unexplained hip pain, and to clarify the role of material loss.
  
  
  METHODS
  We prospectively recruited fifty-five patients who were undergoing revision of a metal-on-metal Birmingham Hip Resurfacing System (BHR) arthroplasty (Smith & Nephew). We collected clinical data preoperatively, intraoperatively, and following the revision arthroplasty. Data included chromium and cobalt levels in whole blood, which were measured with use of inductively coupled plasma mass spectrometry (ICPMS), and component orientation, which was typically measured with use of computed tomography (CT) scans. The wear of the retrieved components was also quantified postoperatively. All parameters were compared with those in a comparable group of patients with a well-functioning BHR arthroplasty.
  
  
  RESULTS
  Sixty-nine percent of the patients who underwent revision arthroplasty did so following a diagnosis of unexplained hip pain. When compared with patients with a well-functioning arthroplasty, patients who underwent revision arthroplasty had a significantly higher acetabular cup inclination angle (p &lt; 0.01), a significantly smaller femoral head diameter (p &lt; 0.01), and significantly higher blood cobalt and chromium ion levels (p &lt; 0.01). However, almost 50% of the patients who underwent revision arthroplasty had blood metal ion levels below the clinical threshold of 7 ppb and low component wear rates of &lt;5 μm/year.
  
  
  CONCLUSIONS
  In a large number of patients with unexplained hip pain leading to revision of a metal-on-metal hip arthroplasty, the acetabular cup orientation was satisfactory and the material loss rate was low. We suspect that patient-specific factors may have been responsible for the failure in a large proportion of these patients.
authors.affiliation: Department of Musculoskeletal Surgery, Imperial College London, Charing Cross Hospital Campus, Fulham Palace Road, London W6 8RF, United Kingdom. a.hart@imperial.ac.uk
authors.forename: Alister J
authors.initials: AJ
authors.lastname: Hart
authors.forename: Ashley
authors.initials: A
authors.lastname: Matthies
authors.forename: Johann
authors.initials: J
authors.lastname: Henckel
authors.forename: Kevin
authors.initials: K
authors.lastname: Ilo
authors.forename: John
authors.initials: J
authors.lastname: Skinner
authors.forename: Philip C
authors.initials: PC
authors.lastname: Noble
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D002857:Chromium / Q000097:blood; D003035:Cobalt / Q000097:blood; D005260:Female; D006622:Hip Prosthesis / Q000009:adverse effects; D006801:Humans; D008297:Male; D008875:Middle Aged; D010146:Pain / Q000097:blood / Q000209:etiology; D011474:Prosthesis Design; D011475:Prosthesis Failure; D012086:Reoperation; D012189:Retrospective Studies; D055815:Young Adult
chemical_list: D002857:Chromium; D003035:Cobalt