title: Which factors determine the wear rate of large-diameter metal-on-metal hip replacements? Multivariate analysis of two hundred and seventy-six components.
abstract: BACKGROUND
  Determining the relationship between clinical factors and engineering analysis of retrieved hip implants can help our understanding of the mechanism of device failure. This is particularly important for metal-on-metal hip arthroplasties because the most common cause of failure is unexplained. We sought to understand the variation in wear rates in a large series of retrieved metal-on-metal hip arthroplasty components.
  
  
  METHODS
  We prospectively recorded preoperative, intraoperative, and postoperative data to study the effect on both head and cup wear rates of the following variables: patient sex, cause of failure, manufacturer type, resurfacing or modular design, blood cobalt and chromium levels, edge-loading, femoral head size, and cup inclination angle. We analyzed 276 components (138 femoral head and acetabular cup couples) retrieved from failed metal-on-metal hip replacements.
  
  
  RESULTS
  We found a high rate of edge-loading (64%), but only forty-three (31%) of 138 hips had a cup inclination angle of >55°. Multivariate analysis showed that the most important factor responsible for the variation in wear rate was the presence or absence of edge-loading, even when adjusted for cup inclination angle. Strong positive correlations were found between acetabular cup and femoral head wear rates and between wear rates and both blood cobalt and chromium ion levels.
  
  
  CONCLUSIONS
  Multivariate analysis of nine factors found that edge-loading was the most important predictor of wear rate and occurred in two-thirds of failed metal-on-metal hip replacements. The majority did not have excessive cup inclination angles: 68% had an inclination angle of ≤55°. This finding, together with the relatively low median wear rate of the components in our study, suggests that cup position and/or wear rate may not be the only outcome related to failure of metal-on-metal hip replacements.
authors.affiliation: Department of Musculoskeletal Surgery, Imperial College (Charing Cross Hospital Campus), Fulham Palace Road, London, UK. a.hart@imperial.ac.uk
authors.forename: A J
authors.initials: AJ
authors.lastname: Hart
authors.forename: S
authors.initials: S
authors.lastname: Muirhead-Allwood
authors.forename: M
authors.initials: M
authors.lastname: Porter
authors.forename: A
authors.initials: A
authors.lastname: Matthies
authors.forename: K
authors.initials: K
authors.lastname: Ilo
authors.forename: P
authors.initials: P
authors.lastname: Maggiore
authors.forename: R
authors.initials: R
authors.lastname: Underwood
authors.forename: P
authors.initials: P
authors.lastname: Cann
authors.forename: J
authors.initials: J
authors.lastname: Cobb
authors.forename: J A
authors.initials: JA
authors.lastname: Skinner
mesh_terms: D019644:Arthroplasty, Replacement, Hip / Q000009:adverse effects / Q000295:instrumentation*; D002857:Chromium; D003035:Cobalt; D015331:Cohort Studies; D019544:Equipment Failure Analysis; D005260:Female; D006622:Hip Prosthesis / Q000009:adverse effects*; D006801:Humans; D008297:Male; D061825:Metal-on-Metal Joint Prostheses / Q000009:adverse effects*; D015999:Multivariate Analysis; D011474:Prosthesis Design / Q000009:adverse effects*; D011475:Prosthesis Failure / Q000009:adverse effects / Q000209:etiology*; D012086:Reoperation; D012307:Risk Factors; D016474:Weight-Bearing
chemical_list: D002857:Chromium; D003035:Cobalt