title: What are the predictors and prevalence of pseudotumor and elevated metal ions after large-diameter metal-on-metal THA?
abstract: BACKGROUND
  Soft tissue masses, or "pseudotumors," around metal-on-metal total hip arthroplasty (MoM THA) have been reported frequently; however, their prevalence remains unknown. Several risk factors, including elevated metal ion levels, have been associated with the presence of pseudotumor, although this remains controversial.
  
  
  OBJECTIVE
  The goals of this study were to (1) determine the prevalence of pseudotumors after large-diameter head MoM THA; (2) identify risk factors associated with pseudotumor formation and elevated metal ion levels; and (3) determine the early failure rate of large-diameter MoM THA.
  
  
  METHODS
  Between December 2005 and November 2012, 258 hips (215 patients) underwent large-diameter head primary MoM THA at our institution. Clinical followup was obtained in 235 hips (91%). Using an inclusion criteria of a minimum followup of 1 year, a subset of 191 hips (mean followup, 4 years; range, 1-7 years) was recruited for high-resolution ultrasound screening for the presence of pseudotumor. Whole blood cobalt and chromium ion levels, UCLA activity level, WOMAC score, patient demographics as well as surgical, implant, and radiographic data were collected. Bivariate correlations and multivariate log-linear regression models were used to compare the presence of pseudotumor and elevated metal ions with all other factors.
  
  
  RESULTS
  Ultrasound detected a solid, cystic, or mixed mass in 20% hips (38 of 191). No correlation was found between the presence of pseudotumor and any risk factor that we examined. After controlling for confounding variables, elevated cobalt ions were correlated (p<0.001, R=0.50, R2=0.25) with smaller femoral head size, the presence of bilateral MoM THA, and female sex. Elevated chromium ions were correlated (p<0.001, R=0.59, R2=0.34) with smaller femoral head size, presence of bilateral MoM THA, and lower body mass index. The overall survival of MoM THA was 96% at a mean followup of 4.5 years (range, 2-8 years).
  
  
  CONCLUSIONS
  With the numbers available, we found no associations between the presence of pseudotumor and the potential risk factors we analyzed, including elevated metal ion levels. Further work is needed to explain why larger femoral head sizes resulted in lower metal ion levels despite being associated with higher early failure rates in joint registry data.
  
  
  METHODS
  Level III, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.
authors.affiliation: Division of Orthopaedic Surgery, St Michael's Hospital, 55 Queen Street East, Suite 800, Toronto, ON, M5C 1R6, Canada, ngbayley@gmail.com.
authors.forename: Nick
authors.initials: N
authors.lastname: Bayley
authors.forename: Habeeb
authors.initials: H
authors.lastname: Khan
authors.forename: Paul
authors.initials: P
authors.lastname: Grosso
authors.forename: Thomas
authors.initials: T
authors.lastname: Hupel
authors.forename: David
authors.initials: D
authors.lastname: Stevens
authors.forename: Matthew
authors.initials: M
authors.lastname: Snider
authors.forename: Emil
authors.initials: E
authors.lastname: Schemitsch
authors.forename: Paul
authors.initials: P
authors.lastname: Kuzyk
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D019644:Arthroplasty, Replacement, Hip / Q000009:adverse effects; D002857:Chromium / Q000097:blood; D003035:Cobalt / Q000097:blood; D005260:Female; D006104:Granuloma, Plasma Cell / Q000000981:diagnostic imaging / Q000453:epidemiology / Q000209:etiology*; D006622:Hip Prosthesis; D006801:Humans; D008297:Male; D061825:Metal-on-Metal Joint Prostheses; D008875:Middle Aged; D015999:Multivariate Analysis; D012307:Risk Factors; D014463:Ultrasonography; D055815:Young Adult
chemical_list: D002857:Chromium; D003035:Cobalt
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