title: Repeated metal ion measurement in patients with bilateral metal on metal (ASR™) hip replacements.
abstract: BACKGROUND
  Blood metal ion measurements have become a cornerstone in the surveillance of metal-on-metal (MoM) hip replacements. Interpretation of these levels in patients with bilateral MoM hip replacements, however, is challenging. We therefore asked (1) if there is a clinically significant change in whole blood (WB) Co or Cr levels in repeated WB assessment in patients operated on with bilateral ASR hip replacements, and (2) what proportion of patients has WB Co or Cr level below the previously established safe upper limits (SUL) (Co<5.0μg/L, Cr<7.4μg/L) in the repeated WB metal ion assessment.
  
  
  OBJECTIVE
  We hypothesized that there is a significant difference between repeated Co and Cr level measurement performed within one year±4 months interval in patients with bilateral ASR hip replacements.
  
  
  METHODS
  We identified all patients (n=139) who had received bilateral ASR hip replacements (278 hips). Patients (n=76, 152 hips) who had undergone two blood metal ion measurements within eight to sixteen months' time interval were included in the study analysis. Study cohort included 38 patients with bilateral ASR hip resurfacings (mean age 53 years, median follow-up 4.6 years, median femoral diameter 51mm) and 38 patients with bilateral ASR XL total hip replacements (mean age 60 years, median follow-up 3.6 years, median femoral diameter 49mm).
  
  
  RESULTS
  There was no significant change in Co and Cr levels between the repeated measurements in the HR cohort. Both Co and Cr were significantly higher in the THR cohort in the repeated measurement (Co: 8.3μg/L vs. 12.6μg/L, Cr: 3.15μg/L vs. 3.4μg/L, both P<0.001). WB Co and Cr levels remained below SUL and within their initial values during a mean one-year measurement interval in the majority of patients with bilateral high risk HR device. In contrast to this, 21.1% of patients with THRs had WB Co ion levels exceeding the SUL in the first measurement. WB Co values significantly increased over the measurement interval in the THR group.
  
  
  CONCLUSIONS
  Annual blood metal ion measurement is not useful in patients with bilateral hip resurfacings who have undergone at least one preliminary screening including both metal ion measurement and cross-sectional imaging. It seems, however, to be beneficial in patients with bilateral high risk MoM THR.
  
  
  METHODS
  Level III (retrospective comparative study).
authors.affiliation: Coxa Hospital for Joint Replacement, Biokatu 6b, 33520 Tampere, Finland. Electronic address: aleksi.reito@fimnet.fi.
authors.forename: A
authors.initials: A
authors.lastname: Reito
authors.affiliation: Coxa Hospital for Joint Replacement, Biokatu 6b, 33520 Tampere, Finland.
authors.forename: O
authors.initials: O
authors.lastname: Lainiala
authors.affiliation: Coxa Hospital for Joint Replacement, Biokatu 6b, 33520 Tampere, Finland.
authors.forename: J
authors.initials: J
authors.lastname: Nieminen
authors.affiliation: Coxa Hospital for Joint Replacement, Biokatu 6b, 33520 Tampere, Finland.
authors.forename: A
authors.initials: A
authors.lastname: Eskelinen
mesh_terms: D000328:Adult; D000368:Aged; D019644:Arthroplasty, Replacement, Hip; D002857:Chromium / Q000097:blood*; D003035:Cobalt / Q000097:blood*; D005260:Female; D006622:Hip Prosthesis; D006801:Humans; D008297:Male; D061825:Metal-on-Metal Joint Prostheses; D008875:Middle Aged; D011475:Prosthesis Failure; D012189:Retrospective Studies; D013997:Time Factors
chemical_list: D002857:Chromium; D003035:Cobalt
keywords: Hip resurfacing; Metal ion levels; Metal-on-metal; Surveillance