title: Rapid polyethylene failure of unicondylar tibial components sterilized with gamma irradiation in air and implanted after a long shelf life.
abstract: BACKGROUND
  The mechanical toughness of polyethylene that has been sterilized by gamma irradiation in air decreases after a long shelf life. The purpose of the present study is to report the high failure rate after unicondylar knee replacements performed with polyethylene bearings that had been sterilized with gamma irradiation in air and implanted after a shelf life of < or = 4.4 years.
  
  
  METHODS
  Between December 1997 and January 2000, seventy-five unicondylar knee replacements were performed in sixty-two patients. All patients were followed both clinically and radiographically. A revision operation was offered when the patient had pain, swelling, and radiographic evidence of rapid polyethylene wear. The effect of aging of the polyethylene during storage was evaluated by dividing the knees into three groups on the basis of shelf life and comparing them with regard to the rate of revision and the observed wear of the polyethylene. Four retrieved components were examined for the presence of oxidation.
  
  
  RESULTS
  At a mean of eighteen months after the arthroplasty, thirty knees had been revised and seven were scheduled for revision. The rate of polyethylene wear increased as the shelf life increased. There was a significant inverse linear correlation between the shelf life of the polyethylene and the time to revision (p < 0.01, r (2) = 0.64). All retrieved components had greater-than-expected wear with pitting and delamination of the surface. Seven components had fractured, and ten had both fractured and fragmented. Analysis of four components confirmed severe oxidation of the polyethylene.
  
  
  CONCLUSIONS
  The present study demonstrated early, severe wear of tibial polyethylene bearings that had been sterilized by gamma irradiation in air and stored for < or = 4.4 years. This risk can be minimized by ensuring that implants have not been sterilized with gamma irradiation in air and stored for several years.
authors.affiliation: Anderson Orthopaedic Research Institute, Alexandria, Virginia 22306, USA.
authors.forename: Thomas F
authors.initials: TF
authors.lastname: McGovern
authors.forename: Deborah J
authors.initials: DJ
authors.lastname: Ammeen
authors.forename: John P
authors.initials: JP
authors.lastname: Collier
authors.forename: Barbara H
authors.initials: BH
authors.lastname: Currier
authors.forename: Gerard A
authors.initials: GA
authors.lastname: Engh
mesh_terms: D019645:Arthroplasty, Replacement, Knee / Q000379:methods; D004869:Equipment Safety; D005720:Gamma Rays; D006801:Humans; D007720:Knee Prosthesis; D008422:Materials Testing; D020959:Polyethylene / Q000528:radiation effects*; D011336:Probability; D011474:Prosthesis Design; D011475:Prosthesis Failure; D012086:Reoperation; D012189:Retrospective Studies; D018570:Risk Assessment; D013242:Sterilization / Q000379:methods; D013314:Stress, Mechanical; D013997:Time Factors
chemical_list: D020959:Polyethylene