title: Very low wear of non-remelted highly cross-linked polyethylene cups: an RSA study lasting up to 6 years.
abstract: OBJECTIVE
  Highly cross-linked polyethylenes (PEs) all appear to reduce wear dramatically in laboratory studies, although there is substantial variation in this respect between manufacturers. Nonremelted cross-linked PE is almost as tough as unirradiated PE, but is not completely stable and can oxidize in vivo, as has been shown in recent retrievals studies. We had reported low wear and good clinical performance after 2 years for 10 non-remelted highly crosslinked PE cups compared to 16 conventional PE cups sterilized by gamma-in-air.
  
  
  METHODS
  Because of possible degradation by free radicals, we followed up both cohorts for 5 years (conventional PE) and 6 years (highly cross-linked PE).
  
  
  RESULTS
  Mean (CI) proximal head penetration over the observation time was linear and measured 0.08 (0.02-0.13) mm for cross-linked PE and 0.42 (0.23-0.62) mm for conventional PE, and total penetration was 0.23 (0.1-0.35) mm and 0.75 (0.05-1.4) mm respectively. After subtracting creep, the annual wear for non-remelted highly cross-linked PE was below 6 microm. The cups had equally low migration and few radiolucencies.
  
  
  CONCLUSIONS
  The theoretical possibility of oxidation in non-remelted highly cross-linked PE may not show clinically. However, it may be that cemented cups with their thicker PE are more forgiving than metal-backed cups with thin PE moving in the locking mechanism. So far, we can conclude that the Crossfire highly crosslinked polyethylene cups performed very well clinically, with extremely low wear even after almost 6 years. This is reassuring, but care should be taken in extrapolating these results to other cross-linked PEs or uncemented cups where toughness of PE is more of an issue.
authors.affiliation: Department of Orthopaedics, Ullevål University Hospital, Division of Neuroscience, Orthopaedics and Rehabilitation, Oslo, Norway. s.m.rohrl@medisin.uio.no
authors.forename: Stephan M
authors.initials: SM
authors.lastname: Röhrl
authors.forename: Ming Guo
authors.initials: MG
authors.lastname: Li
authors.forename: Kjell-Gunnar
authors.initials: KG
authors.lastname: Nilsson
authors.forename: Bo
authors.initials: B
authors.lastname: Nivbrant
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D019644:Arthroplasty, Replacement, Hip / Q000009:adverse effects / Q000379:methods; D020099:Coated Materials, Biocompatible / Q000378:metabolism; D015331:Cohort Studies; D005260:Female; D005500:Follow-Up Studies; D006622:Hip Prosthesis; D006801:Humans; D008297:Male; D008422:Materials Testing; D008875:Middle Aged; D020959:Polyethylene / Q000378:metabolism; D011475:Prosthesis Failure; D013314:Stress, Mechanical; D013997:Time Factors
chemical_list: D020099:Coated Materials, Biocompatible; D020959:Polyethylene