title: Do first-generation highly crosslinked polyethylenes oxidize in vivo?
abstract: BACKGROUND
  Highly crosslinked and thermally treated polyethylenes were clinically introduced to reduce wear and osteolysis. Although the crosslinking process improves the wear performance, it also introduces free radicals into the polymer that can subsequently oxidize. Thermal treatments have been implemented to reduce oxidation; however, the efficacy of these methods with regard to reducing in vivo oxidative degradation remains to be seen. Polyethylene oxidation is a concern because it can compromise the ultimate strength and ductility of the material.
  
  
  OBJECTIVE
  We analyzed the oxidation, oxidation potential, and mechanical behavior of thermally treated highly crosslinked polyethylene retrieved acetabular liners.
  
  
  METHODS
  Three hundred seven acetabular liners were collected from consecutive revision surgeries at six institutions over a 10-year period. Twenty-four were sterilized using nonionizing methods, 43 were sterilized in an inert environment, 80 were highly crosslinked and annealed, and 160 were highly crosslinked and remelted. Oxidation and oxidation potential were assessed by Fourier transmission infrared spectroscopy. Mechanical behavior was assessed by the small punch test.
  
  
  RESULTS
  Oxidation and hydroperoxide (oxidation potential) indices were elevated in the annealed and gamma inert sterilized groups compared with those of the remelted liners and uncrosslinked gas sterilized controls, particularly at the rim. We also detected an increase in oxidation over time at the bearing surface of the remelted group. Ultimate strength of the polyethylene at the bearing surface was negatively correlated with implantation time for the annealed liners.
  
  
  CONCLUSIONS
  Within the first decade of implantation, the clinical outlook for first-generation highly crosslinked polyethylene remains promising. However, ongoing research continues to be warranted for first-generation highly crosslinked polyethylene bearings to monitor the implications of elevated oxidation at the rim of annealed liners as well as to better understand the subtle changes in oxidation at the bearing surface of remelted liners that occur in vivo.
authors.affiliation: Implant Research Center, School of Biomedical Engineering, Science, and Health Systems, Drexel University, and Exponent, Inc, 3401 Market Street, Suite 300, Philadelphia, PA 19104, USA. danwmac@gmail.com
authors.forename: Daniel
authors.initials: D
authors.lastname: MacDonald
authors.forename: Ashlyn
authors.initials: A
authors.lastname: Sakona
authors.forename: Allyson
authors.initials: A
authors.lastname: Ianuzzi
authors.forename: Clare M
authors.initials: CM
authors.lastname: Rimnac
authors.forename: Steven M
authors.initials: SM
authors.lastname: Kurtz
mesh_terms: D003432:Cross-Linking Reagents; D006801:Humans; D008422:Materials Testing; D010084:Oxidation-Reduction; D011095:Polyethylenes / Q000737:chemistry*; D017550:Spectroscopy, Fourier Transform Infrared; D013312:Stress, Physiological; D013499:Surface Properties
chemical_list: D003432:Cross-Linking Reagents; D011095:Polyethylenes; C111601:ultra-high molecular weight polyethylene