title: Multidirectional wear and impact-to-wear tests of phospholipid-polymer-grafted and vitamin E-blended crosslinked polyethylene: a pilot study.
abstract: BACKGROUND
  Modifying the surface and substrate of a crosslinked polyethylene (CLPE) liner may be beneficial for high wear resistance as well as high oxidative stability and excellent mechanical properties, which would be useful in contributing to the long-term performance of orthopaedic bearings. A grafted poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) layer on a vitamin E-blended crosslinked PE (HD-CLPE[VE]) surface may provide hydrophilicity and lubricity without compromising the oxidative stability or mechanical properties.
  
  
  OBJECTIVE
  (1) Will the modifications (PMPC grafting and vitamin E blending) affect the lubrication characteristics of the CLPE surface? (2) Will the modifications affect wear resistance? (3) Will the modifications affect fatigue resistance?
  
  
  METHODS
  We investigated the effects of surface and substrate modifications (PMPC grafting and vitamin E blending) on the wear and fatigue fracture of thin CLPE samples. For each of the untreated and PMPC-grafted CLPE surfaces with and without vitamin E blended (four groups), wettability and lubricity surface analyses were conducted as well as multidirectional wear and impact-to-wear tests using a pin-on-disk testing machine.
  
  
  RESULTS
  The water wettability and lubricity (CLPE [mean ± 95% confidence interval]: 23.2° ± 1.8°, 0.005 ± 0.001; HD-CLPE[VE]: 26.0° ± 2.3°, 0.009 ± 0.003) of the PMPC-grafted surfaces were greater (p < 0.001) than that (CLPE: 90.3° ± 1.2°, 0.067 ± 0.015; HD-CLPE[VE]: 90.8° ± 2.0°, 0.063 ± 0.008) of the untreated surface regardless of vitamin E additives. It was observed that the PMPC grafting (CLPE: 0.23 ± 0.06 mg; HD-CLPE[VE]: 0.05 ± 0.10 mg) was associated with reduced gravimetric wear (CLPE: 0.53 ± 0.08 mg, p = 0.004 HD-CLPE[VE]: 0.23 ± 0.07 mg, p = 0.038) in the multidirectional wear test. The PMPC-grafted surface characteristics did not appear to affect the impact fatigue resistance regardless of vitamin E blending.
  
  
  CONCLUSIONS
  PMPC grafting improved the surface hydrophilicity and lubricity, and it reduced the gravimetric wear in terms of multidirectional sliding. It did not result in differences in terms of the impact-to-unidirectional sliding regardless of vitamin E blending. Further research is needed to evaluate the wear resistance of PMPC-grafted HD-CLPE(VE) in long-term hip simulator tests under normal and severe conditions, which may offer useful clues to the possible performance of these materials in vivo.
  
  
  CONCLUSIONS
  Our preliminary in vitro findings suggest that some improvement in the wear performance of crosslinked polyethylene acetabular liners in total hip arthroplasty could be obtained using PMPC grafting. Further research is needed to evaluate the wear resistance of PMPC-grafted HD-CLPE(VE) in long-term hip simulator tests under normal and severe conditions, which may offer useful clues to the possible performance of these materials in vivo.
authors.affiliation: Department of Materials Engineering, The University of Tokyo, Tokyo, Japan, masayuki.kyomoto@kyocera-md.jp.
authors.forename: Masayuki
authors.initials: M
authors.lastname: Kyomoto
authors.forename: Toru
authors.initials: T
authors.lastname: Moro
authors.forename: Yoshio
authors.initials: Y
authors.lastname: Takatori
authors.forename: Sakae
authors.initials: S
authors.lastname: Tanaka
authors.forename: Kazuhiko
authors.initials: K
authors.lastname: Ishihara
mesh_terms: D001672:Biocompatible Materials / Q000737:chemistry*; D006801:Humans; D008422:Materials Testing; D010865:Pilot Projects; D011095:Polyethylenes / Q000737:chemistry*; D013499:Surface Properties; D014810:Vitamin E / Q000737:chemistry*; D017155:Wettability
chemical_list: D001672:Biocompatible Materials; D011095:Polyethylenes; D014810:Vitamin E