Analysis of Crosslinked and Conventional Polyethylene Explants

• Oxidation levels [ Time Frame: At an average of 5 years in vivo ] [ Designated as safety issue: No ]
  Polyethylene oxidation levels will be quantified as a function of depth away from the articular and rim surfaces using Fourier Transform Infrared Spectroscopy (FTIR). Pre and post-hexane and nitric oxide (NO) exposed post-hexane thin sections will be analyzed.
  
  

• Absorbed lipids [ Time Frame: At an average of 5 years in vivo ] [ Designated as safety issue: No ]
  Absorbed lipids will be evaluated by determining the level of carbonyls before and after hexane extraction using Fourier Transform Infrared Spectroscopy (FTIR).
  
  
• Crosslink density [ Time Frame: At an average of 5 years in vivo ] [ Designated as safety issue: No ]
  Crosslink density will be evaluated using the gravimetric method on thin sections (~300μm) cut co-planar to the articular surfaces.
  
  
• Differential Scanning Calorimeter (DSC) [ Time Frame: At an average of 5 years in vivo ] [ Designated as safety issue: No ]
  Differential Scanning Calorimeter (DSC) will be used to determine the 1st and 2nd heats, and 1st cool crystallinity, peak melting/crystallization points.
  
  
• Hydroperoxides [ Time Frame: At an average of 5 years in vivo ] [ Designated as safety issue: No ]
  Hydroperoxides will be evaluated with FTIR by NO treatment after hexane extraction.
  
  
• Vinyl end-groups [ Time Frame: At an average of 5 years in vivo ] [ Designated as safety issue: No ]
  Vinyl end-groups will be evaluated with FTIR after hexane extraction to quantify chain scission.
  
  
• Trans vinylene (TVI) unsaturations [ Time Frame: At an average of 5 years in vivo ] [ Designated as safety issue: No ]
  Trans vinylene (TVI) unsaturations will be evaluated with FTIR after hexane extraction to determine radiation history and dose.
  
  
• Crystallinity [ Time Frame: At an average of 5 years in vivo ] [ Designated as safety issue: No ]
  Infra-red crystallinity will be evaluated with FTIR after hexane extraction.
  
  
• Gel content [ Time Frame: At an average of 5 years in vivo ] [ Designated as safety issue: No ]
  To quantify the extent of chain scission, gel content will be evaluated by measuring the non-extractable portion of the molecules using thin sections cut co-planar to the articular surfaces with a microtome and extracting the polyethylene chains from these sections in hot xylene.
  
  

For many years, gamma-irradiation in air was a common method for the terminal sterilization of ultrahigh molecular weight polyethylene (UHMWPE) materials used for joint replacement. A serendipitous byproduct of sterilization with gamma-irradiation was polymer crosslinking that tended to improve the wear performance of the polyethylene. During the 1990s, however, the orthopaedic community became aware that gamma-irradiation created free radicals within polyethylene, rendering the material susceptible to oxidative degradation. In view of this potentially deleterious effect, terminal sterilization methods using chemical surface treatments were developed to avoid free radical formation. To induce polyethylene crosslinking without residual free radicals, manufacturers also developed methods to crosslink the polyethylene followed by heat treatments to eliminate free radicals. These crosslinked materials were introduce during the latter part of the 1990s and clinical outcome studies among hip replacement patients have demonstrated substantially reduced wear at early follow-up intervals based on radiographic measurements. However, the long-term performance of crosslinked polyethylene is unknown and some investigators have expressed concerns related to in vivo degradation. Additionally, some crosslinked UHMWPE components have shown high levels of oxidation and chain scission after removal from patients and storage in air. Among these implants, it is unknown whether the oxidation and chain scission occurred during implantation or after the polyethylene was removed from the patient.

This study will characterize the material properties of polyethylene liners retrieved from hip replacement patients. The study population will include crosslinked and conventional UHMWPE liners that were terminally sterilized without the use of radiation. To quantify the potential effects of different storage methods, liners that were vacuum packed and frozen after retrieval and others that were stored at room temperature with access to ambient light and air following explantation will be included in the study population. Analysis techniques will include Fourier Transform Infrared Spectroscopy (FTIR), Differential Scanning Calorimeter (DSC) and crosslink density measurements. Crosslink density measurements will be evaluated as a function of distance away from the articular surface and the rim of the implant. Gel and hydroperoxide contents will also be evaluated. The goal of the analysis is to better understand the changes in polyethylene material properties that occur in vivo and ex vivo.