Analysis of Crosslinked and Conventional Polyethylene Explants

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.

Inclusion Criteria:

- Retrieved total hip arthroplasty liners maintained by the Anderson Orthopaedic
Research Institute

- Crosslinked or conventional liners terminally sterilized without gamma-irradiation

Exclusion Criteria:

- Liners terminally sterilized with gamma-irradiation