229 Radiation Crosslinked UHMWPE for Orthopedic Implants
Orhun K Muratoglu, PhD: Orthopedic Biomechanics and Biomaterials Laboratory Massachusetts General Hospital, Boston
E-mail: omuratoglu@partners.org
Abstract
Highly crosslinked ultra high molecular weight polyethylene (UHMWPE) has become the bearing material of choice for total hip arthroplasty over the past decade and its use in total knees is increasing. Clinical follow-up studies are showing marked reduction in the wear of UHMWPE acetabular liners in comparison with conventional, inert gas gamma sterilized UHMWPE liners, corroborating the findings of the early in vitro hip simulator studies. Crosslinking is exclusively achieved by radiation, which also results in residual free radicals. In the first generation highly crosslinked UHMWPEs, to avoid long-term oxidation residual free radicals are quenched by post-irradiation melting or annealing. Our analysis of surgically explanted components showed that post-irradiation melting is more effective in preventing in vivo oxidation than post-irradiation annealing. Acetabular liners prepared with the latter methods showed presence of residual free radicals and unprecedented levels of oxidation in vivo.
A number of second generation technologies have been advanced recently to address the oxidation related to post-irradiation annealing and the reduction in mechanical properties related to post-irradiation melting. Among these is sequentially crosslinked and annealed UHMWPE (SXL) that has been offered as an alternative to irradiation and annealing with the hypothesis that breaking up the irradiation into three steps with annealing steps in-between would improve the oxidative stability of the polymer over the single step of irradiation and annealing. Other laboratories and we studied the SXL method and found that residual free radicals were present in the SXL treated UHMWPE and that accelerated aging led to the oxidation of the material. Therefore, the long term oxidative instability of sequentially crosslinked UHMWPE may compromise its mechanical and fatigue resistance properties in the long-term.
Another second generation technology is the stabilization of free radicals through impregnation of irradiated UHMWPE with α-tocopherol (αT-PE). α-Tocopherol protects the polymer against oxidation and as melting is no longer necessary the mechanical and fatigue properties are not compromised. Hip simulator studies showed markedly improved adhesive and third body wear resistances with 28 and 36mm femoral heads and knee simulator studies showed improved adhesive wear and delamination resistance with αT-PE than conventional UHMWPE in a cruciate retaining knee design. The fatigue crack initiation resistance as measured by a cantilever bending test showed αT-PE to be effectively equivalent to conventional UHMWPE.