Comparison of the Properties of Annealed Crosslinked (Crossfire) and Conventional Polyethylene As Hip Bearing Materials

In 1998, orthopaedic manufacturers started to introduce highly crosslinked ultra-high molecular weight polyethylene (UHMWPE) for total hip replacement bearings. Today's highly-crosslinked UHMWPEs materials are irradiation processed with a total dose ranging between 50 and 105 kGy, depending upon the manufacturer. Each manufacturer has adopted a different route for producing their highly crosslinked UHMWPE that includes a combination of three important processing steps: an irradiation step, an intra or post-irradiation thermal processing step, and a sterilization step. This paper reviews the choices available to an implant designer when developing a highly-crosslinked UHMWPE as an orthopaedic bearing material. We suggest that the application of annealing rather than re-melting in the thermal processing step allows the retention of important mechanical properties in the finished material. This approach will be illustrated with test data on Crossfire (Stryker Howmedica Osteonics, Mahwah, NJ), an annealed, highly-crosslinked UHMWPE developed specifically for total hip replacements. We compare the physical, mechanical, and wear properties of Crossfire with that of conventional (N2-Vac) UHMWPE and with materials produced using published melt irradiation technology. At the short term (2 and 3 years) the results demonstrated through clinical follow-up, clinical results for Crossfire, are encouraging. Longer follow-up is necessary to confirm the benefits to patients from reduction of debris released from the articulation.