Symposium on Crosslinked and Thermally Treated Ultra-High Molecular Weight Polyethlene for Joint Replacements

Characterization of the Wear Performance of Crosslinked UHMWPE and Relationship to Molding Procedures
St. John, K. R., Poggie, R. L.

Materials and Methods
Using an eight-station hip wear simulator, radiation-crosslinked polyethylenes were tested for their resistance to wear-related weight loss and the results compared to results for studies conducted on the same machine using polyethylene which was being implanted at the time. The materials tested were as shown in the following table:

All materials, except for the GUR 1120 aged samples, were packaged under nitrogen for radiation exposure/sterilization and all materials had been machined using similar procedures at the same facility. There were no as-molded surfaces tested, even in the near net final shape molded groups. Testing was performed at a rate of one cycle per second using a Paul curve with a maximum load of 3000 N. Lubrication was provided by bovine calf serum (SH30073.03, Hyclone Labs). The serum was changed and the parts cleaned and weighed every 500,000 cycles for a total of 3,000,000 to 6,000,000 cycles.

Results
The testing results showed that the wear rate of crosslinked SCM polymer was about 63% lower that standard SCM parts and 69% less than RE 4150 parts. The NFS crosslinked specimens had an 89% reduction in wear rate over the standard NFS parts, regardless of whether GUR 1020 or GUR 1050 crosslinked parts were tested. All wear rates for nitrogen packaged materials were greatly superior to those for parts sterilized in air and for earlier generation GUR 4150 products sterilized and stored in contact with oxygen.

The results confirm that both the SCM and NFS molding processes produce cups with excellent wear properties as compared to previous processes. The crosslinking process provided a major improvement in the wear resistance with either molding process with greater improvement occurring in the NFS specimens.