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

Improved Resistance to Wear, Delamination and Posterior Loading of Electron Beam Irradiated, Melt-annealed, Highly Crosslinked UHMWPE Knee Inserts

Abstract
Recently, a knee prosthesis containing an electron beam irradiated(nominal dose: 65kGy), melt-annealed, highly crosslinked UHMWPE(HXPE) tibial insert has been developed. In the present study,the wear and delamination resistance of the HXPE tibial insertand its fatigue performance under a severe posterior loadingcondition have been evaluated against its conventional gamma-sterilizedUHMWPE counterpart (37kGy, in nitrogen). The test methodologiesused were newly developed with the aim to evaluate this newmaterial under severe testing conditions.

Three sets of experiments were conducted (all samples wereaccelerated aged per ASTM Standard F2003 Method B):

  1. a 20 million cycle knee wear simulator study, wherein 12 HXPE and 6 gamma tibial inserts were wear tested to 5 million gait cycles, of which 8 HXPE and 4 gamma inserts were subsequently wear tested to 20 million cycles;
  2. an 8 million cycle delamination study, wherein 6 HXPE and 6 gamma tibial inserts were delamination tested to 8 million cycles;
  3. a 5 million cycle posterior loading fatigue study, wherein 5 HXPE and 5 gamma tibial inserts were fatigue tested to 5 million cycles under a severe posterior loading condition.

In comparison with the gamma controls, the HXPE inserts:

  1. wore significantly less (Figure 1): achieving wear reductions of 81% and 73% over 5 and 20 million cycles, respectively;
  2. had significantly improved delamination resistance:
    1. a. none of the HXPE inserts delaminated after 20 million gait cycles of wear testing (experiment 1), while one gamma insert underwent extensive delamination damage;
    2. none of the HXPE inserts delaminated in the delamination test (experiment 2), while all gamma controls delaminated (Figure 2).
  3. had significantly improved posterior loading fatigue resistance: none of the HXPE inserts exhibited any indication of fatigue cracking, while all gamma controls developed cracks (Figure 3).