Highly crosslinked ultra-high molecular weight polyethylene (UHMWPE) has shown success in reducing wear in hip arthroplasty but there remains skepticism about its use in Total Knee Replacement (TKR) inserts that are known to experience fatigue loading and higher local cyclic contact stresses.
Two Legacy Posterior-Stabilized (LPS) Zimmer NexGen tibial implants sterilized by gamma irradiation in an inert environment with posts that fractured in vivo were analyzed. Failure mechanisms were determined using optical and scanning electron microscopy along with oxidative analysis via Fourier Transform Infra-Red (FTIR) spectroscopy.
Micrographs of one retrieval revealed fatigue crack initiation on opposite sides of the post and quasi-brittle micromechanisms of crack propagation. FTIR of this retrieval revealed no oxidation. The fracture surface image of the second retrieval indicated a brittle fracture process and FTIR revealed oxidation in the explant.
Our findings suggest that tibial post fractures may be the results of in vivo oxidation and low level crosslinking. These and previous reports of fractured crosslinked UHMWPE devices implores caution when used with high stress concentrations, particularly when considering the potential for in vivo oxidation in TKR.
These two cases suggest that crosslinking of UHMWPE as a manufacturing process or sterilization method in conjunction with designs that incorporate high stress concentrations, such as the tibial post, may reduce material strength. Moreover, free radicals generated from ionizing radiation can render the polymer susceptible to oxidative embrittlement.