Ultra-high molecular weight polyethylene (ultra-high) often limits the longevity of total joint replacements due to excessive wear and associated clinical complications such as osteolysis. To mitigate such wear-related failure, manufacturers produced ultra-high that was highly cross-linked, typically by gamma radiation. Cross-linking was coupled with subsequent re-melting to neutralize free radicals that can lead to oxidative degradation of the material. However, cross-linking and re-melting decreased the resistance to fatigue crack propagation. In an attempt to preserve adequate resistance to fatigue and fracture while maintaining wear resistance and oxidative stability, manufacturers produced ultra-high that was either moderately cross-linked and re-melted, highly cross-linked and annealed below the melting temperature, or sequentially cross-linked and annealed. The success of such treatments remains a subject of debate due to the paucity of full-spectrum mechanical characterization studies that provide controlled comparisons amongst multiple clinically-relevant ultra-high materials.
This dissertation is the first study to simultaneously evaluate fatigue crack propagation, wear, and oxidation in a wide variety of clinically-relevant ultra-high. Results have important clinical implications: primarily, none of the materials was able to excel in all three areas. The moderately cross-linked re-melted material did equally well in all areas, but did not excel in any. With respect to processing treatments, increasing radiation dose increased wear resistance but decreased fatigue crack propagation resistance. Annealing reduced fatigue resistance less than re-melting, but left materials susceptible to oxidation. This appears to occur because annealing below the melting temperature after cross-linking increased the volume fraction and size of lamellae, but failed to neutralize all free radicals. Alternately, re-melting after cross-linking appeared to eliminate free radicals, but, restricted by the network of cross-links, the re-formed lamellae were fewer and smaller in size which resulted in poor fatigue crack propagation resistance.
The trade-off demonstrated is critical to the material's long-term success in total joint replacements: 1) excessive wear is a historical problem that results in large numbers of failures; 2) poor resistance to fatigue crack propagation and fracture has been implicated in recent reports of cross-linked re-melted hip liners fracturing in vivo; and 3) highly oxidized ultra-high cannot adequately withstand in vivo demands. Understanding the shortcomings of the current marketed materials, as well as the relationship of mechanical performance to treatment and microstructure, allows for targeted improvements needed to produce materials and designs that can withstand rigorous in vivo mechanical demands and improve the longevity of total joint arthroplasty.
