UC Berkeley

Total joint replacements (TJRs) are commonplace in the U.S., with over one million primary (i.e. first-time) knee and hip implant surgeries performed per year. The prevalence of primary total knee and total hip replacements is expected to nearly double by 2030, and of those implants, approximately 12% will require revision. The ultimate failure mode causing revision depends on many factors, but breakdown of the plastic component due to damage accumulation from mechanical fatigue is a leading concern. Fatigue failure is especially concerning in knees where cyclic contact stresses are elevated owing to concentrated loading across the condyles during normal articulation. Ultrahigh-molecular-weight-polyethylene (UHMWPE) is the legacy material for the plastic components of TJRs, having been used since the 1960’s and developed continuously since to meet market demands. UHMWPE is a semicrystalline polymer with molecular weights between 2-6 million g/mol, resulting in high chain entanglement and concordant high energetic toughness and abrasion resistance. Modern formulations of ultra-high are typically crosslinked to improve wear resistance, reducing ductility as a tradeoff. They may also have added antioxidants to regain some of the ductility lost from crosslinking, thus improving fatigue crack propagation (FCP) resistance at the cost of overall strength. Understanding the chemical and structural properties of UHMWPE that govern the balancing act between ideal fatigue, wear, and oxidation resistance results in a rich research design space, that, while parts of it have been thoroughly examined, other parts have yet to be documented. While wear and oxidation resistance are major design considerations for UHMWPE, this dissertation focuses on the challenge of fatigue for this orthopedic polymer. Specifically, the two main points of this dissertation are to provide a literature review for fatigue in polymers in general and to add to UHMWPE literature by providing an initial study on the effect of the a/W ratio on cycles to failure and, more importantly, to provide the first record of fatigue crack threshold information across a wide variety of medical formulations of UHMWPE. Chapter 1 serves as an introduction to UHMWPE for TJRs in general and why the studies included are relevant. Chapter 2 is a literature review about fatigue in polymers, providing information on the two primary fatigue design philosophies (total life vs. defect tolerant) and the effects of various factors on fatigue performance. Chapter 3 is an introductory exploration into the effect of notch root radius and a/W ratio on fatigue performance in two formulations of UHMWPE using a total life approach. Chapter 4 is a study using the defect tolerant (i.e. fracture mechanics) approach to understand both Paris regime and threshold behavior across a wide variety of medical formulations of UHMWPE. Finally, Chapter 5 provides concluding remarks and future directions for UHMWPE fatigue research.