Macro- and microscopic evaluation of fatigue in medical grade ultrahigh molecular weight polyethylene

As the first decade of the new millennium is upon us, there has been a thrust to examine and evaluate issues surrounding the performance total joint replacements. This decade has been dubbed the Bone and Joint Decade by the World Health Organization, and there is worldwide focus on improving treatments for joint disorders. One such treatment for arthritic joints is a total joint replacement, in which a metal-polymer system replaces the diseased or damaged bone-cartilage system. The polymer, ultrahigh molecular weight polyethylene (UHMWPE) is the polymer used in an overwhelming majority of total joint replacements. Although UHMWPE remains the material of choice for the bearing surface in total joint replacements it also limits the life of these components due to the generation of mechanically induced polymer debris. Fatigue of UHMWPE is a known damage mode and has been observed to impact the clinical performance of total knee replacements. Understanding the aspects of fatigue damage, specifically initiation of damage, has been the focus of much work of the past decades. The work presented in this dissertation addresses the fatigue performance of UHMWPE to evaluate the initiation and propagation of flaws. First an in-depth and comprehensive analysis of the non-surgical variables that can affect the fatigue performance was carried out to examine the clinical impact and provide a clear picture of the optimal combination of variables. Several analytical tools, including scanning electron microscopy, provided insight into the microscopic mechanisms of the fatigue process. X-ray scattering, in the novel form of ultra small angle x-ray scattering, highlighted the fatigue initiation process from which the delamination fatigue model was improved upon. Several highly crosslinked UHMWPEs have been viewed as improved alternatives for increasing the longevity of the joint replacement. A thorough fatigue investigation of the crosslinked polyethylenes was performed to examine their fatigue fracture resistance along with their resistance to flaw initiation. This work presents a clear picture of the fatigue processes in UHMWPE as it relates to clinical performance, as well as insight into fatigue initiation processes of semicrystalline polymers.