Chapter 32 - Development and Application of the Small Punch Test to UHMWPE

Abstract

This chapter reviews the development and application of miniature specimen mechanical testing techniques, based on the small punch test, to the characterization of ultra-high molecular weight polyethylene (UHMWPE) components for total joint replacement. The development of the small punch test as applied to UHMWPE is motivated by two clinically relevant and related problems—the relationship between process and system variables and polymer degradation, and the relationship between mechanical properties and the ensuing wear of the arthroplasty bearing. To a first order approximation, the polymer degradation problem is motivated by mechanical failure of tibial plateau bearings, and the wear problem is motivated by the clinical need to reduce the prevalence of small particle mediated late-onset osteolysis in total hip arthroplasty systems. The small punch test has been extensively developed to permit direct assay of the mechanical properties of orthopedic UHMWPE bearings. It has proven to be extremely useful in the detection of the mechanical signature of oxidative degradation and the signature associated with elevated levels of irradiation-induced crosslinking. Further testing has enabled associations between wear and mechanical properties to be made from the self-same components, although a quandary remains as to which mechanical property drives wear reduction. The greatest strength of the small punch testing methodology is its minimal requirement for material, thereby permitting mechanical assays to be made from almost any experimental specimen configuration, and one would expect this strength to continue to be leveraged across the mechanical, chemical, and tribological spectra that describe the orthopedic bearing world.