Chapter 13 - Development and Application of the Small Punch Test to UHMWPE

Abstract

This chapter focuses on the development and application of the small punch test for the characterization of ultra-high molecular weight polyethylene (UHMWPE) components for total joint replacement (TJR). The development of the small punch test was influenced 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. The small punch test method, also known as the disk bend test, was originally proposed for the characterization of metallic components from the power industry. In the small punch test for UHMWPE, a disc of material 0.0200 inches thick by 0.250 inches diameter is indented by a hemispherical punch thereby creating a biaxial state of tension in the material. Because of the enclosure of the specimen in the punch-and-die apparatus, the thickness tolerance is only 0.0005 inches. The test has proven extremely useful in the detection of the mechanical signature of oxidative degradation and the signature associated with elevated levels of irradiation-induced crosslinking. 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.