The static and cyclic behavior of UHMWPE and PEEK orthopaedic polymers in the presence of mild stress risers

For over four decades, ultra high molecular weight polyethylene (UHMWPE) has been used as one-half of the metal- or ceramic-on-plastic bearing couple in total joint replacement (TJR) components. In 2004, over 700,000 total hip and total knee replacements were performed. In the absence of severe wear, gross fracture of total joint replacement components tends to originate from stress concentrations such as fillets, undercuts, and grooves. Recently, PEEK has become the subject of interest with regard to its suitability as composite matrix material for bearing surfaces. Therefore, it is of interest to study the behavior of both UHMWPE and PEEK in the presence of a stress concentration.Several formulations of clinically relevant UHMWPE (conventional, moderately crosslinked, and highly crosslinked) were investigated. Their monotonic stress-strain behavior in the presence of a notch was examined using a custom developed video based system, under physiologically relevant conditions. It was found that both an elevation of yield stress and a truncation of orientation hardening took place. Additionally these changes were found to be material and elastic stress concentration factor dependent. Examination of the crystallinity and lamellar thickness distributions revealed that their evolution is stress-state dependent. The micromechanism of fracture was also shown to change drastically in UHMWPEs and PEEK upon notching.Since the fracture of total joint replacement components is likely a fatigue process, the fatigue behavior of UHMWPE and PEEK was also investigated. The fatigue of both materials was modeled using a total life approach (the Basquin relationship). It was found for PEEK that increasing notch severity decreased fatigue life. However, curiously, for the conventional formulations of UHMWPE increasing elastic stress concentration actually led to improved fatigue life. For PEEK a linear elastic fracture mechanics approach was also utilized to estimate the fatigue life spent in initiation vs crack growth. It was shown that the majority of the lifetime is spent in the initiation phase.