Monotonic and fatigue behavior of five clinically relevant conventional and highly crosslinked UHMWPEs in the presence of stress concentrations

Abstract

Five formulations of clinically relevant UHMWPE (conventional, moderately crosslinked annealed and remelted, and highly crosslinked annealed and remelted) were investigated in a physiologically relevant environment. Their monotonic stress-strain behavior in the presence of notches of two different severities and at two different displacement rates was examined using a custom developed video based system. 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. The fatigue behavior of these materials was examined using the same geometries via a stress-life approach with failure defined as fracture of the specimen in the 1,000 to 100,000 cycle lifetime range. The results were modeled using the Basquin relationship (σ=ANb, where σ=stress and N=lifetime, and A and b are experimentally derived constants) via maximum likelihood estimation methods to account for specimen runout (no failure at 250,000cycles). The conventional material was found to have a greater slope, b, and intercept, A, than the crosslinked materials as well as appearing to have less variance in its failure distributions.

Notch fatigue of ultrahigh molecular weight polyethylene (UHMWPE) used in total joint replacements

“…Despite this, few studies have examined the specific role of notch geometry in the fatigue behavior of this polymer. In fact, Sobeiraj and coworkers have presented the only known analysis of notch effects on the fatigue resistance of several modern UHMWPE material formulations using a stress-total life (S-N) approach [24]. However, significant scatter in their results severely limits the statistical interpretation of the data, suggesting that such an S-N approach may not be the best methodology for the notch fatigue evaluation of UHMPWE [24,25].…”

Mechanical in vitro fatigue testing of implant materials and components using advanced characterization techniques

“…The few existing investigations can be divided into uniaxial [20][21][22] and multiaxial 23,24 Through the tests, the fatigue strength was estimated between σ a = 10.17 MPa and σ a = 11.35 MPa. 20…”

“…18,19,22 Investigations with the aim of fatigue life evaluation based on an S-N curve, as addressed in this article, hardly exist. The few existing investigations can be divided into uniaxial [20][21][22] and multiaxial 23,24 Through the tests, the fatigue strength was estimated between σ a = 10.17 MPa and σ a = 11.35 MPa. 20…”

“…Investigations with the aim of fatigue life evaluation based on an S–N curve, as addressed in this article, hardly exist. The few existing investigations can be divided into uniaxial 20–22 and multiaxial 23,24 loading. Urriés et al investigated the effect of different irradiation treatments on the fatigue strength of UHMWPE.…”

“…The tests were performed on notched specimens at a stress ratio of R = 0.04–0.05 at a frequency of f = 2 Hz. Through the tests, the fatigue strength was estimated between σ a = 10.17 MPa and σ a = 11.35 MPa 20 …”