JOURNAL OF THE MECHANICAL BEHAVIOR OF BIOMEDICAL MATERIALS 3 (2010) 331–338 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/jmbbm Research paper An activated energy approach for accelerated testing of the deformation of UHMWPE in artiﬁcial joints Mathias Christian Galetz, Uwe Glatzel ∗ Metals & Alloys, University of Bayreuth, Germany ARTICLE INFO Article history: Received 12 October 2009 Received in revised form 14 January 2010 Accepted 17 January 2010 Published online 25 January 2010 ABSTRACT The deformation behavior of ultrahigh molecular polyethylene (UHMWPE) is studied in the temperature range of 23–80 ◦ C. Samples are examined in quasi-static compression, tensile and creep tests to determine the accelerated deformation of UHMWPE at elevated temperatures. The deformation mechanisms under compression load can be described by one strain rate and temperature dependent Eyring process. The activation energy and volume of that process do not change between 23 ◦ C and 50 ◦ C. This suggests that the deformation mechanism under compression remains stable within this temperature range. Tribological tests are conducted to transfer this activated energy approach to the deformation behavior under loading typical for artiﬁcial knee joints. While this approach does not cover the wear mechanisms close to the surface, testing at higher temperatures is shown to have a signiﬁcant potential to reduce the testing time for lifetime predictions in terms of the macroscopic creep and deformation behavior of artiﬁcial joints. c  2010 Published by Elsevier Ltd 1. Introduction Ultrahigh molecular weight polyethylene (UHMWPE) com- bined with cobalt–chromium–molybdenum (CoCrMo)-alloys has been used as a bearing couple in artiﬁcial knee joints for forty years because of its good balance of properties, like impact behavior, wear resistance and biocompatibility (Kurtz, 2004). Retrieved knee joints show a large variety of wear manifes- tations, while many investigators have noted that polyethy- lene components may exhibit substantial surface damage and creep without signiﬁcant loss of mass. An overview of the extensive literature is given by McKellop (2007). From this survey it follows that any visible or microscopic change in the appearance of the polyethylene surface should be deﬁned as wear damage. ∗ Corresponding author. Tel.: +49 921555555. E-mail address: uwe.galetz@uni-bayreuth.de (U. Glatzel). Creep leads to changes in the form and dimensions of the tibia inlays and acetabular cups that in turn inﬂuences the kinematics of the knee joint and therefore the tribological behavior (Wieleba, 2007). Walking activity studies show that patients with artiﬁcial joints do about 5000 steps per day on average, which extrapolates to approximately 1 million cycles per year for each joint of the lower extremity (Schmalzried et al., 1998). In vitro creep or stress relaxation can only be determined for an experimentally limited duration. The tests to simulate the lifetime of artiﬁcial joints are typically done for some million cycles at 1 Hz and hence are very time consuming and expensive. This shows the need for accelerated testing. For polymers often the principle of testing at higher temperatures is used, for example for the internal “Rupture Test” of polyethylene pipes. By increasing the temperature, 1751-6161/$ - see front matter c  2010 Published by Elsevier Ltd doi:10.1016/j.jmbbm.2010.01.004