* Corresponding author. Tel.: 1-216-844-1133; fax: 1-216-368-3007. E-mail address: cmr10@po.cwru.edu (C.M. Rimnac). Biomaterials 21 (2000) 2081}2087 Cyclic steady state stress}strain behavior of UHMW polyethylene David J. Krzypow*, Clare M. Rimnac Department of Mechanical & Aerospace Engineering and Department of Orthopaedics, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106-7222, USA Received 27 December 1999; accepted 30 March 2000 Abstract To increase the long-term performance of total joint replacements, "nite element analyses of ultra high molecular weight polyethylene (UHMWPE) components have been conducted to predict the e!ect of load on the stress and strain distributions occurring on and within these components. Early models incorporated the monotonic behavior of UHMWPE without considering the unloading and cyclic loading behavior. However, UHMWPE components undergo cyclic loading during use and at least two wear damage modes (pitting and delamination) are thought to be associated with the fatigue fracture properties of UHMWPE. The objective of this study was to examine the fully reversed uniaxial tension/compression cyclic steady state stress}strain behavior of UHMWPE as a "rst step towards developing a cyclic constitutive relationship for UHMWPE . The hypothesis that cycling results in a permanent change in the stress}strain relationship, that is, that the cyclic steady state represents a new cyclically stabilized state, was examined. It was found that, like other ductile polymers, UHMWPE substantially cyclically softens under fully reversed uniaxial straining. More cyclic softening occurred in tension than in compression. Furthermore, cyclic steady state was attained, but not cyclic stability. It is suggested that it may be more appropriate to base a material constitutive relationship for UHMWPE for "nite element analyses of components upon a cyclically modi"ed stress}strain relationship.  2000 Elsevier Science Ltd. All rights reserved. Keywords: Ultra high molecular weight polyethylene; UHMWPE; Cycle-dependent stability; Fatigue; Mechanical performance 1. Introduction Wear damage to ultra high molecular weight polyethy- lene (UHMWPE) total joint components has been recog- nized as a signi"cant clinical problem limiting the lifetime of joint arthroplasties. Long-term complications in total joint replacement, such as loosening and infection, have been correlated to the foreign body reaction to the poly- ethylene debris found in the surrounding tissues and produced from wear damage to the articulating surfaces [1}6]. As part of the e!ort to increase the long-term perfor- mance of total joint replacements, much research has been directed to better understanding the mechanisms that cause wear damage to UHMWPE joint compo- nents. In this context, "nite element analyses of UHMWPE components from both hip and knee replace- ments have been conducted to predict the e!ect of load, geometry and material properties on the stress and strain distributions occurring on and within these components [7}12]. Early models incorporated loading only (mono- tonic behavior) without considering unloading and were based on bilinear elastic or elastic}plastic stress}strain relationships for UHMWPE [8]. However, implant retrieval analyses suggest that pa- tient weight, activity level, and time of implantation are associated with the severity of surface damage of compo- nents [13]. Therefore, UHMWPE fatigue fracture mech- anisms have been suggested to contribute to certain forms (e.g. pitting and delamination) of UHMWPE sur- face damage [14,15]. Thus, modeling the cyclic loading behavior of UHMWPE components is likely to be more clinically relevant than only modeling monotonic behav- ior. In addition, experiments by Blunn et al. [16] sugges- ted that damage to UHMWPE tibial components is also dependent on the kinematics of the interaction with the femoral component. Delamination damage was observed on a #at UHMWPE surface when a metal indenter had been sliding against it, but not for static loading or pure indenter rolling. Estupin  an et al. [8] used "nite element analysis to examine the e!ect of a moving cyclic load on the stress 0142-9612/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 6 1 2 ( 0 0 ) 0 0 1 3 8 - 1