Ultra high molecular weight polyethylene with improved plasticity and toughness by high temperature melting Jun Fu a, b , Bassem W. Ghali a , Andrew J. Lozynsky a , Ebru Oral a, b, * , Orhun K. Muratoglu a, b a Harris Orthopaedic Laboratory, Massachusetts General Hospital, Boston, MA 02114, United States b Department of Orthopedic Surgery, Harvard Medical School, Boston, MA, United States article info Article history: Received 19 January 2010 Received in revised form 30 March 2010 Accepted 3 April 2010 Available online 14 April 2010 Keywords: High temperature melting Wear Total joint implants abstract Our goal was to improve the strength and toughness of ultra high molecular weight polyethylene (UHWMPE), which is the preferred polymeric bearing material in total joint implants. Based on accel- erated diffusion of UHMWPE chains at high temperatures, our hypothesis was that high temperature melting could minimize the structural defects and thus improve the toughness of consolidated UHMWPE. Melting of consolidated medical-grade UHMWPE at 280, 300, and 320  C in inert atmosphere improved the elongation at break, work-to-failure and impact strength, presumably due to chain scis- sioning and structural defect elimination through self-diffusion. An important finding of this study was that the gain in plasticity and toughness did not sacrifice the wear resistance under optimized melting conditions, which may be promising for next generation high performance UHMWPE materials for joint implant bearing surfaces. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Ultra high molecular weight polyethylene (UHMWPE) has been the material of choice for total joint implants for over four decades. The longevity of UHMWPE implants has been limited by its wear and oxidation resistance, especially after radiation sterilization [1,2]. While high dose radiation cross-linking has decreased wear [3] together with the use of antioxidant stabilization to minimize oxidation [4], it has also decreased the mechanical strength of joint implants, limiting its use in high stress applications such as total knee implants, especially in younger and more active patients. Thus, there is an increasing desire to improve the strength and toughness of radiation crosslinked UHMWPEs. In this paper, we present a method by which the toughness of the consolidated UHMWPE is increased for further treatment with cross-linking. Due to its extremely high molecular weight (2e6 million grams per mole), the melt viscosity of UHMWPE is very high, making it practically impossible to process by using conventional processing methods such as injection molding. Currently, compression molding and ram extrusion are two major processing methods to consolidate UHMWPE resins into bars or rods [5]. However, pro- cessing parameters [6] affect the wear and fatigue properties of UHMWPE, which are crucial for its performance as a load-bearing surface in the joint implants [7,8]. During in vivo articulation against (commonly) cobalt-chrome alloy in a total joint implant, UHMWPE components experience cyclic compressive and tensile loading, and abrasive and adhesive shear [9]. Stresses are dissipated through plastic deformation of the polymer, but can also concentrate at structural defects (e.g., crack), causing fatigue crack propagation and increasing risk of fracture [10,11]. Current processing methods, while providing reproducible and adequate mechanical properties for medical implants, inevi- tably leave structural defects inside the UHMWPE components due to the high viscosity and very slow self-diffusion of UHMWPE chains [10,11]. Although type 1 fusion defects (incomplete inter- particle voids) can be removed by properly controlling the consolidation conditions, type 2 fusion defects (incomplete inter- particle cohesion by self-diffusion) are common in UHMWPE. The interface of type 2 fusion defects may have less toughness than that of bulk UHMWPE [12] and thus could be potential sites for failure under cyclic loading. Moreover, such defects could be readily exaggerated by subsequent treatments such as radiation cross- linking for wear reduction [3] and high pressure crystallization for improved strength [13] due to the contraction of the polymer networks, which can lead to a further decrease in the strength and/ or toughness of the material. * Corresponding author. Harris Orthopaedic Laboratory, Massachusetts General Hospital, GRJ 1206, Boston, MA 02114, United States.Tel.: þ1 617 7260657; fax: þ1 617 6432521. E-mail address: eoral@partners.org (E. Oral). Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2010.04.003 Polymer 51 (2010) 2721e2731