October AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 1 8 3 0 *Member of ASM International NOVEL POLYMERS FOR USE IN TOTAL JOINT ARTHROPLASTY Improving the mechanical properties and wear resistance of polymers used as implant bearing materials requires new approaches and modern technology. Sudipta Seal, FASM,* Sushant Singh, Kaitlyn Crawford, Elizabeth Brisbois, and Melanie Coathup University of Central Florida, Orlando B one and joint disease affects mil- lions of people worldwide. Total joint replacement (TJR) is a safe and cost effective medical intervention that restores functional use of the limb, enables pain-free mobility, and improves quality of life. The number of people re- ceiving implants is increasing globally at a significant rate due to both an aging population and inserting implants into younger patients. However, the impact of increased friction and wear generated within thebearing surfaceswhen synthet- ic materials are used is still of concern. Historically, polymeric materials have been used extensively to develop low friction bearing materials for use in orthopedic joint replacement [1] . The introduction and use of high-density polyethylene (HDPE) and self-polymer- izing polymethylmethacrylate (PMMA) in the early 1960s contributed signifi- cantly in reducing surface wear and enhancing implant fixation and survi- vorship. Highly stable polymers such as polytetrafluoroethylene (PTFE), poly- acetal, polyaryletherketones (PAEK), polyetheretherketone (PEEK), and car- bon-reinforced PEEK have also been studied [2,3] . Table 1 shows some of the polymeric materials commonly used in orthopedic applications. The polymer most often used in TJR is ultrahigh mo- lecular weight polyethylene (UHMWPE) and its subsequent variations, mainly used in bearing couples with metal [4] . Its unique properties of high abrasion resistance, impact strength, fatigue re- sistance, low friction and density, and excellent biocompatibility make it an ideal candidate. Over the past 20 years, a number of highly crosslinked polyeth- ylene (HXLPE) materials have also been developed (Fig. 1). POLYMER FAILURE IN TJR Bearing material failure is a sig- nificant factor limiting the longevity of polymer-based orthopedic implants, with the failure rate and type of failure often dependent on joint arthroplasty location (e.g., hip, knee, shoulder). Implant lifetimes average 10 to 20 years, although longer lives are possi- ble. Complications cause implants to fail more rapidly. Primary failure modes for polymeric bearing materials include wear, yielding, creep, fatigue, fracture, delamination, and chemical damage. Wear becomes an increasingly critical concern over the implant lifetime be- cause the bearing materials abrade, generating submicron-sized wear parti- TABLE 1 — POLYMERIC MATERIALS USED IN IMPLANT-BASED ORTHOPEDIC APPLICATIONS Material Characteristics and uses Polymethyl methacrylate (PMMA) • Implant fixation in various orthopedic and trauma surgery [3,4,6] • Knees, shoulders, ankles, elbows, bone tumor implants Polytetrafluoroethylene (PTFE) • Biologically inert and non-biodegradable • Implantable for various conditions including vocal cord defects, arterial grafts, and facial plastic surgery [2] Polyaryletherketone (PAEK) • Semicrystalline thermoplastic with high mechanical strength and high-temperature stability; materials include polyetherketone (PEK), polyetheretherketone (PEEK), and polyetherketoneketone (PEKK) • Applied as biomaterials for trauma, orthopedic, and spinal implants [4,6] Ultrahigh molecular weight polyethylene (UHMWPE) • Semicrystalline polymer with high impact strength, wear resistance, low density, low friction, and high biocompatibility • Used as metal- and ceramic-on-plastic bearing couple in total joint replacement (TJR) • Highly crosslinked PE (HXLPE) used in total knee arthroplasty (TKA) with highly improved wear characteristics and longevity [3]