PEEK biomaterials in trauma, orthopedic, and spinal implants

Abstract

Since the 1980s, polyaryletherketones (PAEKs) have been increasingly employed as biomaterials for trauma, orthopedic, and spinal implants. We have synthesized the extensive polymer science literature as it relates to structure, mechanical properties, and chemical resistance of PAEK biomaterials. With this foundation, one can more readily appreciate why this family of polymers will be inherently strong, inert, and biocompatible. Due to its relative inertness, PEEK biomaterials are an attractive platform upon which to develop novel bioactive materials, and some steps have already been taken in that direction, with the blending of HA and TCP into sintered PEEK. However, to date, blended HA-PEEK composites have involved a trade-off in mechanical properties in exchange for their increased bioactivity. PEEK has had the greatest clinical impact in the field of spine implant design, and PEEK is now broadly accepted as a radiolucent alternative to metallic biomaterials in the spine community. For mature fields, such as total joint replacements and fracture fixation implants, radiolucency is an attractive but not necessarily critical material feature.

Introduction

Following confirmation of its biocompatibility two decades ago [1], polyaryletherketones (PAEKs) have been increasingly employed as biomaterials for orthopedic, trauma, and spinal implants. Commercialized for industry in the 1980s, PAEK is a relatively new family of high-temperature thermoplastic polymers, consisting of an aromatic backbone molecular chain, interconnected by ketone and ether functional groups [2]. Two PAEK polymers, used previously for orthopedic and spinal implants, include poly(aryl-ether-ether-ketone) (PEEK) and poly(aryl-ether-ketone-ether-ketone-ketone (PEKEKK) (Fig. 1). The chemical structure of polyaromatic ketones confers stability at high temperatures (exceeding 300 °C), resistance to chemical and radiation damage, compatibility with many reinforcing agents (such as glass and carbon fibers), and greater strength (on a per mass basis) than many metals, making it highly attractive in industrial applications, such as aircraft and turbine blades, for example [2], [3].

Historically, the availability of polyaromatic polymers arrived at a time when there was growing interest in the development of “isoelastic” hip stems and fracture fixation plates, with stiffnesses comparable to bone [4]. Although neat (unfilled) polyaromatic polymers can exhibit an elastic modulus ranging from 3 to 4 GPa, the modulus can be tailored to closely match cortical bone (18 GPa) or titanium alloy (110 GPa) by preparing carbon-fiber-reinforced (CFR) composites with varying fiber length and orientation [4]. In the 1990s, researchers characterized the biocompatibility and in vivo stability of various PAEK materials, along with other “high performance” engineering polymers, such as polysulphone (PS) and polybutylene terephthalate (PBT) [5]. However, use of these polymers in implants was abandoned for reasons that are not well documented in the literature. Other polyaromatic ketone polymers, such as PEKEKK, were discontinued by their industrial supplier and thus ceased to be available for biomaterial applications.

By the late 1990s, PEEK had emerged as the leading high-performance thermoplastic candidate for replacing metal implant components, especially in orthopedics [6], [7] and trauma [8], [9]. Not only was the material resistant to simulated in vivo degradation, including damage caused by lipid exposure, but starting in April 1998 PEEK was offered commercially as a biomaterial for implants (Invibio, Ltd., Thornton-Cleveleys, United Kingdom). Facilitated by a stable supply, research on PEEK biomaterials flourished and is expected to continue to advance in the future [10].

Numerous studies documenting the successful clinical performance of PAEK polymers in orthopedic and spine patients continue to emerge in the literature [11], [12], [13], [14], [15], [16]. Recent research has also investigated the biotribology of PEEK composites as bearing materials and flexible implants used for joint arthroplasty [17], [18], [19], [20]. Due to interest in further improving implant fixation, PEEK biomaterials research has also focused on compatibility of the polymer with bioactive materials, including hydroxyapatite (HA), either as a composite filler, or as a surface coating [21], [22], [23], [24], [25], [26]. As a result of ongoing biomaterials research, PEEK and related composites can be engineered today with a wide range of physical, mechanical, and surface properties, depending upon their implant application.

The versatility of PEEK biomaterials necessarily translates into increased complexity, both for implant designers, as well as for researchers seeking to explore new modifications of PEEK for novel implant applications. In recent years, advances in the processing and biomaterials applications of PEEK have been progressing steadily. However, much of the previous research on PEEK implants has been fragmented in the materials science, composites, biomaterials, and application-specific literature. Consequently, the primary goal of this review is to synthesize the disparate repositories of data to provide a comprehensive, state-of-the-art assessment of PEEK and PEEK composites as a family of biomaterials. As background for this review, we first provide a summary of polyaromatic ketones as the basis for understanding the chemical, physical, and mechanical properties for this family of polymeric biomaterials. The second part of this paper summarizes the biocompatibility and in vivo stability of PEEK. Although a thorough treatment of all types of PEEK implants is beyond the scope of this paper, we conclude with an overview of the clinical applications of PEEK and related polyaromatic ketones in the orthopedics, trauma, and spinal literature.