**The Use of PEEK in Spine Arthroplasty**

T. Brown1, Qi-Bin Bao1 and Hansen A. Yuan2

*1Pioneer Surgical 2SUNY Upstate, New York USA* 

### **1. Introduction**

210 Recent Advances in Arthroplasty

Williams S, et al. (2004). Comparative wear under different conditions of surface-engineered

Wimmer MA, et al. (2003). Tribochemical reaction on metal-on-metal hip joint bearings: a comparison between in-vitro and in-vivo results. *Wear*, 255, 2003: 1007–1024 Yoshitomi H, et al. (2009). Manufacturers Affect Clinical Results of THA with Zirconia Heads A Systematic Review. *Clin Orthop Relat Res*, 467, 2009: 2349–2355 Ziaee H, et al. (2007). Transplacental transfer of cobalt and chromium in patients with metal-

112-117

metal-on-metal bearings for total hip arthroplasty. *J Arthroplasty*, 19 (Suppl 3), 2004:

on-metal hip arthroplasty: a controlled study. *J Bone Joint Surg Br*, 89, 2007: 301-305

Cervical and lumbar disc arthroplasty are one component in the continuum of treatment for symptomatic degenerative disc disease (DDD) that is unresponsive to conservative care. In the lumbar spine, this may be accomplished via nucleus replacement or total disc replacement, and in the cervical spine by total disc replacement. The goal of both lumbar disc arthroplasty treatments are the same; relieving discogenic back pain through removing the pain source and restoring or maintaining motion segment function. For cervical arthroplasty, the goal is to relieve radicular pain as a result of nerve root compression, and/or myelopathy as a result of spinal cord compression, in addition to preserving motion. From a design standpoint, nucleus replacement technology consists of elastomers and nonelastomers, both preformed and *in-situ* cured, and can incorporate articulation similar to total disc replacements, with the intent of replicating to various extents the natural nucleus and preserving most of the annulus, thereby relying on a biomechanically intact annulus to share the compressive load. Also, most artificial nucleus devices are not fixed to the vertebral endplates, and therefore allow small, relative motion between their external surfaces and the vertebral endplates. In contrast, the majority of total disc replacement technology consists of articulating designs and material combinations that have been developed based upon the wealth of scientific and clinical information produced by the success of total joint arthroplasty. An artificial disc is designed to replace the entire disc tissue by excising almost all the disc materials, and therefore removing all the natural constraints in the anterior column. In addition, all artificial discs have a superior plate and an inferior plate, which are fixed to the two adjacent vertebrae. These represent key design differences between the two technologies.

A key challenge for a disc arthroplasty device is selecting the proper material(s) for the various components that consitute its design. Unlike total joint replacement, a candidate for disc arthroplasty is on average 40 years of age with a target indication of 18 to 60 years (Zigler et al., 2007; Murrey et al., 2009). As a consequence, these devices are expected to last much longer than those of total joint recipients, whose average age is 70 years (Bergen, 2011, Garellick et al., 2010). Therefore, there are stringent requirements for long term implantable materials, and this will significantly limit the selection of materials available. Biocompatibility and biodurability, otherwise known as the abilities of a material to maintain its physical and chemcial integrity under *in vivo* applications without eliciting an aggressive host immune response for a given application, are essential for permanent

The Use of PEEK in Spine Arthroplasty 213

(Rousseau et al., 2007) and non-fusion applications (Balsano, 2011; Senegas, 2002) due to its excellent mechanical strength, biostability, biocompatibility and radiolucency (Kurtz & Devine, 2007), and has replaced to a significant extent the use of metallic cages. It is also commonly used in the cervical spine in the form of interbody cages for fusion. Therefore, it potentially has most of the material characteristics deemed to be required for use as material in spine arthroplasty applications. However, unlike traditional polymers and metals, common materials used in the spine arthroplasty arena, the use of PEEK represents a new material for disc arthroplasty applications in the cervical and lumbar spine, and as such, its

PEEK has recently been used in the form of a nucleus replacment device and in the form of a cervical total disc replacement. Both devices utilize a ball and socket articulation for motion. As part of evaluating the safety and effectiveness profile of both devices, several preclinical tests were successfully performed to satisfy FDA and the Essential Requirements of the Medical Device Directive (MDD) for CE mark requirements prior to clinical use. Each device is CE marked and currently in clinical use. Worldwide clinical results as measured by validated measures such as Oswestry Disability Index (ODI), Visual Analog Scale (VAS), along with patient satisfaction, suggest that both devices can relieve the symptoms of their respective degenerative processes, with no adverse events occurring as a direct result of the device not performing as expected from a biomaterial perspective. The methodology, results and interpretation of these preclinical studies that have allowed for advancement to the clinical stage will be discussed, along with the clinical results in light of both devices' clinical

The cervical arthroplasty device incorporates a unique, Ti cam blade fixation system that can be described as a rotating shaft with blades for primary fixation (Figure 1). Given that PEEK is a relatively bio-inert material, bone apposition does not readily occur onto the material. Therefore, a plasma-sprayed hydroxyapatite (HA) coating was added to the outer endplates. The design objectives are to achieve secure, long term fixation within the disc space, exhibit the necessary strength and durability for the lifetime of the patient for its intended use, and restore or maintain the range of motion (ROM) at the operative level while simultaneously

use as a material for this technology has largely gone unexplored.

performance.

**2. Preclinical studies and results** 

Fig. 1. PEEK cervical arthroplasty device.

**2.1 PEEK cervical total disc replacement device** 

not adversely affect the biomechanics at the adjacent levels.

medical implants. Since a central tenet of all arthroplasty devices is the preservation of motion, it is therefore expected that these devices will produce wear particulate, and the subsequent potential biologic activity needs to be evaluated. This is in addition to exhibiting sufficient strength and fatigue performance over the expected lifetime of the device within the *in vivo* environment. Subsequently, disc arthroplasty devices should be introduced into clinical use only after a successful preclinical evaluation of their mechanical and biological properties within the scope of their intended use, as failure can lead to a technically challenging revision, increased economic burden and more importantly, a significant risk to the patient (Devin et al., 2008, Cavanaugh et al., 2009, Francois et al., 2007, Guyer et al. 2011, Lucina & Thorpe, 2004, Punt et al., 2009).

Initially, much of the preclinical testing of candidate materials is not conducted on the device but rather the material itself. Logical progression then leads to testing of the device itself, since the design of the device will also affect a given materials performance. These tests should include bench top testing such as static and fatigue assessments, wear testing, and evaluation in human cadaver and animal models. The wear, mechanical durability and potential resultant biologic activity of disc arthroplasty devices are a key component that needs to be addressed prior to clinical use. This can be assessed to a large extent by the appropriate wear tests for articulating devices and fatigue tests in the case of devices that utilize elastomers for the basis of their motion. However, in the absence of significant implant retrievals, the appropriate tests to perform in assessing a respective devices' durability properties becomes critical. Device fatigue and wear may decrease the devices' lifespan for its intended application. In addition, the wear particulate generated may result in a particle mediated inflammatory response potentially leading to osteolysis or delayed type hypersensitivity. Evaluation of the particulate generated can allow for assessment of potential biologic reactivity. Successful animal models can act as a final spring board for initiation of clinical studies by evaluation of the mechanical behavior and elicited histopathological response of the device component materials following long-term implantation. Analyses can based upon gross necropsy, MRI radiography, plain X-ray, microradiography, multi-directional flexibility testing and biocompatibility assays (local and systemic histology). The overall successful completion and evaluation of these studies allows for advancement to the clinical stage.

Traditionally metal alloys, ceramics and polymers have been the materials of choice for arthroplasty applications. Ceramics are noted for their high wear resistance and strength, and metal alloys, such as titanium (Ti) and cobalt chrome (CoCr) for their high strength, fracture toughness and hardness; properties that are necessary for long term implantable materials. Polymers are a more compliant material, with low-friction properties ideal for articulating surfaces which produce relatively inert wear debris. These materials and combinations thereof have a long clinical history in total joint arthroplasty. Therefore, material combinations such as metal on UHMWPE and metal on metal are typically used for articulating disc replacements based upon the established scientific and clinical history of these bearing couples in total joint arthroplasty. Poly-ether-ether-ketone (PEEK) is a linear, aromatic thermoplastic with proven biocompatibility and biostability. It can be repeatedly steam and gamma sterilized with no detrimental effects on its bulk material properties. It is an exceptionally strong engineering thermoplastic that retains its mechanical properties even at very high temperatures (Rae, 2007). The material is tough and abrasion resistant with high-impact strength and excellent flexural and tensile properties. It has a low coefficient of friction and resists attack by a wide range of organic and inorganic chemicals and solvents. To this end, PEEK is widely used in the lumbar spine for both fusion

medical implants. Since a central tenet of all arthroplasty devices is the preservation of motion, it is therefore expected that these devices will produce wear particulate, and the subsequent potential biologic activity needs to be evaluated. This is in addition to exhibiting sufficient strength and fatigue performance over the expected lifetime of the device within the *in vivo* environment. Subsequently, disc arthroplasty devices should be introduced into clinical use only after a successful preclinical evaluation of their mechanical and biological properties within the scope of their intended use, as failure can lead to a technically challenging revision, increased economic burden and more importantly, a significant risk to the patient (Devin et al., 2008, Cavanaugh et al., 2009, Francois et al., 2007, Guyer et al. 2011,

Initially, much of the preclinical testing of candidate materials is not conducted on the device but rather the material itself. Logical progression then leads to testing of the device itself, since the design of the device will also affect a given materials performance. These tests should include bench top testing such as static and fatigue assessments, wear testing, and evaluation in human cadaver and animal models. The wear, mechanical durability and potential resultant biologic activity of disc arthroplasty devices are a key component that needs to be addressed prior to clinical use. This can be assessed to a large extent by the appropriate wear tests for articulating devices and fatigue tests in the case of devices that utilize elastomers for the basis of their motion. However, in the absence of significant implant retrievals, the appropriate tests to perform in assessing a respective devices' durability properties becomes critical. Device fatigue and wear may decrease the devices' lifespan for its intended application. In addition, the wear particulate generated may result in a particle mediated inflammatory response potentially leading to osteolysis or delayed type hypersensitivity. Evaluation of the particulate generated can allow for assessment of potential biologic reactivity. Successful animal models can act as a final spring board for initiation of clinical studies by evaluation of the mechanical behavior and elicited histopathological response of the device component materials following long-term implantation. Analyses can based upon gross necropsy, MRI radiography, plain X-ray, microradiography, multi-directional flexibility testing and biocompatibility assays (local and systemic histology). The overall successful completion and evaluation of these studies

Traditionally metal alloys, ceramics and polymers have been the materials of choice for arthroplasty applications. Ceramics are noted for their high wear resistance and strength, and metal alloys, such as titanium (Ti) and cobalt chrome (CoCr) for their high strength, fracture toughness and hardness; properties that are necessary for long term implantable materials. Polymers are a more compliant material, with low-friction properties ideal for articulating surfaces which produce relatively inert wear debris. These materials and combinations thereof have a long clinical history in total joint arthroplasty. Therefore, material combinations such as metal on UHMWPE and metal on metal are typically used for articulating disc replacements based upon the established scientific and clinical history of these bearing couples in total joint arthroplasty. Poly-ether-ether-ketone (PEEK) is a linear, aromatic thermoplastic with proven biocompatibility and biostability. It can be repeatedly steam and gamma sterilized with no detrimental effects on its bulk material properties. It is an exceptionally strong engineering thermoplastic that retains its mechanical properties even at very high temperatures (Rae, 2007). The material is tough and abrasion resistant with high-impact strength and excellent flexural and tensile properties. It has a low coefficient of friction and resists attack by a wide range of organic and inorganic chemicals and solvents. To this end, PEEK is widely used in the lumbar spine for both fusion

Lucina & Thorpe, 2004, Punt et al., 2009).

allows for advancement to the clinical stage.

(Rousseau et al., 2007) and non-fusion applications (Balsano, 2011; Senegas, 2002) due to its excellent mechanical strength, biostability, biocompatibility and radiolucency (Kurtz & Devine, 2007), and has replaced to a significant extent the use of metallic cages. It is also commonly used in the cervical spine in the form of interbody cages for fusion. Therefore, it potentially has most of the material characteristics deemed to be required for use as material in spine arthroplasty applications. However, unlike traditional polymers and metals, common materials used in the spine arthroplasty arena, the use of PEEK represents a new material for disc arthroplasty applications in the cervical and lumbar spine, and as such, its use as a material for this technology has largely gone unexplored.

PEEK has recently been used in the form of a nucleus replacment device and in the form of a cervical total disc replacement. Both devices utilize a ball and socket articulation for motion. As part of evaluating the safety and effectiveness profile of both devices, several preclinical tests were successfully performed to satisfy FDA and the Essential Requirements of the Medical Device Directive (MDD) for CE mark requirements prior to clinical use. Each device is CE marked and currently in clinical use. Worldwide clinical results as measured by validated measures such as Oswestry Disability Index (ODI), Visual Analog Scale (VAS), along with patient satisfaction, suggest that both devices can relieve the symptoms of their respective degenerative processes, with no adverse events occurring as a direct result of the device not performing as expected from a biomaterial perspective. The methodology, results and interpretation of these preclinical studies that have allowed for advancement to the clinical stage will be discussed, along with the clinical results in light of both devices' clinical performance.
