**1. Introduction**

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Back pain is the second most common reason for physician visits in the United States, and affects up to 84 percent of patients at some point in their lives (Deyo & Tsui-Wu 1987). For many patients, neck and back pain is often due to injury or spinal instability through trauma, disease or degeneration. This can cause impingement of neural structures resulting in pain, numbness, or weakness. Also, this can be a consequence of changes in the vertebral bodies or degenerative changes in the intervertebral cartilaginous discs. Treatment of such disorders may require surgery if the pain or neurologic symptoms prove to be intractable to conservative treatments such as physical therapy or pain medications.

The spine is a load-bearing structure made of ligaments, tendons, and bone that allows for a functional range of motion while protecting the spinal cord. The vertebral bodies are joined by two bilateral facet joints in the posterior aspect of the spinal column and are separated at each level by a cartilaginous intervertebral disc. This three-joint complex, illustrated in Figure 1, comprises the smallest mobile segment of the spine, commonly referred to as the functional spinal unit (FSU). The posterior joints are diarthrodial joints, which include articular cartilage, synovial membrane, and a joint capsule (Yong-Hing et al. 1980).

Fig. 1. Illustration of Three-Joint Complex

The Performance Envelope of Spinal Implants Utilizing Thermoplastic Materials 321

Modern spinal fixation devices have been made of biocompatible metallic alloys such as stainless steel and titanium. While these have become standard due to their strength and fatigue resistance, they possess radiographic challenges and biomechanical limitations. First, and foremost, metallic materials are radiologically problematic. Artifacts created by metals in computerized tomography (CT) and magnetic resonance (MR) imaging, as well as obfuscation of bone by metals in planar x-rays, inhibit visualization of new bone growth thus, making it difficult for physicians to assess progression of arthrodesis. Second, implants that are stiffer than natural bone are subject to so-called stress shielding. When two bones are fused together, the state of compressive forces between them determines the extent of modeling or remodeling. According to Wolff's law, bone apposition within a fusion graft is governed in part by the state of compressive stresses within it. Because of this it has been proposed that stiff metallic implants may shield the graft from the stress required for fusion, delaying or preventing the process. This can induce iatrogenic effects such as device-related osteopenia, intervertebral device protrusion into a neighboring vertebral body, and fracture or instability (Lippman et al. 2004). Implants developed out of less stiff materials may prevent this stress shielding and foster better clinical outcomes compared to traditional

Thermoplastic polyetheretherketone (PEEK) polymers, a subset of the polyaryletherketone or PAEK class of polyaromatic polymers, were initially considered for use in spinal applications to overcome the aforementioned limitations due to their inherent material properties, such as biocompatibility, radiolucency, and the ability to tailor their elastic moduli with fiber reinforcement such as carbon filament. Neat or unfilled PEEK has an elastic modulus of approximately 3.6 GPa, however this can be tailored to closely match cortical bone (18 GPa) or even titanium alloy (110 GPa) (Kurtz & Devine 2007). These properties, along with validation of their strength, wear, creep and fatigue resistance have pushed PEEK implants to be the most viable and attractive alternatives to metallic spinal implants to date. (Brown et al. 1990; Brantigan & Steffee 1993; Kurtz & Devine 2007). Figure 3 is an x-ray taken from a patient following implantation of a PEEK cage along with a pedicle screw and titanium rod system. Notice the radiolucency of the PEEK cage; only the

Fig. 2. PEEK Lumbar Interbody Cage (Left) alongside Titanium Lumbar Interbody Cage

devices.

(Right)

**3. Thermoplastic spine implants** 

radiopaque markers inside the implant are visible.

Spinal stability refers to the ability of the spine segment to limit excessive displacement that can possibly result in incapacitating deformity or neurologic symptoms due to impingement on the spinal cord and nerve roots (White & Panjabi 1990). Factors that can contribute to instability may affect the bones, disc, joints, or ligaments. These include trauma, tumors, infections, inflammatory diseases, connective tissue disorders, congenital disorders and degenerative disorders (Yong-Hing & Kirkaldy-Willis 1983). It is important to note that the surgical procedures performed in the interest of relieving neurologic symptoms may themselves contribute to instability.

Degenerative changes may affect the vertebrae and the intervertebral disc as well. The viscoelastic properties of the cartilaginous disc allow it to act as a shock absorber for the spine. As the disc degenerates, the ability for it to handle such stresses also changes (Panagiotacopulos et al. 1987). As peripheral fibers of the disc are stressed, rupture may occur resulting in herniation of disc material outwards to compress nerve roots or the spinal cord, potentially causing neurological symptoms including pain and weakness (Benzel 2004).

Surgical treatment of the spine often involves widening of the spinal canal and decompression of the spinal cord, the removal of osteophytes or disc material impinging on neurologic structures, and careful consideration of alignment of the spinal segment (Benzel 2004). These procedures may lead to destabilization of the FSU. Vertebral fusion helps provide stabilization of the spine, and can prevent further deformation and additional neurologic symptoms (Gibson & Waddell 2005). If spinal fusion is performed, the intervertebral disc is removed and the endplates of the vertebral bodies are bridged by a bone graft, often with an interbody device that allows for preservation of disc height and appropriate sagittal balance. Bony union connects the two vertebrae together into a continuous structure providing the needed stability; this structure along with any new bone is often referred to as a "fusion mass." Clinically, if the fusion is successful, it is referred to as an arthrodesis. Immobilization of the vertebral bodies has been shown to significantly enhance the success of fusion (Bridwell et al. 1993). Currently, internal spinal instrumentation is thought to be the best therapy to ensure a good clinical outcome in this regard.
