Porous and Bioactive PEEK Implants for Interbody Spinal Fusion

Back and radicular pain is the leading cause of activity and productivity loss amongst adults of any age in the United States [Ref. 1]. In many cases, surgical intervention is required, including at least 5,000 interbody spinal fusion cages implanted each month in the U.S. alone [Ref. 2]. The total U.S. market for spinal fusion implants was valued at nearly $4 billion in 2008 [Ref. 3]. Interbody spinal fusion is used to alleviate pain caused when a herniated, bulging, or flattened intervertebral disc impinges on the spinal cord or nerve root. The disc and vertebral endplates are removed and an interbody fusion cage is inserted in the disc space (Fig. 1) to restore vertebral height, promote fusion of bone tissue between adjacent vertebrae and, thus, mechanically stabilize the spine [Ref. 2]. Polyetheretherketone (PEEK) cages (Fig. 2) have many attractive characteristics for spinal surgeons and patients [Ref. 4,5]. Radiolucent PEEK enables post-operative radiographic assessment of fusion, which is inhibited by the high x-ray attenuation of titanium. PEEK also exhibits a modulus of elasticity similar to bone, enhancing load transfer to tissue in the cage, and seems to demonstrate a relative lack of subsidence compared to alternatives. Finally, PEEK implants eliminate the need for limited human allograft sources and the possibility of donor source contamination. Despite the above advantages, current PEEK (and titanium) fusion cages are dense and bioinert, which limits incorporation into the fusion mass and the subsequent implant stability. Moreover, the center cavity of the cage must be augmented with osteoinductive agents, such as autograft, demineralized bone matrix and/or bone morphogenetic protein (BMP) in order to promote fusion. PEEK interbody fusion cages used in conjunction with BMP have been shown to produce high fusion rates [Ref. 4]. However, surgeons are left to question the necessary amount of fusion mass to be achieved around the PEEK implant for clinical success as bioinert materials are encapsulated by a layer of fibrous tissue. Allograft and autograft remain the only available option capable of being incorporated into the fusion mass through direct apposition of bone tissue. For these reasons, some surgeons remain reluctant to use PEEK implants. Porous and Bioactive PEEK Implants for Interbody Spinal Fusion