Conceptual Design of a Cervical Disc and Facet Joint Arthroplasty System Made From PEEK

INTRODUCTION The cervical spine provides structural support and motion through flexible intervertebral discs along with articulating synovial facet joints. In response to severe disc degeneration, arthroplasty devices attempt to both replace the disc and restore normal motion. However, after disc arthroplasty, facet joint loads can increase during extension [1]. The clinical significance of these increased forces is not known but disc arthroplasty has yet to show significantly improved clinical outcomes compared to fusion and debate continues regarding the reasons [2]. It is hypothesized that expanding the arthroplasty approach to include the facet joints may be one way to eventually improve clinical performance. Natural cervical spinal motion is produced by disc and facet interaction that occurs about a transient helical axis whose center of rotation changes [3]. Current disc replacements do not completely accommodate this motion and thus could be modified along with the option of added facet joint implants to give a cervical total level arthroplasty system (CTLAS). The purpose of the present study was to initiate the development of CTLAS by proposing a conceptual design. METHODS An engineering design procedure was applied to develop a system to restore natural kinematics, load sharing and motion of the cervical spine via CTLAS. Importance was also placed on post-operative imaging but the issue of fixation in bone was not addressed at this time. The instantaneous centers of rotation and the helical axes of the cervical spine were mapped. While developing the conceptual design of CTLAS, ongoing wear tests of PEEK-on-PEEK articulations were being performed using an Ortho-POD pin-on-disc apparatus (AMTI, Watertown, MA, USA) to explore the suitability of using this material. RESULTS A modular CTLAS was developed to treat disc and facet degeneration (Fig. 1). A semi-constrained bearing, that replicated the motion produced by the helical axis, was proposed for the disc while a surface replacement design was proposed for the facet joints. Axial rotation and lateral bending are coupled, as in the natural spine, by permitting motion about non-fixed instantaneous centers of rotation with some anterior-posterior translation. In this conceptual CTLAS design, carbon fiber reinforced PEEK was proposed as the sole material because, among other advantageous features, it has low wear [4, 5, 6] and does not interfere with post-operative imaging. DISCUSSION Proactive design improvement to include the unique characteristics of the cervical spine has the possibility of improving outcomes for spinal disc arthroplasty. The ability to replace facets could allow some candidates for fusion to be treated with arthroplasty. Provided adequate fixation can be achieve, the CTLAS conceptual design has the potential to offer better replication of natural kinematics, sequential application of facet joint implants and excellent post-operative imaging.

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