Chronic back pain and related nerve damage is a significant contributor to the use of the healthcare system generating an estimated 13 million doctor and 50 million chiropractor visits annually in the United States and almost certainly proportional numbers in Canada. In particular, the prevalence of neck pain and associated nerve impingement has been reported to be within the range of 9% to 12% of the general population with the combined effects of neck and back pain accounting for up to 65% of cases of disability. Neck pain is specifically related to the cervical spine. This represents notable cost to society in terms of healthcare expenses plus lost work time and suffering for those afflicted.

The cervical spine must perform somewhat contradictory roles. It must provide adequate structural support for the head while allowing range of motion to complete everyday activities. Also, it must protect one of the most vital components of the central nervous system, the spinal cord and its related branching nerves. The motion and protection of the cervical spinal cord with its branching nerves depends a large extent on the condition of the joints. There are two types of joints:​ the intervertebral discs which are a flexible connection and the facets, which are articulating synovial joints. Both joints carry axial load and torque during rotational motion with the discs carrying most of the axial load and providing axial shock absorption. Both degenerate with age. A recent study of the lumbar spine suggests that disc degeneration usually occurs before facet joint degeneration. This sequence is likely to occur in the cervical spine as well and it has been estimated that up to 62% of neck pain cases arise from the facet joint degeneration.

Current surgical procedures to alleviate severe dysfunction and pain associated with disc degeneration include spinal arthrodesis (fusion) and disc arthroplasty (replacement with a joint). If severe dysfunction and pain are also associated with the facet joints, spinal fusion is the only option. Occasionally, when only the facet joints seem to be the cause of pain and there is little dysfunction (major nerve impingement), the small nerves to the facet joints can be killed. However, the continued structural collapse of the facet joints is likely to cause disc degeneration and thus eventually fusion may be required.

In spinal fusion the intervertebral disc is removed and, with the in vivo spacing preserved, the adjacent vertebrae are fused together with a combination of bone grafts and implants. Fusion prevents or severely limits the intervertebral motion, which causes abnormally high stress levels in adjacent discs.

In disc replacement, the damaged disc is removed and an articulating device is inserted in place of the disc and allows intervertebral motion that attempts to mimic the healthy spine. Disc arthroplasty is a significant improvement over spinal fusion, particularly in the cervical region, with regards to the reduction in adjacent level motion and stresses. However, the lack of rotational constraint causes increased facet contact pressure at both the surgical and adjacent levels especially in extension with axial rotation when the facets are the most heavily loaded.

Current FDA (U.S. Food and Drug Administration) approved devices for cervical disc arthroplasty are the Prestige® and Bryan® implants both from Medtronic Spinal & Biologics and the ProDisc™-C from Synthes Spine, Inc.. Metallic alloys provide the main structural components. These metallic alloys introduce medical imaging problems that would disappear if a polymer such as medical grade polyetheretherkeytone (PEEK) could be substituted. There are also several facet arthroplasty devices that have been proposed and are under current investigation/promotion but they too include metallic alloys and none of them have been specifically designed to function in conjunction with cervical disc arthroplasty.

Thus, opportunities exist for improving disc arthroplasty and developing facet arthroplasty in the cervical spine. Efforts should be made, in disc arthroplasty, to reduce postoperative facet pressures. This could involve a disc implant design that better restores the load sharing between the disc and facet joints. Furthermore, there may be merit in an approach that integrates both geometry and structure to give a combined replacement of the disc and facets for a cervical total level arthroplasty system (CTLAS) solution for patients afflicted with both disc and facet degeneration. The only current treatment option in this case is fusion. Thus, there is a need for modular CTLAS implant system that has a disc implant specifically designed to preserve the facet joints and an implant or set of implants for facet arthroplasty that can act together with the disc implant in an integrated and co-operative manner. In some cases, there may be a need for one or two stand-alone facet joint implants.

The design of a modular cervical CTLAS implant system is proposed that would replace both the disc and the facet joints. PEEK (polyetheretherkeytone) was proposed as the main structural material to improve medical imaging and some thought was given to using shock absorbing materials to protect the branching nerves. An allpolymer PEEK articulation was specified that reproduced the natural helical axis of the cervical spine. The facet joints were to be replaced with an all-polymer PEEK surface replacement. The proposed CTLAS design was considered an important first step in realizing such a technological advance in clinical practice. The proposed design was incomplete because there were still fixation issues to consider along with optimization for manufacturing. Furthermore, pre-clinical investigations would be required and probably include animal testing of fixation methods involving PEEK surfaces plus extensive simulator testing to show acceptably low levels of both fatigue and wear.

In the present thesis, pre-clinical testing was initiated with pin-on-plate wear testing of PEEK-on-PEEK to explore the tribology of this rather uncommon contact configuration. This testing contributed to an existing body of knowledge on this material pairing that was obtained in previous pin-on-plate and simulator studies done at various research centres. The present study used a protocol of testing under an adverse loading regime to investigate the sensitivity of all-polymer PEEK articulations to previous wear damage and extreme conditions. A gravimetric wear assessment protocol was used that avoided vacuum drying. The lower wear of polyacrylonitrile carbon fibre reinforced (CFR) PEEK compared with unfilled PEEK, as found by many other studies, was confirmed. PEEK was shown to have remarkably low wear despite previous adverse load testing. CFR PEEK was found to have much lower and more predictable wear than the unfilled PEEK, under a variety of test conditions. The wear of unfilled PEEK pairs seemed to show sensitivity to lubricant protein concentration. The coefficient of friction in all testing was found to be quite high (up to 0.5) and this might have clinical implications. Finally, it was noted that the comparison of wear factors and wear amounts from the present study (and other previous studies at Waterloo by Austin et al) with those of the very similar study by Scholes and Unsworth (Durham University, UK) suggested that their laboratory model might be overstating the wear of all-polymer PEEK articulations for potential applications in orthopaedic tribology. Also, the present study suggested that PEEK-on-PEEK articulations could carry loads for a certain number of cycles up to some threshold condition after which the surfaces could no longer resist the energy input without sustaining a severe and cascading damage. This situation may or may not occur in clinical application.

The CFR PEEK had a higher elastic modulus, smaller contact areas, higher contact stresses and lower wear than the unfilled PEEK. While this clearly made the CFR look better than the unfilled PEEK, the possibility of subsurface fatigue exposing carbon fibers to direct contact and wear remains a concern. If this concern is adequately addressed with further wear test data including simulator studies of the proposed design, then CFR is clearly better than unfilled PEEK for use in a spinal disc and facet arthroplasty device because its low wear under a variety of conditions.