Bridging Outgrowths: Development of a Tribological Test Module for Cervical Facet Joint Evaluation

Zygapophyseal joints are derived from the Greek zygos, meaning bridge, and physeal, meaning outgrowth, "the bridging of outgrowths" or facet joints. These bridging outgrowths are located on either side of the vertebra, creating a three-joint complex in the spine and the intervertebral disc. The primary role of these joints is limiting motion and carrying 25 % of the axial loads under nondegenerative conditions. Following their degeneration, first-line therapy includes painkillers, steroids, radio-frequency denervation, muscle therapy and physical therapy, often providing short-term solutions. For more severe cases, the gold standard therapy is fusion, limiting motion while altering the spine biomechanics. Current conceptualisations indicate that limited research was conducted on relieving pain through minimally invasive methods such as facet joint resurfacing devices. Similarly, there has been limited work on safety assessments using essential tribological testing methods under conditions representing complex spine environments. Although it is well established that high frictional moments result in clinical failure as they cause the displacement of the uncemented hip implants. International regulatory bodies’ guidelines for wear assessment of facet joint replacements focus on the lumbar spine, leaving cervical spine wear or friction testing unexplored. Furthermore, no advanced testing rigs are available globally. This thesis focuses on the tribology of minimally invasive, motion-preserving cervical facet joint replacements. A target-profile analysis was conducted to design facet joint replacement surfaces for advanced tribological testing essential to implant safety assessment. A ball-in-socket joint replacement surface of Low-Carbon Cobalt-Chrome-Molybdenum (LC CoCrMo)and GUR1050 Ultra-High Molecular Weight Polyethylene (UHMWPE) was designed and manufactured for advanced friction and wear testing. As there are no devices presently, this study offers the opportunity to consider what these joint replacement devices may entail. It thus enables a subsequent pre-clinical testing protocol to be prescribed to test the tribology of these devices from a design specification perspective that will inform the manufacture of a novel testing rig. Thus, investigations were completed to define/inform the design specifications for simple sliding tests. Simple sliding tests were performed using LC CoCrMo-on-UHMWPE pin-on-plate configuration to identify the friction force and torque transducer sensing range requirements for a facet joint replacement friction and wear simulator. These tests were conducted under 0.65, 2.3 and 3.9 MPa pressures and 1,3 and 10 mm stroke lengths, inherent to the ball-in-socket design’s Hertzian pressures and stroke lengths. The friction tests were completed under two different lubricants. Osteoarthritis-oriented synovial fluid was used to mimic the synovial fluid of a patient with osteoarthritis (31 g/L protein), while the diluted bovine serum (with 20 g/L protein content) as per ASTM F2694-16. The X-ray photoelectron spectroscopy analysis quantified the differences between surface interactions on LC CoCrMo and lubricants with different protein contents outside and in the sliding area. An UHMWPE particle isolation method was developed; the short-term testing wear debris was isolated from diluted bovine serum and Phosphate Buffered saline-based osteoarthritis-oriented synovial fluid. The chemical content of the final solution was assessed using advanced characterisation methods: Fourier transform infrared and Energy Dispersive X-ray. The influence of the cleaning procedure using the ultracentrifugation method on reducing the endotoxin concentration was investigated. The stroke length, lubricant and axial load influence on the particle size, roundness and aspect ratio were analysed. A novel tribological testing module was designed and manufactured to accommodate the facet joint resurfacing samples and F/T transducer. The testing allowed the crucial design specification of the tribological test rig to be defined and subsequently manufactured. Such modules measuring tribological characteristics of facet joint replacements are not commercially available globally. The module can measure friction force and torques under static and dynamic loads during flexion-extension and axial rotation. The friction factor was 0.14 ± 0.003 under 200 N axial loading during ± 7.5 flexion-extension for dry conditions. Misalignment error may stem from manufacturing tolerances (0.01-0.1 mm) estimated to vary from 0.9 to 9.1 % of the frictional torques generated under 200 N axial load. The research presented in this thesis has enabled comprehension of the requirements for designing motion-preserving surfaces for cervical facet joint implants and comparing the design requirements of the cervical FJ implants with those of other commercially available joint replacements. The findings from designing the facet joint replacements and conducting simple sliding tests informed the development of a tribological testing rig, enabling safety assessments of future implants. The initial results indicated that misalignment has three times more impact on the generated frictional torques than the hip implants. Furthermore, the author developed a novel particle isolation method and isolated UHMWPE particles from two types of lubricants, which was used to reveal the link between tribological testing parameters and particle size and shape.