Surgical interventions for the treatment of chronic neck pain, which affects 330 million people globally, include fusion and cervical total disc replacement (CTDR). Most of the currently clinically available CTDRs designs include a metal-on-polymer (MoP) bearing. Numerous studies suggest that MoP CTDRs are associated with issues similar to those affecting other MoP joint replacement devices, including excessive wear and wear particle-related inflammation and osteolysis.
The aim of this study was to investigate the biotribology of a novel metal-on-metal (MoM) design of cervical total disc replacement device in its pristine form and coated with chromium nitride or silicon nitride, in order to understand the influence of loading conditions upon the tribological performance of the implant, and to investigate biological effects of the wear debris produced by the implants. To achieve this, a series of studies were carried out.
Chromium nitride and silicon nitride coatings have been characterised for their mechanical properties, chemical composition and surface finish. Whilst some of the experiments showed minor differences between the mechanical properties and adhesion of the coatings, there was no indication of significant differences between the chromium nitride and silicon nitride coated samples.
Functional testing in the six-station spine wear simulator showed that MoM CTDRs produced wear volumes significantly lower than those of the commercially available MoP devices. The wear volumes were reduced further by three-fold, following testing under altered ISO-18192-1:2011 kinematics, whereby, reduced ranges of motions were applied. Whilst the silicon nitride coated CTDRs failed catastrophically early in the test, chromium nitride coated CTDRs produced an eight-fold reduction in wear volumes, when compared to the pristine devices tested under the same conditions.
Investigation of potential biological effects of the particles generated in wear testing showed that that high concentrations (5-50µm3 per cell) of CoCrMo particles resulted in significant reduction of cell viability of the L929 fibroblast cells, but not the dural fibroblasts, which were used in this study. No ceramic coating particles, at any concentrations, caused significant reduction of cell viability.
In summary, results presented in this thesis showed that whilst the MoM CTDR device exhibited significantly lower wear rates than those of the commercially available MoP devices, the cytotoxic wear particles could potentially lead to adverse biological reactions, particularly in patients with metal hypersensitivity, and lead to devastating consequences similar to those of failed MoM THRs. Currently, the consequences of similar failure, leading to metalosis or pseudotumour formation in the vicinity of the spinal cord are unknown. During the investigation of the ceramic coatings, it was also found that chromium nitride ceramic coating could not only lower wear rates further, but it also has the potential to reduce the cytotoxic potential of the wear particles.
