Total ankle replacement (TAR) is an alternative to ankle arthrodesis (AA), which consists of replacing the degenerated joint with a mechanical motion-preserving alternative. TAR is still not considered as clinically successful as hip and knee replacements, with approximately 45-91% survivorship at 15 years. This is primarily due to implant loosening, which has been associated with wear-mediated osteolysis. The development of a successful TAR has been further restricted by limited pre-clinical testing and biomechanical analyses compared to hip and knee joints. The research in this thesis aimed to assess the biomechanical and wear performance of a third generation mobile-bearing TAR.
The wear performance of the BOX® ankle (MatOrtho Ltd, Leatherhead, UK) was determined through a series of four implant and simulator parameter based studies: (1) implant size; (2) accelerated artificial ageing; (3) simulator type; and (4) simulator input profiles. Different sizes of the device were evaluated in two types of mechanical knee simulators (pneumatic and electromechanical) for up to 5 million cycles (Mc), under varying loads and kinematics (University of Leeds and ISO 22622:2019), aiming to replicate an ankle gait cycle. Gravimetric measurements of polyethylene wear were taken every Mc, while simulator input following, topographic changes, and visual damage wear modes were also reported. No statistically significant differences in the mean wear rate were determined between implant sizes, simulator type or simulator input profiles, with each wear rate comparing well to the ankle simulator literature. The artificially aged inserts exhibited increased wear rates during the steady-state wear phase, but showed no indication of structural failure. The biomechanical performance of the implant was investigated through three-dimensional gait analysis. A BOX® ankle patient cohort (n = 6) was compared against AA patients (n = 9) and a mixed TAR patient group (n = 6). A multi-segment foot model (MFM) was used to calculate peak and ROM kinematics of the hindfoot, midfoot and forefoot segments, while kinetic and kinematics parameters were determined for the hip and knee joints. Spatiotemporal parameters and patient reported outcome measures were also assessed. The range of hindfoot sagittal motion was greater in both TAR cohorts compared to the AA cohort, while the AA patients displayed hypermobility at the distal foot segments. The outcome of this study further emphasizes the clinical relevance of using a MFM and suggests surgical decision making should consider the effect of treatment options on the distally located foot joints.
To summarise, the thesis highlighted the influence of a variety of implant and simulator parameters on TAR wear simulation, alongside the improved biomechanical function of TAR when compared to AA. The study provided a useful benchmark for future TAR wear simulations, which should aim to develop a clinically relevant wear simulation protocol, under a variety of activities of daily living, to further understand the biomechanical and wear performance of a TAR.
