A computational investigation of the long-term performance of bioabsorbable magnesium-based implants in orthopaedic fracture applications
Quinn, Conall
Quinn, Conall
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Publication Date
2023-06-20
Type
Thesis
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Abstract
Current orthopaedic fixation devices made from stainless steel and titanium can lead to a range of long-term complications that include bone reabsorption due to stress shielding, implant loosening and/or infection. Bioabsorbable magnesium-based metals offer a promising alternative to permanent metallic devices due to their comparable mechanical properties to the native tissue. These devices have the potential to support the fracture region during the healing period, while subsequently being absorbed once their functional load-bearing role has been completed. However, the non-uniform and localised pitting corrosion mechanisms associated with magnesium-based alloys presents significant design challenges when it comes to predicting in-vivo performance. The objective of this thesis was to develop a computational framework to investigate the short- and long-term performance of bioabsorbable magnesium fixation devices in orthopaedic fracture applications. This thesis presents the development, implementation and integration of multiple computational algorithms that describe (i) the mechanobiological aspects of bone tissue repair and (ii) non uniform surface-based corrosion processes of degradable magnesium-based implants. Firstly, a coupled framework for bone fracture repair was developed using the finite element method to predict both short-term fracture healing and long-term remodelling phases of the tissue repair. Additionally, an enhanced surface-based phenomenological magnesium corrosion framework was developed to robustly predict the spatiotemporal features of non-uniform galvanic and pitting corrosion. Finally, a fully integrated computational model was developed, whereby the framework for bone tissue repair was coupled with the enhanced surface-based magnesium corrosion model to predict the long-term performance of bioabsorbable magnesium fixations devices. Through this combined framework, the model was used to investigate the fracture healing and remodeling performance of a tibial fracture model in the presence of both permanent titanium and bioabsorbable magnesium fixation plates. Through the development of an enhanced surface-based corrosion framework, this thesis provided a robust and flexible framework that could predict the phenomenological features of non-uniform pitting corrosion in magnesium-based materials. This model was fully implemented in three dimensions and provided a significant advance on existing models as it enabled the prediction of intricate features of corrosion such as multi-directional pitting and a wide range of pit morphologies. The corrosion model also eliminated unwanted effects of mesh and model parameter sensitivity. In parallel, a coupled framework for bone fracture repair was used to investigate the short- and long-term performance of permanent fixation devices in tibia fracture model for a range of loading conditions and fracture gap sizes over a period of 12 months. It was found that the introduction of a titanium plate stabilised the fracture region resulting in accelerated fracture healing. However, in the longer-term, the high mechanical properties of titanium relative to native bone disrupted the normal stress distribution within bone resulting stress shielding proximal to the fixator and stress concentrators at interfacial boundaries, causing bone reabsorption, increasing the likelihood of implant loosening and failure. By combining both algorithms, the performance of magnesium fixation devices for long-bone fracture repair was investigated. The results showed that fixation was only required to provide mechanical stability to the fracture region for approximately the first 30 days for successful fracture union to occur. Model corrosion rate and pit severity heavily influenced the mechanical support provided by the corroding magnesium fixator. While the stiffer titanium fixator slightly outperformed the less rigid magnesium fixator regarding fracture healing, magnesium fixation displayed highly favourable outcomes regarding long-term bone remodelling, displaying no stress shielding or interfacial damage-based reabsorption. The computational methods and outcomes developed within this thesis form a platform for the future design and optimisation of magnesium fixation devices.
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NUI Galway