Abstract
Magnesium (Mg) and its alloys were initially investigated as resorbable orthopaedic biomaterials more than a century ago. However, their use was abandoned due to rapid corrosion resulting in mechanical failure and excess hydrogen production. Whilst recent advances in manufacturing processes have led to a renewed interest in the field of Mg based biomaterials, the corrosion behaviour remains the feature that is the most difficult to control and characterise. In particular, the development of these materials is hindered by an inability to use in vitro techniques to accurately predict the behaviour of Mg alloys in an in vivo environment. Furthermore, the biocompatibility of the materials is often underemphasised in favour of the assessment of corrosion behaviour. The purpose of this thesis was therefore two-fold. Firstly, the aim was to develop and optimise a series of both in vitro and in vivo methodologies that would allow the prediction of corrosion rate, and the assessment of the biocompatibility of Mg materials. Secondly, these methods would be used to identify Mg alloys that could be investigated for use as orthopaedic biomaterials.
The initial portion of this study focussed on the development of an in vitro immersion method that could be used to cautiously predict the in vivo corrosion rates of Mg alloys. The in vitro protocol involved the use of three different solutions with varying degrees of physiological relevance, the results from which were compared to the corrosion occurring in an in vivo subcutaneous environment. The solution providing the most comparable corrosion rates was identified and subsequently utilised to select four slowly corroding Mg alloys for further investigation.
Preliminary analysis of the biocompatibility of these four alloys was carried out using an in vitro cell culture technique. The results of this investigation indicated that the use of a closed cell culture environment is likely to reduce cellular viability due to the increased pH and osmolality associated with rapid Mg corrosion. Accordingly, it was concluded that whilst cell culture techniques can provide results indicative of the degree of biocompatibility, they must be used in conjunction with the results of in vivo investigations for a comprehensive understanding of the true biocompatibility of Mg based materials. Consequently, an in vivo soft tissue study was undertaken involving the implantation of the four Mg alloys in both intermuscular and intramuscular locations in Lewis rats. This investigation indicated minimal hydrogen production associated with the Mg alloys, and biocompatibility equivalent to that seen with the implantation of a biomedical Ti alloy (Ti-6Al-4V).
The results from both the in vitro and in vivo studies identified Mg-1Zn and Mg-0.4Ca as the alloys exhibiting optimal characteristics for further investigation as orthopaedic implant materials. An intraosseous study was carried out involving the implantation of these alloys into the shaft and epiphyseal region of the ovine tibia. The results indicated both alloys were biocompatible and corrosion resistant, indicating their potential as promising resorbable orthopaedic biomaterials.