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Title: Micromechanical characterisation of fatigue failure in acrylic bone cement
Author: Shearwood-Porter, Natalie
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 2013
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Acrylic bone cement has been used for fixation of load-bearing orthopaedic implants for over five decades, and continues to be the 'gold standard' for elderly patients and those with systemic disease. Aseptic loosening remains a major indication for revision of cemented hip implants, and has been associated with mechanical degradation of the cement mantle via the initiation and coalescence of fatigue micro-cracks. Microstructural defects such as voids and agglomerates of radiopacifier particles have been implicated in this damage accumulation process. Improved understanding of the relative effects of these features on the mechanisms of fatigue crack initiation and failure within the cement is required in order to inform the development of more robust cement formulations and thus increase the longevity of the cement mantle in vivo. The present study utilised micro-computed tomography (μ-CT) and scanning electron microscopy (SEM), in conjunction with mechanical testing, to provide a systematic, quantitative assessment of the effect of cement formulation and microstructure (including voids and radiopacifiers) on the in vitro fatigue failure of four commercial, vacuum-mixed cement formulations. Results were compared with μ-CT data and fractographic analysis of an ex vivo cement specimen. This novel 'data rich' methodology enabled non-destructive, three-dimensional analysis of defect populations in terms of the size, morphology and spatial density of individual microstructural features, and the identification and characterisation of crack-initiating defects. The inclusion of barium sulphate as a radiopacifier was found to have a negative effect on the fatigue life of cement; radiopacifier particles showed a tendency to form numerous large agglomerates, which readily initiated fatigue cracks; furthermore, fatigue life scaled consistently with initiating defect size. In contrast, cement containing zirconium dioxide as a radiopacifier demonstrated superior fatigue performance, and failure in these cement formulations was dominated by crack initiation from voids. In all four cement formulations, void populations were found to be bi-modal, and the largest voids (> 0.5 mm equivalent spherical diameter) were surrounded by secondary satellite voids in both in vitro and ex vivo cement specimens. Extensive void formation was also noted in both moulded specimens and cement mixing gun stubs, in addition to ex vivo cement. Optimisation of cement formulations and vacuum-mixing techniques may therefore be advantageous in order to reduce the formation of barium sulphate agglomerates and large voids, and thus minimise their potential crack initiation effects in vivo.
Supervisor: Browne, Martin ; Sinclair, Ian Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QM Human anatomy ; R Medicine (General) ; RD Surgery ; TJ Mechanical engineering and machinery