Characterization and computational modelling of acrylic bone cement polymerisation

Abstract

Total joint replacement is one of the most successful surgical procedures and is a proven treatment for arthritis. Despite low failure rates, the wide application of the treatment means that large numbers of prostheses fail and must be revised. Improved pre-clinical testing methods for these orthopaedic devices may assist in developing new prostheses with improved clinical results. Computational modelling of biological systems is becoming increasingly accurate and is a much quicker and cheaper alternative to physical testing, but continued development is necessary to ensure computational models produce accurate and reliable predictions of implant behaviour. Acrylic bone cements have been used as a method of fixation for over 50 years but despite improvements in cement handling techniques and numerous attempts to improve the mechanical properties of the cement in other ways, the cement is often highlighted as the weak link in the joint replacement system. Aseptic loosening is cited as the cause for the majority of revision operations and cement degradation has been shown to be a contributor to the loosening process. In-vivo, cement is subject to cyclic loads and these are the primary cause of cement damage. Residual stresses generated during the polymerisation of the cement are now thought to play a significant role in cement failure. This thesis examines the development of residual stresses as a result of thermal and chemical changes during polymerisation of the cement. Experimental techniques for characterising the evolution of materials properties during the polymerisation reaction are discussed. Differential scanning calorimetry was used to measure the reaction variables such as the activation energy of polymerisation. The development of an ultrasonic rheometry technique for monitoring the mechanical property evolution within a bone cement specimen is discussed. Computational models were generated to predict the reaction behaviour of the cement in terms of the heat produced and the evolution of the physical properties of the curing mass. Some advantages and disadvantages of candidate mathematical models have been evaluated and are discussed, along with applications in several implant fixation scenarios.. The model compared well with experimental data and was used to predict thermal necrosis in the bone surrounding both a hip resurfacing implant and a knee replacement. Using the output reaction path produced by the thermal model a mechanical model was also produced simulating the shrinkage and mechanical property evolution exhibited by the polymerising cement. Two material models were compared with and without the effects of plasticity. Residual stress magnitudes were assessed in comparison with published values and showed better agreement when plasticity was included. Peak stresses were observed to occur during polymerisation. The location of the peak stresses were compared with experimental data on pre-load crack locations in the literature and showed good agreement.

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