Main, KAI; (2013) Development of Composites for Bone Repair. Doctoral thesis , UCL (University College London).

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Abstract

Current bone repair materials can be too stiff compared to native bone and with insufficient strength for load bearing applications often with brittle fracture behaviour. In addition they can have unacceptably high polymerization shrinkage and decline in strength with time. This thesis describes the development of chemical cure, silica reinforced dimethacrylate composites and discusses their viability as alternatives for bone repair. The intended applications of these composites include hip fracture screw augmentation and vertebroplasty. Characterization and comparison of cement materials was central to this thesis. The handling of the materials was assessed qualitatively via repeated delivery and mixing whilst the rate and degree of cure was determined from Fourier Transform Infrared (FTIR) spectroscopy. This cure data was used with composition to calculate and compare the volumetric shrinkage of materials. Biaxial testing gave the fracture behaviour, flexural strength and Young’s modulus of cements. The properties of several existing bone cement materials (including composite cements Cortoss and Comp06 and PMMA cements Palacos R and Simplex P) were compared using the techniques above. Subsequently a variety of (di)methacrylate monomers, silica fillers and silica and polymer fibres were screened, and monomer and filler systems selected for further testing. The levels of each component in the selected systems were varied systematically and the effects on trends in mechanical properties were quantified. The interaction between level of initiator and the chosen monomer system was investigated and the polymerization reaction described in terms of kinetic theory. It was found that PPG DMA, combined with UDMA and HEMA monomers, and silane treated silica glass particles and fibres could produce materials that cured more extensively than existing bone cements (69 – 97 % methacrylate conversion compared to 86 % and 64 % for Palacos and Cortoss respectively). A PPG DMA composite achieved a flexural strength of 93 MPa that was maintained even after 20 weeks storage in hydrated conditions, unlike Cortoss which declined in strength by 40 % following hydrated storage. Maintenance of good mechanical properties is essential for load bearing applications. This PPG DMA composite had a stiffness of 2.2 GPa, whilst PMMA cements and Cortoss had Young’s moduli 1.6 and 3.4 GPa respectively. Moreover these materials were found to perform well in-vivo in a lapine femoral condyle model.

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