Alqadi, SF; (2016) Effects of replacing calcium phosphate with strontium phosphate in remineralising and antibacterial releasing dental composites. Doctoral thesis , UCL (University College London).

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Abstract

Introduction: The main cause of dental composite restoration failure is recurrent caries. This occurs due to micro-gap formation upon polymerisation shrinkage allowing bacterial penetration and continuing demineralization of dental tissue beneath the restoration. The aim of this study was to compare the effect of replacing calcium phosphate with strontium phosphate on water sorption induced expansion, surface hydroxyapatite layer formation and release of two different antimicrobial agents from new composite materials being developed to address these issues. Methods: Composite pastes were prepared by mixing dimethacrylate monomers with dental glass filler at 3:1 ratio. The glass contained 10 wt% chlorhexidine diacetate (CHX), 10 wt% mono-calcium phosphate monohydrate, 10 wt% tricalcium or tri-strontium phosphate and PLS (2 or 10 wt %). Monomer conversion of the experimental composite was assessed using Fourier transform Infra-red Spectroscopy. The change in mass and volume of light cured composite discs were determined in water and simulated body fluid (SBF) at different time points. Surface microstructure and chemistry were assessed using Scanning Electron (SEM), Raman Microscopy and EDX. CHX and PLS release was measured through UV spectroscopy. The biaxial flexural strength of the experimental composites (BFS) and modulus of elasticity were assessed up to one month of water storage. Results: Monomer conversion of all the dental formulations was between 70.6% and 76.2% and higher than 61.1% for Z250. Higher monomer conversions were obtained with higher PLS content. In all formulations MCPM played an essential role in promoting water sorption and subsequent diffusion of the remineralising ions and antimicrobial agents from the set materials. Effects of replacing TCP by TSrP on water sorption were minimal. Addition of PLS at higher percentages, however, encouraged greater water sorption and more release of CHX. A calcium phosphate layer precipitated on the composite surfaces that were soaked in SBF. The Ca/P ratio was close to that of mineral in dentine. Whilst high levels of antibacterial and remineralising agents may be beneficial for the prevention of recurrent caries the levels must be optimised to ensure mechanical properties are not severely compromised. Conclusion: To sum up, this work proved that replacing TCP by TsrP had limited effect on the properties of dental composites investigated in this thesis. Conversely, addition of MCPM and different percentages of PLS played a more important role in generating a remineralising and antibacterial releasing dental composite; however high percentages of antimicrobial drugs decreased the strength of the material dramatically.

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