Alkhouri, Nabih; (2020) Optimising novel dental composites for paediatric patients. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Since amalgam use to restore children`s teeth is no longer an option due to Minamata agreement and with the absence of a viable strong and easy to apply material, a need for a novel restoration has become essential. The aim of this research was therefore to develop a novel children’s composite that would enable a simpler and pain-free restorative method and be a feasible alternative for the difficult to place resin restorations and the weak glass ionomers. The initial eight formulations studied incorporated an adhesion promoting monomer, 4META (3wt%) within a base monomer, UDMA (72wt%) combined with low shrinkage diluent monomer PPGDMA (24wt%) and the photoinitiator camphorquinone (1wt%). This was mixed with a glass powder phase at two Powder/ Liquid ratios (PLR, 5:1 or 3:1). The glass filler contained different levels of two new novel additives to enable placement on caries affected dentine following minimal tooth excavation; antibacterial PolyLysine (PLS, 5 or 2wt%) and a remineralising agent Monocalcium Phosphate monohydrate, (MCP, 8 or 4wt%). Tests were performed to determine maximum levels of active agents that could be added whilst maintaining properties that would enable compliance with composite ISO and safety requirements. Studies included light curing kinetics, interaction of cured material with water (sorption and solubility, mass and volume changes, and release of agents) and mechanical properties (Flexural strength and modulus). These studies lead to a predicted optimal formulation with PLS of 4wt%, MCPM of 8wt% and PLR 3:1. The optimal formulation was large scale manufactured and the above studies repeated. Furthermore, testing of adhesion to tooth structure by means of microleakage, adaptation to cavity walls, shear bond strength and formation of resin tags within carious dentine was assessed. Finally, the ability to precipitate minerals and inhibit enzymatic activity at the adhesion interface was tested. Results showed that the proposed formulations were stable when aged at 60ºC for 6 months and had high monomer conversion (~>65%) when light-cured for 40s even with samples of 3mm thickness. Formulations of higher PLS and MCP content also exhibited higher water sorption and solubility values due to the hydrophilicity and release of these components. This was expected and observed to reduce the early mechanical properties, but the results showed that flexural strength and modulus levelled off after 3 months of soaking cured discs in water. The flowability of the paste and the PLS and MCP content were all positive factors in achieving better penetration and resin tags formation within the collagen mesh. This collagen mesh was a standardised caries-like model which was created during this study. The final optimised formulation results showed the material to be radio-opaque. It achieved >75% monomer conversion and was mechanically strong (120MPa biaxial flexural strength and 3.5GPa modulus). The water sorption and solubility values were just higher than the ISO recommended maximum values that were set for composites of non-releasing ability. The material also outperformed commercial comparators in terms of self-etching the enamel and adapting to the sound cavity walls. Furthermore, it also formed long and extensive tags (more than 200µm long) within collagen mesh (formic acid demineralised coronal tooth slices) and naturally carious dentine. Finally, the optimised material exhibited precipitation of minerals at the adhesion interface. Tags and mineral precipitation helped explain observation of matrix metalloproteinase (MMP) inhibition at the tooth / carious dentine interface. The results were used to support applications to MHRA for clinical trials and a notified body for CE mark. In conclusion, the research presented has achieved its aim to develop a marketable material that has the potential to address problems arising with amalgam ban.

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