Enhancing the efficacy of NovaMin® for Dentine hypersensitivity through particle size reduction and enhanced microbranch tubule mapping

Abstract

Dentine contains fluid-filled tubules, which carry stimuli from the outer enamel to the nerves in the pulp cavity through the movement of the fluid. Dentine hypersensitivity arises from the exposure of the tubules to the oral environment, through enamel loss or gum recession. Bioactive glasses such as NovaMin® can be incorporated into at-home dentifrices to occlude patent tubules with a hydroxyapatite-like (HA-like) layer and reduce symptoms. Dentine also contains a complex network of microbranches diverting from the main tubules. However, this has not been studied on a large scale around the tooth in 3D, meaning the influence of these microbranches on the tissue porosity, and therefore fluid flow between adjacent or nearby tubules, is unknown. The first objective of this project was to use Serial Block Face Scanning Electron Microscopy (SBF SEM) to observe the microbranches in 7 regions of interest in three teeth and quantify key morphological parameters. Tubule diameter and density decreased moving away from the pulp cavity. Microbranch diameter did not show any regional variation. Microbranch density was higher in areas of low tubule density. The network was extremely tortuous, with up to half of the microbranches per volume leaving the field of view. The microbranches significantly increased the tissue’s porosity. Another objective was to investigate the effect of NovaMin® particle size on tubule occlusion and the sensitivity-causing fluid flow; 5 diameters were chosen (2, 4, 7, 10 and 15 µm), plus a NovaMin®-free and an artificial saliva control. The mechanical properties of the HA-like layer were tested using nanoindentation, and the crystallographic properties using Glancing Angle X-Ray Diffraction (GA-XRD). 28 bovine dentine samples (n=4 per group) were brushed twice daily for 5 days. Samples were indented while fully hydrated, and the hardness and reduced modulus calculated. Neither were found to significantly change with changing particle size. The 26 °2θ peak of the GA-XRD spectra was compared. The average crystallite size did not significantly vary between particle sizes. The level of tubule occlusion after 24 and 48 hours was investigated by brushing samples of human dentine twice daily, then imaging with SBF SEM. The smallest particle sizes showed the highest levels of surface occlusion, and the greatest depth of penetrated material into the tubules. Finally, the reaction rate of NovaMin® powder of each particle size was compared by reacting with artificial saliva for 1, 4, 8, 12, 24 and 36 hours, then investigating with X-Ray Diffraction. At ≤12 hours, the smallest particle sizes were the most reactive, showing sharper apatite peaks. However, by the 24- and 36-hour timepoints, the difference between groups reduced until there was no significant difference. The crystallite size was compared at 12, 24 and 36 hours, with no trend. In conclusion, reducing the particle size of NovaMin® increased the reactivity of the bioglass. It had no significant effect on the mechanical or crystallographic properties of the HA-like surface layer formed by the dentifrice. It did however result in greater tubule occlusion and penetration of material into the tubules after only 24 and 48 hours. SBF SEM was a valuable tool for investigating both the tubule occlusion and the microbranch network within the tissue, allowing their regional properties to be quantified and compared. This work has proven the capabilities of this novel technique, providing a high throughput of high-resolution 3D images, beyond the resolution offered by traditional volume imaging techniques, such as micro computed tomography.

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Identifiers

ORCID for Bethany Harding: orcid.org/0000-0002-8871-3683

ORCID for Richard Cook: orcid.org/0000-0002-2468-5820

ORCID for Katy Rankin: orcid.org/0000-0002-8458-1038

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