Dynamic monitoring of refractive index change through light-activated dental fillings

The majority of dental patients prefer filling materials that match the aesthetics of their natural teeth. However, the durability of large 'white'-fillings (resin-based composites; RBCs) is generally considered to be inferior to the non-aesthetic 'silver'-filling types (dental amalgams). This may relate to the increased number of technique-sensitive application steps in addition to any inherent material shortcomings of RBCs.Modern RBC filling materials are supplied in a paste form. They are moulded directly by the dentist in the tooth cavity and subsequently set to a rigid material 'on command' with a high-intensity blue light. However, the RBC filling material has a limited depth of cure (approximately 2mm) and therefore the dentist must use multiple (incremental) layers to fill large cavities, unlike the use of dental amalgam whereby the cavity can be filled in bulk, reducing technique sensitivity, procedure time and cost. Consequently, the development of RBCs with improved depth of cure is highly desirable which may improve the shortcomings of such materials and ultimately improve its clinical success in order to benefit the patient community.There remain many unanswered questions relating to the change in optical properties of RBC filling materials as they harden under high-intensity light, which may provide a greater insight into the setting reaction to ultimately improve the depth of cure. By altering the components of the RBC, which include a resin matrix filled with reinforcing ceramic particles, it is possible to change the extent with which the curing light is transmitted through the material. This unique proposal offers, for the first time, the capacity to measure the change in optical properties throughout the hardening reaction of RBC materials, using optical coherence tomography (OCT). As the RBC hardens, the resin component becomes more dense and shrinks. Consequently the refractive index of the resin component increases (a property which changes the speed of light through the material). As the refractive index of the resin approaches that of the reinforcing filler particles (which remain constant throughout hardening) the amount of light scattering will decrease. The design of the OCT equipment in this proposal will measure the change in refractive index as the material hardens and will establish the relationship between refractive index change and volumetric shrinkage. The total amount of light scatter is also dependent on the size and shape of the filler particles and the absorption of chemical components within the resin responsible for hardening the material. Therefore, by testing RBCs with known composition (model formulations) a greater comprehension of the effect of refractive index change on hardening will provide a platform with which to improve the depth of cure and potentially the clinical durability of white-filling materials.This proposal aims to validate the use of a novel OCT technique to determine the change in optical properties of RBCs as they harden. Subsequently, the effect of material components (resin type and filler particle size and shape) on the change in refractive index in three-dimensions through the bulk of the RBC specimen can be established. This will facilitate a greater understanding of the setting characteristics, which ultimately has the potential to drive forward the development of light-activated RBCs.