Text

theses

Dental caries are a global oral health disease caused by acidogenic bacterial on teeth whereby acid causes demineralization, loss of ions, from the tooth into the saliva. The resulting cavities can be filled with light-cured, composite, resin restorations which are less toxic than amalgam, less expensive than gold and the resin can match the natural tooth color. However, composite resins tend to suffer from polymerization shrinkage after exposure to light, this can result in a gap forming between the adhesive layer at the base of the composite restoration and the tooth surface. The gap occurs due to the combination of polymer shrinkage and weak bonding between the adhesive layer and dental tissue, this then permits microleakage and eventually secondary caries to develop.

A Dental adhesive system acts as the key intermediate layer between a composite resin restoration and the tooth tissues (enamel and dentin). Ideally, the adhesive is able to penetrate into a roughened, porous enamel surface and into open tubules in dentin, hence forming a strong interlocking bond call a hybrid layer. The properties of the hybrid layer depend on the chemical composition of the adhesive system, including the monomers, photointiators, and additives, as well as its mechanical and rheological properties.

The aim of this thesis was to develop a new photo-cured dental adhesive with improved rheological and mechanical properties to enable a good interlocking hybrid layer and reduce gap formation. Firstly, commercially available products, including self-etch and three-step adhesive systems, were investigated, applied to cow teeth, and evaluated. Graphene and hydroxyapatite were blended with the three-step adhesive as an additive in an attempt to improve performance of the hybrid layer. Secondly, a new adhesive system was developed composed of high and low molecular weight monomers, diurethane dimethacrylate (UDMA) and methyl methacrylate (MMA), respectively. Thirdly, graphene and hydroxyapatite were added in different weight percentages to UDMA-MMA blends to optimize the mechanical properties of the photo-cured adhesive system. The viscosity, mechanical properties (using nanoindentation and tensile test), and degree of conversion (using FTIR and micro-Raman) were characterized for the UDMA-MMA blends with and without additives. Samples were imaged using optical microscopy, helium ion microscopy, and scanning electron microscopy, and surface topography was viewed using AFM. Thermography was used to monitor curing rate and temperature during curing.

For the commercial adhesive systems, results indicate that gap formation occurs between the adhesive and tooth tissues, which is related to the weak adhesive and hybrid layers. For the UDMA-MMA blends, results indicate that the viscosity and mechanical properties may be tuned and optimized to improve adhesion. For example, low weight % UDMA in MMA provides low viscosity, which allows penetration into tubules in the dentin and enamel forming a good hybrid layer while still providing good mechanical properties. For UDMA-MMA blends addition of even a small weight % of graphene or hydroxyapatite gave results indicating that mechanical properties are significantly increased while viscosity can be tailored for the application. The enhanced mechanics are due to changes in bonding when the additives are present. Graphene provided a more significant increase in mechanical properties than hydroxyapatite as the additive. It is concluded that UDMA-MMA blends or UDMA-MMA blends with suitable additives are potentially a new and improved option for photo-cured dental adhesive system with the potential to avoid gap formation and microleakage.

ETD_11078

Ph.D.

Includes bibliographical references

ETD doctoral

doi:10.7282/t3-mqda-7j03

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