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Magister Scientiae Dentium - MSc(Dent) / Restorative dentistry aims to replace lost or damaged tooth structure with durable and life-like alternatives. To accommodate the inherent limitations and weakness of the restorative materials, preparation techniques often require the sacrifice of healthy tooth structure to create enough restorative space. This can lead to weakening of the remaining tooth structure, with subsequent damage or catastrophic failure. When using indirect restoratives, the development of adhesive luting agents (adhesive cements) and stronger allporcelain restorations (lithium disilicate) has contributed to the development of “minimally invasive” preparation techniques and concepts such as cavity design optimization (CDO) and bio-substitution. With these techniques, resin materials are combined with ceramic restoratives in an attempt to not only produce strong restorations, but also increase the longevity of the remaining tooth. The clinician needs to therefore find the ideal preparation design that combine such materials to produces a clinically performing restoration while increasing the strength and longevity of the underlying tooth.

The research objectives are to develop experimental and computational techniques to characterize and to study the influence of polymer coating on the mechanical response of walnut shell particles to be used as proppants. E3-ESEM and Zeiss Axiophot LM are used to study the cellular microstructure and feasibility of polymer infiltration and uniform coating. Three main testing procedures; single particle compression, heating tests on coated and uncoated walnut shell particles and 3-point flexure tests are undertaken. In in-situ ESEM observations on both the coated and uncoated particles showed signs of charring at about 175 – 200 ºC. Single particle compression test are conducted with random geometry particles and subsequently with four distinct shape categories to minimize the statistical scatter; flat top, round top, cone top, and high aspect ratio. Single particle tests on uniformly cut cuboid particles from walnut shell flakes are used to capture the nonlinear material response. Furthermore cyclic compression loads are imposed on flat top particles which reveal that significant permanent deformation set in even at low load levels. Computational models include Hertzian representation, 2D and 3D finite element models to simulate single coated and uncoated particles under compression. The elastic material with geometric nonlinear representation is not able to simulate the compression response observed during testing. The inelastic material representation is able to significantly improve the compression response and address the influence of geometric shape on particle response. A single uniform layer of polymer coat is introduced on the 3D models with nonlinear material definition. Coating provides a marginal improvement in load vs displacement response of the particles while increasing the ability of the particle to withstand higher loads.

<p><font face="Times New Roman" size="4"><font face="Times New Roman" size="4"> <p align="left">The aim of this study is to investigate the repair bond strength of composite resin following micromechanical and chemical means of retention in improving the repair of composite resin specimens.</p> </font></font></p>

The research objectives are to develop experimental and computational techniques to characterize and to study the influence of polymer coating on the mechanical response of walnut shell particles to be used as proppants. E3-ESEM and Zeiss Axiophot LM are used to study the cellular microstructure and feasibility of polymer infiltration and uniform coating. Three main testing procedures; single particle compression, heating tests on coated and uncoated walnut shell particles and 3-point flexure tests are undertaken. In in-situ ESEM observations on both the coated and uncoated particles showed signs of charring at about 175 – 200 ºC. Single particle compression test are conducted with random geometry particles and subsequently with four distinct shape categories to minimize the statistical scatter; flat top, round top, cone top, and high aspect ratio. Single particle tests on uniformly cut cuboid particles from walnut shell flakes are used to capture the nonlinear material response. Furthermore cyclic compression loads are imposed on flat top particles which reveal that significant permanent deformation set in even at low load levels. Computational models include Hertzian representation, 2D and 3D finite element models to simulate single coated and uncoated particles under compression. The elastic material with geometric nonlinear representation is not able to simulate the compression response observed during testing. The inelastic material representation is able to significantly improve the compression response and address the influence of geometric shape on particle response. A single uniform layer of polymer coat is introduced on the 3D models with nonlinear material definition. Coating provides a marginal improvement in load vs displacement response of the particles while increasing the ability of the particle to withstand higher loads.

A major cause for dental resin composite restoration replacement is secondary caries attributed to Streptococcus mutans. Salivary esterases were shown to degrade resin composites. Hypothesis: S. mutans contain esterase activities that degrade dental resin composites and adhesives. Esterase activities of S. mutans were measured using synthetic substrates. Standardized specimens of resin composite (Z250), total-etch (Scotchbond-Multipurpose, SB), and self-etch (Easybond, EB) adhesives were incubated with S. mutans UA159 for up to 30 days. Quantification of a bisphenol-glycidyl-dimethacrylate (BisGMA)-derived biodegradation by-product, bishydroxy-propoxy-phenyl-propane (BisHPPP) was performed using high performance liquid chromatography. Results: S. mutans were shown to contain esterase activities in levels comparable to human saliva. A trend of increasing BisHPPP release throughout the incubation period was observed for all materials and was elevated in the presence of bacteria vs. control for EB and Z250 (p&lt;0.05) but not SB. Conclusion: biodegradation by cariogenic bacteria could compromise the resin-dentin interface and reduce the longevity of the restoration.

A major cause for dental resin composite restoration replacement is secondary caries attributed to Streptococcus mutans. Salivary esterases were shown to degrade resin composites. Hypothesis: S. mutans contain esterase activities that degrade dental resin composites and adhesives. Esterase activities of S. mutans were measured using synthetic substrates. Standardized specimens of resin composite (Z250), total-etch (Scotchbond-Multipurpose, SB), and self-etch (Easybond, EB) adhesives were incubated with S. mutans UA159 for up to 30 days. Quantification of a bisphenol-glycidyl-dimethacrylate (BisGMA)-derived biodegradation by-product, bishydroxy-propoxy-phenyl-propane (BisHPPP) was performed using high performance liquid chromatography. Results: S. mutans were shown to contain esterase activities in levels comparable to human saliva. A trend of increasing BisHPPP release throughout the incubation period was observed for all materials and was elevated in the presence of bacteria vs. control for EB and Z250 (p&lt;0.05) but not SB. Conclusion: biodegradation by cariogenic bacteria could compromise the resin-dentin interface and reduce the longevity of the restoration.

<p><font face="Times New Roman" size="4"><font face="Times New Roman" size="4"> <p align="left">The aim of this study is to investigate the repair bond strength of composite resin following micromechanical and chemical means of retention in improving the repair of composite resin specimens.</p> </font></font></p>

A major cause for dental resin composite restoration replacement is secondary caries attributed to Streptococcus mutans. Salivary esterases were shown to degrade resin composites. Hypothesis: S. mutans contain esterase activities that degrade dental resin composites and adhesives. Esterase activities of S. mutans were measured using synthetic substrates. Standardized specimens of resin composite (Z250), total-etch (Scotchbond-Multipurpose, SB), and self-etch (Easybond, EB) adhesives were incubated with S. mutans UA159 for up to 30 days. Quantification of a bisphenol-glycidyl-dimethacrylate (BisGMA)-derived biodegradation by-product, bishydroxy-propoxy-phenyl-propane (BisHPPP) was performed using high performance liquid chromatography. Results: S. mutans were shown to contain esterase activities in levels comparable to human saliva. A trend of increasing BisHPPP release throughout the incubation period was observed for all materials and was elevated in the presence of bacteria vs. control for EB and Z250 (p&lt;0.05) but not SB. Conclusion: biodegradation by cariogenic bacteria could compromise the resin-dentin interface and reduce the longevity of the restoration.

A major cause for dental resin composite restoration replacement is secondary caries attributed to Streptococcus mutans. Salivary esterases were shown to degrade resin composites. Hypothesis: S. mutans contain esterase activities that degrade dental resin composites and adhesives. Esterase activities of S. mutans were measured using synthetic substrates. Standardized specimens of resin composite (Z250), total-etch (Scotchbond-Multipurpose, SB), and self-etch (Easybond, EB) adhesives were incubated with S. mutans UA159 for up to 30 days. Quantification of a bisphenol-glycidyl-dimethacrylate (BisGMA)-derived biodegradation by-product, bishydroxy-propoxy-phenyl-propane (BisHPPP) was performed using high performance liquid chromatography. Results: S. mutans were shown to contain esterase activities in levels comparable to human saliva. A trend of increasing BisHPPP release throughout the incubation period was observed for all materials and was elevated in the presence of bacteria vs. control for EB and Z250 (p&lt;0.05) but not SB. Conclusion: biodegradation by cariogenic bacteria could compromise the resin-dentin interface and reduce the longevity of the restoration.