Cure performance of light-activated composites by differential thermal analysis (DTA)

A technique which utilizes differential thermal analysis and a divided fibre optic system is a valuable aid to the study of the curing behaviour of lightactivated composites. Degree of cure and temperature rise can readily be evaluated as a function of irradiation time. Shade of material, thickness, sample temperature and the presence of porosity all have a significant effect on cure behaviour. The temperature rise for a 60 mm³ sample of material varies between 20°C and 40°C for the materials tested.     

Sterilization of complete dentures with sodium hypochlorite

It is important to give the patient a denture that is clean and free from cross-contamination. This study was made to determine if Clorox could be used as a rapid, safe, and clinically effective way to sterilize complete dentures. The data obtained from this study indicate that a 5-minute immersion of dentures in undiluted Clorox accomplished sterilization against a variety of microorganisms, including a spore-forming bacteria and C. albicans.     

Fibrous defect (nonossifying fibroma) of the mandible

Two cases of fibrous defect (nonossifying fibroma) of the mandible are presented. Each case occurred in the mandibular ramus of an 11-year-old girl. The usual fibrous defect of bone is an intraosseous fibrous, nonneoplastic, reactive lesion which appears as a well-delineated defect in the metaphysis of long bones. This report reviews the literature, histopathology, and classification of the fibrous defect of bone.     

Analyses of layer-thickness effects in bilayered dental ceramics subjected to thermal stresses and ring-on-ring tests.

OBJECTIVES: The purpose of this study was to analyze the stress distribution through the thickness of bilayered dental ceramics subjected to both thermal stresses and ring-on-ring tests and to systematically examine how the individual layer thickness influences this stress distribution and the failure origin. METHODS: Ring-on-ring tests were performed on In-Ceram Alumina/Vitadur Alpha porcelain bilayered disks with porcelain in the tensile side, and In-Ceram Alumina to porcelain layer thickness ratios of 1:2, 1:1, and 2:1 were used to characterize whether failure originated at the surface or the interface. Based on (1) the thermomechanical properties and thickness of each layer, (2) the difference between the test temperature and the glass transition temperature, and (3) the ring-on-ring loading configuration, the stress distribution through the thickness of the bilayer was calculated using closed-form solutions. Finite element analyses were also performed to verify the analytical results. RESULTS: The calculated stress distributions showed that the location of maximum tension during testing shifted from the porcelain surface to the In-Ceram Alumina/porcelain interface when the relative layer thickness ratio changed from 1:2 to 1:1 and to 2:1. This trend is in agreement with the experimental observations of the failure origins. SIGNIFICANCE: For bilayered dental ceramics subjected to ring-on-ring tests, the location of maximum tension can shift from the surface to the interface depending upon the layer thickness ratio. The closed-form solutions for bilayers subjected to both thermal stresses and ring-on-ring tests allow the biaxial strength of the bilayer to be evaluated.     

Application of analytical stress solutions to bi-axially loaded dental ceramic–dental cement bilayers

OBJECTIVE: Increased consideration of dental ceramics and dental cements as a singular structural unit for in-vitro mechanical testing has resulted in the reporting of a wide range of analytical methods to calculate the failure stresses. Therefore a comparison of observations between studies is complicated by the use of dissimilar stress solutions despite the employment of a similar testing methodology and specimen geometry. MATERIALS AND METHODS: Three analytical solutions to calculate failure stresses in bi-axially loaded dental ceramic-dental cement bilayers were appraised for a commonly utilized testing geometry (ball-on-ring). Clinically relevant datasets were generated from the bi-axial flexure testing of uncoated and dental cement coated aluminous core ceramic exposed to differing ceramic surface preparations. A Weibull statistical approach was utilized in order to provide insight into the impact of the analytical method on both the scale (sigma(0)) and distribution (m) of the failure stress data. RESULTS: Calculation of the bi-axial flexure stress utilizing Timoshenko's analysis resulted in an increase in sigma(0) for the uncoated (6%) and dental cement coated (11-12%) aluminous core ceramic, when compared with the bilayered solutions reported by Rosenstiel and Hsueh. However, the shape of the failure distributions illustrated by the consistency of m and associated 95% confidence intervals was not influenced by the analytical stress solution employed. SIGNIFICANCE: The choice of the analytical method chosen to calculate failure stresses in bi-axially loaded dental ceramic-dental cement bilayers will impact on the magnitude of the reported strength. Comparison between the failure stresses of uncoated and cement coated dental ceramics is more accurately represented by bilayer solutions, which account for the mismatch between the elastic constants of dissimilar materials. However, within the context of dental cement coated dental ceramics of clinically relevant dimensions, the choice of solution is unlikely to impact on the interpretation of the observations previously reported in the dental literature.     

Analysis of the degradation of a model dental composite

Dental composites undergo material property changes during exposure to the oral environment and may release compounds of potential toxicity, such as bisphenol A. Degradation of dental composites was studied in a simplified overlayer model in which bisphenol A diglycidyl methacrylate (BisGMA) was covalently bound to a porous silicon oxide surface. It was hypothesized that the chemical structure of this overlayer would allow release of bisphenol A, BisGMA, and the decomposition products thereof, upon exposure to water for an extended period. Liquid chromatography mass spectrometry found leaching of intact BisGMA and several degradation products that contained the bisphenol A moiety from the overlayer into distilled water after 2 wks of aging. The absence of bisphenol A release from the overlayer reduces concerns regarding its potential health risk in dental composites. Nevertheless, health concerns might arise with respect to BisGMA and the leached degradation products, since they all contain the bisphenol A moiety. Abbreviations: BisGMA, bisphenol A diglycidyl methacrylate; HPLC, high-performance liquid chromatography; LCMS, liquid chromatography mass spectrometry; MA, methacrylic acid; MPS, 3-(trimethoxysilyl) propyl methacrylate; m/z, mass-to-charge ratio; and TIC, total ion chromatogram.     

Marginal fit of 14-unit zirconia fixed dental prosthesis retainers

Summary  The marginal fit of 14-unit fixed dental prosthesis retainers and single-crown copings fabricated by the same computer-aided design (CAD)/computer-aided manufacturing (CAM) system were evaluated in vitro hypothesizing that the marginal opening might be independent of the type of restoration. Eight ivorine maxillary teeth (FDI locations 27, 25, 23, 21, 11, 13, 15 and 17) were prepared to accommodate a 14-unit prosthesis. Ten fixed dental prosthesis retainers and 40 single-crown-copings were fabricated using Zeno CAD/CAM on 20 master dies. Four cross-sections were made from each tooth and the marginal gap dimensions were measured. One-way anova was used to test the difference between the experimental groups and two-way anova and a post hoc test (Student–Newman–Keuls) were carried out to determine the influence of the location of the abutment tooth and the measurement location within the abutment tooth in the experimental groups ( P  < 0·05). Mean marginal gap dimensions and standard deviations for fixed dental prosthesis retainers and single-crown copings were 25 ± 29 and 13 ± 12 μm, respectively. The type of restoration showed a significant influence ( P  < 0·001) on the marginal gap. The location of the abutment tooth ( P  < 0·001) and the measurement location ( P  < 0·001) exhibited significant influence on marginal gaps of fixed dental prosthesis retainers while no influence on single-crown copings could be detected. The highest marginal gaps were found at the palatal surface of the incisor and canine of the 14-unit fixed dental prosthesis retainers. Fourteen-unit fixed dental prosthesis showed significantly higher marginal openings than single crowns fabricated under the same conditions. However, both restorations showed clinically acceptable marginal openings.