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Abstract: Objectives: The main objective of this study was to assess the feasibility of using X-ray microcomputed tomography (μCT) as a viable technique for accurate measurements of composite volumetric changes due to polymerization. Methods: μCT, a non-destructive 3D imaging technique that measures the intensity of content at each voxel, was used to determine the volume of experimental and commercial polymeric dental composites before and after photopolymerization. The experimental composites consisted of various mass ratios of 2,2-bis(4-(2-hydroxy-3-methacryloxypropoxy)-phenyl]propane (BisGMA) and triethylene glycol dimethacrylate (TEGDMA) filled with various amounts of barium boroaluminosilicate glass and fumed amorphous silica. Polymerization shrinkage was determined using μCT and shrinkage calculated based on density measurements. Results: μCT was successfully used to calculate volumetric shrinkage of dental composites. Results agreed well with shrinkage obtained via density measurements of the same sample. The addition of radio opaque filler was necessary to achieve sufficient contrast between the sample and background. Appropriate image analysis procedures were needed to obtain the volumes of sample specimens. Significance: μCT enables the characterization of polymerization shrinkage under clinically relevant conditions. In particular, the sample geometry and the physical state (liquid or solid) are flexible. The current test allowed multiple samples to be measured at the same time. In addition, the ability to accurately measure the size and shape of the object will allow multiple properties to be determined simultaneously. [Copyright &y& Elsevier]

Abstract: Objectives: The increased usage of composite dental restorations underscores the need for continued improvements in material properties. Well-controlled sample fabrication and reproducible methods to quantify and compare material properties will accelerate material design and optimization. Our objectives were to fabricate samples encompassing a range of processing parameters and develop techniques that systematically quantify multiple properties of these samples, thus reducing sample-to-sample variation while concurrently testing numerous processing conditions. Methods: Gradient samples were prepared to evaluate the effects of composition and irradiation time. Comonomer ratio of 2,2-bis[p-2′-hydroxy-3′-methacryloxypropoxy]-phenyl]propane (BisGMA) and triethylene glycol dimethacrylate (TEGDMA) was varied discretely, and irradiation time was varied continuously across each composition. Degree of conversion was measured using infrared spectroscopy, mechanical properties were evaluated using nanoindentation, and cell viability and density were quantified using fluorescence microscopy. Results: Higher BisGMA contents increased elastic moduli while higher TEGDMA contents increased conversions. Cell response depended only on irradiation time and not composition, with conversions of at least 52% and 60% required for unaffected viability and cell density, respectively. A single composition–irradiation combination to achieve all of the ‘best’ properties (highest conversion, highest elastic modulus, lack of cytotoxicity) was not identified, illustrating the necessity of testing all combinations for multiple relevant properties. Significance: Simultaneously screening composition and conversion increased the experimental throughput and allowed for the quantification of chemical, mechanical, and biological properties in a controlled, reproducible fashion. This 2D gradient approach is useful for optimizing compositions and processing parameters to achieve the desired combination of properties. [Copyright &y& Elsevier]

Abstract: Objectives: Previous studies have developed calcium phosphate and fluoride releasing composites. Other studies have incorporated chlorhexidine (CHX) particles into dental composites. However, CHX has not been incorporated in calcium phosphate and fluoride composites. The objectives of this study were to develop nanocomposites containing amorphous calcium phosphate (ACP) or calcium fluoride (CaF2) nanoparticles and CHX particles, and investigate Streptococcus mutans biofilm formation and lactic acid production for the first time. Methods: Chlorhexidine was frozen via liquid nitrogen and ground to obtain a particle size of 0.62μm. Four nanocomposites were fabricated with fillers of: nano ACP; nano ACP+10% CHX; nano CaF2; nano CaF2 +10% CHX. Three commercial materials were tested as controls: a resin-modified glass ionomer, and two composites. S. mutans live/dead assay, colony-forming unit (CFU) counts, biofilm metabolic activity, and lactic acid were measured. Results: Adding CHX fillers to ACP and CaF2 nanocomposites greatly increased their antimicrobial capability. ACP and CaF2 nanocomposites with CHX that were inoculated with S. mutans had a growth medium pH>6.5 after 3 d, while the control commercial composites had a cariogenic pH of 4.2. Nanocomposites with CHX reduced the biofilm metabolic activity by 10–20 folds and reduced the acid production, compared to the controls. CFU on nanocomposites with CHX were three orders of magnitude less than that on commercial composite. Mechanical properties of nanocomposites with CHX matched a commercial composite without fluoride. Significance: The novel calcium phosphate and fluoride nanocomposites could be rendered antibacterial with CHX to greatly reduce biofilm formation, acid production, CFU and metabolic activity. The antimicrobial and remineralizing nanocomposites with good mechanical properties may be promising for a wide range of tooth restorations with anti-caries capabilities. [Copyright &y& Elsevier]

Abstract: Objective: Use X-ray microcomputed tomography (μCT), to test the hypothesis that composite shrinkage and sites of potential leakage in human teeth are non-uniformly distributed and depend on cavity geometry and C-factor. Methods: Two holes of equal volume but different dimensions were drilled into the exposed dentin of extracted human molars. The cavities were filled with composite and teeth were scanned, before and after curing, using μCT. Three-dimensional (3D) reconstructions of the data were prepared and analyzed using image analysis software. Results: 3D reconstructions showed that cavity geometry did not affect the polymerization shrinkage. The shrinkage for all restorations was 2.66±0.59%, and cavity dimensions did not affect the volume lost, either in quantity or location on the sample. Potential leakage sites were identified by gap formations and found to be non-uniformly distributed along the tooth–composite interface. Leakage in regions calculated by μCT was confirmed by visualization of sectioned samples with confocal laser scanning microscopy. Significance: μCT evaluation will add tremendous value as part of a suite of tests to characterize various properties of dental materials. The non-uniform distribution of potential leakage sites about the cavities that was determined by μCT emphasizes the inadequacy of traditional methods of determining leakage, which are capable of analyzing only limited areas. Additionally, μCT evaluation can produce quantitative analyses of shrinkage and leakage, compared to the conventional methods, which are qualitative or semi-quantitative. Finally, experimentally determined shrinkage and leakage of composite in extracted teeth agrees with the results of similar experiments in model cavities, confirming the validity of those models. [Copyright &y& Elsevier]