Physicochemical and Microbiological Assessment of an Experimental Composite Doped with Triclosan-Loaded Halloysite Nanotubes.

This study is aimed at evaluating the effects of triclosan-encapsulated halloysite nanotubes (HNT/TCN) on the physicochemical and microbiological properties of an experimental dental composite. A resin composite doped with HNT/TCN (8% w/w), a control resin composite without nanotubes (HNT/TCN-0%) and a commercial nanofilled resin (CN) were assessed for degree of conversion (DC), flexural strength (FS), flexural modulus (FM), polymerization stress (PS), dynamic thermomechanical (DMA) and thermogravimetric analysis (TGA). The antibacterial properties (M) were also evaluated using a 5-day biofilm assay (CFU/mL). Data was submitted to one-way ANOVA and Tukey tests. There was no significant statistical difference in DC, FM and RU between the tested composites (p > 0.05). The FS and CN values attained with the HNT/TCN composite were higher (p < 0.05) than those obtained with the HNT/TCN-0%. The DMA analysis showed significant differences in the TAN δ (p = 0.006) and Tg (p = 0) between the groups. TGA curves showed significant differences between the groups in terms of degradation (p = 0.046) and weight loss (p = 0.317). The addition of HNT/TCN induced higher PS, although no significant antimicrobial effect was observed (p = 0.977) between the groups for CFUs and (p = 0.557) dry weight. The incorporation of HNT/TCN showed improvements in physicochemical and mechanical properties of resin composites. Such material may represent an alternative choice for therapeutic restorative treatments, although no significance was found in terms of antibacterial properties. However, it is possible that current antibacterial tests, as the one used in this laboratory study, may not be totally appropriate for the evaluation of resin composites, unless accompanied with aging protocols (e.g., thermocycling and load cycling) that allow the release of therapeutic agents incorporated in such materials.

Bond strength of universal adhesives to air-abraded zirconia ceramics.

The bond strength of universal adhesives to air-abraded zirconia ceramic was evaluated. Overall, 40 zirconia ceramic blocks with dimensions of 6 × 6 × 4 mm were cut from pre-sintered blanks. The sintered blocks were embedded in self-cured acrylic resin. The zirconia blocks were then randomly allocated to four groups (n = 10) in which different universal adhesives were used, except for the control group in which no universal adhesive was used. A silicon mold was used to build the resin cement. All specimens were stored in distilled water for 24 h at 37°C and mounted on a universal testing machine. They were then subjected to shear bond strength testing at a cross-speed of 0.5 mm/min until failure occurred. The failure modes were analyzed using a digital microscope at 50× magnification. Univariate one-way analysis of variance and Tukey's post-hoc test were used for statistical analysis. Compared with the control group, the groups with universal adhesives showed statistically significant differences (P < 0.05). In addition, there was no statistically significant difference in the bond strengths of the groups with universal adhesives (P > 0.05) . After 24 h of storage, the cementation bond to air-abraded zirconia ceramic was improved by the application of a universal adhesive.

A method for calculating the compliance of bonded-interfaces under shrinkage: validation for Class I cavities.

OBJECTIVE: The compliance for tooth cavity preparations is not yet fully described in the literature. Thus, the objectives were to present a finite element (FE) method for calculating compliance and to apply this to peak shrinkage stress regions in model cavities restored with resin-composite. METHODS: Three groups of FE-models were created, with all materials considered linear, homogeneous, elastic and isotropic: (a) a pair of butt-joint bonded cubic prisms (dentin/resin-composite), with dentin of known compliance (0.0666 µm/N). Free ends were fixed in the Z-axis direction. A 1% volumetric shrinkage was simulated for the resin-composite. Mean displacements in the Z direction at each node at the dentin-resin interface were calculated and divided by the sum of normal contact forces in Z for each node. (b) A series of more complex restored cavity configurations for which their compliances were calculated. (c) A set of 3D-FE beam models, of 4 mm × 2 mm cross-section with lengths from 2 to 10mm, were also analyzed under both tensile and bending modes. RESULTS: The compliance calculated by FEM for the butt-joint prisms was 0.0652 µm/N and corresponded well to the analytical value (0.0666 µm/N). For more accurate representations of the phenomenon, such as the compliance of a cavity or any other complex structure, the use of the displacement-magnitude was recommended, as loading by isotropic contraction also produces transversal deformations. For the beam models, the compliance was strongly dependent upon the loading direction and was greater under bending than in tension. SIGNIFICANCE: The method was validated for the compliance calculation of complex structures subjected to shrinkage stress such as Class I 'cavities'. The same FEM parameters could be applied to calculate the real compliance of any interface of complex structures. The compliance concept is improved by considering specific load directions.

A method to investigate the shrinkage stress developed by resin-composites bonded to a single flat surface.

OBJECTIVES: To purpose a method for predicting the shrinkage stress development in the adhesive layer of resin-composite cylinders that shrink bonded to a single flat surface, by measuring the deflection of a glass coverslip caused by the shrinkage of the bonded cylinders. The correlation between the volume of the bonded resin-composite and the stress-peak was also investigated. METHODS: A glass coverslip deflection caused by the shrinkage of a bonded resin-composite cylinder (diameter: d=8 mm, 4 mm, or 2 mm, height: h=4 mm, 2 mm, 1 mm, or 0.5 mm) was measured, and the same set-up was simulated by finite element analysis (3D-FEA). Stresses generated in the adhesive layer were plotted versus two geometric variables of the resin-composite cylinder (C-Factor and volume) to verify the existence of correlations between them and stresses. RESULTS: The FEA models were validated. A significant correlation (p<0.01, Pearson's test) between the stress-peak and the coverslip deflection when the resin-composites were grouped by diameter was found for diameters of 2 and 4 mm. The stress-peak of the whole set of data showed a logarithmic correlation with the bonded resin-composite volume (p<0.001, Pearson's test), but did not correlate with the C-Factor. SIGNIFICANCE: The described method should be considered for standardizing the stress generated by the shrinkage of resin-composite blocks bonded to a single flat surface.

Finite element analysis of bonded model Class I 'restorations' after shrinkage.

OBJECTIVES: The C-Factor has been used widely to rationalize the changes in shrinkage stress occurring at the tooth/resin-composite interfaces. Experimentally, such stresses have been measured in a uniaxial direction between opposed parallel walls. The situation of adjoining cavity walls has been neglected. The aim was to investigate the hypothesis that: within stylized model rectangular cavities of constant volume and wall thickness, the interfacial shrinkage-stress at the adjoining cavity walls increases steadily as the C-Factor increases. METHODS: Eight 3D-FEM restored Class I 'rectangular cavity' models were created by MSC.PATRAN/MSC.Marc, r2-2005 and subjected to 1% of shrinkage, while maintaining constant both the volume (20 mm(3)) and the wall thickness (2 mm), but varying the C-Factor (1.9-13.5). An adhesive contact between the composite and the teeth was incorporated. Polymerization shrinkage was simulated by analogy with thermal contraction. Principal stresses and strains were calculated. Peak values of maximum principal (MP) and maximum shear (MS) stresses from the different walls were displayed graphically as a function of C-Factor. The stress-peak association with C-Factor was evaluated by the Pearson correlation between the stress peak and the C-Factor. RESULTS: The hypothesis was rejected: there was no clear increase of stress-peaks with C-Factor. The stress-peaks particularly expressed as MP and MS varied only slightly with increasing C-Factor. Lower stress-peaks were present at the pulpal floor in comparison to the stress at the axial walls. In general, MP and MS were similar when the axial wall dimensions were similar. The Pearson coefficient only expressed associations for the maximum principal stress at the ZX wall and the Z axis. SIGNIFICANCE: Increase of the C-Factor did not lead to increase of the calculated stress-peaks in model rectangular Class I cavity walls.

Sequential software processing of micro-XCT dental-images for 3D-FE analysis.

OBJECTIVES: The aim was to describe a sequential software processing of mu-XCT molar-images for 3D-FE tooth/restoration model geometries based on a representative molar tooth, giving attention on each step of data-processing. This paper first gives an overview of a sequential processing and then applies the resulting model to the particular case. METHODS: An intact mandibular molar was scanned using a micro-XCT instrument (1072, SkyScan, Belgium) in which 960 slices were obtained. Sixty-three non-adjacent bitmap slices were then optimally selected for model-creation. Enamel/dentin boundaries were clarified, for each slice, using image control-system software (ScanIP, Simpleware), generated a file which was sequentially converted into a mesh in a reconstruction software (ScanFE, Simpleware) and posteriorly converted into a STL-file (triangulated-2D-stereolithography). This was imported into a FE-software package (Patran, MSC.Software, USA) and all elements were re-meshed. From these elements, surfaces were created and exported to another FE-software (Hypermesh, Altair Hyperworks) to build the dental-cavities. Finally, the volumetric-mesh was created and the model was imported back to FE-software to apply the boundary-conditions, material-properties and initiate post-processing (using Patran and Marc, MSC Software). To demonstrate the use of the resulting model, this was applied to the particular case of a Class I restoration subjected to distributed loading. The analysis was performed as linear and structural and outputs of maximum principal (MP) and maximum shear (MS) stresses were then evaluated. RESULTS: A 3D-model of a mandibular molar was processed without generating errors in the FE-package used. The maximum deviation between the tooth and the model was less than 0.1%. Stress concentrations were found at the surface where the load was applied and in the vicinity of the tooth-composite interface. SIGNIFICANCE: The described procedure is a successful method able to produce a highly detailed 3D finite element model of restored molar teeth with any cavity configuration and combination of restorative materials and this method can also be used for other biological or biomaterials applications.

Colour-stability and gloss-retention of silorane and dimethacrylate composites with accelerated aging.

OBJECTIVES: To evaluate the colour-stability and gloss-retention of silorane versus dimethacrylate composites exposed to accelerated aging from daylight radiation. METHODS: Five disc-shaped specimens of photo-cured resin-composites were prepared and manually polished for each material (Filtek Silorane, Herculite XRV, Tetric Evoceram and QuiXfil). Colour and gloss were evaluated before and after periods (baseline, 24, 72, 120 and 192 h) of accelerated photo-aging in xenon light following ISO 7491:2000. Colour measurements were performed with a colourimeter according to the CIE-Lab colour-space. The colour change (DeltaE) for each time was calculated. The surface gloss was measured using a glossmeter. Results were evaluated using one-way ANOVA and Tukey tests (alpha=0.05). Correlations between logtime, DeltaE and gloss were evaluated using Pearson's correlation (alpha=0.05). RESULTS: Materials generally decreased in L and a and increased in b. The strong exception was Filtek Silorane which maintained a and b. DeltaE was found to be a positive linear function of logtime for all materials. Materials varied in the magnitude and rate of increase of DeltaE with logtime: QuiXfil>Tetric EvoCeram>(Filtek Silorane>or=Herculite XRV). DeltaE remained<3.3 for Filtek Silorane and Herculite XRV. Gloss was found to be a negative linear function of logtime. Gloss was maximal in the sequence: Filtek Silorane approximately Tetric EvoCeram>Herculite XRV>QuiXfil. CONCLUSIONS: Silorane gave the best overall performance in stability over time, compared to a set of representative dimethacrylate composites.

The suitability of different FEA models for studying root fractures caused by wedge effect.

Finite element analysis (FEA) utilizing models with different levels of complexity are found in the literature to study the tendency to vertical root fracture caused by post intrusion ("wedge effect"). The objective of this investigation was to verify if some simplifications used in bi-dimensional FEA models are acceptable regarding the analysis of stresses caused by wedge effect. Three plane strain (PS) and two axisymmetric (Axi) models were studied. One PS model represented the apical third of the root entirely in dentin (PS-nG). The other models included gutta-percha in the apical third, and differed regarding dentin-post relationship: bonded (PS-B and Axi-B) or nonbonded (PS-nB and Axi-nB). Mesh discretization and material properties were similar for all cases. Maximum principal stress (sigma max) was analyzed as a response to a 165 N longitudinal load. Stress magnitude and orientation varied widely (PS-nG: 10.3 MPa; PS-B: 0.8 MPa; PS-nB: 10.4 MPa; Axi-B: 0.2 MPa; Axi-nB: 10.8 MPa). Axi-nB was the only model where all sigma max vectors at the apical third were perpendicular to the model plane. Therefore, it is adequate to demonstrate the tendency to vertical root fractures caused by wedge effect. Axi-B showed only part of the sigma max perpendicular to the model plane while PS models showed sigma max on the model plane. In these models, sigma max)orientation did not represent a situation where vertical root fracture would occur due to wedge effect. Adhesion between post and dentin significantly reduced sigma max.

Influence of local factors on composite shrinkage stress development--a finite element analysis.

PURPOSE: Using finite element analysis (FEA), to determine the nominal shrinkage stress of a composite under different restriction conditions defined by the longitudinal compliance (LC) and C-factor (C) of the testing system, and by the elastic modulus of the bonding substrate (E). MATERIALS AND METHODS: Eight axisymmetric models representing an experimental setup used to determine composite shrinkage stress were simulated. Composite thicknesses of 0.5 mm and 4 mm were tested, defining different C and volumes (C = 6 and vol = 14 mm3 or C = 0.8 and vol = 113 mm3, respectively). The E of the substrate was tested in two levels, 12 GPa and 207 GPa. Two LC values (1 x 10(-6) or 28 x 10(-6) mm/N) were defined for each E value by varying the length of the rods used as bonding substrate (0.3 mm and 9.5 mm for E = 12 GPa; 6.0 mm and 163.9 mm for E = 207 GPa). Materials were considered elastic, homogeneous, and isotropic. Shrinkage was simulated by thermal analogy. Nominal stress (nodal force/cross-sectional area) was calculated for each condition. Results were analyzed using Taguchi's method. RESULTS: Nominal stress values varied between 1.7 MPa and 30.3 MPa. The main variables were statistically significant (LC: p = 0.0046; C: p = 0.0153; E: p = 0.0155), as well as the LC x E interaction (p = 0.0354). Stress reduction between low and high LC was more pronounced for E = 207 GPa compared to E = 12 GPa. Stress was lower for the high C conditions for both compliance levels. CONCLUSION: Not only the C-factor of the testing assembly, but also its LC and the E of the bonding substrate influence stresses generated by composite shrinkage.