Eduardo Coutinho

Influence of joint component mechanical properties and adhesive layer thickness on stress distribution in micro-tensile bond strength specimens

OBJECTIVESnDifferences in mechanical properties between adherents and adhesive result in stress singularity at the free edge for tensile test specimens. Beyond causing premature failures, these also trigger fracture initiation at the bond edge, where the use of the maximum stress at failure as a measure of strength is not a reliable parameter for obtaining bond strength. The aim of this study is to describe the pattern of stress concentration along the interface for micro-tensile bond strength (microTBS) specimens. Also, possible sources of variation in stress concentration (adhesive layer thickness and mechanical properties) were evaluated using the finite element method.nnnMETHODSnOne-fourth models of typical microTBS specimens were created using axisymmetric elements. A 15MPa load was applied at the outer edge and finite element calculations were performed within the elastic deformation range. In the first set of runs, the adhesive thickness was varied and in the second set, different nu/E ratios between the adhesive and the adherent were assigned.nnnRESULTSnWhen adhesive thickness increased, a significant increase in stress concentration at the free edge and in the length of the stress-affected area was observed. Increasing the nu/E ratio between adhesive and adherent lead to an increase in stress concentration at the free edge area without change in length of the stress-affected area.nnnSIGNIFICANCEnIf mechanical properties and adhesive thickness expected for a specific composite-dentin bond combination in microTBS specimens are known, stress concentration at the interface can be predicted. The adhesive thickness should remain constant when comparing different joints, as it facilitates the interpretation of nominal strength values.

Mechanical properties and micro-morphology of fiber posts

OBJECTIVESnTo evaluate flexural properties of different fiber posts systems and to morphologically characterize their micro-structure.nnnMETHODSnSix types of translucent fiber posts were selected: RelyX Post (3M ESPE), ParaPost Taper Lux (Colthéne-Whaledent), GC Fiber Post (GC), LuxaPost (DMG), FRC Postec Plus (Ivoclar-Vivadent), D.T. Light-Post (RTD). For each post system and size, ten specimens were subjected to a three-points bending test. Maximum fracture load, flexural strength and flexural modulus were determined using a universal loading device (5848 MicroTester(®), Instron). Besides, for each system, three intact posts of similar dimensions were processed for scanning electron microscopy to morphologically characterize the micro-structure. The following structural characteristics were analyzed: fibers/matrix ratio, density of fibers, diameter of fibers and distribution of fibers. Data were statistically analyzed with ANOVA.nnnRESULTSnType and diameter of posts were found to significantly affect the fracture load, flexural strength and flexural modulus (p<0.05). Regarding maximum fracture load, it was found to increase with post diameter, in each post system (p<0.001). Regarding flexural strength and flexural modulus, the highest values were recorded for posts with the smallest diameter (p<0.001). Finally, structural characteristics significantly varied among the post systems tested. However, any correlation has been found between flexural strength and structural characteristics.nnnSIGNIFICANCEnFlexural strength appeared not to be correlated to structural characteristics of fiber posts, but it may rather be affected by mechanical properties of the resin matrix and the interfacial adhesion between fibers and resin matrix.

Influence of notch geometry and interface on stress concentration and distribution in micro-tensile bond strength specimens

OBJECTIVESnDescribe stress distribution and compare stress concentration factor (K(t)) for homogeneous micro-specimens with different notch geometries and stick-shaped homogeneous and bimaterial specimens by means of finite element (FE) analysis.nnnMETHODSnAxisymmetric models were created for homogeneous specimens with different notches and for stick-shaped homogeneous and bimaterial specimens. FE mesh was refined at areas of expected stress concentration and boundary conditions included an applied tensile stress in the axial direction. Linear elastic analysis was used.nnnRESULTSnFor hourglass homogeneous specimens, K(t) equaled 1.32 and 1.12 for a notch radius of 0.6mm and 3.3mm, respectively. A non-uniform axial (sigma(zz)) stress distribution was found in the notch cross-section, with values at the outer edge being 78% and 25% larger than at the center. In addition, a triaxial stress state was generated. Stick-shaped and dumbbell homogeneous specimens presented K(t)=1 and a uniform, uniaxial stress distribution along the entire cross-section. Shear stresses were zero for all homogeneous specimens. When an adhesive interface was added to the stick-shaped specimen, an area of localized axial stress concentration (K(t)=1.55) was detected at the bimaterial joint near the outer edge. Normal stresses sigma(rr) and sigma(thetatheta) and shear stress tau(zr) were also non-zero at the free-edge.nnnCONCLUSIONSnDumbbell or stick-shaped specimens are favored for muTBS testing, as they do not present stress concentrations due to geometry. However, dissimilar mechanical properties of joint components will lead to stress concentrations and non-uniform multi-axial stresses, although to a lesser extent.

3D-microleakage assessment of adhesive interfaces: exploratory findings by μCT.

OBJECTIVESnTo explore the feasibility of a 3D-microleakage protocol for the evaluation of various configurations of adhesive-tooth interfaces.nnnMETHODSnThree different kinds of specimens were prepared: (1) a Class-I composite restoration placed without any bonding to maximize gap formation at the interface; (2) a glass-fiber post cemented with a self-adhesive composite cement into a prepared root canal; and (3) inlay MOD composite restorations placed with either a 1-step self-adhesive or a 2-step etch-and-rinse composite cement. After silver-nitrate (AgNO3) infiltration, the specimens were scanned using a Skyscan 1172 X-ray microtomograph (μCT; Skyscan Bruker) at 100kV, 100μA and 7.8-9.5μm resolution. Projection images were reconstructed, after which maximum-intensity projections (MIPs) and 3D-volumetric renderings were produced. For the inlays, an additional conventional stereomicroscopic (SM) microleakage evaluation was accomplished after specimen sectioning.nnnRESULTSnMIPs and 3D-renderings from specimens (1) and (2) revealed strongly varying microleakage patterns along the marginal gap/interface. For the specimens of setup (3), the high radiopacity of the 2-step composite cement hindered evaluation of the MIPs. MIP-microleakage patterns along the enamel margin of the restoration cemented with the 1-step composite cement corresponded well to the stereomicroscopic images.nnnSIGNIFICANCEnThe reported μCT-protocol revealed good sensitivity to detect AgNO3 infiltration at the adhesive-tooth interface when considerable microleakage was present. When microleakage was less apparent and spread in a more diffuse pattern, evaluation with μCT was less sensitive compared to stereomicroscopic evaluation.

Microstructural analysis of demineralized primary enamel after in vitro toothbrushing

The aim of this study was to investigate, in vitro, the morphological characteristics of demineralized primary enamel subjected to brushing with a dentifrice with or without fluoride. In order to do so, 32 enamel blocks were divided in 4 different groups containing 8 blocks each. They were separately immersed in artificial saliva for 15 days. The experimental groups were: C - control; E - submitted to etching with 37% phosphoric acid gel (30 s); EB - submitted to etching and brushing 3 times a day with a non-fluoridated dentifrice; EBF = submitted to etching and brushing 3 times a day with a fluoridated dentifrice. The toothbrushing force was standardized at 0.2 kgf and 15 double strokes were performed on each block. After the experimental period, the samples were prepared and examined under SEM. The control group (C) showed a smooth surface, presenting scratches caused by habitual toothbrushing. The etched samples (E) exhibited different degrees of surface disintegration, but the pattern of acid etching was predominantly the type II dissolution. The brushed surfaces were smooth, with elevations which corresponded to the exposure of Tomes process pits and depressions which corresponded to interrod enamel. Particles resembling calcium carbonate were found in the most protected parts of the grooves. No morphological differences were observed between brushing with fluoridated (EBF) and non-fluoridated (EB) dentifrice. The results suggest that the mechanical abrasion caused by brushing demineralized enamel with dentifrice smoothes the rough etched surface, and the presence of fluoride does not cause morphological modifications in this pattern.

Titanium implant functionalization with phosphate‐containing polymers may favour in vivo osseointegration

AIMnOsseointegration of titanium implants is predictable, but can be improved via surface functionalization.nnnMATERIALS AND METHODSnOne hundred and twenty implants were installed in parietal bone of 12 domestic pigs and left to heal for 1 or 3xa0months. Five groups were defined according surface treatments: immersion in water (H2 O), 10% polyphosphoric acid (PPA10), 1% phosphorylated pullulan (PPL1), 10% phosphorylated pullulan (PPL10) or 10% phosphorylated pullulanxa0+xa01xa0μg bone morphogenetic protein-2 (PPL10 BMP). As primary outcome, implant osseointegration was evaluated by quantitative histology, namely peri-implant bone formation (B/T in %) and bone-to-implant contact (BIC in %) for each healing period. The Wilcoxon signed-rank test and Mann-Whitney U-test with αxa0=xa00.05 were performed.nnnRESULTSnPPL10 and PPA10 groups showed significantly higher B/T and BIC results than the control (H2 O) group at 1-month (pxa0<xa0.05). No significant difference was found between PPL1 and H2 O or between PPL10 BMP and H2 O, irrespective of healing time (1 or 3xa0months) or investigated parameter (B/T and BIC; pxa0>xa0.05). After 3xa0months, no experimental group showed a significant difference compared to the control group (H2 O) for both investigated parameters (B/T and BIC; pxa0>xa0.05).nnnCONCLUSIONnFunctionalizing titanium implants with inorganic or organic phosphate-containing polymers at 10xa0wt% concentration may stimulate peri-implant bone formation and implant osseointegration at early healing times.