Glass-ceramic Flexural Strength after Hydrofluoric Acid and Unfilled Resin Treatment

poster abstractThe use of hydrofluoric (HF) acid is considered one of the most effective methods for achieving durable resin bond to glass-ceramics. Nonetheless, HF acid etching effect on glass-based ceramics strength remains uncertain and only a few contradictory studies have reported the influence of an unfilled resin (UR) application on the ceramic strength. Objectives: To investigate the effect of HF acid etching followed by silane and UR applications on the biaxial flexural strength of a nanofluorapatite glass-ceramic. Methods: 144 disc-shaped (15±1mm in diameter and 0.8±0.1mm in thickness) nanofluorapatite ceramic specimens were allocated into 12 groups: G1-control (no etching), G2-30s, G3-60s, G4-90s, G5-120s, G6-60s+60s. Meanwhile, G7-G12 were treated in the same fashion as G1-G6, but followed by silane and UR applications. Surface morphology of G1-G12 was assessed by scanning electron microscopy/SEM. The flexural strength was determined by biaxial testing per ISO 6872. Statistical analyses were two-way ANOVA and the Sidak multiple comparisons procedure (α=0.05). Additionally, Weibull statistics and finite element analysis (FEA) were carried out. Results: A significant effect of etching time (p=0.0290) on flexural strength was observed. G4 led to a significantly (p=0.0392) higher flexural strength than G1. Correspondingly, G10 revealed a considerably higher flexural strength than G7 (p=0.0392). Furthermore, flexural strength was significantly higher for G7-G12 than for G1-G6 (p<0.0001). For G1-G6, G4 showed the highest Weibull characteristic strength and G10 also presented the highest Weibull characteristic strength among G7-G12. FEA showed lower stress concentration in G7-G12 with the gradient stress supporting the fracture types of the biaxial test. Finally, the SEM data revealed that the HF acid etching affected the surface of ceramic specimens by generating pores and irregularities and more importantly that the UR was able to penetrate into the ceramic microstructure. Conclusions: HF acid etching followed by silane and UR applications enhanced the ceramic biaxial flexural strength

The Effect of Resection Angle on Stress Distribution after Root-End Surgery

Introduction: This study aimed to investigate the influence of the resection angle on the stress distribution of retrograde endodontic treated maxillary incisors under oblique-load application.  Methods and Materials: A maxillary central incisor which was endodontically treated and restored with a fiber glass post was obtained in a 3-dimensional numerical model and distributed into three groups according to type of resection: control; restored with fiber post without retrograde obturation, R45 and R90 with 45º and 90º resection from tooth axial axis, respectively and restored with Fuji II LC (GC America). The numerical models received a 45º occlusal load of 200 N/cm2 on the middle of lingual surface. All materials and structures were considered linear elastic, homogeneous and isotropic. Numerical models were plotted and meshed with isoparametric elements, and the results were analyzed using maximum principal stress (MPS). Results: MPS showed greater stress values in the bone tissue for control group than the other groups. Groups with apicectomy showed acceptable stress distribution on the fiber post, cement layer and root dentin, presenting more improved values than control group. Conclusion: Apicectomy at 90º promotes more homogeneity on stress distribution on the fiber post, cement layer and root dentin, which suggests less probability of failure. However, due to its facility and stress distribution also being better than control group, apicectomy at 45° could be a good choice for clinicians.Keywords: Apicectomy; Cut Angle; Dental Stress Analysis; Endodontics; Finite Element Analysis; Resection Angl

The importance of correct implants positioning and masticatory load direction on a fixed prosthesis

Through the biomechanical study of dental implants, it is possible to understand the dissipation effects of masticatory loads in different situations and prevent the longevity of osseointegration. Aims: To evaluate the microstrains generated around external hexagon implants, using axial and non-axial loads in a fixed four-element prosthesis with straight implants and implants inclined at 17°. Three implants were modeled using CAD software following the manufacturer?s measurements. Then, implants were duplicated and divided into two groups: one with straight implants and respective abutments, and the other with angled implants at 17° and respective abutments. Both groups were arranged inside a block simulating bone tissue. A simplified fixed prosthesis was installed on both groups and the geometries were exported to CAE software. Five loads of 300N were performed at axial and non-axial points on the fixed prosthesis. Stress on the implants and strain on the block were both analyzed. An in vitro experiment was performed following all structures made in FEA in order to validate the model. In each experimental block, 4 strain gauges were linearly placed between the implants and the same loads were repeated with a loading applicator device. The deformations computed by the gauges were correlated with the FEA results, showing that the group with inclined implants had more damaging biomechanical behavior and was significantly different from the group with straight implants (P<0.005). The mathematical model used is valid and inclined implants can induce unwanted bone remodeling

Influence of substrate design for in vitro mechanical testing

The goal of this study was to evaluate the influence of dental substrate simulator material, and the presence of root and periodontal ligament on the stress distribution in an adhesively-cemented monolithic crown. Five (5) 3D models according to the substrate simulator material and shape were modeled with CAD software for conducting non-linear finite element analysis (FEA): Tooth with and without periodontal ligament - subgroup ?pl? (groups Tooth+pl and Tooth-pl), machined tooth in epoxy-resin with and without pulp chamber - subgroup ?pc? (ER+pc and ER-pc) and simplified epoxy-resin substrate without pulp chamber and roots (SiER). Next, adhesively-cemented monolithic crowns in zirconia reinforced lithium silicate were modeled over each substrate. The solids were then imported in STEP format to the analysis software and the contact between teeth and cylinder was considered perfectly bonded; whereas, the contacts involving the resin cement were considered as non-separated. The materials were considered isotropic, linearly elastic, and homogeneous. An axial load (600 N) was applied to the occlusal surface and results of maximum principal stress (MPa) on the restoration were required. FEA revealed that all evaluated subtracts showed the crown intaglio surface as the most stressed region. The average stress and stress peaks were similar for restorations cemented onto Tooth+pl, Tooth-pl and ER+pc substrates, but, 13% higher in comparison to ER-pc and SiER substrates. Simplified substrates can be used to evaluate posterior full crown behavior without periodontal ligaments and roots, since the rigidity of the specimen is taken into account

Properties of type IV plaster considering variation in the mold/model position during setting stage

Objective: To assess the influence of the position of the mold during the setting stage of type IV stone plaster Durone (Dentsply Ind. Com., Rio de Janeiro, Brazil), on the following properties: surface hardness and roughness. Methods: For the roughness test, two groups (n=6) in the form of pellets were prepared. In the first group, the surface of the base of the device was turned down during the plaster setting stage (N), in the second group this position was inverted, which has been described as an act of capsize it (E). For analysis, a roughness meter with reading precision of 0.01 μm was used. With regard to the hardness analysis, two groups with conical-shaped samples were obtained. The plasters were left to set under the same conditions of the mold/model position described for the previous experiment. Hardness measurement was performed in a durometer with a spherical penetrating tip for Rockwell readout. Three measurements were performed for each test specimen in both tests. Results: The hardness (N - 39.8, standard deviation = 3.3, E - 30.8, standard deviation = 5.6) and roughness data (N - 0.67, standard deviation = 0.17, E - 0.74, standard deviation = 0.13) submitted to the Student’s-t test (5%) showed no statistically significant differences for the roughness test (0.489), but showed statistically significant differences for the hardness test (0.014). Conclusion: The variation in the mold/model position influenced the final characteristics of the specimens in terms of hardness, since those obtained with the capsize technique showed lower surface hardness, whereas for roughness these differences were not statistically significant

A study on stress distribution to cement layer and root dentin for post and cores made of CAD/CAM materials with different elasticity modulus in the absence of ferrule

To evaluate the stress distribution in a maxillary central incisor with different post and cores made of six CAD/CAM materials with different elastic modulus in the absence of ferrule using the finite element analysis. A three-dimensional endodontically treated maxillary central incisor restored with an all-ceramic crown was modelled in Rhinoceros (5.0 SR8, McNeel). The geometries were analyzed in ANSYS 17.2 (ANSYS Inc.) considering isotropic, homogeneous, linearly elastic materials with perfectly bonded contacts. The elastic moduli (E) of the post-and-cores defined the groups to be compared: nanoceramic resin (E=12.8GPa); composite resin (E=16GPa); hybrid ceramic (E=34.7GPa); lithium disilicate (E=95GPa); titanium (Ti-Al6-V4) (E=112GPa); and Y-TZP material (E=209.3GPa). The set was constrained in the cortical bone and loaded (45°/100 N) on the incisor palatine face. Stress distribution was analyzed by Maximum Principal Stress criteria for the crown-core cement line, Post-and-core?s cement line, Post-and-core system and Dentin. The stress distribution at the crown-core cement line (11.4 ? 13.2 MPa) was inversely proportional to the increase of the elastic modulus of the post-core approaches, while it was direct proportional on the post-and-core (4.7 ? 40 MPa) and cement line (4.1 ? 6.2 MPa). Stress distribution on the dentin was similar for all groups (24.7 - 25.3). Post-and-core made by CAD/CAM seems to be an efficient treatment alternative, since it is a conservative approach, promotes better aesthetic quality and it allows the control of the cement line thickness

Influence of cavosurface angle on the stress concentration and gaps formation in class V resin composite restorations

The study aimed to evaluate the influence of cavosurface angle on stress concentration and gap formation in class V restorations. Cylindrical cavities 3 mm in diameter were prepared in forty-five bovine incisors, changing only the angle of the bur in relation to the flat surface of the tooth. The cavities maintained the same volume (17.67 mm³). The samples were divided according to the cavosurface angle, into three groups (n = 15): 90°, 120°, 135°. After adhesive application (Futurabond U, VOCO), the cavity was filled with bulk placement of a resin composite (GrandioSO, VOCO). The teeth were analyzed with stereomicroscopy. Data of marginal gap formation were statistically analyzed with a one-way analysis of variance (ANOVA) followed by Tukey tests (significance level: α = 0.05). Finite element analysis (FEA) was used to study residual stress in these geometries and to correlate those stresses with experimentally measured gap formation. The elastic modulus and polymerization shrinkage were determined for FEA. Residual shrinkage stresses were expressed in maximum principal stress (MPS). There was a significant difference in the gap formation among the groups (p = 0.001). A significantly lower marginal gap formation was found for 120° and 135° angles, with no significant difference between them. The cavosurface angle at 90° caused substantially higher stresses, in the restoration interface, with greater marginal gap. For the 120° and 135° angles, the stress concentrations were smaller and were located in the dental structure. The cavosurface angle influenced the marginal gap formation and stress concentration

3D Finite Element Analysis of Rotary Instruments in Root Canal Dentine with Different Elastic Moduli

The aim of the present investigation was to calculate the stress distribution generated in the root dentine canal during mechanical rotation of five different NiTi endodontic instruments by means of a finite element analysis (FEA). Two conventional alloy NiTi instruments F360 25/04 and F6 Skytaper 25/06, in comparison to three heat treated alloys NiTI Hyflex CM 25/04, Protaper Next 25/06 and One Curve 25/06 were considered and analyzed. The instruments’ flexibility (reaction force) and geometrical features (cross section, conicity) were previously investigated. For each instrument, dentine root canals with two different elastic moduli(18 and 42 GPa) were simulated with defined apical ratios. Ten different CAD instrument models were created and their mechanical behaviors were analyzed by a 3D-FEA. Static structural analyses were performed with a non-failure condition, since a linear elastic behavior was assumed for all components. All the instruments generated a stress area concentration in correspondence to the root canal curvature at approx. 7 mm from the apex. The maximum values were found when instruments were analyzed in the highest elastic modulus dentine canal. Strain and von Mises stress patterns showed a higher concentration in the first part of curved radius of all the instruments. Conventional Ni-Ti endodontic instruments demonstrated higher stress magnitudes, regardless of the conicity of 4% and 6%, and they showed the highest von Mises stress values in sound, as well as in mineralized dentine canals. Heat-treated endodontic instruments with higher flexibility values showed a reduced stress concentration map. Hyflex CM 25/04 displayed the lowest von Mises stress values of, respectively, 35.73 and 44.30 GPa for sound and mineralized dentine. The mechanical behavior of all rotary endodontic instruments was influenced by the different elastic moduli and by the dentine canal rigidit

Marginal integrity of restorations produced with a model composite based on polyhedral oligomeric silsesquioxane (POSS)

Marginal integrity is one of the most crucial aspects involved in the clinical longevity of resin composite restorations.Objective To analyze the marginal integrity of restorations produced with a model composite based on polyhedral oligomeric silsesquioxane (POSS).Material and Methods A base composite (B) was produced with an organic matrix with UDMA/TEGDMA and 70 wt.% of barium borosilicate glass particles. To produce the model composite, 25 wt.% of UDMA were replaced by POSS (P25). The composites P90 and TPH3 (TP3) were used as positive and negative controls, respectively. Marginal integrity (%MI) was analyzed in bonded class I cavities. The volumetric polymerization shrinkage (%VS) and the polymerization shrinkage stress (Pss - MPa) were also evaluated.Results The values for %MI were as follows: P90 (100%) = TP3 (98.3%) = B (96.9%) >; P25 (93.2%), (

Effect of framework type on survival probability of implant-supported temporary crowns : an in vitro study

This in vitro study evaluated the effect of framework type on the survival probability of temporary implant-supported crowns and on the implant platform structure after dynamic fatigue. Thirty (30) external hexagon implants (3.75 x 10 mm) were embedded in acrylic resin following the ISO-14801. Standardized temporary crowns (n=10, N=30) were manufactured in acrylic resin and divided according to the framework type: Total plastic, Plastic with CoCr base and Titanium. The crowns were installed onto the implants (20N.cm) and fatigued (100N, 2 Hz) to determine the crowns? survival probability for missions of 300.000 and 600.000 cycles. Fatigue data were submitted to the Kaplan-Meier test followed by Wilcoxon and Log Rank, all with ? = 5%. The implant platforms were parametrically inspected based on the scanning before and after the fatigue to evaluate the damage. The strain values were analyzed using One-way ANOVA and Tukey test, all with ? = 5%. ANOVA revealed that the Total plastic showed less implant damage (-0.07 ± -0.03 mm) than the Plastic with CoCr base (-0.08 ± -0.04 mm) and the Titanium (-0.10 ± -0.01 mm) frameworks. Therefore, the framework type to manufacture implant-supported temporary crowns influences the fatigue survival of the restoration and the implant platform damage. The Plastic with CoCr base and Titanium frameworks showed superior reliability than the Total plastic framework which could not survive 600,000 cycles. The Plastic with CoCr base and the Titanium framework are suitable for restorations over 3 months in use, without a difference in the implant platform damage