Efeito de diferentes protocolos de envelhecimento e do sistema de fresagem na rugosidade superficial de uma cerâmica de zircônia

The zirconia has found wide application in dentistry due to its high mechanical strength and excellent aesthetic properties. However, the surface degradation suffered in hydrothermal environment is of great concern. Purpose: To evaluate the influence of milling and aging protocols on the surface roughness of ceramic zirconia system. Methods: A standard metal cast was made to prepared standardized zirconia crowns systems (N=40). The copings were divided into 8 groups (n=5), in accordance with the factors "aging" and "milling system" (CAD / CAM-Neodent and MAD / MAM Zirkonzahn) ": CADct and MADct (control groups); CADaut and MADaut (autoclave); CADph and MADph: pH cycling during eight days and CADtc and MADtc: thermal cycles between 5° and 55°C (12,000 cycles). After treatments, the surface roughness (Ra) was obtained through a digital optical profiler and analysis of x-ray diffraction was performed on two samples of each group (n=16). The data (nm) obtained were statistically analyzed by 2- Ways ANOVA. Results: aging protocols (p=0.5255) and milling systems (p=0.1126) did not influence the surface roughness of ceramic zirconia. Mean roughness (nm) for the experimental groups (CADct, MADct, CADaut, MADaut, CADph, MADph, CADtc and MADtc) were as follows: 1.0620A, 1.0780A, 1.0280A, 0.8060A, 0.8460A, 1.0340A, 1.020A, 1,0220A. Conclusion: The aging methods that simulate the oral environment and milling systems did not influenced the values of surface roughness.A zircônia encontrou ampla aplicação na Odontologia devido à sua alta resistência mecânica e excelentes propriedades estéticas. No entanto, a degradação superficial sofrida em ambiente hidrotérmico é de grande preocupação. Objetivo: Avaliar a influência do sistema de fresagem e de protocolos de envelhecimento sobre a rugosidade superficial da cerâmica de zircônia. Metodologia: Um molde metálico fundido foi fabricado para se confeccionar os copings de zircônia padronizados para os dois sistemas de fresagem (N=40). Os copings foram divididos em 8 grupos (n=5), de acordo com os fatores: "envelhecimento" e "sistema de fresagem" (CAD/CAM-Neodent e MAD/MAM Zirkonzahn)": CADct e MADct (grupos controles); CADaut e MADaut (autoclave); CADph e MADph: ciclos de pH durante 8 dias. E, CADtc e MADtc: termociclagem entre 5° e 55°C (12.000 ciclos). Após os tratamentos, a rugosidade superficial (Ra) foi obtida através de um perfilômetro óptico digital e a análise de difração de raios-x foi realizada em duas amostras de cada grupo (n=16). Análise de Dados: Os dados (nm) obtidos foram analisados estatisticamente por ANOVA – 2 fatores. Resultados: Os protocolos de envelhecimento (p=0,5255) e os sistemas de fresagem (p=0,1126) não influenciaram significativamente a rugosidade superficial da cerâmica de zircônia. A média de rugosidade (nm) para os grupos experimentais (CADct, MADct, CADaut, MADaut, CADph, MADph, CADtc e MADtc) foram respectivamente: 1.0620A, 1.0780A, 1.0280A, 0.8060A, 0.8460A, 1.0340A, 1.020A, 1,0220A. Conclusão: Os envelhecimentos que simulam o meio bucal e os sistemas de fresagem não modificaram a rugosidade superficial da zircônia

Evaluation of shear bond strength and shear stress on zirconia reinforced lithium silicate and high translucency zirconia

This study evaluated the shear stress distribution on the adhesive interface and the bond strength between resin cement and two ceramics. For finite element analysis (FEA), a tridimensional model was made using computer-aided design software. This model consisted of a ceramic slice (10x10x2mm) partially embedded on acrylic resin with a resin cement cylinder (Ø=3.4 mm and h=3mm) cemented on the external surface. Results of maximum principal stress and maximum principal shear were obtained to evaluate the stress generated on the ceramic and the cylinder surfaces. In order to reproduce the in vitro test, similar samples to the computational model were manufactured according to ceramic material (Zirconia reinforced lithium silicate - ZLS and high translucency Zirconia - YZHT), (N=48, n=12). Half of the specimens were submitted to shear bond test after 24h using a universal testing machine (0.5 mm/min, 50kgf) until fracture. The other half was stored (a) (180 days, water, 37ºC) prior to the test. Bond strength was calculated in MPa and submitted to analysis of variance. The results showed that ceramic material influenced bond strength mean values (p=0.002), while aging did not: YZHT (19.80±6.44)a, YZHTa (17.95±7.21)a, ZLS (11.88±5.40)b, ZLSa (11.76±3.32)b. FEA results showed tensile and shear stress on ceramic and cylinder surfaces with more intensity on their periphery. Although the stress distribution was similar for both conditions, YZHT showed higher bond strength values; however, both materials seemed to promote durable bond strengt

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

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

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

The impact of restorative material and ceramic thickness on CAD\CAM endocrowns

Endocrown restorations as a conservative approach to restore endodontically treated teeth still need in vitro investigation under fatigue and made in different materials. This study evaluated the effect of restorative material and restoration thickness on the maximum fracture load of endocrowns subjected to cyclic loading. Sixty (60) third molar teeth received an endocrown preparation with three different heights of remaining dental tissue (1.5, 3.0 or 4.5 mm). A leucite-based ceramic (LEU) and a lithium disilicate (LD) based ceramic were selected to manufacture the CAD/CAM endocrown restorations, totaling 6 groups (n=10). The specimens were subjected to fatigue loading (200N, 2 x 106 cycles, water) and then to the single load to failure test (1 mm/min crosshead speed). Data were analyzed by using two-way ANOVA and Tukey tests (p< 0.05). All endocrowns survived the fatigue test. The thickness did not influence the restoration?s fracture load (p=0.548) instead the restorative material (p=0.003). LD showed higher mean values (1714.43 N)A than LEU (1313.47 N)B. Endocrowns manufactured with CAD/CAM lithium disilicate blocks showed superior fracture load than the leucite-based blocks after mechanical fatigue. Nevertheless, both materials presented acceptable survival and fracture load as long as the material?s minimum thickness and the enamel adhesion are respected

Stress distribution of complete-arch implant-supported prostheses reinforced with silica-nylon mesh

This study evaluated the presence of a silica-nylon mesh and two cantilever lengths on the biomechanical behavior of complete-arch implant-supported prostheses. Twenty-four (24) complete mandibular arch implant-supported prostheses were divided into 4 groups according to the presence of reinforcing mesh (with or without) and the cantilever length (molar ? 15 mm or premolar ? 5 mm). The specimens were submitted to strain gauge analysis (30-kgf, 10 s) at different points (molar and premolar). Three-dimensional models were created based on the in vitro specimens, and the results in the bone (microstrain), prostheses (tensile stress), implants and prosthetic screws (von-Mises stress) were evaluated using the finite element method (FEM). All materials were considered homogeneous, isotropic and linear. Strain gauge data were submitted to 3-way analysis of variance and the Tukey test (?=.05). FEM results were qualitatively analyzed using colorimetric graphs. The microstrain magnitude for the prostheses with reinforcement was 519.91±359 and 583.33±661 without reinforcement (p=.001). The microstrain values for loading on the molar was 867.49±784 and on the premolar was 235.75±145. FEM corroborated with the in vitro findings for the bone behavior. The load application in the premolar showed reduced stress concentration, and a significant difference was observed between the presence or absence of the reinforcement for the prostheses. Silica-nylon mesh reduced the peri-implant microstrain and the prosthesis stress regardless of the cantilever extension. For temporary complete-arch implant-supported prostheses, the limitation of the cantilever to the premolar region improves the biomechanical response during load application

Biaxial flexural strength and Weilbull characteristics of adhesively luted hybrid and reinforced CAD/CAM materials to dentin: effect of self-etching ceramic primer versus hydrofluoric acid etching

This study evaluated the influence of the surface treatment and aging on the biaxial flexural strength of ceramic materials cemented to a dentin analogue. One hundred twenty disc-shaped specimens were allocated into 12 groups considering three study factors: ceramic material (lithium disilicate, leucite-based ceramic and hybrid ceramic), surface treatment (10% hydrofluoric acid etching + silane or self-etching glass-ceramic primer) and Aging (with 10,000 thermocycles of 5–37–55 °C or without). A tri-layer assembly was designed to mimic a cemented restoration (Variolink N) into a dentin analogue. All samples were submitted to the biaxial flexural strength assay. The flexural strength in MPa was calculated using the finite element method for each sample considering thickness, material properties, and the load to fracture during the in vitro test. Fractographic analysis was also performed. The data was evaluated using three-way ANOVA and Tukey test (α = 5%). ANOVA showed influence for the Material*Treatment*Aging interaction on the flexural strength (p = 0.011). The highest strength was calculated for lithium disilicate ceramic + self-etching ceramic primer without aging (499 ± 17 MPa)A and the lowest value for hybrid ceramic material + acid etching with aging (424 ± 48 MPa)E. According to the Weibull modulus, the most predictable strength was calculated for lithium disilicate + acid etching after aging. Acid etching or self-etching ceramic primer promotes similar immediate biaxial flexural strength for each evaluated ceramic. In the long-term, superior strength was observed using acid etching for lithium disilicate and the self-etching ceramic primer for the hybrid ceramic while no difference was observed for leucite-based ceramic

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