Does glaze firing affect the strength of advanced lithium disilicate after simulated defects?

OBJECTIVE: To study the influence of glazing on strength repair of lithium disilicate glass-ceramics after defect incorporation in different production processing phases. MATERIALS AND METHODS: Bar-shaped specimens (1 × 1 × 12 mm, n = 280; 20/group) made from different lithium disilicate ceramics (IPS e.max CAD, Ivoclar, "LD" or advanced lithium disilicate CEREC Tessera, Dentsply Sirona, "ALD") were exposed to 7 different protocols: crystallized without (c) and with glaze layer (cg), with a defect incorporated before crystallization without (ic) and with glaze layer (icg), with a defect after crystallization without (ci) or with glaze layer (cig), and defect incorporated after the glaze layer (cgi). The flexural strength was determined using the three-point bending test. Analysis of indented areas and fractured specimens was performed by scanning electron microscopy. Flexural strength data were evaluated by two-way ANOVA followed by Tukey tests (α = 5%). RESULTS: Two-way ANOVA revealed a significant influence of ceramic (p < 0.001; F = 55.45), protocol (p < 0.001; F = 56.94), and the interaction protocol*ceramic (p < 0.001; F = 13.86). Regardless of ceramics, defect incorporation as final step resulted in the worst strength, while defects introduced before crystallization did not reduce strength. Glaze firing after defect incorporation led to strength repair for ALD, whereas such an effect was not evident for LD. CONCLUSIONS: The advanced lithium disilicate must receive a glaze layer to achieve its highest strength. Defects incorporated in the pre-crystallized stage can be healed during crystallization. Defects should not be incorporated after glazing. CLINICAL RELEVANCE: Clinical adjustments should be performed on pre-crystallized or crystalized restorations that receive a glazer layer afterwards.

Silica-Nylon Reinforcement Effect on the Fracture Load and Stress Distribution of a Resin-Bonded Partial Dental Prosthesis.

This study investigated the influence of silica-nylon reinforcement on the stress distribution and fracture load of a resin-bonded fixed partial dental prosthesis (RBFDP). Three-unit RBFDPs (N = 60) were inserted between the first premolar and the first molar of a maxillary model. The groups were divided according to the nylon reinforcement (n = 20/group): conventional fixed prosthesis (without reinforcement), prosthesis with silica-nylon reinforcement positioned vertically, and prosthesis with silica-nylon reinforcement positioned horizontally. Half of the specimens were tested after 24 hours in a universal testing machine until fracture (1,000 kgf; 1 mm/minute) to determine the single load to fracture. The other half was submitted to mechanical aging during 106 cycles (100 N, 2 Hz), totaling 6 groups (n = 10/group). The results were analyzed by two-way analysis of variance (ANOVA) (α = 5%). The stress distribution for non-aged groups was simulated using finite element analysis. The numeric prostheses were modeled similarly to the in vitro assay. ANOVA showed no statistical difference between groups (P < .05) for load to fracture. However, the use of the reinforcement provided stability even after the failure, as the parts did not separate. The computational analysis showed similar biomechanical behavior among the groups. The use of the nylon reinforcement does not influence the fracture load or the stress distribution, but it does enable the prosthesis to remain in position after failure.

Effect of different repair methods on the bond strength of resin composite to CAD/CAM materials and microorganisms adhesion: An in situ study.

OBJECTIVE: To evaluate the influence of different repair protocols andin situ aging on colony forming units (CFU) and shear bond strength (SBS) between CAD/CAM materials and resin composite. METHODOLOGY: 150 blocks (6 × 5 × 2.5 mm) were made out of each restorative material (CAD/CAM resin composite -RC), polymer-infiltrated ceramic - PIC and glass ceramic (VS), totaling 450 blocks. Fifty blocks of each material were submitted to a 60-day in situ aging by fixing the blocks into cavities prepared in the posterior region of the base of complete dentures. The aged and non-aged blocks were randomly divided into 30 groups of 10 (N = 300) according to the following factors: "Restorative material", "Surface Treatment", and "Aging". Z350 resin composite cylinders were made on the surface of the blocks, and the specimens were submitted to thermocycling, shear test, failure mode analysis, and complementary analyses of roughness, fungal and bacterial CFUs, SEM, and EDS. The SBS (MPa) and CFU/mL data were statistically analyzed by ANOVA and Tukey's test (5%). The other variables were analyzed by qualitative analyzes. RESULTS: The "aging" factor was significant for RC and VS and the "surface treatment" factor was significant for the three restorative materials. Hydrofluoric (HF) acid etching followed by silanization was the best protocol for PIC and VS and diamond bur + SBU was the best protocol for RC. CFU/mL was similar among the restorative materials. CONCLUSION: In situ aging reduced the bond strength between the resin composite repair and RC and VS materials. CLINICAL SIGNIFICANCE: The repair protocol for each restorative material is different and may be influenced by the time of clinical use.

Effect of Framework Type on the Biomechanical Behavior of Provisional Crowns: Strain Gauge and Finite Element Analyses.

The aim of this study was to evaluate the effects of different frameworks on the biomechanical behavior of implant-supported provisional single crowns to indicate or not the use of plastic framework as infrastructure. For finite element analysis, a hemi-jaw stone model was scanned and modeled with an external hexagon implant. A framework was screwed onto the implant and a crown was constructed over it. The set was made in triplicate according to framework type: plastic, cobalt-chromium (CoCr), and titanium. Models were exported in volumetric format to analysis software where structures were considered isotropic, linear, elastic, and homogeneous. Axial loads (100, 200, and 300 N) were applied to the fossa bottom, and the system's fixation occurred on the bone base. For strain-gauge analysis, the same hemi-jaw model was built in polyurethane and an implant was placed on it. Three crowns were made, each one with a different framework. Four strain gauges were glued around the implant to obtain microstrain values. The data were analyzed by three-way analysis of variance (ANOVA) and Tukey tests (P < .05). Finite element analysis exhibited microstrain results for bone, von Mises stress values for the implant and screw, and maximum principal stress values for the crown. For computational method, as the applied load increased, so did the stress generated. Titanium frameworks concentrated more stress in the crown and bone, while plastic ones concentrated more in the implant and screw. ANOVA showed that the higher the load value and the framework elastic modulus, the higher the generated microstrain in bone. It can be concluded that all evaluated framework types can be used in the manufacturing of provisional crowns.

Capacity to Maintain Placement Torque at Removal, Single Load-to-Failure, and Stress Concentration of Straight and Angled Abutments.

Because the main complication of implant-supported prostheses is torque loosening and/or fixation screw fracture, the goal of this study was to evaluate the torque before and after fatigue (screw placement and removal, respectively), single load-to-failure (compression test), and stress concentration of straight and angled abutments. Eighty implants were included in polyurethane cylinders. Half of the implants received straight abutments (group S, n = 40) and the other half received angled abutments (group A, n = 40). The abutments for cemented prostheses were installed with a torque of 20 Ncm. Eighty titanium structures were machined and cemented on the abutments with zinc-phosphate cement. After storage for 24 hours, half of the specimens had their torque loosening evaluated and were then immediately submitted to a compressive test in a universal testing machine (1 mm/minute, 1,000 kgf), while the other half were subjected to cyclic fatigue (200 N at 2 Hz for 2 × 106 cycles at 37°C) as an aging protocol (n = 20 from each group). The aged samples then had their torque loosening measured and were also submitted to the compression test. Representative samples were evaluated by scanning electron microscopy. Two bidimensional models similar to the in vitro specimens were created and analyzed using the finite element method to evaluate the stress concentration. Data from the in vitro tests were submitted to two-way analysis of variance and Tukey test, both with significance at P = .5. The results show that angled abutments are less capable of maintaining the installation torque and are less resistant during the single load-to-failure test. The von Mises stress concentration was higher for group A in the cervical region. The straight abutments have better prognosis than angled abutments and less susceptibility to mechanical failures.

Influence of custom-made and stock mouthguard thickness on biomechanical response to a simulated impact.

BACKGROUND/AIMS: Mouthguards (MGs) are devices that can reduce the risks of facial trauma. However, the large variety of MG types and thicknesses raises the question of which type is the most effective and beneficial for the athletes. The aim of this study was to evaluate stress distribution in the skull, teeth, and jaws as a consequence of a direct impact. MATERIAL AND METHODS: Using modeling software, a human skull was modeled and a human jaw was created with all teeth inserted into the respective alveolus. The models were divided according to the MG type (custom-made or stock) and thickness (1, 2, and 4 mm). Two models without MG were evaluated with and without teeth contact. The geometries were exported to analysis software and the materials were considered ideal. Fixation occurred at the base of the foramen magnum. The load (500 N) was applied on the canine tooth with a ball. Maximum principal (MPa) and Von-Mises results were obtained. RESULTS: Without any protection, the generated tensile stress was of greater magnitude causing more damage in the absence of teeth contact. The presence of a MG significantly reduced the generated stress in all structures, and the customized/individualized type was more efficient than stock MGs. CONCLUSIONS: In extreme situations when it is impossible to use a MG, keeping the teeth in maximum intercuspal position is less harmful. Despite this, the use of any MG is beneficial and assists in dampening the generated stress. The thicker the device, the greater the capacity for decreasing the damage in all structures. The use of individual protectors for each patient is even more beneficial for preventing trauma during at-risk activities of impact.

Effect of primer-cement systems with different functional phosphate monomers on the adhesion of zirconia to dentin.

PURPOSE: The objective of this study was to evaluate the effect of primer-cement systems with different functional phosphate monomers on the adhesion of zirconia to dentin with and without aging protocols. MATERIALS AND METHODS: Bovine teeth (N = 180) were embedded in acrylic resin after sectioning their roots with with their coronal parts exposed. The buccal surface of each tooth was polished with silicon carbide papers (#200, 400, 600) until dentin exposure. Sintered zirconia cylinders (N = 180) (Ø: 3.4 mm; height: 4 mm) (Vita In-Ceram 2000) were prepared and distributed into 18 groups (n = 10 per group) considering the following factors: "Cementation System" (Panavia F - PAN; RelyX Ultimate - ULT, Multilink N - MULT) and "aging" (water storage in distilled water at 37 °C for 24 h (control, C); 30 days (30D); 6 months (6 M) and thermocycling for 5000 (5TC), 10,000 (10TC) and 20,000 (20TC) thermal cycles (5-55 °C; dwell time: 30 s)". Zirconia and dentin cementation surfaces were conditioned according to the recommendations of the manufacturers of each resin cement. The cylinders were adhesively cemented to the dentin surfaces and the specimens were submitted to the aging protocols. After aging, the specimens were subjected to shear bond strength test (SBS) (1 mm/min) in a Universal Testing Machine and failure types were analyzed. The data (MPa) were statistically using Kruskal-Wallis followed by the Dunn test (α = 5%). The degree of conversion (DC) rates of the cementing systems were also measured. RESULTS: While without aging (24 h) no significant difference was found between the cement systems (p > 0.05), after 30D (4.3-5.4), the highest decrease in all groups were observed after 5TC (1.5-2.3) (p < 0.05). Overall, MULT and ULT presented significantly higher results than that of PAN (p < 0.05). Pre-test failures during TC were more frequent in the PAN group. Complete adhesive failures at the cement/dentin interface were more frequent for MULT (30-80%) and PAN (10-70%) and for ULT (20-90%) at the cement/ceramic interface. DC of the tested cements did not show significant difference. CONCLUSION: Adhesion performance of the primer-cement systems with different functional phosphate monomers on zirconia-dentin complex varied as a function of aging strategies with MULT and ULT delivering higher bond strength values. When failure types considered, none of the cement systems performed well on both ceramic and dentin.