Repair of monolithic zirconia restorations with different direct resin composites: effect on the fatigue bonding and mechanical performance.

OBJECTIVE: The study aims to evaluate the shear bond and flexural strength fatigue behavior of yttrium-stabilized zirconia (4YSZ) repaired using different resin composites. MATERIALS AND METHODS: Cylindric specimens of 4YSZ were obtained for the bond strength (Ø = 6 mm, 1.5 mm of thickness) and biaxial flexural strength (Ø = 15 mm, 1 mm of thickness) fatigue tests and divided into 3 groups according to the repair resin composite: EVO (nanohybrid), BULK (bulk-fill), and FLOW (flowable). The zirconia surface was air-abraded with alumina particles, a 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP) primer was applied, and the resin composite was build-up over the zirconia. Fatigue shear bond strength and flexural fatigue strength tests were performed (n = 15). One-way ANOVA and Tukey post hoc tests were carried out for both outcomes, besides scanning electron microscopy and finite element analysis. RESULTS: The repair material affected the fatigue shear bond strength of zirconia ceramic. The BULK group (18.9 MPa) depicted higher bond strength values than FLOW (14.8 MPa) (p = 0.04), while EVO (18.0 MPa) showed similar results to both groups. No effect was observed for the mechanical behavior (p = 0.53). The stress distribution was similar for all groups. CONCLUSION: The repair of yttrium-stabilized zirconia (4YSZ) ceramics with bulk-fill resin composites was the best option for high fatigue bond strength. However, the fatigue mechanical performance was similar regardless of the applied repair material. CLINICAL RELEVANCE: The repair of yttrium-stabilized zirconia (4YSZ) monolithic restorations may be performed with nanohybrid and bulk-fill resin composites in order to promote longevity in the treatment.

Enhancing mouthguard longevity: Impact of surface treatment against aging from brushing and disinfectant exposure.

BACKGROUND/AIMS: The study aimed to assess the surface characteristics of sports mouthguards under mechanical stresses during cleaning, either by brushing or immersion in disinfectant solutions. MATERIAL AND METHODS: Ethylene-vinyl acetate samples, 4 mm thick, were randomly assigned to cleaning methods: control (C-no cleaning), brushing with water (B.W), brushing with neutral liquid soap (B.S), brushing with toothpaste (B.T), immersion in distilled water for 10 min (I.W), immersion in 2.25% sodium hypochlorite solution for 10 min (I.SH), and immersion in sodium bicarbonate solution for 5 min (I.SB). All cleaning methods were applied for 28 days. Surface roughness average (Ra) and wettability were measured at baseline for the control group (n = 9), and after cleaning for all the other groups. RESULTS: One-way ANOVA with Tukey tests (5% significance) indicated significant differences among groups (p < .05). The I.SB group had higher surface roughness than B.S and B.T (p < .05). B.W showed the lowest wettability, significantly lower than B.T, I.W, and I.SB (p < .05). I.SB exhibited the highest wettability, significantly different from sodium hypochlorite, neutral liquid soap, brushing with water, and control groups (p < .05). The sodium bicarbonate immersion group (I.SB) demonstrated greater statistical variation, displaying higher susceptibility to aging compared to brushing with neutral liquid soap. CONCLUSION: Cleaning mouthguards with a toothbrush, water, and neutral liquid soap emerged as the most promising method, causing minimal surface changes in the material.

Influence of CAD-CAM milling strategies on the outcome of indirect restorations: A scoping review.

STATEMENT OF PROBLEM: The influence of computer-aided manufacturing (CAM) parameters and settings on the outcomes of milled indirect restorations is poorly understood. PURPOSE: The purpose of this scoping review was to summarize the current CAM systems, parameters, and setting changes, and their effects on different outcomes of milled indirect restorations and aspects related to their manufacture. MATERIAL AND METHODS: The protocol of this review is available online (https://osf.io/x28ps/). Studies that used at least 2 different parameters (CAM units, number of axes, digital spacers, or protocols with different rotatory instruments, grit-sizes, milling speed, or others) for milling indirect restorations were included. A structured search up to July 2023 was performed by 2 independent reviewers for articles written in English in LILACS, MEDLINE via PubMed, EMBASE, Web of Science, and Scopus. RESULTS: Of 1546 studies identified, 22 were included in the review. Discrepancies were found between the planned and actual measured cement space, with a decreasing linear relationship impacting restoration adaptation at different points. The CEREC MC XL milling machine was the most used system in the included studies, with variations in bur types, milling modes, and number of burs uses affecting internal fit and surface trueness. The results demonstrated the better adaptation of restorations made with 5-axis over 3-axis milling machines. Lithium disilicate and zirconia were the most commonly used materials, and crowns and inlays were popular designs. Marginal and internal adaptation were the primary outcomes assessed using the various techniques. CONCLUSIONS: The study presented a comprehensive exploration of CAM systems and parameters, and their influence on indirect restorations. The planned cement space was not properly reproduced by the milling. Bur characteristics can affect restoration fit and trueness. The 5-axis units seem to result in better-adapted restorations compared with 3- and 4-axis units.

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.

Endocrown restorations in premolars: influence of remaining axial walls of tooth structure and restorative materials on fatigue resistance.

OBJECTIVES: To evaluate the effect of the remaining tooth structure and different CAD/CAM materials on the fatigue performance and failure mode of endodontically treated premolars restored with endocrowns. MATERIALS AND METHODS: Ninety maxillary premolars were endodontically treated and assigned into 6 groups (n = 15) according to the number of remaining axial walls (four, three, and two) and restorative materials (ultra-translucent zirconia 5Y-PSZ [KATANA UTML] and lithium disilicate [IPS e.max-CAD]). The specimens were subjected to cyclic fatigue loading test (initial load 200 N; 20 Hz). An incremental step load of 100 N per 10,000 cycles was applied until failure. The fatigue failure load (FFL) and number of failure cycles (CFFs) data were statistically analyzed with two-way ANOVA and Kaplan-Meier test (α = 0.05). Failed specimens were examined under a stereomicroscope 25 × and failure modes were determined. RESULTS: FFL and CFF were significantly influenced by restorative material (p < 0.05). 5Y-PSZ endocrowns showed significantly higher FFL when compared with lithium disilicate. The number of remaining walls did not affect the fatigue behavior or failure mode of the specimens. Of the lithium disilicate restorations, 51% had repairable failures, while 95% 5Y-PSZ restorations had non-repairable failures. CONCLUSIONS: Zirconia endocrowns showed better FFL than lithium disilicate endocrowns, regardless of the number of remaining axis walls. Lithium disilicate and 5Y-PSZ endocrowns showed FFL higher than the normal masticatory loads. CLINICAL RELEVANCE: Restoring endodontically treated premolars with endocrown could be a promising treatment, regardless of the remaining axial walls. However, precaution should be taken in material selection since it affects the fatigue resistance and failure mode.

The ability of mouthguards to protect veneered teeth: A 3D finite element analysis.

BACKGROUND/AIMS: Professional and amateur athletes might have veneer restorations. The aim of this study was to investigate the protective effect of mouthguards on veneered anterior restorations. METHODS: A nonlinear dynamic analysis was performed to simulate conditions during an impact with or without a custom-made mouthguard. Using a computer-aided design (CAD) software, a slice of a human maxilla was designed containing an upper right central incisor. The model was composed of mucosa, cortical bone, trabecular bone, periodontal ligament, dentin, enamel, and pulp tissue. The enamel was prepared (feather design), restored with an indirect veneer (1.0 mm thickness), and duplicated to simulate both conditions with or without a mouthguard (4 mm thickness). Both models were subdivided into finite elements using the computer-aided engineering (CAE) software. Frictionless contacts were used, and an impact was simulated in which a rigid sphere hit the model at 1 m s-1 . Fixation was defined at the base of the bone. The elastic modulus of the veneer was assessed by using five different restorative materials (resin composite, hybrid ceramic, zirconia-reinforced lithium silicate, lithium disilicate, and zirconia). Von Mises stress, minimal principal stress, and maximum principal stress (in MPa) were obtained and plotted for visual comparison. RESULTS: Von-Mises results showed higher stress concentrations in the veneer's cervical labial region for models without a mouthguard. Observing the quantitative results for each model, the highest compressive (709 MPa) and tensile (58 MPa) stresses occurred in the situation without a mouthguard with a zirconia veneer, while the lowest occurred in resin composite veneer with a mouthguard (8 and 5 MPa). The mouthguard was able to reduce the stresses in the tooth structure and it also reduced the risk of fracture in all conditions. CONCLUSIONS: Mouthguards were beneficial in reducing the effects of dental trauma regardless of the restorative material used to manufacture the indirect veneer, since they act by dampening the generated stresses during the trauma event. Equal impact stresses on a mouthguard will lead to higher stresses in veneered teeth with more rigid restorative materials leading to a less protective effect.

Fracture resistance and biomechanical behavior of different access cavities of maxillary central incisors restored with different composite resins.

OBJECTIVES: The aim of this study was to compare the effect of three different access cavities on the tissue removal, deflection, fracture resistance, and stress distribution of extracted maxillary central incisors. MATERIALS AND METHODS: Forty human maxillary central incisors were randomly assigned in four experimental group (n = 10) including conservative access cavity "CAC," traditional access cavity "TAC," invasive access cavity "IAC," and without access cavity (control group). Cone-beam computed tomography "CBCT" scans were used to evaluate the tissue removal during the different access cavities. All specimens were restored with composite resin (Admira Fusion, Voco, Cuxhaven, Germany) and embedded in acrylic resin blocks after simulating the periodontal ligament using red wax, then the specimens were submitted to the deflection test applying a load of 250 N and to the load-to-fracture test after artificial aging in a mechanical cycling machine (150 N, 5 × 106 cycles, 10 Hz). Lastly, stress distribution was assessed by three-dimensional finite element analysis (3D-FEA), simulating the specimens restoration by two types of composite resins of low and high elastic modulus (8 and 18 GPa respectively) after the access cavities. The data were submitted to Shapiro-Wilk and KS normality tests. Then, they were analyzed by one-way ANOVA and Tukey tests with a significance level (α ≤ 0.05). RESULTS: CBCT scans showed a significant difference of worn tissues in CAC and TAC when compared to the IAC (P < 0.0001). In deflection test, CAC showed lower deformation values than the TAC and IAC. Load-to-fracture test presented no significant difference among the three experimental groups (P = 0.6901). 3D-FEA showed that the more conservative the access cavity, the higher the stress magnitude. CONCLUSIONS: CAC promote less worn tissue; however, this does not improve the stress distribution or fracture resistance of endodontically treated maxillary incisors. CLINICAL RELEVANCE: Clinicians should reconsider the pros and cons of the conservative access cavity.

Fracture resistance and stress distribution of weakened teeth reinforced with a bundled glass fiber-reinforced resin post.

OBJECTIVES: To make an in vitro assessment of fracture resistance of weakened and non-weakened teeth receiving intraradicular reinforcement using Rebilda bundled glass fiber-reinforced composite posts (GT), Rebilda conventional glass fiber posts (RP), or both systems combined (GT + RP). MATERIALS AND METHODS: Eighty sound bovine incisors were prepared and divided randomly into eight groups as follows: (a) nWnR: without simulating weakness, and without intraradicular reinforcement; (b) WnR: simulating weakness, but without intraradicular reinforcement; (c) nWGT: without simulating weakness, but with GT; (d) WGT: simulating weakness, and with GT; (e) nWRP: without simulating weakness, but with RP; (f) WRP: simulating weakness, and with RP; (g) nWGTRP: without simulating weakness, but with GT + RP; (h) WGTRP: simulating weakness, and with GT + RP. The specimens were subjected to the load-to-fracture test using the DL-2000MF universal testing machine. The finite element method assessed the mechanical behavior and stress distribution in endodontically treated teeth. RESULTS: The groups nWGTRP and WGTRP presented the best results in the load-to-fracture test, with the former being better than the latter, but with no statistically significant difference (P > 0.05). However, there was a significant difference between these and the other groups (P < 0.05), except for nWRP. Stress distribution inside the canal wall was different among the groups, with promising mechanical behavior for nWGTRP and nWRP. CONCLUSIONS: The Rebilda conventional fiber post (RP), combined with the Rebilda bundled glass fiber-reinforced composite post (GT) improves the resistance and stress distribution of immature teeth. CLINICAL RELEVANCE: Longitudinal fracture is less frequent in teeth restored with GT and RP posts.

Fatigue behavior and stress distribution of molars restored with MOD inlays with and without deep margin elevation.

OBJECTIVES: This study evaluated the effect of deep margin elevation (DME) and restorative materials (leucite-reinforced glass-ceramics [C] vs. indirect resin composite [R]) on the fatigue behavior and stress distribution of maxillary molars with 2-mm deep proximal margins restored with MOD inlay. METHODS: Fifty-two extracted human third molars were randomly assigned into four groups (n = 13): C; DME + C; R; and DME + R. Inlays were fabricated in CAD-CAM and bonded to all teeth. The fatigue behavior was assessed with the stepwise stress test (10,000 cycles/step; step = 50 N; 20 Hz; initial load = 200 N). Fatigue failure loads and the number of cycles were analyzed with 2-way ANOVA and Tukey's test (p < 0.05) and Kaplan-Meier survival plots. The stress distribution was assessed with finite element analysis. The models were considered isotropic, linear, and homogeneous, and presented bonded contacts. A tripod axial load (400 N) was applied to the occlusal surface. The stress distribution was analyzed with the maximum principal stress criterion. RESULTS: For fatigue, there was no difference for DME factor (p > 0.05). For the material factor, the load and number of cycles for failure were statistically higher in the R groups (p < 0.05). The finite element analysis showed that resin composite inlays concentrated more stress in the tooth structure, while ceramic inlays concentrated more stress in the restoration. Non-reparable failures were more frequent in the resin composite inlays groups. CONCLUSIONS: DME was not negative for fatigue and biomechanical behaviors. Resin composite inlays were more resistant to the fatigue test, although the failure mode was more aggressive. CLINICAL SIGNIFICANCE: DME does not impair mechanical behavior. Resin composite inlays failed at higher loads but with a more aggressive failure mode.

Canine guidance reconstruction with ceramic or composite resin: A 3D finite element analysis and in vitro wear study.

STATEMENT OF PROBLEM: Wear of the maxillary canine cusps is a common clinical condition which can affect function and esthetics and, in some situations, lead to occlusal pathology. The mechanical behavior of different restorative techniques for the condition is unclear. PURPOSE: The purpose of this in vitro study was to evaluate the biomechanical behavior of direct or indirect restorative techniques used in restoring canine guidance. MATERIAL AND METHODS: Three-dimensional models obtained from 10 extracted undamaged maxillary canines were modeled by using reverse engineering. Each model received 2 different preparations: incisal (I) or incisal and labial (IL), restored with composite resin (CR) or ceramic (C), compared with unrestored teeth (U). Finite element analysis was used to evaluate the total deformation and the maximum principal stress. For the in vitro wear test, 30 teeth were divided into groups: U (control), I-CR, and IL-C. The teeth were wear tested for 240 000 cycles with a 2-mm sliding contact, a load of 49 N applied, with a 4-Hz cycle with a composite resin piston as antagonist. The wear of restorations and antagonists was quantified by the digital image correlation technique. The 1-way analysis of variance test for total deformation and the Tukey test for the maximum principal stress were used (α=.05) to statistically analyze the data. The Friedman test was applied in the comparison between wear cycles, and the Tukey test was used in the comparison among groups. RESULTS: No significant difference was found among groups (P>.05) for the total deformation. IL-CR showed a higher failure probability, reaching stress peaks which exceeded the tensile strength of the material. I-CR showed greater wear in the in vitro test than IL-C (P=.02). No difference was found among groups in antagonist wear (P=.074). CONCLUSIONS: Ceramic restorations with labial involvement show biomechanical behavior closer to that of unrestored teeth in restoring canine guidance compared with composite resin.

Impact of different complete coverage onlay preparation designs and the intraoral scanner on the accuracy of digital scans.

STATEMENT OF PROBLEM: The trueness and precision of intraoral scanners (IOSs) and the effect of intracoronal restorations have been reported. However, studies addressing the accuracy of IOSs in reproducing different complete coverage onlay preparation designs are lacking. PURPOSE: The purpose of this in vitro study was to evaluate the influence of complete coverage onlay preparation design and intraoral scanning devices on the accuracy of digital scans in terms of trueness and precision. MATERIAL AND METHODS: Three preparation designs on the mandibular first molar were considered: a traditional preparation design with isthmus reduction (IST), a traditional preparation design without isthmus reduction (wIST), and simplified nonretentive preparation (nRET). Digital scans of epoxy resin mandibular arch reference models of the preparations (containing second premolar, first molar, and second molar) were obtained by using 3 IOSs (iTero Element 2 [ELE], Trios 3 [TRI], and Primescan [PRI]) (n=10). Trueness (µm) and precision (µm) were analyzed by superimposing the digital scan on the digital reference models obtained with a high-accuracy industrial scanner (ATOS Core 80) in a tridimensional metrology software program. Accuracy was quantified by the absolute deviation (µm). Local and overall mean positive and negative deviations for trueness were also obtained. Data were analyzed by using the Kruskal-Wallis and Dunn tests with a statistical software program (α=.05). RESULTS: The nonretentive preparation groups obtained higher trueness (3.8 µm) and precision (2.7 µm) than the IST and wIST groups (trueness=7.5 to 6.3 µm, precision=5.5 to 4.6 µm). Trueness values were lower with ELE×IST (16 µm), followed by ELE×wIST (13 µm), and PRI×IST (7.8 µm). In general, no difference was found between PRI and TRI scanners (6.3 to 5.9 µm), with lower performance for ELE (13 µm). Positive deviations were higher on the proximal box of the IST and wIST preparation and on the occlusal box of the IST group. Negative deviation was higher on the ELE×IST occlusal box. CONCLUSIONS: Different intraoral scanners and preparation designs influenced the accuracy of digital scans. A more complex preparation such as IST and wIST showed higher deviation. The iTero Element 2 scanner exhibited higher deviation for both trueness and precision.

Effect of surface treatment and glaze application on shade characterized resin-modified ceramic after toothbrushing.

STATEMENT OF PROBLEM: The extrinsic characterization with pigments and glaze application on the surface of ceramic restorations promotes individualization and esthetics. However, whether this characterization is resistant to toothbrushing abrasive wear is unclear. PURPOSE: The purpose of this in vitro study was to investigate the abrasive wear resistance of characterized and glazed resin-modified ceramic with different surface treatments before characterization. MATERIAL AND METHODS: Eighty rectangular specimens (10×8×6 mm) were precision cut from Vita Enamic and randomly divided in 4 groups as per the surface treatment before the characterization technique. These groups included the following: no additional treatment or polishing only, acid etching, airborne-particle abrasion, and self-etching silanization. For each surface treatment, 2 subgroups were created considering the presence or absence of the glaze layer after shade characterization, totaling 8 groups (n=10). The specimens were submitted to toothbrushing simulation (150 000 cycles, 2.45-N, 180 strokes per minute) with a soft and straight toothbrush and a solution of toothpaste and distilled water (250 g/L). After each 50 000 cycles, the wear of the characterized surface was measured by using a contact profilometer in µm which was equivalent to 5, 10, and 15 years of simulated toothbrushing. The rates were analyzed by using 3-way ANOVA and the post hoc Tukey test (α=.05). Scanning electron microscopy analysis was performed to access the surface profile between the evaluated periods. RESULTS: The mean wear rate (Rz) of the characterized surface was affected by the isolated factors, including surface treatment, glaze, and time (P<.001), with no interactive effects noted. A higher mean value of Rz was observed for polishing only (12.9 ±4.4 µm), followed by airborne-particle abrasion (10.2 ±4.5 µm), acid etching (9.4 ±2.4 µm), and then self-etching silanization (7.6 ±2.9 µm). Mean wear rate was also increased by the glaze (11.1 ±4.7 µm) compared with its absence (9 ±3.2 µm) and by the length of time, with 15 years (11.3 ±4.5 µm) followed by 10 years (9.8 ±4.0 µm) and 5 years (9 ±3.6 µm). The scanning electron microscopy images showed similar worn surfaces for each subgroup, glaze removal after 150 000 cycles, and surface homogeneity in function of time. CONCLUSIONS: Self-etching silanization was found to be the treatment suitable for reducing the wear of characterized resin-modified ceramic during prolonged brushing of more than 5 years. Glaze application did not protect the characterization from surface wear, regardless of the ceramic surface treatment performed on the specimens.

Effect of Biologically Oriented Preparation Technique on the Stress Concentration of Endodontically Treated Upper Central Incisor Restored with Zirconia Crown: 3D-FEA.

The aim of this study was to evaluate the effect of biologically oriented preparation technique on the stress concentration of endodontically treated upper central incisors restored with zirconia crown (yttria-stabilized zirconia polycrystalline ceramic) through finite element analysis (FEA). Four models of maxillary central incisors containing enamel, dentin, periodontal ligament, cortical and medullary bone were created in CAD. Each model received a polymeric core-build up with nanofilled dental resin composite. The evaluated models were SM-preparation in shoulder 90°; CM-chamfer preparation; BOPT-biologically oriented preparation technique and BOPTB-BOPT preparation 1 mm below the cement-enamel junction. All models received zirconia crowns (5Y-TZP), fiberglass post and 1 mm ferrule. The models were imported into the analysis software with parameters for mechanical structural testing using the maximum principal stress and the tensile strength as the analysis criteria. Then, load of 150 N was applied at the cingulum with 45° slope to the long axis of the tooth, with the fixed base for each model. The type of marginal preparation affected the stresses concentration in endodontically treated teeth and in the zirconia crown margin. Considering the stress magnitude only, BOPT is a viable option for anterior monolithic zirconia crowns; however, with the highest stress magnitude at the restoration margin.

Effect of material types on the fracture resistance of maxillary central incisors restored with CAD/CAM post and cores.

AIM: To evaluate the fracture resistance and failure pattern of endodontically treated teeth with post and cores made of different CAD/CAM materials in comparison with cast post and cores. MATERIALS AND METHODS: A total of 40 maxillary central incisors were divided into four groups. Post and cores were fabricated using the following materials: fiber-reinforced composite, high-density polymer, polymer-infiltrated ceramic network, and nonprecious metal alloy as a control group. Specimens were covered with lithium disilicate crowns and subjected to thermomechanical cyclic loading followed by fracture resistance testing until failure. One-way ANOVA followed by Bonferroni multiple comparison tests were used to determine significant differences between the four groups. The significance level was set at 0.05. 3D finite element analysis was then performed, and results were analyzed based on the von Mises stress distribution criteria and the maximum principal stress for the possible failure areas. RESULTS: No statistically significant differences were found in the fracture resistance between the four groups. In terms of unrestorable failures, no significant differences were found among the tested groups. CONCLUSION: The tested post and cores have comparable fracture resistance to that of metallic cast post and cores in the anterior region. CLINICAL SIGNIFICANCE: Post and cores made of CAD/CAM materials offer an acceptable alternative for the restoration of anterior teeth.

Full-Crown Versus Endocrown Approach: A 3D-Analysis of Both Restorations and the Effect of Ferrule and Restoration Material.

PURPOSE: To assess stress distribution in full-crowns with a composite buildup and endocrowns under axial or oblique loads, both with different ferrules (1 or 2 mm) and ceramic materials (glass ceramic or hybrid ceramic). MATERIALS AND METHODS: Sixteen models were analyzed with finite element analysis. No-separation contacts were considered between restoration/resin cement and resin cement/tooth. The contact between the fixation cylinder and the root was considered perfectly bonded. The axial load was applied to the occlusal surface and the oblique load was applied to the buccal cusp. The resulting tensile stresses were shown for the crown, the cement layer and the tooth. RESULTS: Almost all factors influenced the stress distribution significantly in the crown and the cement layer, as well as the tooth. The only exception was found under oblique loading by the restoration material and the type of crown that were of no significant influence on the stress distribution in the tooth. CONCLUSIONS: Under axial load, the endocrown showed the least tensile stresses in the tooth, but under oblique loads, the full-crown showed less tensile stresses than the endocrown. With the hybrid ceramic material, lower stresses were found in the crown, but higher stresses were present in the cement layer. The 2 mm ferrule is beneficial for reducing the resulting tensile stresses in all modalities.

Mouthguard use and TMJ injury prevention with different occlusions: A three-dimensional finite element analysis.

BACKGROUND/AIMS: There is a lack of data regarding the mechanical responses of the temporo-mandibular joints during an impact to the orofacial region. The aim of this study was to analyze the biomechanical effects of wearing a mouthguard (MG) on the impact response of the mandibular condyle and articular disk according to the type of occlusion. The hypothesis was that the MG would minimize the effect in those structures, regardless of the occlusion type. METHODS: Using modeling software, a human skull with jaw, teeth and articular disk was created. The models were divided according to the occlusion type (Class I, II, or III) and the presence of a mouthguard (with or without). The geometries were exported to analysis software, and the materials were considered ideal. Fixation occurred at the base of the foramen magnum. The load (0-500N, 1s) was applied to the upper central incisors with a steel ball. Maximum principal stress and Von Mises results (MPa) were obtained in the mandibular condyle and articular disk. Minimum principal stress and maximum shear stresses were also recorded in the articular disk. RESULTS: For both structures, the MG caused a decrease in stress concentration regardless of the occlusion and stress criteria. The condyle neck was the most tensile-stressed area while for the articular disk, both the superior and inferior surfaces were the most stressed areas. The highest stress peaks in the disk were found for compression followed by tensile and then shear stress. CONCLUSION: This biomechanical analysis of the effects of using a mouthguard exhibited considerably decreased stresses on the mandibular condyle and articular disk, regardless of the occlusion type.

Different combinations of CAD/CAM materials on the biomechanical behavior of a two-piece prosthetic solution.

AIM: This study evaluated the stress distribution of implant-supported prostheses, varying the different combinations of computer-aided design/computer-aided manufacturing (CAD/CAM) materials between the hybrid abutment and the monolithic crown by three-dimensional (3D) finite element analysis (FEA). MATERIALS AND METHODS: Nine models were designed with Rhinoceros 3D and Ansys software. Each model contained a bone block of the molar area, including an implant (IH; Ø 3.75 × 11 mm) supporting a hybrid abutment (ceramic mesostructure (MS) cemented onto a titanium [Ti] base) and a monolithic crown. The occlusal load was applied to the fossa bottom (300 N; 30 degrees). The results were analyzed using the von Mises stress for each separated prosthetic structure and microstrain for the bone tissue. RESULT: Von Mises maps of the crown, ceramic MS, implant, screw, and cement layers showed a decreased stress concentration as the elastic modulus (E modulus) of the ceramic crown (CR) associated with a rigid ceramic MS decreased. No differences in bone tissue regarding microstrain were observed. CONCLUSION: Implant-supported crowns present less stress concentration when a rigid abutment is associated with resilient crowns.

Do Mechanical Advantages Exist in Relining Fiber Posts with Composite Prior to its Cementation?

PURPOSE: To evaluate two different techniques for glass fiber-reinforced resin post (FRC) insertion by assessing the stress distribution under polymerization shrinkage or masticatory loading and measuring the pull-out bond strength to dentin. MATERIALS AND METHODS: A model of an endodontically treated maxillary central incisor was used for three-dimensional simulation with two conditions: conventionally cemented (FRC) and relined (RFRC). The volumetric solids were exported to analysis software (ANSYS 17.2, ANSYS) in STEP (Standard for the Exchange of Product Data) format. All contacts were considered perfectly bonded between the geometries. Numerical models received a load of 100 N (45 degrees) on the lingual surface. The composite cement polymerization shrinkage was simulated by thermal analogy to obtain Von Mises, maximum principal stress, and shear stress. For in vitro evaluation, a pull-out bond strength test (n = 20/N = 40) was performed (50 Kgf, 1 mm/min) after mechanical cycling (in water at 37°C, 84 N, 2 bar, 45 degrees, 106 cycles, 4 Hz), and failure analysis was subsequently performed. The results were analyzed using one-way ANOVA and Tukey's test (α < 0.05). RESULTS: The FRC group showed more stress concentration in the cement layer. The RFRC group [(32 ± 13); (288 ± 129)] presented better performance than the FRC group [(6 ± 7); (152 ± 87)] for stress distribution and bond strength (p < 0.05). Adhesive and mixed failures occurred in both groups. CONCLUSION: Relined fiberglass posts reduced the stress generated by polymerization shrinkage and showed greater bond strength to dentin.

Validation of a Simplified Implant-Retained Cantilever Fixed Prosthesis.

PURPOSE: To evaluate the stress and strain generated in a fixed four-element prosthesis under the application of axial and nonaxial loads using a simplified implant-supported fixed prosthesis model. MATERIALS AND METHODS: A 3-dimensional model was constructed containing 3 implants with a conventional anatomical prosthesis (G1). The second model with the same implant system received the simplified prosthesis (G2). A load of 300 N was applied at an axial point and a nonaxial point through finite element analysis software. RESULTS: The G2 group showed different values of stress concentration in the prosthesis, fixation screw, retention screw, and abutments when compared with G1. Within a limit of 10% degrees of acceptability, the stress on the implants and the bone strain were enclosed for both models of prostheses. CONCLUSION: The simplified fixed prosthesis evaluated presents biomechanical behavior similar to an anatomical prosthesis in the implants and in the surrounding bone structure.

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.