Loading stress distribution in posterior teeth restored by different core materials under fixed zirconia partial denture: A 3D-FEA study.

PURPOSE: To evaluate the effect of different substrate stiffness [sound dentin (SD), resin composite core (RC) or metal core (MC)] on the stress distribution of a zirconia posterior three-unit fixed partial denture (FPD). METHODS: The abutment teeth (first molar and first premolar) were modeled, containing 1.5 mm of axial reduction, and converging axial walls. A static structural analysis was performed using a finite element method and the maximum principal stress criterion to analyze the fixed partial denture (FPD) and the cement layers of both abutment teeth. The materials were considered isotropic, linear, elastic, homogeneous and with bonded contacts. An axial load (300 N) was applied to the occlusal surface of the second premolar. RESULTS: The region of the prosthetic connectors showed the highest tensile stress magnitude in the FPD structure depending on the substrate stiffness with different core materials. The highest stress peak was observed with the use of MC (116.4 MPa) compared to RC and SD. For the cement layer, RC showed the highest values in the molar abutment (14.7 MPa) and the highest values for the premolar abutment (14.4 MPa) compared to SD (14.1 and 13.4 MPa) and MC (13.8 and 13.3 MPa). Both metal core and resin composite core produced adequate stress concentration in the zirconia fixed partial denture during the load incidence. However, more flexible substrates, such as composite cores, can increase the tensile stress magnitude on the cement. CLINICAL SIGNIFICANCE: The present study shows that the choice of the cast core and metallic post by the resin composite core and fiberglass post did not improve the biomechanical behavior of the FPD. This choice must be performed based on clinical criteria (other) than mechanical.

Influence of different post-endodontic restorations on the fatigue survival and biomechanical behavior of central incisors.

PURPOSE: To evaluate the influence of different post-endodontic techniques on the fatigue survival and biomechanical behavior of crowned restored central incisors. METHODS: The crowns of 69 bovine incisors were cut, and the roots were treated endodontically and assigned randomly into three groups (n=23): resin composite buildup (BUP), glass fiber post-retained resin composite buildup (GFP), and cast post-and-core (CPC). They received full crown preparation with 2 mm ferrule, and a leucite-reinforced ceramic crown was cemented adhesively. Three specimens from each group were tested monotonically. The remaining specimens were subjected to the stepwise stress fatigue test until fracture or suspension after 1.5 x 106 cycles in a chewing simulator. The load and step at which each specimen failed were analyzed by Kaplan-Meier and Mantel-Cox (log-rank test) statistics, followed by multiple pairwise comparisons, at 5% significance level. The three groups tested (BUP, GFP, and CPC) were 3D modeled (Rhinoceros 4.0) and the maximum principal stress (MPa) criteria were used to calculate the results using FEA. RESULTS: There was no statistical difference between the treatments regarding the load or the number of cycles (Mantel-Cox log-rank test for trend, X²= 0.015, df=1, P= 0.901, X²=3.171, df=1, P= 0.995). Crown cracks were the predominant failure mode, and oblique root fractures were only observed in groups GFP and CPC. In endodontically treated incisors with a 2-mm ferrule, the post-endodontic treatment had no significant effect on fatigue survival. Non-restorable fractures only occurred in teeth restored with posts. CLINICAL SIGNIFICANCE: Although the clinical significance of laboratory studies has some limitations, this study suggests that composite buildups without posts may be an option for restoring endodontically treated incisors with 2 mm ferrule height.

Mechanical behavior of conceptual posterior dental crowns with functional elasticity gradient.

PURPOSE: To evaluate the biomechanical behavior of monolithic ceramic crowns with functional elasticity gradient. METHODS: Using a CAD software, a lower molar received a full-crown preparation (1.5 mm occlusal and axial reduction). The monolithic crown was modeled with a resin cement layer of 0.1 mm. Four groups were distributed according to the full crown elastic modulus (E) :(a) Bioinspired crown with decreasing elastic modulus (from 90 to 30GPa); (b) Crown with increasing elastic modulus (from 30 to 90 GPa); (c) Rigid crown (90 GPa) and (d) Flexible crown (30 GPa). The model was exported to the analysis software and meshed into 385.240 tetrahedral elements and 696.310 nodes. Materials were considered isotropic, linearly elastic, and homogeneous, with ideal contacts. A 300-N load was applied at the occlusal surface and the base of the model was fixed in all directions. The results were required in maximum principal stress criterion. RESULTS: Crowns consisting of layers with increasing elastic modulus presented intermediate results between the rigid and flexible crowns. Compared to the flexible crown, the bioinspired crown showed acceptable stress distribution across the structure with lower stress concentration in the tooth. In dental crowns the multilayer structure with functional elasticity gradient modifies the stress distribution in the restoration, with promising results for bioinspired design. CLINICAL SIGNIFICANCE: The manufacturing of posterior crowns with functional elasticity gradient should be considered due to its promising results on the stress concentration behavior.

Mechanical behavior of Class I cavities restored by different material combinations under loading and polymerization shrinkage stress. A 3D-FEA study.

PURPOSE: To examine the influence of different bulk and block composite and flowable and glass-ionomer material combinations in a multi-layer technique and in a unique technique, in deep Class I dental restorations. METHODS: 3D CAD of the sound tooth were built-up from a CT scan dataset using reverse engineering techniques. Four restored tooth models with Class I cavity were virtually created from a CAD model of a sound tooth. 3D-finite element (FE) models were created and analyzed starting from CAD models. Model A with flowable resin composite restoring the lower layer and bulk-fill resin composite restoring the upper layer, model B with glass-ionomer cement (GIC) restoring the lower layer and bulk-fill resin composite restoring the upper layer, model C with block composite as the only restoring material and model D with bulk-fill resin composite as the only restoring material. Polymerization shrinkage was simulated with the thermal expansion approach. Physiologic masticatory loads were applied in combination with shrinkage effect. Nodal displacements on the lower surfaces of FE models were constrained in all directions. Static linear analyses were carried out. The maximum normal stress criterion was used to assess the influence of each factor. RESULTS: Considering direct restoring techniques, models A, B and D exhibited a high stress gradient at the tooth/restorative material interface. Models A and D showed a similar stress trend along the cavity wall where a similar stress trend was recorded in the dentin and enamel. Model B showed a similar stress trend along enamel/restoration interface but a very low stress gradient along the dentin/restoration interface. Model C with a restoring block composite material showed a better response, with the lowest stress gradient at the dentin, filling block composite and enamel sides. CLINICAL SIGNIFICANCE: Bulk resin-based composite materials applied in a multilayer technique to deep and large Class I cavities produced adverse stress distributions versus block resin composite. Polymerization shrinkage and loading determined high stress levels in deep Class I cavities with bulk multi-layer restorations, while its impact on adhesion in block composite restorations was insignificant.

Influence of Restoration Height and Masticatory Load Orientation on Ceramic Endocrowns.

AIM: Endocrown restorations are an alternative to restore end-odontically treated teeth. Due to the fact that in the literature it is recommended a remnant of 1.5 mm, different heights of endocrown were elaborated and analyzed, obtaining possible faults and their location. This study aimed to evaluate the mechanism of stress distribution in the tooth/restoration set, varying two factors: "restoration height"-three levels, and load application-two levels (oblique or axial), totaling six groups. MATERIALS AND METHODS: For finite element analysis (FEA), a maxillary premolar was modeled with an endodontic treatment. Then, this template was triplicated and each copy received an endocrown restoration of different heights: G6 (4.5 mm), G7 (5.5 mm), and G8 (6.5 mm). The models were exported in STEP format to analysis software (ANSYS 17.2, ANSYS Inc.). During preprocessing, the solids were considered isotropic, linearly elastic, and homogeneous. Initially, a load (300 N) was axially applied in the central fossa region. For a second evaluation, an oblique load (300 N) was applied on the grinding slope of functional cusp. System fixation occurred at the base of poly-urethane cylinder. Results were evaluated through maximum principal stress (MPS). RESULTS: For axial load, lower stress values were generated in all groups. For oblique load, G8 showed a higher stress concentration in the cement layer and root dentin. CONCLUSION: When an endocrown restoration is performed, there is a tendency of failure in the cement line and in the root directly proportional to its size. However, regardless of the size of the element to be reconstituted, the axial direction of the masticatory loads tends to decrease stress concentration. CLINICAL SIGNIFICANCE: When performing an endocrown restoration, care must be taken with its high regardless the tooth remnant high, altering even the anatomical angulations of the occlusal face, when necessary, to avoid stress concentration in thick areas.