Effect of crown retention systems and loading direction on the stress magnitude of posterior implant-supported restorations: A 3D-FEA.

This study aimed to investigate the effect of four retention systems for implant-supported posterior crowns under compressive loading using three-dimensional finite element analysis. A morse-taper dental implant (4.1 × 10 mm) was designed with Computer Aided Design software based on non-uniform rational B-spline surfaces. According to International Organization for Standardization 14,801:2016, the implant was positioned at 3 mm above the crestal level. Then four models were designed with different crown retention systems: screw-retained (A), cement-retained (B), lateral-screw-retained (C), and modified lateral-screw-retained (D). The models were imported to the analysis software and mesh was generated based on the coincident nodes between the juxtaposed lines. For the boundary conditions, two loads (600 N) were applied (axial to the implant fixture and oblique at 30°) totaling 8 conditions according to retention design and loading. The von-Mises stress analysis showed that different retention systems modify the stress magnitude in the implant-supported posterior crown. There is a similar stress pattern in the implant threads. However, models C and D presented higher stress concentrations in the crown margin in comparison with A and B. The oblique loading highly increased the stress magnitude for all models. In the simulated conditions, part of the stress was concentrated at the lateral screw under axial loading for model C and oblique loading for model D. The results indicate a possible new failure origin for crown retained using lateral screws in comparison to conventional cement-retained or screw-retained systems.

Cyclic fatigue vs static loading for shear bond strength test of lithium disilicate and dentin substrates: A comparison of resin cement viscosities.

OBJECTIVE: To explore the effect of resin cement viscosities on the shear bond strength under static and fatigue load of lithium disilicate and dentin substrates. METHODS: Bonded tri-layer samples (lithium disilicate ceramic cylinder, resin cement, and substrate - ceramic or dentin) was performed considering 2 factors (n = 15): "resin cement viscosity" (high, HV; or low, LV) and "loading mode" (static, s-SBS; or fatigue shear bond strength, f-SBS). The specimens were subjected to s-SBS (1 mm/min, 1 kN load cell) and f-SBS (cyclic fatigue, initial load: 10 N; step-size: 5 N; 10,000 cycles/step; underwater). Failure mode, topography, and finite element analysis (FEA) were performed. RESULTS: The resin cement viscosity did not influence the s-SBS and f-SBS of lithium disilicate substrate; however, it affected the bond strength to dentin, with HV presenting the worst fatigue behavior (f-SBS = 6.89 MPa). Cyclic loading in shear testing induced a notorious detrimental effect with a relevant decrease (16-56 %) in bond strength and survival rates, except for dentin substrate and LV. Most failures were adhesive. A distinct pattern comparing the disilicate and dentin was identified and FEA demonstrated that there was a stress concentration on the top of the cement layer. SIGNIFICANCE: Cyclic fatigue loading in shear testing has detrimental effects on the adhesive behavior and survival probabilities of bonded lithium disilicate sets, regardless of resin cement viscosity. In contrast, resin cement viscosity affects the bond strength and the survival rates of dentin substrate submitted to cyclic loading mode, in which a low viscosity results in better performance.

Mechanical characterization of a multi-layered zirconia: Flexural strength, hardness, and fracture toughness of the different layers.

This study compared the flexural strength under monotonic (static - sσ) and cyclic load application (fatigue - fσ), hardness (H) and fracture toughness (KIC) of different layers of a multi-layered zirconia (IPS e.max ZirCAD MT Multi, Ivoclar). Each layer was sectioned, classified into three groups according to yttria content (4-YSZ, 4/5-YSZ and 5-YSZ), and shaped on samples for flexural strength and fracture toughness tests (bars: 1.0 × 1.0 × 11 mm); and Vickers hardness test (plates: 1.5 × 4.0 × 5.0 mm). Flexural strength under monotonic load application (sσ; n = 10) was obtained through two different devices (three-point-bending and ball-in-hole device) and fatigue flexural strength (fσ; n = 15; initial load: 10 N; step-size: 5 N; 10,000 cycles/step) was assessed using a ball-in-hole device under cyclic load application. Vickers hardness test (n = 5), fracture toughness test (n = 10), and additional analyzes (Finite Element Analysis - FEA, Energy-dispersive X-ray spectroscopy - EDS and Scanning Electron Microscopy - SEM) were also performed. No differences were found between the different devices in the monotonic flexural strength test, and FEA showed similar tensile stress distribution for the two devices. 4-YSZ showed higher values of flexural strength under monotonic and cyclic load application modes (sσ = 1114.73 MPa; fσ = 798.84 MPa), and fracture toughness (KIC = 3.90 MPa√m). 4/5-YSZ had an intermediate sσ; however, fσ was similar to 5-YSZ (404.00-429.36 MPa) and KIC similar to 4-YSZ (KIC = 3.66 MPa√m). No statistical differences were found for hardness and Weibull modulus for fatigue flexural strength data. The amount of yttria in the layer compositions increased from 4-YSZ to 5-YSZ, and larger zirconia crystals were observed in the topographic images of 5-YSZ. Failures in the flexural strength and toughness tests started from the face subjected to tensile stress. Different layers of the multi-layered zirconia blank presented distinct mechanical properties. 4-YSZ (cervical layer) presented the highest flexural strength under monotonic and cyclic loads (fatigue), and higher fracture toughness even similar to the transition layer (4/5-YSZ). Hardness was similar between the layers. The ball-in-hole device performed similarly to the three-point bending device and can be used as an alternative to the traditional method.

The relation between impact strength and flexural strength of dental materials.

AIM: The aim of this study was to investigate whether there is a relation between impact strength and flexural strength of different composite and ceramic materials used in dental restorations. MATERIALS AND METHODS: The three-point-bending test was used to determine the flexural strength and flexural modulus, and the Dynstat impact test was used to determine the impact strength of different composite and ceramic dental materials. The relation between the flexural strength and impact strength was mathematically investigated and a three-dimensional finite element analysis model of the impact test set-up was created to verify these results. RESULTS: We found a relation between the impact strength, adU, the flexural strength, σ, and the flexural modulus, E, which can be represented by the formula: adU=λDK(σ2/E), where λDK is a constant dependent on the test set-up. CONCLUSION: The obtained impact strength of materials is specific to the test set-up and dependent on the geometric configuration of the test set-up and the specimen thickness. The clinical significance of this investigation is that roughness and fatigue have far more influence on the impact strength than the flexure strength.

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.

Effect of different materials and undercut on the removal force and stress distribution in circumferential clasps during direct retainer action in removable partial dentures.

OBJECTIVES: This study aimed to evaluate the effect of different materials and undercut on the removal force and stress distribution in the supporting tooth and in the circumferential clasp used in removable partial prosthesis. METHODS: Upper molars prepared for Akers circumferential clasp with retention and opposing arm were modeled, scanned, elaborated with CAD software and the geometries imported in FEA and analyzed. Six different materials were selected for the clasp (Polyamide, Polyoxymethylene, Polyetheretherketone - PEEK, Gold alloy, Titanium and CoCr) and 3 different undercuts (0.25, 0.50 and 0.75mm), totaling 18 groups. RESULTS: The clasps presented greater stress in their structure and potentially greater damage to the dental enamel when made with rigid materials and with more undercut; however, they presented greater ability to remain in position. SIGNIFICANCE: Polyamide with a higher undercut is an esthetic alternative to rigid metallic clasps. It showed promising behavior because it strongly reduces the damage to the enamel, and even with an undercut of 0.75, the retention is lower than for CoCr with a 0.25 undercut, and this retention might still be sufficient. Polyoxymethylene and Polyetheretherketone (PEEK) are not suitable materials for the clasps, because the maximum stress occurring during removal with higher undercuts is higher than the material strength.

Properties of an In Vivo Fractured Poly(Methyl Methacrylate) Cranioplasty After 15 Years.

BACKGROUND: In 2001, a 27-year-old man was diagnosed with a meningioma with skull bone involvement. A craniectomy was performed and a CMW-3 poly(methyl methacrylate) cranioplasty was manually manufactured to reconstruct the remaining cranial defect. In 2016, he complained about progressive neurologic impairment. A computed tomography scan revealed that the cranioplasty had fractured into 4 dislocated pieces. Removal was indicated, and during the same operation a polyetheretherketone patient-specific implant was inserted. METHODS: The fractured cranioplasty was compared with freshly prepared CMW-3 specimens to determine whether the material properties had changed during 15 years in vivo. Gel permeation chromatography, microcomputed tomography, and flexural strength tests were performed. The fracture itself was analyzed using finite element analysis. RESULTS: The polydispersity index and molecular weight were not significantly different for the fractured cranioplasty and CMW-3. The fractured cranioplasty contained a total porosity of 10.7%, fresh CMW-3 cured at atmospheric pressure contained 4.1%, and 0.06% when cured at 2.2 bar. The flexural strength of the CMW-3 cured at 2.2 bar was significantly higher than both the fractured cranioplasty and CMW-3 cured at atmospheric pressure. Finite element analysis showed stress of 12.2 MPa under a load of 100 N on a weak spot. CONCLUSIONS: This ex vivo study shows that CMW-3 after 15 years in vivo was not influenced in molecular weight or flexural strength. However, the design of the implant and the handling of the poly(methyl methacrylate) seem to be important factors to improve mechanical properties of cranial reconstructions.

Does Luting Strategy Affect the Fatigue Behavior of Bonded Y-TZP Ceramic?

PURPOSE: To evaluate the effect of different luting strategies on the fatigue failure load (FFL) and stress distribution of Y-TZP disks luted to epoxy resin substrate. MATERIALS AND METHODS: Y-TZP disks (diameter = 10 mm; thickness = 0.7 mm) were assigned to five groups according to the luting strategy (n = 15): CC: no zirconia surface treatment, composite cement; G_CC: application of a thin glaze layer on zirconia followed by hydrofluoric acid etching and silanization, composite cement; Al_CC: air abrasion of the zirconia surface with 125-µm Al2O3 particles, composite cement; Si_CC: tribochemical silica coating (30-µm SiO2 particles), composite cement; ZP: air abrasion of the zirconia surface with 125-µm Al2O3 particles, zinc-phosphate cement. The disks were luted to the epoxy resin substrate. The FFL was evaluated by the step-test method. The load was applied in stages of 10,000 cycles, starting with 600 N, followed by increments of 200 N. Samples were loaded until fracture or to a maximum of 100,000 cycles. FFL data were submitted to Kaplan-Meier (α = 0.05) and Weibull analyses. Conditions simulating a strong and a weak bond between Y-TZP and epoxy resin were evaluated in the finite element analysis (FEA). RESULTS: Luting strategy influenced the FFL (p < 0.001) and the stress distribution of Y-TZP disks. Al_CC (2227 ± 149 N) and Si_CC (2133 ± 235 N) showed the highest FFL values, followed by CC (1800 ± 293 N) and G_CC (1280 ± 147 N), while ZP showed the lowest FFL value (680 ± 101 N). The highest Weibull modulus occurred in Al_CC (18.2). A strong bond reduced the tensile stress concentration in the Y-TZP luting surface, while a weak bond between Y-TZP and the epoxy resin favored the concentration of tensile stresses. CONCLUSION: Air abrasion with aluminum oxide and silica-coated alumina particles improves the FFL of bonded Y-TZP. Zinc-phosphate cement should be used very carefully in the cementation of zirconia restorations, since it results in lower values of FFL than composite cements.

Elastic Properties of Lithium Disilicate Versus Feldspathic Inlays: Effect on the Bonding by 3D Finite Element Analysis.

PURPOSE: To determine the elastic properties of five ceramic systems with different compositions (lithium disilicate vs. feldspathic ceramics) and processing methods and compare the stress distribution in premolars in the interface with inlays made with these systems loaded with the maximum normal bite force (665 N) using 3D finite element analysis (FEA). MATERIALS AND METHODS: The elastic properties of five ceramic restoration materials (IPS e.max Press, IPS e.max CAD, Vita PM9, Vita Mark II, Vita VM7) were obtained using the ultrasonic pulse-echo method. Three-dimensional FEA simplified models of maxillary premolars restored with these ceramic materials were created. The models were loaded with a load at the two nodes on the occlusal surface in the middle of the tooth, 2 mm from the outside of the tooth, simulating a loading ball with a radius of 6 mm. RESULTS: The means values of density (g/cm³), Young's modulus (GPa), and Poison's ratio was 2.6 ± 0.3, 82.3 ± 18.3, and 0.22 ± 0.01 for IPS e.max Press; 2.3 ± 0.1, 83.5 ± 15.0, and 0.21 ± 0.01 for IPS e.max CAD; 2.5 ± 0.1, 44.4 ± 11.5, and 0.26 ± 0.08 for PM9; 2.4 ± 0.1, 70.6 ± 4.9, and 0.22 ± 0.01 for Vitamark II; 2.4 ± 0.1, 63.3 ± 3.9, and 0.23 ± 0.01 for VM7, respectively. The 3D FEA showed the tensile stress at the interface between the tooth and the inlay was dependent on the elastic properties of the materials, since the Vita PM9 and IPS e.max CAD ceramics presented the lowest and the highest stress concentration in the interface, respectively. CONCLUSIONS: The elastic properties of ceramic materials were influenced by composition and processing methods, and these differences influenced the stress concentration at the bonding interface between tooth and restoration. The lower the elastic modulus of inlays, the lower is the stress concentration at the bonding interfaces.

The effect of internal roughness and bonding on the fracture resistance and structural reliability of lithium disilicate ceramic.

OBJECTIVE: To evaluate the effect of internal roughness and bonding on the load to failure and structural reliability (Weibull analysis) of a lithium disilicate-based glass ceramic under different testing scenarios. METHODS: IPS e.max CAD blocks (Ivoclar Vivadent AG) were shaped into cylinders (N=100), crystalized according to the manufacturer's instructions, and randomly assigned into two surface conditions: (1) polished surface (600-grit SiC polish papers), and (2) a roughened surface (air-abrasion with 50µm Al2O3). Two assemblies were investigated: a ceramic disc isolated (to isolate the effect of roughness); and a simplified tri-layer setup simulating the restoration of a posterior tooth (ceramic+cement+epoxy resin) to evaluated the influence of bonding isolated and the associated effect of both factors. Four different scenarios were tested: (1) isolated disc under static load (n=10); (2) disc bonded to an epoxy resin substrate and tested under a static load (n=10); (3) disc bonded and tested under fatigue (n=20); and (4) simulated-bonding tested statically (n=10). The data of load to failure were submitted to One-way ANOVA and Weibull analysis. RESULTS: At a non-bonded scenario (isolated disc and simulated-bonding) a polished internal surface presented a higher characteristic strength. However, when bonding was present this difference became inexistent. No difference was found in terms of structural reliability (Weibull moduli) among the groups. FEA analysis shows that with bonding the tensile stress is better distributed, while in a non-bonded scenario higher tensile stresses occur at the bonding interface. SIGNIFICANCE: A rough internal surface impacted deleteriously the mechanical properties of lithium disilicate ceramic when it was not properly bonded to the substrate. However, bonding to the substrate appeared to play a more significant role in the fracture resistance than internal roughness.

Effect of a retention groove on the shear bond strength of dentin-bonded restorations.

STATEMENT OF PROBLEM: With the increasing use of minimally invasive restorations, effective adhesion becomes more important. Applying mechanical retention to a flat dentin surface might improve the adhesion of ceramic and composite resin restorations. PURPOSE: The purpose of this in vitro study was to evaluate the effect of a groove in a flat dentin surface on the bond strength of various restorative materials. MATERIAL AND METHODS: Dentin specimens of bovine teeth were prepared with or without a groove. Identical disks were fabricated from direct composite resins (Filtek Supreme XTE [FS] and Clearfil AP-X [AP]) and indirect ceramics (Vita Mark II [VM] and IPS E.max CAD [EM]). These materials were bonded directly or cemented adhesively to the dentin specimens. Shear bond strength was tested with a universal testing machine. Finite element analysis (FEA) models of the test arrangement were made to further analyze the stress distribution. RESULTS: VM (no groove, 5.1 ±3.0 MPa; groove, 8.7 ±1.5 MPa) and EM (no groove, 11.4 ±3.7 MPa; groove, 17.7 ±5.2 MPa) showed significant effect of a groove on the shear bond strength. FS (no groove, 18.6 ±4.9 MPa; groove, 16.3 ±4.3 MPa) and AP (no groove, 25.8 ±3.8 MPa; groove, 24.2 ±7.2 MPa) showed no significant effect of a groove. For the composite resins, the retention groove increased the shear stress along the dentin-restoration interface, and debonding at the contact surface started at lower load values than for the specimens without a groove. CONCLUSIONS: Application of a groove to a flat dentin surface improved the shear bond strength for ceramic restorations. For direct composite resin restorations, exhibiting a lower elastic modulus, a groove had no significant effect on the shear bond strength, while it increased the shear stress along the dentin-restoration interface for composite resin.

Mechanical performance of implant-supported posterior crowns.

STATEMENT OF PROBLEM: The fracture of implant-supported restorations, especially of the veneering layer, is a common problem in dentistry. Monolithic ceramic or resin restorations might help solve this problem. PURPOSE: The purpose of this in vitro study was to obtain additional insight into the risk of fracture of implant-supported restorations. MATERIAL AND METHODS: Identical crowns (n=10) of 10 different ceramic and composite resin materials were cemented on conventional abutments on implant replicas embedded in polymethyl methacrylate blocks. The specimens were subjected to compressive load in a universal testing machine to record initial load to failure (ILF). Additionally, the flexural strength (FS), compressive strength (CS), and elastic modulus (E) of the investigated materials were determined. These results were used in a finite element analysis model of a composite resin and a lithium disilicate crown. RESULTS: Anatomic contour zirconia (Lava Plus) crowns had the highest ILF (6065 N), followed by lithium disilicate (IPS e.max) (2788 N) and the composite resin materials (Protemp 4, Majesty Flow, Telio CAD, Estenia C&B, Lava Ultimate, VITA Enamic) (2386 to 1935 N). Veneered zirconia (Lava) crowns showed the lowest ILF (1477 N). The highest FS, CS, and E were found for Lava Plus and IPS e.max. No direct relationship was found between ILF and the FS, CS, or E. The finite element analysis showed stresses that did not exceed the FS or CS of IPS e.max. The surface roughness of these crowns might have caused initial failure at relatively low stresses. CONCLUSIONS: In this laboratory study, monolithic implant-supported crowns showed a higher ILF than conventional veneered ceramic crowns. Monolithic ceramic restorations might perform better than composite resin crowns.

In vitro debonding of orthodontic retainers analyzed with finite element analysis.

OBJECTIVE: The aim of this in vitro study was to determine the load and deflection at failure of different lingual retainers bonded with composite to enamel in a standardized three-point bending test. The results were rationalized with finite element analysis (FEA) models. MATERIALS AND METHODS: Four types of multistranded wires, Dead Soft Respond, Twisted ligature, Penta-One, Gold-plated Penta-One, and two glass fibre-reinforced composite retainers, Fibre 07 and Fibre 09, were bonded to enamel with composite and submitted to a three-point bending test. The load and deflection at failure and the mode of debonding were recorded. The stiffness of the wires was determined and all experimental data were used in FEA models to rationalize the observed values and mode of debonding. RESULTS: Significant higher load and deflection were found for the most flexible retainers Twisted ligature and Dead Soft Respond. All retainers failed between the wire and composite, which was confirmed by FEA showing the highest stress in the composite around the retainer. The FEA models showed that the amount of composite used for bonding the retainers should be 2-4mm. CONCLUSIONS: Based on the in vitro results, optimal bonding of lingual retainers can be achieved by flexible retainers, bonded with intrinsically strong composites. According to the FEA models the retainer should be bonded with 2-4mm composite, leaving the critical 'free-wire' length for the success of the retainer system.

The fracture resistance of a CAD/CAM Resin Nano Ceramic (RNC) and a CAD ceramic at different thicknesses.

OBJECTIVES: This study aimed to investigate the influence of restoration thickness to the fracture resistance of adhesively bonded Lava™ Ultimate CAD/CAM, a Resin Nano Ceramic (RNC), and IPS e.max CAD ceramic. METHODS: Polished Lava™ Ultimate CAD/CAM (Group L), sandblasted Lava™ Ultimate CAD/CAM (Group LS), and sandblasted IPS e.max CAD (Group ES) discs (n=8, Ø=10 mm) with a thickness of respectively 0.5 mm, 1.0 mm, 1.5 mm, 2.0 mm, and 3.0 mm were cemented to corresponding epoxy supporting discs, achieving a final thickness of 3.5 mm. All the 120 specimens were loaded with a universal testing machine at a crosshead speed of 1 mm/min. The load (N) at failure was recorded as fracture resistance. The stress distribution for 0.5 mm restorative discs of each group was analyzed by Finite Element Analysis (FEA). The results of facture resistances were analyzed by one-way ANOVA and regression. RESULTS: For the same thickness of testing discs, the fracture resistance of Group L was always significantly lower than the other two groups. The 0.5 mm discs in Group L resulted in the lowest value of 1028 (112) N. There was no significant difference between Group LS and Group ES when the restoration thickness ranged between 1.0 mm and 2.0 mm. There was a linear relation between fracture resistance and restoration thickness in Group L (R=0.621, P<0.001) and in Group ES (R=0.854, P<0.001). FEA showed a compressive permanent damage in all groups. SIGNIFICANCE: The materials tested in this in vitro study with the thickness above 0.5 mm could afford the normal bite force. When Lava Ultimate CAD/CAM is used, sandblasting is suggested to get a better bonding.

Mechanical behavior of a bi-layer glass ionomer.

OBJECTIVE: A high-viscosity consistency of the glass-ionomer cement (GIC) may lead to poor adaptation into the cavity. The use of a flowable GIC layer seemed to improve its adaptation in approximal restorations in vitro. In this study we assessed the flexural strength of a two-layered GIC, using a flowable GIC as a liner (two-layer technique). Additionally, finite element analysis on standardized bar-shaped models and on a representative tooth model was performed to rationalize the obtained results. METHODS: The flexural strength and Young's modulus were calculated from the results of a three-point-bending test. Bar-shaped specimens were prepared either with a conventional GIC, with a flowable GIC (powder/liquid ratio 1:2), or with two-layers (either with the flowable layer down or on the top of the specimen). Three dimensional FEA models of the bar-shaped specimens and a model of tooth 46 provided information on the stress distribution of each component of the specimen and on the restoration. RESULTS: The apparent flexural strength and Young's modulus of both two-layered groups were significantly lower than that of the conventional group. FEA showed that the layers of the two-layer specimens with the flowable GIC down separated from each other under load. The tooth model showed better stress distribution for the two-layer restorations. SIGNIFICANCE: The two-layer GIC showed inferior flexural strength, which might be explained by the detachment of the layers under load. Nevertheless the tooth model showed that the two-layer GIC provides a lower stress concentration on the occlusal surface of the material.

Shear bond strength of three dual-cured resin cements to dentin analyzed by finite element analysis.

OBJECTIVES: To determine the shear bond strength to bovine dentin of dual-cured resin cements cured in different circumstances, the contraction stress and volumetric shrinkage in both polymerization modes, and to review the failure stress distribution at the cement-tooth interface with finite element analysis. METHODS: The volumetric shrinkage of RelyX Unicem, Panavia F 2.0 and DC Core Automix was determined by mercury dilatometry. Polymerization contraction stress was determined using a constraint tensilometer set-up. For the shear bond strength test, cement discs on bovine root dentin (self-cured and dual-cured), composite discs cemented to dentin (self-cured and dual-cured), and dentin cemented to dentin (self-cured) specimens were fabricated. Specimens were stored in water for 24h (37°C, 100% humidity) and tested (crosshead speed 1mmmin(-1)). FE modeling of the specimens was carried out in order to calculate the maximum shear stresses in the cement-dentin interface. Differences between groups were determined using two-way ANOVA with Tukey post hoc tests, and paired samples t-tests (α<0.05). RESULTS: Panavia F2.0 showed significantly lower volumetric shrinkage than the other cements. Dual-curing lead to higher contraction stresses for all tested cements compared to self-curing. RelyX Unicem showed higher volumetric shrinkage when dual-cured. Shear bond strength and maximum shear stress was positively influenced by dual-curing. DC Core Automix performed best and Panavia F2.0 worst in terms of shear bond strength and maximum shear stress. SIGNIFICANCE: Curing mode may play an important role in the final bond strength to dentin of indirect restorations, depending on the material used.

The influence of pigments on the slow crack growth in dental zirconia.

OBJECTIVES: Partially yttria stabilized zirconium oxide was introduced as core material for core-veneered full ceramic dental restorations, because of its biological inertness, high mechanical strength, and toughness. In order to improve the esthetical possibilities pigments in the core are introduced, that might influence the stabilization by yttrium. METHODS: Double torsion tests were performed to study the influence of the pigments in the core ceramics on its fracture toughness. RESULTS: A significant difference was observed in the stress intensity factor (K(10)) as well as in the R-curve behavior between the ceramic with and without pigment. SIGNIFICANCE: The lower stress intensity factor for the ceramic with pigment could affect the clinical performance of dental zirconia restorations with this material.

The influence of rotating fatigue on the bond strength of zirconia-composite interfaces.

OBJECTIVES: To evaluate the effect of cyclic loading on the bond strength of resin composite to zirconia framework material. METHODS: Bar shaped zirconia/composite specimens (2 mm x 2 mm x 25 mm) were prepared using three different resin cements and placed in a four-point bending test setup. The flexure strength (F(s)) was calculated by placing the bars (n=10) fixed between the four supports (at 10 and 20 mm) with the interface centered between the inner rollers and subsequently loaded (1 mm/min crosshead speed) until fracture. Rotating fatigue resistance (RFR) was determined in a rotating bending cantilever test setup (104, 1.2 Hz) with the highest stress located at the interface (n=20). The RFR was determined by the staircase method and the mean RFR was calculated using logistic regression analysis. RESULTS: Resin cement composition had no significant influence on the bond strength value obtained by both F(s) (F=0.6, P> or =0.5) and RFR (F=1.1, P> or =0.3) tests. However, after rotating fatigue testing there was a significant reduction in bond strength between 46 and 50% of the three resin cements. CONCLUSION: Zirconia resin bond strength is liable to deterioration under the influence of fatigue.

Influence of retainer design on two-unit cantilever resin-bonded glass fiber reinforced composite fixed dental prostheses: an in vitro and finite element analysis study.

PURPOSE: The aim of this study was to evaluate in vitro the influence of retainer design on the strength of two-unit cantilever resin-bonded glass fiber-reinforced composite (FRC) fixed dental prostheses (FDP). MATERIALS AND METHODS: Four retainer designs were tested: a proximal box, a step-box, a dual wing, and a step-box-wing. Of each design on 8 human mandibular molars, FRC-FDPs of a premolar size were produced. The FRC framework was made of resin impregnated unidirectional glass fibers (Estenia C&B EG Fiber, Kuraray) and veneered with hybrid resin composite (Estenia C&B, Kuraray). Panavia F 2.0 (Kuraray) was used as resin luting cement. FRC-FDPs were loaded to failure in a universal testing machine. One-way ANOVA and Tukey's post-hoc test were used to evaluate the data. The four designs were analyzed with finite element analysis (FEA) to reveal the stress distribution within the tooth/restoration complex. RESULTS: Significantly lower fracture strengths were observed with inlay-retained FDPs (proximal box: 300 +/- 65 N; step-box: 309 +/- 37 N) compared to wing-retained FDPs (p < 0.05) (step-box-wing: 662 +/- 99 N; dual wing: 697 +/- 67 N). Proximal-box-, step-box-, and step-box-wing-retained FDPs mainly failed with catastrophic cusp fracture (proximal box 100%, step-box 100%, and step-box-wing 75%), while dual-wing-retained FDPs mainly failed at the adhesive interface and/or due to pontic failure (75%). FEA showed more favorable stress distributions within the tooth/restoration complex for dual wing retainers. CONCLUSION: A dual-wing retainer is the optimal design for replacement of a single premolar by means of a two-unit cantilever FRC-FDPs.

Prestresses in bilayered all-ceramic restorations.

INTRODUCTION: A general trend in all ceramic systems is to use veneering ceramics of slightly lower thermal expansion coefficients compared with that of the framework resulting in a positive mismatch in thermal expansion coefficient (+DeltaTEC). The concept behind this TEC mismatch is to generate compressive stresses in the weaker veneering ceramic and thus enhance the overall strength of the restoration. This technique had excellent results with porcelain fused to metal restorations (PFM). However, there are concerns to apply this concept to all-ceramic restorations. The aim of this research was to determine the stresses in bilayered all-ceramic restorations due to the mismatch in TEC. MATERIALS AND METHODS: Two commercial veneering ceramics with a TEC lower than that of zirconia (+DeltaTEC); NobelRondo zirconiatrade mark and Lava Ceramtrade mark, plus one experimental veneering ceramic with an identical TEC that matches that of zirconia (DeltaTEC = 0) were used to veneer zirconia discs. The specimens were loaded in biaxial flexure test setup with the veneer ceramic in tension. The stresses due to load application and TEC mismatch were calculated using fractography, engineering mathematics, and finite element analysis (FEA). RESULTS: In this study, the highest load at failure (64 N) was obtained with the experimental veneer where the thermal mismatch between zirconia and veneering ceramic was minimal. For the two commercial veneer ceramics the magnitude of the thermal mismatch localized at the zirconia veneer interface (42 MPa) exceeded the bond strength between the two materials and resulted in delamination failure during testing (ca. 50 MPa). SIGNIFICANCE: For all-ceramic zirconia veneered restorations it is recommended to minimize the thermal mismatch as much as possible.