Niek de Jager

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.

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.

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.

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.

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.

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 Youngs 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 Youngs 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.

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), Youngs modulus (GPa), and Poisons 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 manufacturers 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.