To investigate the influence of the convergence angle of tooth preparation on the fracture load of Y-TZP-based ceramic (YZ – Vita YZ) substructure (SB) veneered with a feldspathic porcelain (VM9 – Vita VM9).
Finite element stress analysis (FEA) was performed to examine the stress distribution of the system. Eighty YZ SB were fabricated using a CAD–CAM system and divided into four groups ( n = 20), according to the total occlusal convergence (TOC) angle: G6 – 6° TOC; G12 – 12° TOC; G20 – 20° TOC; and G20MOD – 20° TOC with modified SB. All SB were veneered with VM9, cemented in a fiber reinforced epoxy resin die, and loaded to failure. Half of the specimens from each group ( n = 10) were cyclic fatigued (10 6 cycles) before testing. Failure analysis was performed to determine the fracture origin. Data were statistically analyzed using Anova and Tukey’s tests ( α = 0.05).
The greatest mean load to fracture value was found for the G20MOD, which was predicted by the FEA. Cyclic fatigue did not significantly affect the load of fracture. Catastrophic failure originating from the internal occlusal surface of the SB was the predominant failure mode, except for G20MOD.
The YZ–VM9 restorations resisted greater compression load than the usual physiological occlusal load, regardless of the TOC angle of preparations. Yet, the G20MOD design produced the best performance among the experimental conditions evaluated.
The introduction of high crystalline content ceramic materials increased the use of metal-free restorations. The yttria partially stabilized zirconia (Y-TZP) has aroused the greatest interest among the metal-free ceramic systems . Several studies reported on the properties of Y-TZP ceramic , broadening clinical applications. However, the clinical success of these restorations is also dependent on other factors, such as tooth preparation and restoration design that have shown little evolution over the time. Tooth preparation for prosthetic restorations was among the first aspects to receive specific recommendations. In 1923, Prothero indicated that the total occlusal convergence (TOC), which consists of the convergence angle between two opposing axial surfaces, should range from 2° to 5° . The ideal TOC values are considered to be between 2° and 22° . Nevertheless, studies have shown the difficulty in obtaining preparations with low mean TOC values , which ranged from 14.1° to 27.3° varying according to the preparation purpose (extra-oral training or intra-oral practice), type and location of the tooth and dental surfaces involved. Yet, preparations with 12° TOC are recommended for metal-free crowns machined by CAD–CAM systems mainly because of the marginal and internal adaptation resulting from this TOC.
The preparation TOC influences on the amount of restorative material required for the rehabilitation. It seems that the design and the amount of the porcelain and ceramic layers may influence the fracture resistance of the restoration, since these factors produce different stress distribution throughout the structure . The few clinical studies on Y-TZP-based restorations reported that chipping failures are a significant problem of this system . The ceramic–porcelain bond strength seems to be another inherent problem of zirconia-based restorations and may facilitate porcelain chipping .
Understanding the clinical failure mode of dental ceramics is crucial to adequate indication of such restorations. Principles of fractography are used to study the fracture surfaces of a ceramic restoration . In an attempt to reproduce the clinical failure of brittle materials in laboratory experiments, some researchers have developed innovative methodologies, identifying the fracture origin, which is usually located at porcelain subsurface or at cementation surface .
Finite element analysis (FEA) is a useful tool for a nondestructive approach, since it has the ability to predict the mechanical and structural behavior of the materials, evaluating different designs, load types and elastic characteristics of the experimental components . Thus, the purpose of this study was to investigate the influence of tooth preparation TOC and cyclic fatigue on the fracture load of Y-TZP-based ceramic substructure (SB) veneered with a feldspathic porcelain, testing the hypotheses that (1) the maximum principal stress varies according to the TOC angle of the FEA models, (2) the cyclic fatigue decreases the fracture load of the restorations, and (3) the preparation TOC angle and the SB design of the restorations influence the load to fracture.
Materials and methods
The materials used in this study are shown in Table 1 .
|VITA In-Ceram ® YZ||Vita Zahnfabrik, Bad Sackingen, Germany||Densely sintered zirconia-based ceramic partially stabilized by yttria, indicated for bridge and crown substructures (CTE ≈ 10.5 × 10 −6 K −1 )|
|VITA VM ® 9||Vita Zahnfabrik, Bad Sackingen, Germany||Feldspathic porcelain indicated for veneer zirconia-based substructures (CTE ≈ 9 × 10 −6 K −1 )|
|VITA VM ® Modeling Liquid||Vita Zahnfabrik, Bad Sackingen, Germany||Liquid indicated for mixing with the porcelain powder|
|VITA In-Ceram ® YZ COLORING LIQUID (color LL1)||Vita Zahnfabrik, Bad Sackingen, Germany||Pigmented liquid used for complete or partial coloring of frameworks made from VITA In-Ceram YZ zirconium dioxide|
|NEMA grade G10||International Paper, Hampton, SC, USA||Dentin analog material – epoxy filled with woven glass fibers|
|Aquasil Easy Mix Putty and Aquasil Ultra Low Viscosity||Dentsply, Petropolis, RJ, Brazil||Hydrophilic addition reaction silicone|
|Porcelain conditioner||Dentsply, Petropolis, RJ, Brazil||10% hydrofluoric acid|
|Panavia F 2.0||Kuraray, Tokyo, Japan||Dual resin cement containing phosphate monomer (MDP)|
|Monobond S||Ivoclar Vivadent, Schaan, Liechtenstein||Silane coupling agent|
Finite element analysis (FEA)
The stress distribution of ceramic restorations cemented on preparations with different TOC angles was investigated using FEA. The models simulated the experimental tests and had the following parameters: base of 8 mm (diameter) × 6 mm (height), preparation of 6 mm in height, rounded shoulder finish line (radius = 0.5 mm) and TOC angles varying according to the models (experimental groups). FEA models named G6, G12 and G20 had preparations with 6°, 12° and 20° TOC angles, respectively ( Fig. 1 ). The substructure (SB) thickness was uniform, 0.5 mm in the axial walls and 0.7 mm in the occlusal surface, for all three models ( Fig. 2 ). The G20MOD model simulated a modified SB cemented on a 20° TOC preparation. The aim of the G20MOD modified SB was to compensate for the convergence angle, resulting in a porcelain layer similar to G6 model ( Fig. 2 ). The cement thickness was set to 100 μm for all models and the external design of the restorations was identical regardless of the TOC angle of the preparations. The models were created (Rhinoceros 4.0, Seattle, WA, USA) and exported to a simulation program (Ansys, Canonsburg, PA, USA), where the material properties were inserted ( Table 2 ). It was assumed that all solids were homogeneous, isotropic and linear elastic. The components were considered perfectly bonded and flawless. A mesh composed by tetrahedral dominant elements was generated after the convergence test, which was done by changing the size of elements to reach less than 10% of variation. The number of nodes ranged between 509,226 (G20) and 548,382 (G6), and the number of elements were from 293,067 (G20) to 314,910 (G6). The support base was constrained in the three axes ( x , y and z ) and the stress was generated from a load of 1000 N applied to the center of the occlusal surface of the restoration (area of 0.03 mm 2 ) in the axial axis direction ( Fig. 2 ). Maximum principal stress (MPS) analysis was used to verify the stress distribution in the materials.
|Feldspathic veneering porcelain||67||0.21|
|Dentin analog material||14.9||0.31|
The dies were fabricated with a dentin analog material (epoxy filled with woven glass fibers; NEMA grade G10). This material has elastic and adhesion properties similar to hydrated dentin . The dimensions of the dies were described for the FEA models ( Fig. 1 ). The restorations were made using the In-Ceram YZ system, which is a Y-TZP-based ceramic (YZ) SB veneered with a feldspathic porcelain (VM9) ( Table 1 ). The experimental groups were divided ( n = 20) according to the TOC angle of the preparation:
G6 – 6° TOC preparation and SB with uniform thickness (0.5 mm for the axial walls and 0.7 mm for the occlusal);
G12 – 12° TOC preparation and SB thickness as described for G6;
G20 – 20° TOC preparation and SB thickness as described for G6;
G20MOD – 20° TOC preparation and modified SB compensating the convergence, resulting in an external SB design similar to G6.
Fig. 2 schematically shows the preparation and SB design and the position and direction of load application for FEA and laboratory experiments. A representative die from each experimental group was duplicated in a type IV high-strength dental stone (CAM-base, Dentona AG, Dortmund, Germany). The dental stone dies were scanned by the CAD–CAM system (Cerec inLab, Sirona Dental Systems, Bensheim, Germany) and the substructures (SB) were generated by the software (inLab 2.9, Sirona). The G20MOD SB design was modified to increase the axial wall thickness compensating for the convergence angle and resulting in an external wall design similar to G6 SB. After machining, all SB were sonically cleaned in distilled water, subjected to the cleaning firing cycle ( Table 3 ) and dipped in coloring liquid ( Table 1 ) for 2 min, as per manufacturer’s instructions. The substructures were sintered ( Table 3 ) and a laboratory technician applied the porcelain (VM9). First, a thin layer of porcelain was applied (wash) and sintered ( Table 3 ), then the main porcelain layer (body) was applied to achieve the external design of the restoration, which was sintered in accordance with the manufacturer’s instruction ( Table 3 ). External wall standardization of the restorations was obtained using abrasive burs and controlled measurements (Digimatic caliper, Mitutoyo Corp., Tokyo, Japan), followed by a glaze cycle ( Table 3 ). All restorations had a final thickness of 1.8 mm at the occlusal surface (SB – 0.7 mm; veneer – 1.1 mm) and 1.6 mm at the finish line (SB – 0.5 mm; veneer – 1.1 mm). The restoration thickness at the axial wall varied as described above. All restorations were sonically cleaned with isopropyl alcohol and cemented onto the woven glass fiber-filled epoxy dies using a resin cement system (Panavia F) containing a phosphate monomer (MDP) ( Table 1 ). The bonding area of the dies were etched with 10% hydrofluoric acid for 1 min, washed in water, dried using oil-free air, silanated and the adhesive system (ED Primer A + B) was applied ( Table 1 ) prior to cementation. Cement pastes were mixed and applied to the internal surface of the restorations, which were placed onto the dies. A constant cementation load of 750 g was applied to the occlusal surface and the excess cement was removed from the finishing line prior to light curing (Radii-cal LED curing light, SDI, Victoria, Australia; 1200 mW/cm 2 ) for 20 s from each restoration surface. The cemented restorations were stored in 37 °C distilled water for 24 h and randomly divided into two subgroups ( n = 10): cyclic fatigued (c) or water stored.
|Type of cycle||Furnace||Firing parameters|
|Starting temperature (°C)||Holding time (min)||Heating rate (°C/min)||Firing temperature (°C)||Holding time (min)|
|YZ sintering cycle||Zyrcomat T, Vita Zahnfabrik, Bad Sackingen, Germany||40||–||17||1530||120|
|YZ cleaning firing||Vacumat 6000 MP, Vita Zahnfabrik, Bad Sackingen, Germany||600||3||33||700||5|
|Base dentin wash firing||500||2||55||950||1|
To cyclic fatigue the restorations, the dies were hold by their base and submitted to 10 6 cycles at 4 Hz and load of 88 N (ERIOS ER-11000, São Paulo, Brazil) in 37 °C distilled water to simulate 1 year of oral service . Cyclic fatigued restorations were inspected for surface damages under the stereomicroscope (Zeiss Stemi 2000-C, Edmund Optics Inc., Barrington, NJ, USA; 20–100×). The non-fatigued specimens were stored in 37 °C distilled water while the remaining specimens were cycled fatigued (approximately 7 days). All restorations were submitted to a compressive load (crunch the crown test) applied by a sphere-shaped stainless steel piston (1.5 mm curvature radius) to the center of the occlusal surface ( Fig. 3 ). The test was performed in a 37 °C water environment using a universal testing machine (EMIC DL-1000, EMIC, Sao Jose dos Pinhais, PR, Brazil) at a cross-head speed of 0.5 mm/min. The load to fracture ( L , in N) was registered and the contact pressure ( P ) at fracture (MPa) was calculated according to the following equations :
P = 3 E 1 4 k r 2 / 3 L 1 / 3 π