High probability of survival was observed even for 0.5 mm restorations.
Models with thinner restorations accumulated more stress in the structures.
Resin-matrix ceramic materials seem up-and-coming systems for occlusal veneers.
To evaluate the influence of resin-matrix ceramic material and thickness on reliability and stress distribution of occlusal veneers (OV).
One hundred and twenty-six OV of a mandibular first molar were milled using a CAD/CAM system and allocated according to materials (resin nanoceramic (RNC) or polymer-infiltrated ceramic network (PICN)) and thicknesses (0.5, 1.0 and 1.5 mm), totaling six groups (RNC0.5, RNC1, RNC1.5, PICN0.5, PICN1, and PICN1.5). Step-stress accelerated-life testing was performed (n = 21/group) with the load applied at the distobuccal cusp tip of the occlusal veneer until failure or suspension. The use level probability Weibull curves and reliability were calculated and plotted (90% CI). Finite element analysis evaluated the stress distribution according to maximum principal stress (σ max ) on the restoration and maximum shear stress (τ max ) on the cement layer.
There was no difference in the probability of survival for the estimated missions among the groups, except at 600 N in which the results were significantly lower to PICN1.5 (6%) compared to RNC1 (55%) and RNC1.5 (60%). The σ max values were higher for PICN (31.85–48.63 MPa) than RNC (30.78–33.09 MPa) in the same thicknesses. In addition, 0.5 mm groups concentrated more stress in the restoration (33.09–48.63 MPa) than 1.0 mm (31.11–35.36 MPa) and 1.5 mm (30.78–31.85 MPa) groups in the same material.
Both resin-matrix ceramic materials seem up-and-coming restorative systems for occlusal veneers irrespective of the thicknesses as a consequence of the high reliability.
The increase and/or reestablishment of the occlusal vertical dimension is essential to prevent the progression of the detrimental effects caused by pathological wear of the dental structure [ ]. Conventionally, the treatment involves non-conservative rehabilitations with metal-ceramic or all-ceramic crowns, demanding a significant reduction of healthy dental tissues [ ]. Nevertheless, less invasive techniques, such as occlusal veneers (OV), may be indicated as a restorative approach for patients where the dental structure has been selectively lost by wear [ , ].
The restoration thickness is an important factor influencing its longevity, with a proportional increase in the load required to initiate a fracture as a function of restoration thickness [ ]. Conventional occlusal surface rehabilitation of posterior teeth requires a minimum thickness of 1.5 mm to support chewing loads [ ]. However, the possibility to treat severe tooth wear using ceramic or composite resin OV with reduced thicknesses of approximately 0.5 mm has shown promising results [ , ]. Hence, the dental structure can be preserved from conventional preparation, eventually reducing the risk of endodontic complications and preserving enamel for adhesive cementation [ ]. Although the use of OV seems a promising restorative alternative, the material and minimum thickness combination for OV fabrication still requires further investigation.
Resin-matrix ceramics (RMCs) are a group of CAD/CAM restorative materials that combines the characteristics of polymers, which have a similar modulus of elasticity to dentin, with the reinforcement mechanism of ceramics [ ]. One of the RMCs is the resin nanoceramic (RNC), consisting of zirconia nanoparticles and zirconia-silica nanoclusters linked by a highly cured resin matrix (Bis-GMA, UDMA, Bis-EMA, TEGDMA) [ ]. RNC presents a 12 GPa dentin-like elastic modulus and a higher flexural strength (∼150 MPa) and fracture toughness (∼1.2 MPA.m 1/2 ) when compared to light-cured composite resins (∼130 MPA and ∼0.9 MPA. m1/2 ) [ ]. Another RMC is the polymer-infiltrated ceramic network (PICN), consisting of a feldspathic ceramic matrix in which an organic phase of dimethacrylate resin (UDMA and TEGDMA) is infiltrated (159 MPa and 1.3 MPA 1/2 ) [ , ]. PICN shows an elastic modulus of approximately 37 GPa, which is also similar to dental structures when compared to conventional ceramic systems [ , ].
Considering that different thicknesses and resin matrix ceramic material used for OV are yet to be investigated, this study characterized the reliability and stress distribution of RNC and PICN occlusal veneers of 0.5, 1.0, and 1.5 mm thicknesses. The null postulated hypothesis was that the resin-matrix ceramic system and restoration thickness would not influence the probability of survival (reliability) and the stress distribution of occlusal veneers.
Materials and methods
A CAD-based three-dimensional (3D) model of a mandibular first molar was designed (SolidWorks 2013; SolidWorks Corporation, Concord, MA, USA). A uniform reduction on the occlusal surface was performed to produce by boolean operations, three CAD models of restorations with different thicknesses (0.5, 1.0, and 1.5 mm). For the milling parameters of the restorations, a marginal gap of 0 μm and an internal gap of 50 μm were established between the tooth and the restoration. The restorations were milled (Ceramill Motion 2; Amann Girrbach, Koblach, Austria) in two different materials (n = 21/material): resin nanoceramic (Lava Ultimate; 3 M Oral Care, St Paul, MN, USA) and polymer-infiltrated ceramic network (Enamic; Vita Zahnfabrik, Bad Sackingen, Germany), totaling 126 occlusal veneers. Finishing and polishing were performed according to the manufacturers’ recommendations. The CAD-modeled preparation replicas, simulating advanced wear of the occlusal surfaces were milled in fiber-reinforced epoxy resin (n = 21/group) (G10; Protec, São Paulo, SP, Brazil) due to its similarity in terms of adhesion (9 MPa) and elastic modulus (18.6 GPa) to hydrated dentin (6.5 MPa and 18 GPa, respectively) [ , , , ].
All OV were cemented to aged (stored in distilled and deionized water at 37 °C for 21 days) [ , ] fiber-reinforced epoxy resin preparation preparations using a dual-cure resin cement (RelyX Ultimate; 3 M Oral Care, St Paul, MN, USA). Epoxy resin preparations were etched with 10% hydrofluoric acid (Condac porcelana; FGM Produtos Odontológicos Ltda, Joinville, SC, Brazil) for 120 s [ ]. The pretreatment of the OV followed the manufacturers’ instructions: RNC was sandblasted with aluminum oxide 50 μm grain size (Óxido de Alumínio, Bio-Art, São Carlos, SP, Brazil), at a distance of ∼1 cm for 5 s at 2 bar, and PICN was etched using 5% hydrofluoridric acid (Condac porcelana; FGM Produtos Odontológicos Ltda, Joinville, SC, Brazil) for 60 s [ , ]. Then, both epoxy resin preparation and OV were cleaned in an ultrasonic bath with distilled water for 2 min. The adhesive (Single Bond Universal; 3 M Oral Care, St Paul, MN, USA) was applied on both ceramic and epoxy resin preparation for 20 s, and then resin cement was applied on the internal OV surface. After the restoration-to-preparation setting, the assembly was maintained under a load of 10 N [ , ] to standardize the cementation process thus promoting a uniform cement spreading and thickness within the intaglio surfaces. The cement excess was removed with a micro-brush. The margins were light-cured (radii-cal; SDI Limited Bayswater, Victoria, Australia) for 20 s on each surface. After that, the set was vertically embedded in acrylic resin (Self-curing acrylic resin, Artigos Odontológicos Clássico Ltda, Campo Limpo Paulista, SP, Brazil).
Step stress accelerated life testing (SSALT)
Three specimens per group were subjected to the single load-to-failure (SLF) test in a universal testing machine (Instron 4411, Corona, CA, USA) with a load applied axially through a tungsten carbide indenter on the central fossa of the occlusal surface of the OV using a 5 kN load cell at a loading rate of 1 mm/min. Based on the SLF data, three different step-stress profiles, mild, moderate, and aggressive, were designed.
The remaining specimens (n = 18/group) were allocated to the three fatigue profiles: mild (n = 9), moderate (n = 6) and aggressive (n = 3), following the distribution ratio of 3:2:1, respectively, considering that the accuracy of the statistic prediction is inversely proportional to its cycling length [ ].
The SSALT test was performed in an electrodynamic fatigue testing machine (ElectroPlus E3000 Linear-Torsion Test InstrumentTM; Instron, Norwood, GA, USA) in the presence of distilled water at 20 Hz [ ]. In each cycle, the tungsten carbide indenter contacted 0.5 mm lingual to distobuccal cusp tip of the occlusal veneer in an axial direction. The test was considered finished when samples failed (considered as chipping or delamination) or when suspended for no event being detected throughout the designed cycles and 1500 N maximum load.
Based on the SSALT failure distribution, the use level probability Weibull curves were calculated and plotted (Synthesis 9, Alta Pro; Reliasoft, Tucson, AZ, USA) using the Weibull distribution and the inverse power law life-stress relationship for damage accumulation. The reliability was calculated for completion of a mission of 100,000 cycles at 200, 300, 400, 500 and 600 N, and the differences between groups were identified based on the non-overlap of the 90% two-sided confidence interval (CI). This analysis provides the beta (β) value, which describes the behavior of the failure rate over time (β <1: values indicate that the failure rate has decreased over time, β ∼ 1: failure rate does not vary over time, and β> 1: means that the failure rate has increased over time) [ ]. If the calculated beta value was <1 for any group, then a Weibull 2-Parameter Contour plot (Weibull modulus – m vs. characteristic strength eta – η) was calculated using the final load to failure data of all groups.
Representative fractured specimens were sputter-coated with gold (Emitech K650; Emitech Products Inc, Houston, TX, USA) for scanning electron microscopy (SEM) analysis (JSM-5600LV, Jeol, Boston, MA, USA) and fractographic evaluation.
Finite element analysis
The CAD models of preparation replicas and OV were imported into the finite element analysis software (Ansys Workbench 15.0; Ansys, Inc, Canonsburg, PA, USA). A convergence test of 10% [ ] was performed to guarantee that no further mesh refinement was required to achieve accurate numerical results of the FEA model and, subsequently, the element size was determined (0.5 mm). The three-dimensional solid models meshed with tetrahedral and hexahedral elements were automatically generated. The number of elements and nodes varied depending on the model (33,744 to 36,853 elements and 81,738 to 88,153 nodes). All structures were considered isotropic, homogeneous, and linearly elastic [ ]. To simulate the influence of different structures, physical and mechanical properties (modulus of elasticity and Poisson’s ratio) of each structure of the model were obtained from the literature ( Table 1 ) and introduced on the software. All contacts were considered perfectly bonded. An axial load of 600 N was applied as a normal surface pressure load on the distobuccal cusp tip of the occlusal veneer models. The stress distribution was evaluated according to the criteria of maximum principal stress (σ max ) on the OV restoration and maximum shear stress (τ max ) on the cement layer. A scale of values was determined to compare stress patterns in the models.
|Material||Modulus of elasticity||Poisson’s ratio||References|
|Resin Nanoceramic||12.7||0.45||Wendler et al. [ ]|
|Polymer-infiltraded ceramic network||37.8||0.24||Wendler et al. [ ]|
|Epoxy Resin||14.9||0.31||Yi & Kelly [ ]|
|Resinous Cement||18.3||0.33||Li-li et al. [ ]|
Use level probability Weibull curves at a use load of 300 N for the occlusal veneers are plotted in Fig. 1 . The mean beta (β) values were lower than 1 for PICN0.5, PICN1, RNC0.5, and RNC1 groups, indicating that failure rate decreased over time and was controlled by material strength. However, for PICN1.5 and RNC1.5 beta (β) values were higher than 1, indicating that failure rate increased over time and fatigue damage accumulation dictated failures ( Table 2 ).
|0.5 mm||1.0 mm||1.5 mm||0.5 mm||1.0 mm||1.5 mm|
|200 N||99 aA||100 aA||100 aA||100 aA||99 aA||99 aA|
|300 N||93 aAB||97 aAB||99 aAB||95 aAB||96 aAB||93 aAB|
|400 N||79 aBC||90 aABC||94 aABC||79 aBC||88 aBC||72 aBC|
|500 N||53 aCD||77 aBC||82 aBC||45 aCD||70 aCD||35 aCD|
|600 N||24 abD||55 aC||60 aC||11 abD||45 abD||6 bD|