| Abstract|| |
Aims: Considering new ceramic systems, doubts about the appropriate combination of ceramics and cement are common. Settings and Design: To evaluate the influence of the elastic modulus (E) of cement agents associated with different indirect veneers on the stress distribution using finite element analysis. Methods and Materials: The finite element analysis was applied to evaluate the stress distribution on the structures. For that, a computer-aided design software was used for a three-dimensional (3D) modeling of an upper central incisor with preparation for an indirect veneer. The model was imported into the analysis software in STEP (Standard for Exchange of Product data) format. Tetrahedral elements formed the mesh. Solids were considered isotropic, linearly elastic, homogeneous, and with ideal contacts. Load application (100N, 45°) occurred on the lingual face. Cement agents have their E classified as low, intermediate, and high. The ceramic materials used were a hybrid ceramic, a zirconia reinforced lithium silicate and a lithium disilicate. Results: It was observed that none of the factors significantly influenced the stress concentration in dentine. Groups with high E cementing agent showed the highest stress peaks. The E of restorative material was significant for the stress generated in the veneer, and groups with hybrid ceramic presented more homogeneous stress results. Conclusions: The higher E of the cement agent and the ceramic, the higher the stress concentration, suggesting that hybrid ceramic associated with low elastic modulus resinous cement has superior biomechanical behavior.
Keywords: Cementing agent, ceramic, finite element analysis, Young's modulus
|How to cite this article:|
Penteado MM, Mendes Tribst JP, Dal Piva AM, Archangelo KC, Bottino MA, Souto Borges AL. Influence of different restorative material and cement on the stress distribution of ceramic veneer in upper central incisor. Indian J Dent Res 2020;31:236-40
|How to cite this URL:|
Penteado MM, Mendes Tribst JP, Dal Piva AM, Archangelo KC, Bottino MA, Souto Borges AL. Influence of different restorative material and cement on the stress distribution of ceramic veneer in upper central incisor. Indian J Dent Res [serial online] 2020 [cited 2020 May 30];31:236-40. Available from: http://www.ijdr.in/text.asp?2020/31/2/236/284566
| Introduction|| |
Ceramic veneer exhibit resistance to wear, favorable aesthetics, chemical stability, and biocompatibility. Therefore, they are considered as conservative alternatives for the tooth color, position, and shape (conoid tooth and diastema), which shows compromised aesthetics.,,,
The veneer allows preserving greater dentine remnant, and provides a minimally invasive treatment., A ceramic and cement combination provides a favorable prognosis for a wide range of clinical cases, but some factors such as dental surface, ceramic thickness, cement type, preparation geometry, and occlusion are decisive factors for clinical success.,,
Clinical investigation researches on ceramic restorations showed survival rates of 93.5% and 82.93% after 10 and 20 years, respectively. The occurrences of catastrophic failures have been less reported, mainly because of the evolution of aesthetic ceramic materials. Despite accentuated enhancement of these materials, there is still a possibility of cracks and chipping., Limitations of veneer restorative systems are due to extensive preparation which exposes dentine and compromises the bond strength, increasing the occurrence of debonding or fracture. Therefore, conservative preparations to preserve enamel (higher bond strength values than dentin) are the best technique choice whenever possible.,, This technique can only be indicated due to the ceramic quality and manufacturing techniques that allow restorations with minimum thickness.,
The occurrence of failures at incisal and cervical areas are common in veneer restorations. It was observed as stress concentration in those areas. Therefore, to improve the stress profile preparation design is fundamental, even so indirect veneer is not indicated to overload through unbalanced occlusal, parafunctional activity, or deleterious habits.,
On the other hand, cementing agents have a chemical composition which ensure good adhesion to both tooth and ceramic material when associated with adhesive systems. Many studies have been conducted for this type of investigation,,, as well as for surface treatments that could improve the adhesive interface. However, there are no reports that have evaluated the combination of the cement and ceramic elastic modulus and their influence on the stress distribution in the veneer, cement line, and dentine.
To evaluate the influence of each structure mechanical properties, the finite element method can be used. This computational simulation allows visualizing the fields of stress generated during the incidence of masticatory load, with limited variables but enough sensitivity to allow modifications in the colorimetric scale to visualize the differences between the groups. Although it is a theoretical analysis, the 3D simulation by Finite Element Analysis (FEA) method is widely used in dentistry, and its findings may contribute to support future laboratory or even clinical studies. As every research method, FEA has limitations, so the results are valid but must be interpreted with care.
Therefore, the purpose of this study was to evaluate the combination of different elastic modulus of the cementing agent (three levels) and the ceramic (three levels) using the finite element analysis, which allows analyzing the biomechanical behavior mathematically, without the need for recruiting natural teeth and investing in restorative materials., The null hypothesis was that different combinations of the materials would not interfere in the veneers performance.
| Subjects and Methods|| |
Computer-Aided Design (CAD) Rhinoceros 5.0 software (McNeel North America, USA) was used for 3D modeling of an upper central incisor for preparing an indirect veneer [Figure 1]c. The final model [Figure 1]f with enamel, dentine, periodontal ligament [Figure 1]d, ceramic [Figure 1]a, and cementing agent [Figure 1]b positioned in the fixation cylinder [Figure 1]e was acquired from the Laboratory of Bioengineering at São Paulo State University (Unesp/São José dos Campos) [Figure 1].
|Figure 1: Schematic illustration of modeling sequence. (a) Indirect veneer. (b) Cementing agent. (c) Veneered tooth. (d) Periodontal ligament. (e) Cylindrical fixing base. (f) Complete system|
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Cementing agents and ceramics varied respectively between low, intermediate, and high elastic modulus [Table 1]. The elastic modulus of Hybrid ceramic (Vita Enamic, VITA Zahnfabrik, Bad Säckingen, Germany), Zirconia-reinforced lithium silicate (Vita Suprinity, VITA Zahnfabrik, Bad Säckingen, Germany), and Lithium disilicate (IPS Emax press, Ivoclar Vivadent, Liechtenstein) were used. The veneer was assumed to have 0.5 mm of thickness and a cement line of 0.3 mm. Then, the model was imported in STEP format to Ansys (version 15.0), a Computer-Aided Engineering (CAE) software. Tetrahedral elements formed the mesh (639.065 elements and 1.097.954 nodes), and the solids were considered isotropic, linearly elastic, and homogeneous. All contacts were considered perfectly bonded and the fixation occurred at the base of the cylinder. A mesh convergence test (10%) was performed to guarantee that it would not interfere with the results. A load (100 N) was applied on the lingual face of the central incisor at 45° for structural static analysis, simulating an occlusal contact situation. The mechanical properties (Elastic modulus and Poisson ratio) required for the analysis were summarized in [Table 1]. Results of maximum principal stress were requested for failure criteria. Results with difference larger than 10% were considered significant different.
|Table 1: Mechanical properties of materials used in the study: Elastic modulus (E) and Poisson's ratio|
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| Results|| |
Considering the maximum principal stress results, it was observed that the elastic modulus of the cement and the restorative material did not influence the stress distribution in dentine [Figure 2]a. The lingual region near the cement–enamel junction is the site of the highest stress concentration in all groups.
|Figure 2: Maximum principal stress for (a) dentine, (b) cement line, and (c) veneer, according to material's (hybrid ceramic, zirconia reinforced lithium silicate and lithium disilicate, from left to right) and cementing agent's elastic modulus (low, intermediate, and high elastic modulus, top to bottom)|
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For the cement line, its elastic modulus minimally influenced the stress concentration. The group with high E value presented the highest stress concentration near the restoration's margins, regardless of the type of ceramic [Figure 2]b. Stress generated in the veneer was influenced by the “restorative material.” Groups restored with hybrid ceramic (the lowest E) showed less stress concentration in the vestibular face and interproximal margins [Figure 2]c. In [Figure 3], maximum principal stress peaks (in the dentine, cement layer, and veneer) for each cement agent is shown according to the evaluated restorative material. According to the gradual increase of the cementing agents' elastic modulus, similar behavior is observed for the dentine; however, there is an increase in stress for both cement line and veneer.
|Figure 3: Maximum principal stress peaks in dentine, cement layer and venner for: (a) hybrid ceramic, (b) zirconia-reinforced lithium silicate and (c) lithium disilicate. The abscissa axis shows the different (E) for cement layer|
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| Discussion|| |
It was possible to correlate ceramic materials and different cementing agents through finite elements analysis (FEA), observing the stress generated in three regions – dentine, cement, and ceramic veneer. The null hypothesis was rejected because the cementing agent's and the restorative material's elastic modulus significantly influences the stress distribution in the rehabilitation.
FEA analysis is widely used in dentistry to study different situations such as partial restorations, total crowns, dental implants, intraoral devices, and mechanical responses after clinical interventions. Here, the simulation was performed using a validated 3D model,, following a compliance that evaluated other variables in indirect veneer restorations using FEA.
During the chewing process, a load is directly applied on the lingual face of upper central incisors. When a tooth is submitted to veneer restorative treatment, the first substrate found by the load vector is the enamel, which behaves in a similar manner, regardless of the ceramic and cement agent, reflecting similar results for dentine between all groups. These results can probably be explained due to the thickness of remaining dentine being the same for all groups and close enough to the load to avoid different behavior between groups. The thickness of the veneer (0.5 mm) and the cement (0.3 mm) add up to 0.8 mm of dental wear, assuming conventional necessary wear parameters.,,, It is possible that increased wear interferes in stress distribution in dentine and more invasive cases require endodontic treatment. The amount of tooth wear (conservative or conventional) does not interfere with a tooth's fracture rates, but more invasive preparations dissipate more stress to the veneers.
Three different elastic modulus corresponding to three ceramic alternatives available on the market were used: a hybrid ceramic with a polymeric matrix having a low elastic modulus, a long-term successful lithium disilicate based ceramic already proven in the literature,, and finally a zirconia-reinforced lithium silicate as an alternative to lithium disilicate with similar strength.
It is known that stress behavior is strongly associated with laminate design. Literature shows that butt joint restoration at incisal face dissipate lower stress to the teeth.,, Therefore, the model was designed without chamfer preparation modeled with 0.5 mm of thickness also to improve the adhesive performance.
Ceramic veneers' adhesive cementation consists of a crucial step because if the complete polymerization of the resin cement is not reached, the restoration's color stability will be compromised in the long term, as well as the mechanical properties of fracture resistance and adequate adherence to remaining dentine.,,
To improve mechanical properties of resin cements, the composite resins were started to be used as cementing agents due to its large number of charge. Its preheating allows better adaptation of the restoration and better monomer conversion. This use of a high charge containing cementing agent was simulated in the group with high elastic modulus. Thus, when the stress in the cement line was observed, it was possible to observe that the larger the elastic modulus of the cement material, the higher the concentration of stress near the edges of the restoration. As shown in [Figure 2]c, small regions of stress concentration were assumed to be insufficient to detach the veneers due to the large area that remains homogeneous. However, because it is the marginal region, it is possible to suggest the occurrence of marginal infiltration and pigmentation. It can also be observed that the higher the stress concentration in the cement line, the more stress transmitted to the ceramic, which is not enough to fracture it due to the flexural strength of the restorative materials currently available on the market. Therefore, the adhesive interface remains the critical region of this study because the restorations' success is attributed to the quality of the adhesive interface.
Although ceramic materials have high elastic modulus and high stiffness, hybrid ceramics (one of evaluated materials with the lowest elastic modulus) presented the most favorable behavior in relation to the load distribution for both cement line and veneer. It is suggested that veneer cementation with a composite resin may be unfavorable according to the ceramic materials used. This is contrary to the findings of Addison et al. (2007), and Fleming et al. (2012), wherein high elastic modulus favored the ceramics' performance. It is possible that the results obtained by these studies are linked to the type of ceramics used, and that possible adhesive failures will cause future problems not immediately observed.
Marginal adaptation of hybrid ceramic compared to feldspathic is higher; however, bond strength is lower than feldspathic after fatigue. In principal, it was observed that the gradual increase of the cement's elastic modulus did not favor the performance of the veneers. In literature, there is no long-term monitoring of cementation with heated composite resin,, but in using FEA's mathematical results it is believed that pre-heating composite resin with low elastic modulus could allow to postpone marginal infiltration.
It is important to emphasize that this is a theoretical study. In this manner, an ideal condition was absent from variables such as temperature, pH, and mechanical fatigue present in the oral environment., In addition, the materials simulated herein are isotropic, absent from defects with a homogeneous cementation layer without bubbles, and with controlled thickness. The findings herein are valid due to the consolidated methodology.,,,,,, However, future laboratory and clinical studies should be conducted to prove the theoretical results presented.
| Conclusion|| |
Within this study's limitations, the combination of both ceramic and cement with low elastic modulus suggests the best biomechanical behavior. Therefore, clinicians should be aware regarding the best combination of materials for a better clinical prognosis.
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Conflicts of interest
There are no conflicts of interest.
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Amanda Maria de Oliveira Dal Piva
Laboratory of Dental Materials and Prosthodontics, 777 Avenida Eng. Francisco José Longo, São José dos Campos, SP - 12245000
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3]