Mechanical and thermal stress analysis of hybrid ceramic and lithium disilicate based ceramic CAD-CAM inlays using 3-D finite element analysis

Objective: The aim of this study was to analyze mechanical and thermal stresses of hybrid ceramic and lithium disilicate based ceramic of CAD/CAM inlays using 3D Finite element analysis. Material and Methods: A three dimensions finite element model of permanent maxillary premolar designed according to standard anatomy with class II cavity preparation for inlay restored with two different ceramic materials: 1Hybrid ceramic (Vita Enamic), 2Lithium disilicate based ceramic (IPS e.max CAD). Totally six runs were performed on the model as: One loading case for each restorative material was tested in stress analysis; seven points of loading with 140N vertically applied at palatal cusp tip and cusp slop, marginal ridges and central fossa while the models base was fixed as a boundary condition in the two cases. Two thermal analysis cases were performed for each restoration material by applying 5oC and 55oC on the crown surface including the restoration surface. Results: The results of all structures were separated from the rest of the model to analyze the magnitude of stress in each component. For each group, maximum stresses on restorative materials, cement, enamel, and dentin were evaluated separately. Both ceramic materials generated similar stress RESUMO


INTRODUCTION
P reclinical in vitro testing of dental materials is crucial to prove their mechanical capability and compatibility to service in the oral cavity. The oral environment surrounding dental restorations incorporates many challenging conditions like humidity, acidic or basic pH, and cyclic loading [1]. Accordingly, laboratory testing should simulate different aspects of the oral environment to produce failure modes like those seen clinically [2]. Clinically, mechanical failure of dental restorations occurs after many years in service, indicating a fatigue failure rather than acute overload [3].
Recently, finite element analysis (FEA) has been used in biomechanical researches of clinical situations and various areas of dentistry [4]. In fact, some dental and medical studies have been conducted on living subjects. They are costly and ethically skeptical. Using virtual models and simulations can help improve research results and reduce the cost of in vitro and in vivo experiments [5]. Up to date, inconsistent and conflicting results have been reported from studies investigating the effect of inlay or onlay cavity design and restorative materials on stress distribution in the tooth-restoration complex [6]. Therefore, it was speculated that it would be of importance to analyze the stresses of hybrid ceramic and lithium disilicate based ceramic CAD/CAM inlays. distribution patterns for all groups when a total occlusal load of 140 N was applied. Conclusion: Thermal fluctuations of temperature have a great influence on the stresses induced on both restoration and tooth structure. IPS e.max CAD produced more favorable stresses on the tooth structure than Vita Enamic.
The finite element analysis is a method in which geometry of the subject to be analyzed is described through a certain number of geometrical entities (elements) connected in nodes. The stress and strain distribution are derived from displacements calculated in each node. This method permits the response evaluation of a natural system under various loads and conditions [7]. The human teeth cannot be represented through the two finite element analysis dimensional model. Therefore, the actual and structural response cannot be simulated without considering the third dimension.
Finite element analysis results allow a better knowledge in comprehending the actual process which conducts to failure, while in vitro studies reveal the maximum load to failure for the system being tested [8]. The aim of this study was to analyze mechanical and thermal stresses of hybrid ceramic and lithium disilicate based ceramic CAD-CAM inlays using 3D finite element analysis.

Materials
Two types of ceramic blocks and one type of adhesive system were utilized in this study.

B-Cement system
Rely X Unicem, Dual cure self-adhesive universal resin cement.
All materials used in the present study, composition, manufacture, batch number and website are listed in Table I.

Three-Dimensional finite element analysis
The simulated clinical design was class II cavity preparation of upper second premolar for inlay with two different materials: -Lithium disilicate material (IPS e.max CAD) -Hybrid ceramic (Vita Enamic) The generalized steps to perform a finite element analysis can be summarized as follows:     5. Application of load, and boundary conditions. 6. Obtaining the data of resultant stresses and comparing the results.

Planned design details were as follows
Class II cavity preparation on the second premolar with pulpal floor depth 2.5 mm and 1.5 mm axial wall creating a class II box which is 4 mm occlusogingivally and gingival floor of 2 mm.

Model scanning
A Three dimensional (3D) finite element model was constructed by 3D scanning of a sample tooth (second premolar). The tooth geometry was acquired by using laser scanner (Geomagic Capture, 3D Systems, Cary, NC, USA). Such scanner produced data file containing a cloud of points coordinates. An intermediate, software was required (Rhino 3.0 -McNeel inc., Seattle, WA, USA) to trim a newly created surface by the acquired points. Then, the solid (closed) tooth geometry was exported to finite element program as STEP file format.

Geometric model preparation
First, the directions were set up (top, bottom, mesial, distal, anterior, posterior) followed by the mask thresholds to define the mask of enamel and the mask of dentin and to define tooth tissues with its mechanical properties lately and finally, we calculate 3D object.
We used "cut orthogonal to screen" tool to cut through the tooth to reproduce the inlay of the premolar, then we formed the pulpal extension part by cutting in the facial aspect and proximal surface of the premolar, then the two parts were merged to form the whole inlay ( Figure 1).
Then all the dentin parts were merged followed by enamel part construction to be applied in the finite element analysis test with its mechanical properties.
A cement layer of 60 μm was created by using a set of vertical and horizontal planes and applying set of Boolean operations (divide, cut, add, etc.) to keep the cement layer separated from the tooth and the restoration. All these parts were exported in the form of STL files then imported into the software (3-matic 7.01 (Materialise NV).

Meshing
All materials used in this study assumed to be homogenous, isotropic and to possess linear elasticity. Each of the model components (bone, cement and restoration) was assigned to a material property on the finite element package ANSYS Workbench version 16 (ANSYS Inc., Canonsburg, PA, US). Materials properties used in this study are presented in Table II. The parabolic tetrahedral element was used for meshing the model, that adequate mesh density was selected to ensure results accuracy for the discrete model. Mesh density of all model components is presented in Table 3. The model components on ANSYS screen after meshing was illustrated ( Figure 2).
Totally six runs were performed on the model as: One loading case for each restoration material was tested in stress analysis; seven points of loading with 140N vertically applied at palatal cusp tip and cusp slop, marginal ridges and central fossa while the models base was fixed as a boundary condition in the two cases. Two thermal analysis cases were performed for each restoration material by applying 5ºC and 55ºC on the crown surface including the restoration surface, while maintaining bone at 37ºC. The solid modeling and finite element analysis (linear static and thermal analysis) were performed on Laptop (Dell Inspiron 5000, with Core I7, 2.4 GHz processors, 6G).

RESULTS
The results of all structures were separated from the rest of the model to analyze the distribution and magnitude of stress in each component. For each group, maximum stresses on restorative materials, cement, enamel, and dentin were evaluated separately. Both ceramic materials generated similar stress distribution patterns for all groups when a total occlusal load of 140 N was applied onto functional cusp, marginal ridges, and central fossa.

1.2) Mechanical loading for IPS e.max CAD group
Von Mises stresses of restoration, cement, enamel and dentin of order 303.12 MPa, 33.63 MPa, 859.98 MPa and 121.61 MPa respectively ( Figure 4).

2.1) Thermal loading at 5 o C for Vita Enamic group
Von Mises stresses of restoration, cement, enamel and dentin of order 35.339 MPa, 14.106 MPa, 65.974 MPa and 27.4480 MPa respectively ( Figure 5).

2.2) Thermal loading at 5 o C for IPS e.max CAD group
Von Mises stresses of restoration, cement, enamel and dentin of order 22.462MPa, 9.1694 MPa, 58.694 MPa and 27.877 MPa respectively ( Figure 6).

2.3) Thermal loading at 55 o C for Vita Enamic group
Von Mises stresses of restoration, cement, enamel and dentin of order 64.2650 MPa, 29.0320 MPa, 124.04 MPa and 33.498 MPa respectively (Figure 7).

2.5) Thermal loading at 55o C for IPS e.max group
Von Mises stress of restoration, cement, enamel and dentin of order 50.648 MPa, 24.389 MPa, 95.463 MPa and 51.86 MPa respectively ( Figure 8).
Results analysis for mechanical and Thermal loadings at on Vita Enamic and IPS e.max CAD restorations (Figure 9-12).

DISCUSSION
Ceramic materials are sensitive to tensile forces and their mechanical resistance is highly influenced by the presence of superficial scratches and internal voids. Such defects may serve as sites of initiation of cracks. This phenomenon is also influenced by factors such as marginal design, thickness of restoration, residual pressure, porosity, intensity, direction and frequency of applied loads, modulus of elasticity of restoration components, interfacial defects between the restoration and cement and oral conditions [9].
Hot and cold liquid drinks in the mouth cause a temperature gradient that results in thermal stresses because of the different physical and thermal properties of different materials in the restored tooth. The thermal stresses which occur in the restored tooth are dependent on many factors such as the properties of restorative materials, preparation design and adhesive resistance between tooth and restorative materials [10].
Considering the above explained parameters, the thermal loads were determined as 5 o C (for cold liquids) and 55 o C (for hot liquids). In heat variations, the heat on the outer surface of the material can be lower or higher than the temperature of the environment. The reason for this is the transition of heat by convection to the outer surface. The heat transfer among the materials occurs via conduction. Thus, increase or decrease in temperature results in thermal stresses.
The thermal stresses could result in tension stresses that facilitate the initiation of cracks and/ or subcritical crack growth within the ceramic specimens, thereby enabling catastrophic failure. In addition, the thermal stresses could cause interfacial degradation and debonding due to differences in coefficient of thermal expansion and contraction of the tooth, cement and restoration [11].
Values from Finite Element Analysis are divided as von mises stress, maximum principle stress (tensile stress), minimum principle stress (compressive stress) and shear stress. However, in the majority of finite element studies presented in the literature, von mises stress is used as analysis criteria and there is also evidence that the selection of the von mises criteria seems to be reliable because brittle materials, which the tooth is a member of, fail primarily because of tensile and compressive types of stress [12]. Therefore, it was decided to use the von mises stress, once it is the type of stress most used and referred in the literature. In the present study, qualitative analysis of stress distribution was performed using von mises stress diagrams. Von mises stress is essentially an aggregated stress which combines tensile, compressive and shear stresses [13].
The 140 N loads used in this study were chosen, as average chewing force, which is supposed to be the one third of the maximum biting force [14].
The elastic modulus of the material that will support the ceramic restoration should be assessed when considering the use of base materials or selecting a material for a buildup. Materials with lower elastic modulus may lead to decreased strength of the restoration [15].
A finite element analysis was arranged to thermal stress distribution in a restored tooth. The factors which may account for the differences in stress levels include crown geometry, boundary conditions, type, size and number of elements and loading condition. To analyze the stress distribution in our finite element analysis, the influence of the pulp chamber on the stress distribution was ignored, all materials were assumed to be linearly elastic and isotropic and they remained elastic under applied thermal loads. The cement was assumed to bond perfectly to ceramic and dentine [16].
The results of this study revealed that the total von mises, shear stresses, tensile stresses, compressive stresses and total deformation of Vita Enamic is higher than that of IPS e.max CAD in enamel, dentin and cement at the cervical area, occlusal enamel and marginal ridge . This agreed with Gulec and Ulusoy in 2017 [17] who found that the highest von mises stress values were observed in Vita Enamic, while the lowest stress value was observed in feldspathic ceramic. These findings are consistent with the results of the study which defended that materials with low elastic moduli transferred more stress to dental tissues [18].
Also, our results agreed with Yin et al. in 2019 [19]. They stated that these results may be related to the fact that polymer infiltrated ceramic network has low fracture resistance and flexural strength than Glass ceramics which case more stresses on the restoration and surrounding structure at the applied magnitude of load. The results of our study also showed that tensile stresses were smaller than the compressive stresses which was also in agreement with Lin et al. in 2001 [20], Ausiello [5]. These results may be attributed to the combination of axial and bending stresses in compression. The compressive stresses were considerably higher at the palatal cervical area and smaller at the buccal cervical area, this might be the result of tooth bending caused by a perpendicular component of loadings to the longitudinal axis.
The results contradicted with those of Dejak and Mlotkowski in 2008 [24] who reported that the restorative material influences adhesive and cohesive failures in adhesive resin. In their study, the stress values in the composite resin model were lower than the ceramic models. This result may be attributed to the similar elastic moduli of composite resin used compered to both the dentin and adhesive resin. Despite the relatively higher thermal expansion, composite resin absorbs loads without conducting stresses to adhesive resin.
The influence of simultaneous thermomechanical loads on stress distribution associated with two types of inlays was investigated by finite element analysis. The different properties of the restorative materials did not affect the stress distribution. However, the stresses resulting from combined thermomechanical loads were considerably higher than those caused by mechanical loading alone [25].
Temperature changes also create thermal stress on restored teeth. This study demonstrated that tensile stresses and total deformation created by cold exposure was greater than that with hot exposure. This result agreed to the studies of Oskui et al. in 2014 [26] and Gungor et al. in 2010 [27]. These findings may be related to the daily and natural temperature difference between drinkable materials and the oral environment. The temperature difference between 37ºC and 5ºC was significantly higher than 37 ºC to 55ºC.
The stress distribution within the tooth was reversed by the type of thermal exposure. Cold conditions caused tensile stress on the enamel and restoration surface, and compressive one in the dentin, whereas hot conditions caused compressive stress on the enamel and restoration surface, and tensile one in the dentin. These Polymer infiltrated ceramic network had the highest thermal expansion coefficient and exhibited higher stress values compared with lithium disilicate ceramic because of the change in volume. A mismatch between the thermal expansion coefficients of the restorative materials and the tooth will result in differential expansion and contraction during intraoral temperature changes. Several studies have reported that thermal stress concentrations occur at biomaterial interfaces [31].
This study had some limitations such as all specimens were tested using vertical loads, even though lateral forces are the most damaging, so clinical implications of the current study must be limited to that application. Also, comparison of stress distribution between various finite element analysis studies is difficult because of different morphology and/or teeth tested, as well as different loadings and vectors used. 2. IPS e.max CAD produced more favorable stresses on the tooth structure than Vita Enamic.