1. Introduction
The primary function of teeth is biting, which involves the blending, cutting, and grinding of food to allow the tongue and oropharynx to shape it into a bolus that can be easily swallowed. The most common dental problem observed in children, teenagers, and older individuals is cervical cavities, which are caused primarily by a variety of factors such as oral bacteria, frequent snacking, consuming sugary beverages, inadequate tooth cleaning, and occlusal stresses due to wasting diseases [
1]. Preparing a cavity at the neck of the tooth (junction of crown and root surface) is a typical form of treatment modality for restoration of cervical lesions for filling a cavity on the buccal and lingual surfaces (Class V cavities termed by Dr. G V Black) [
2]. It is a crucial activity as the dental restorations should last long in the biological setting. Since the restorative material and the dental substrate have different material properties, dentists need to determine the anticipated mechanical performance of a restored tooth [
3]. Mandibular teeth, especially those with high occlusal loading as seen by the presence of wear facets, have been observed to have a greater failure rate of class V restorations when high-modulus macro-filled materials are used [
4]. Finite element analysis (FEA) studies have shown that varying the mechanical properties of restorations and the restoration/cavity anatomy leads to variations in the stress distribution patterns when the same boundary condition and mastication load are applied on occlusal regions for different models [
4,
5].
The incidence of class V lesions that are noncarious in nature is 31–58%. The location of the lesion can make it more challenging to accomplish a long-lasting and stable restoration, which is the fundamental challenge in restorative treatment [
6]. Amalgam, resin composites, and glass ionomer cement are usually used to restore the cavity [
7]. However, these restorative materials have shortcomings in their ability to sustain the thermal stress and temperature variations (between 0 °C to 67 °C) that possibly occur in the oral cavity. This can cause the contraction and expansion of the cavity after the consumption of hot or cold beverages [
8]. The properties of the materials, the procedures, cavity design, and the influences of the thermal stress on the restored tooth will decide how effectively the material adheres to the tooth surface. Tensile stress is seen on the silver amalgam restorative material when cold liquid is consumed, while compression happens in the resin composite restorative material. The opposite phenomenon takes place when a hot liquid is drunk, putting compression on the amalgam but tensile stress on the composite [
9].
FEA has recently been employed to simulate the clinical context in several biomechanical studies. The finite element assessment is increasingly used to model and simulate dental treatment procedures, including the procedures involved and their effects post-treatment [
10]. The expense of in vitro and in vivo studies can be decreased, and research outcomes can be improved by using these virtual models and simulations [
11]. FEA plays an important role in the assessment of relevant treatment procedures by performing force analysis and material evaluation and approximating the tooth geometry with a finite number of points, three-dimensional (3D) imaging, and mapping of the tooth topology [
12]. Subsequently, structural and thermal stress, compression, and strain calculations are performed for the elemental body [
13]. This study aims to evaluate the causal effect of thermal and thermomechanical stimuli on the thickness of dental restorative materials, geometry, and material properties of cervical restorations.
4. Discussion
Due to the various physical and thermal characteristics of various restorative materials, a temperature gradient caused by hot and cold liquid drinks in the mouth leads to thermal stresses. The heat transfer among the materials occurs due to conduction. Thus, an increase or decrease in temperature results in thermal stresses. The thermal stresses could result in tension stress, which leads to crack initiation or growth within the restorative materials, thereby causing catastrophic failure. The study conducted by Swathi Pai et al. [
1] has shown that a cervical trapezoidal cavity will undergo the least deformation and von Mises stress when Group 5 materials (1 mm GIC, 0.06 mm hybrid layer, 2 mm composite resin) are used, and it is also shown that a cervical elliptical-shaped cavity will undergo the least deformation and von Mises stress when Group 6 (2 mm GIC, 0.06 mm hybrid layer, 1 mm composite resin) materials are used. However, this study did not focus on thermal analysis. The chances of deformation would be higher because of high thermal stress due to temperature differences while drinking hot or cold beverages. In the present study, the main focus was on the behavior of the materials under different thermal and thermomechanical loading conditions.
Several studies have reported that thermal stress concentrations occur at biomaterial interfaces [
16,
17,
18,
19,
20,
21]. Therefore, it is very necessary to achieve good adhesion between the two layers to resist the applied load [
21]. Clinically, this can be achieved by using good isolation techniques. Different stress levels may be caused by variations in the crown shape, boundary conditions, type, size, and several parts, as well as loading conditions. In our finite element analysis, we neglected the effect of the pulp chamber on the stress distribution and assumed that all materials were linearly elastic and isotropic, remaining elastic under applied thermal loads [
22]. These results obtained in the study of Anusavice et al. [
23] supported the outcomes of Gulec and Ulusoy [
24], who argued that materials with low elastic moduli put additional stress on dental tissues. In their study, Gulec and Ulusoy [
24] found that interbedded ceramic had the lowest stress value, and Vita Enamic had the greatest von Mises stress values.
From this study, it can be concluded that the stresses induced in the elliptical cavity are slightly lower than those in a trapezoidal cavity. The study conducted by Nabih et al. [
25] has shown the mechanical and thermal stresses of Vita Enamic and IPS e.max CAD. Compared to Vita Enamic, the IPS e.max CAD produced more valuable stresses on the tooth structure. From the results obtained, it was observed that the materials that have higher elastic modulus will undergo the least deformation. The temperature variations significantly affect the stresses induced on both restoration and tooth structures. This study was not focused on thermomechanical loading, which is a crucial real-time condition where both thermal and mechanical loads interact. However, the present study focused on both thermal and thermomechanical conditions where a concentrated load of 140 N was applied on three occlusion points [
19], along with a temperature of 5 °C and 55 °C applied to the dentin–enamel junction while maintaining cementum temperature at 35 °C. The least von Mises stress and deformation for the trapezoidal cavity were observed by Group 6 (2 mm GIC, 0.06 µm hybrid layer, 1 mm composite resin), and the least von Mises stress and deformation for the elliptical cavity were also observed by Group 1 (1 mm GIC, 0.03 µm hybrid layer, 2 mm composite resin). The least amount of deformation and von Mises stress was observed in two distinct groups, although identical materials were used for the restoration of the trapezoidal and elliptical cavities. In the current study, the deformation observed in both shapes is very negligible; therefore, it would not be significant to compare them entirely based on deformation. Analyzing the region of maximum stress, it is observed that the higher stress values are experienced at the interface between the hybrid layer and GIC. According to a study conducted by Nabih [
25], the strains placed on both the restoration and the tooth structure are greatly influenced by thermal temperature variations, and IPS e.max CAD produced more favorable stresses on the tooth structure than Vita Enamic. The findings of Nabih [
25] concurred with those of Yin et al. [
26]. They claimed that the low fracture resistance and flexural strength of the polymer-infiltrated ceramic network compared to glass ceramics, which causes higher stresses on the restoration and surrounding structure at the applied magnitude of load and may be the cause of these results. In accordance with Lin et al. [
27], Ausiello et al. [
28], Rees et al. [
29], Federlin et al. [
30], and Ausiello et al. [
12], the results of their investigation likewise demonstrated that tensile stresses were lower than compressive stresses.
Dejak and Mlotkowski [
31] used a three-dimensional (3D) finite element analysis involving contact elements in the research. Seven identical 3-D replicas of primary molars were modelled. Intact tooth (IT), unrestored tooth (UT), tooth with a cavity prepared using the Modified Open Technique (MOT); tooth restored with composite resin inlays (CRIT) (True Vitality; 5.4 GPa elastic modulus); tooth restored with composite resin inlays (CRIH) (Herculite XRV; 9.5 GPa elastic modulus); tooth restored with composite resin inlays (CRIC) (Charisma; 14.5 GPa elastic modulus); tooth restored with composite resin inlays (CRIZ) (Z100) The occlusal surface of each model was subjected to 200 N of stress. Calculations were made to determine the stresses experienced by the inlays, composite resin cement layer, and tooth tissues during testing. The Mohr-Coulomb failure criterion was employed to assess material toughness. Dejak and Mlotkowski [
31] measured the tensile and shear binding strengths of luting cement to enamel and dentin, and compared them to contact stresses in the cement-tissue adhesive interface. It was found that the Mohr-Coulomb failure criterion values were lower in teeth restored with composite resin and ceramic inlays compared to those of unrestored teeth with a preparation (UT), but were still 2.5 times higher than those of an undamaged tooth (IT). The Mohr-Coulomb failure criterion values were nearly three times higher for the ceramic inlay (CI) than for the composite resin inlays. These numbers were 2–4 times smaller for the ceramic inlay model’s luting agent compared to those of the composite resin inlay models’ luting agents. Contact tensile and shear stresses were lower at the adhesive interface between the cement and tooth surrounding the ceramic inlays than they were around the composite resin inlays. It was shown that stresses exceeded tissue strength in the cervical enamel adjacent to the inlays’ proximal surface.
Toparli et al. [
8] found that composite resin shows better behavior than amalgam when cold liquid (15 °C) is used. On the contrary, amalgam is more satisfactory when hot liquid (60 °C) is used [
6]. However, we found that the lowest von Mises stress was at the tooth–restorative interface at both 5 °C and 55 °C. The results of our study are not in agreement with those of Toparli et al. [
8] This difference may be related to the different experimental conditions of MS Guler [
32] study. MS Guler [
32] observed that when thermal changes at the interface of tooth materials are taken into account, the smallest stress and maximum stress were found in amalgam and glass ionomer cement, respectively. The varying mechanical and thermal qualities of restorative materials could be the cause. As a result, amalgam could be employed in class V cavities to minimize stress on the restorative material and lower the chance of material loss. The results reported here need to be confirmed by more in vivo and in vitro research.
Temperature fluctuations in the mouth cause cyclic changes that may cause the thermal fatigue of the adhesive process [
32,
33]. The tensile stresses were produced at the regions of load application on the occlusal surface in both restored cases. Therefore, it is essential to control this surface roughness by polishing it to avoid stress concentration spots and the development of fatigue cracks, which might lead to fracture [
34,
35,
36,
37,
38].
The present study revealed that the stress induced in the trapezoidal cavity is slightly higher than in the elliptical cavity. For example, Group 1 of elliptical-shaped cavities generated von Mises stresses of about 14.65 MPa (at 5 °C), 41.84 MPa (at 55 °C), 14.83 MPa (at 5 °C and 140 N), and 28.89 MPa (55 °C and 140 N), while the trapezoidal cavity generated 36.27 MPa (at 5 °C), 74.44 MPa (at 55 °C), 34.14 MPa (at 5 °C and 140 N), and 75.57 MPa (55 °C and 140 N), which is significantly higher than the stress in elliptical cavities. This could be the result of sharp edges present in a trapezoidal cavity as stress concentration occurs because of the sudden change in the geometry. Further study on fatigue analysis is required to predict the number of cycles to the failure. This analysis can be applied to all kinds of cavity restorations to predict the life of the filler material.
5. Conclusions
The least deformation and von Mises stress for an elliptical-shaped cavity were shown by Group 1 (1 mm GIC, 0.03 µm adhesive layer, and 2 mm composite layer), and the highest was shown by Group 5 (1 mm GIC, 0.06 µm adhesive layer, and 2 mm composite layer), whereas, in the trapezoidal-shaped cavity, the highest deformation and stress were observed in Group 1 (1 mm GIC, 0.03 µm adhesive layer, and 2 mm composite layer), and the least stress in Group 6 (2 mm GIC, 0.06 µm adhesive layer, and 1 mm composite layer). It was observed that maximum deformation occurred at the upper right end of the cavity. From this study, we can conclude that the stresses induced in the elliptical cavity are slightly lower when compared to the trapezoidal cavity. The transfer of load between the layers is largely governed by the cavity shape.