Background: The dental filling is mainly used to restore the partially lost dental structure caused by external factors such as trauma or dental caries and is exposed to a similar assortment of loads as the sound tooth. These loads can be due to mastication, biting, swallowing, chewing, clenching, bruxism, speech and by the action of the tongue, perioral and circumoral musculature too. Aim: This study aimed to determine the effect of three different types of class IV with different amounts of tooth preparation destruction (butt joint, 2mm bevel, and plain chamfer) on the stress profile in and around labial and palatal class V restoration in the maxillary central incisor under three loading conditions, including masticatory, parafunctional, and traumatic case.

Methodology:  A Three-dimensional finite element model was constructed by 3D scanning of a sound maxillary central incisor. Changes were made in the crown region to create two groups of class V restoration including labial and palatal; each of which contains four models depending on class IV restoration. A static force of 190 N was delivered at three different loading conditions including masticatory, parafunctional, and traumatic cases. Then stress distribution was analysed in the structures of the models in the cervical area separately.

Results: The maximum Von Mises stresses concentration in both groups of class V restoration were found with plain chamfer type of class IV restoration with a higher amount of stress percentage increasing in the palatal side that led to failure in the enamel-restoration interface. Furthermore, according to the loading conditions, the higher values were reported under the second loading condition followed by the third loading and then the first loading conditions.

Conclusion: This study confirms that the amount of the remaining tooth structure has an effect on the stress distribution on class V restoration in the maxillary central incisor, so this point should take into consideration when selecting of class IV preparation type in the presence of class V restoration in the same tooth.

The dental filling is subjected to a similar range of loads as the sound tooth and is mostly used to restore the partially destroyed dental structure brought on by external sources such as trauma or dental caries. Mastication, biting, swallowing, chewing, clenching, bruxism, speaking, as well as the motion of the tongue, perioral, and circumoral musculature, can all result in these loads [1]. In a perfect occlusion, the anterior teeth protrude outward to shield the back teeth. Additionally, it has the ability to tear food, and the stresses produced by these abilities are crucial for the long-term success of restorations. The most widely recognized theory explaining the development of the abreaction lesions as a result of tooth deflection forces is mechanical stress from high occlusal forces, according to numerous research [2,3]. During tooth deflection, enamel tissue near the cemento-enamel junction (CEJ) is subjected to high stresses because the forces have to flow into and through it to the root of the tooth and subsequently into the supporting bone [4]. Therefore, the restorations in the cervical area can be subjected to a high amount of stress even though these regions are not liable to direct contact during mastication [5,6]. Furthermore, according to a study by the tensile stress in the enamel of the tooth increases from the incisal margin towards the cervical line, also the shear stress is higher at the incisal margin and decreases towards the cervical line [7]. According to the findings of these research, class IV and class V restorations on the maxillary incisor tooth are more likely to be located where occlusal loads from biting and protrusive movement are concentrated. Therefore, high fracture resistance is needed for restorations in anterior teeth where high impact stresses are present [8]. The use of a suitable restoration compound, a suitable adhesive, and ideal dental cavity preparation are methods to improve the biomechanics of restorations. Composite resin performs superbly both aesthetically and mechanically when used as a restorative compound [9,10]. Additionally, bevelling the cavity margin during cavity preparation has been shown to improve restoration retention [11,12]; similarly, chamfer preparation has been demonstrated to improve fracture resistance in class IV restoration [8]. Since its inception, composite resins have played a significant role in the field of restorative materials [13]. The development of the acid-etching process by Buonocore produced a significant advance in conservative dentistry [14]. Additionally, over the past few decades, the technology behind composites has advanced steadily, making it the material of choice for both anterior and posterior tooth restoration [15]. Therefore, numerous studies using a variety of different methods must be applied in an effort to study the internal stresses in teeth and various dental materials. Growing interest in aesthetic dental restorations has led to the development of innovative materials for aesthetic restorations of teeth. The two approaches that are most frequently employed are the experimental technique and finite element analysis (FEA). Additionally, to predict a tooth's resistance to fracture, stress analyses using photoelastic and computer simulation methods are also carried out on healthy and restored teeth. However, these methods fall short of accurately predicting the type and distribution of the teeth stresses [5]. Due to its capacity to resolve intricate biomechanical issues for which other study methods are insufficient, the FEA Method is the most suitable for assessing stress distribution. At any point along the structure, strain, stress, and other properties can be calculated. In order to avoid the need for costly and time-consuming actual experiments, which are often necessary during the design phase, FEA is also being used to simulate potential structural failure [16]. The effect of occlusal restoration on a buccal Class V restoration in posterior teeth and the stress distribution with different class IV designs in the maxillary central incisor has already been studied, but no published studies have been conducted on the influence of the class IV restoration presence on the stress distribution around class V restoration of the maxillary central incisor. Thus, the purpose of this study was to investigate the effect of three different designs of class IV  restoration, (Butt joint, Bevel and Plain chamfer preparations), on the stress profile in and around labial and palatal class V restoration in the maxillary central incisors.

A three-dimensional (3D) static linear finite element analysis study was conducted to determine the effect of class IV restoration in the stress distribution on and around labial and palatal class V restoration in the maxillary central incisor using FEA software in the Faculty of Dentistry, Sana'a University.

Inclusion criteria

1. Sound right maxillary central incisor with mature root.

2. Good quality micro-computed tomographic (CT) image.

3. Composite resin with good physical and mechanical properties to resist fracture and initial failure due to the stresses that generated by the polymerization shrinkage.

4. Adhesive with low modulus of elasticity to reduce composite restoration deterioration during its polymerization.

Exclusion criteria

1. Maxillary central incisor with caries, operative or crown restoration(s).

2.  Endodontically treated tooth.

3. Tooth with open immature root apex or other defects (resorption, fracture… etc.)

4. Tooth with inherited or developmental anomalies.

5. Poor quality CT image.

6. Any stresses that are likely to be interfere during the tooth preparation has been ignored.

Geometric model

A Three-dimensional (3D) finite element model was constructed by 3D scanning of a freshly extracted sound tooth (central incisor) due to periodontal disease after patient acceptance. The tooth geometry was acquired by using a high-resolution Cone Beam Computed Tomography (CBCT) machine (Planmeca ProMax 3d MID; Planmeca, Helsinki, Finland), operating at 90 KV, 12mA with a voxel dimension of 75?m generating a total of 668 images. Images were processed using the materialize interactive medical image control system (MIMICS 15.0; Materialise, Leuven, Belgium) to produce a data file containing a cloud of points coordinates (STL file) (Figure 1). An intermediate, software was required "3 Matic version 15.01 (Materialize, NV, USA)" to trim newly created surfaces by the acquired points (Figure 2). Then, the solid tooth geometry was exported to finite element program as IGES file format. Geometry modification to create the study models: Five proposed cavities (two types of class V and three types of class IV) were created in "Autodesk Inventor" Version 8 (Autodesk Inc., San Rafael, CA, USA), then exported as STEP files. Another set of Boolean operation (subtract and overlap) was used to generate restorations and to create adhesive layer of 30?m [17]. Class V cavity was prepared in the labial and palatal position with dimensions of 2 mm gingivo-occlusally, 3 mm mesiodistally and 1.5 mm depth with the gingival margin of the cavity placed 1 mm coronal to the CEJ. The internal line angles of the cavity were rounded, in order to prevent any stress concentration [18]. Moreover, class IV cavity was prepared with a standardized dimension of 4mm gingivally and 4mm distally from the incisal angle, then the two points joined together to form the fracture line of class IV. Two groups of models were created according to the position of class V restoration (Group A-tooth with labial class V restoration, and Group B-tooth with palatal class V restoration) each of them consist of four models, the first one is the control case and the other three models depend on the type of class IV preparation. Then, a twenty-four runs were analysed as each model has been studied under the three loading conditions— masticatory (first loading condition), parafunctional (second loading condition), and traumatic (third loading condition).

All of the materials employed in this investigation were presumptively homogeneous, isotropic, and elastic along a linear direction. The ANSYS Workbench version 16 (ANSYS Inc., Canonsburg, PA, USA) finite element package's material properties were given to each component of the eight models.

Meshing: Mesh density is yet another important factor, which due to the complexity of the geometries, increases the discrete model's results accuracy (raising the accuracy of the generated stress levels in areas with significant stress gradients). The eight models were meshes using the parabolic tetrahedral element, and a suitable mesh density was chosen to guarantee the accuracy of the results for the discrete model.

Loads and boundary conditions

However, the validity of linear static analysis is questionable for more realistic situations such as immediate loading, in this study, the endurance limit (fatigue failure limit) governs most cases of dental analysis. In the case of having static stresses lower than the endurance limit under the worst case of extreme loads, there will be no risk of fatigue failure. The final model was verified against similar studies and showed very good agreement. The mean force for central incisors has been found to be 189.3 N for normal teeth and 181 N after implantation so a static force of 190 N in magnitude was delivered at the three different conditions that mentioned earlier [19-20].

The study's findings are shown in Figure 3. According to the study's findings, class IV restoration in the maxillary central incisor under three different load scenarios masticatory , parafunctional, and traumatic has the following effects on and surrounding class V restoration: The different types of class IV restorations have minor value changes in the total deformation on all the parts of the study. The changes in the values of Von Mises stress on and around class V restoration on both sides were varied from minor to significant depending on the type of class IV preparation and loading condition. The compressive and tensile stresses were higher on the side on which the load is applied. Depending on the type of class IV restoration, the more destructive type of class IV preparation (plain chamfer) led to more Von Mises stress concentration in the cervical region than other types (butt joint and 2mm bevel).  The increase in values of the stresses were higher on the palatal side, which led to failure in the enamel-restoration interface with plain chamfer type of class IV restoration. According to the loading conditions, class V restoration and all studied parts around it with all types of class IV restoration showed the highest values of stress concentration under the second loading condition followed by the third loading and finally the first loading conditions.

In the current work, the stress profile on class V restorations was examined in-depth qualitatively using the finite element technique (FEM). The biomechanical loads on tooth structures and various types of restorative materials have been estimated using a variety of methodologies. An approximate numerical technique called the finite element method (FEM) can offer in-depth qualitative information regarding the stress profile on class V restorations. Finite element analysis has a number of advantages over other techniques, including low cost, excellent reproducibility of the results, and the capacity to investigate anatomical areas that are essentially unreachable in vivo [21]. In the current study, the central incisor was chosen as the study subject. A 3D model of the tooth was created using a CT stl file retrieved using Materialize software (MIMICS), and the geometry of any cavities or restorations was modeled using information from the literature and the author's personal experience. 

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Figure 1: Tooth geometry pictures; (a) scanned tooth, (b) resulted STL file.  

Figure 2: 3-Matic screen during correcting STL file errors.

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Figure 3: Results obtained in the first Run - complete model; (a) directional deformation in Y axis, (b) directional deformation in Z axis, (c) total deformation, (d) Von Mises stress, (e) Maximum principal stress "tensile", and  (f) Minimum principal stress "compressive".

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