Chewing is the intake of food in a systematic cycle and begins by cutting and/or crushing the food, which is then destroyed, crushed, and ground on the hard surfaces of the teeth. Occlusion dentary parts are expressed in a three-dimensional alignment (noninterference), with the largest loading area on the stamping cusps rather than on the cutting cusps, which properly distributes vertically applied occlusal loads [1]. During swallowing, the teeth come into contact with antagonists; when swallowing saliva or when the bolus is completely fragmented occlusively with a three-dimensional alignment (noninterference), tripoidism (contact points A, B, C) occurs in the subcontact area of the cusps, there is a greater loading area on the stamping cusps than on the cutting cusps limited to a point of contact on the cutting cusp (distal rim), and the loads are applied vertically [1,2]. 

Deflection is the detour of the path jaw, and an offset to eccentric is created. Axial impacts between anterior and horizontal teeth on posterior teeth originate from microfractures as a result of inappropriate occlusal forces. There is no tripoidism, only one point of contact, which is located in the supercontact area of the stamping cusp, and a noxious horizontal load for the periodontal ligament and tooth [2]. Resins are restorative materials used in front and back teeth, and their proper mechanical behavior gives them the ability to withstand axial and tension forces generated during chewing [3]. Micromovements on restoration accompanied by a lack of marginal adaptation generate microfiltration factors that influence the failure of resin restorations. To increase the success rate, it is vital to maintain the integrity of the marginal sealing, which depends on cavity configuration, physical-mechanical properties, composite elasticity, adhesion, restoration technique, photopolymerization method and tooth occlusion type [4]. Photopolymerization contraction control limits the tension restoration-tooth interface, avoiding cracking, micro bacterial filtration, recurrent cavities, and postoperative sensitivity [5].

The incremental technique or the use of bulk resins has helped to minimize failures, especially in the bottom of the proximal class II restoration box, where access to light is limited [6,7]. The longer the restoration, the greater the risk of fractures, and layering of resin (2 mm) is recommended, but even so there are risks such as incorporation of air bubbles, contamination and failure of the joint between layers [8,9].

Cross-cutting, descriptive and comparative research, models I, II, III and finite element analysis were digitally designed with SOLIDWORKS CAD Software®.

Healthy Premolar Tooth Modeling (DS)

Geometric structures similar to a real dental piece were designed, and lines, curves, and joints in different planes were joined together to obtain the final shape, a 3D virtual model of a second healthy mangrove premolar, Model I (Table 1). It was imitated in its natural geometry (enamel, dentin and pulp tissue), including periodontal and bone structures (cortical bone and spongy bone). The gum and periodontal ligament were not modeled, and their elastic modules were low and did not influence the stress distribution [10-12]. 

Cavity Modeling and Restorations with Nanocomposite Resin and Bulk (DB, DN)

There were two copies of Model I, named Model II and Model III, respectively. Class II OD cavities were designed, and Filtek Z250 composite resin was chosen for Model II (3M ESPE, MN, USA); Filtek Bulk Fill resin (3M-ESPE, MN, USA) was chosen for Model III (Table 1).

• Axial force of 200 N in functional chewing area (Table 2)

• Definition of axial force of 200 N at 5 stable points on occlusal face in swallowing (Table 3)

• Definition of horizontal force of 600 N at 5 stable points on occlusal face in deflection (Table 4)

Table 1: Models I, II, and III Subjected to 200 N or 600 N, Depending on the Situation (Chewing, Swallowing, Deflection)

Table 2: Places with Stress Load

Table 3: Site-Specific Probability of Failure (Fracture).

Table 4: Places with a High Stress Load

Von Misses' values of the healthy tooth are uniform in chewing, swallowing and deflection. The distribution of stresses is balanced, starting from the crown to the root, with average stress values, without generating high stresses on the tooth or on the bone. During chewing, the maximum stress peak of the healthy tooth is located in the middle third of the clinical crown and is a safe place because at this level, a greater thickness of the enamel and dentin exists comparedto the occluse/cervical third of the clinical crown. The bulk resin tooth (BT**) [10,11] has peaks of stress in the occlusal third, where the enamel is stronger and more suitable for chewing loads than the third. Notably, there is a large difference between the healthy tooth value (HT*) (26.01 MPa) and the value of the tooth with nanocomposed resin (NT***) (132,272 MPa), justifying that NT experiences great effort at the cervical level and peak stress that makes it prone to failure. In BT, there is a red zone at the level of marginal sealing, showing risk at this point. Von Misses' maximum values do not focus on the crown; they focus on the root [13]. In swallowing, the HT has its maximum stress peak in the occlusal third (332.83 MPa), the BT (396.13 MPa) in the cervical third of the clinical crown, and the NT (343.51 MPa) in the middle third of the clinical crown; the greatest risk of being cervical is BT. In HT, deflection (582.65 MPa) [14] has its peak in the occlusal third, BT (593.65 MPa) in the occlusal third, and NT (572.54 MPa) in the cervical third, where cervical damage is more likely to occur [15]. In normal chewing cycles, occlusal forces are generated bilaterally, and even upon cyclic closure of the jaw, the force is greater on the working side. Additionally, the movement made by the mandibular teeth on the maxillary teeth affects the distribution of stress [16]. The second lower premolar was taken into account separately from the other teeth in the organic occlusion, and the model was programmed with the characteristics of the trabecular and spongy jawbone. For the safety factor of the healthy tooth, bone is the main element for the distribution of stress. During chewing, swallowing and deflection, the bone absorbs the highest stress, allowing the tooth not to overload. During chewing, the BT, even though its highest level of stress is in the area of the bone, moves away from the parameters of the HS by presenting a greater area of stress in the clinical crown compared to the bone [17]. NT has more stress on the bone than in a clinical crown; in its neck, there is a reddish tone compared to the neck of the BT, which means that this area of the neck of the NT is more prone to fractures. Marginal sealing in NT is endangered at the level of the occlusal third of the clinical crown; in BT, the hazard is located in almost all marginal sealing. During swallowing, HT and NT have their highest stress point in the bone, while BT has it in the occlusal third of the clinical crown. BT is most stressed in both the crown and neck and bone relative to NT. Marginal sealing in NT is endangered at the occlusal third level of the clinical crown, while in BT it is endangered throughout the marginal sealing. During deflection, the greatest stress is placed in the bone of the three model teeth. In the cervical third, the three teeth experience stress, which is why the back teeth experience wear on their necks when subjected to axial loads in the presence of interference [18]. Marginal sealing of the HT cavity floor has less stress than BT. The BT bone has less stress than the NT bone, but the NT crown carries more stress compared to NT. Derchi et al., in a study on mechanical behavior in amalgam and resin restorations, applied a force of 291.36 N at five contact points on 3D models of the crown of an upper premolar with class I or II cavities, restored with amalgam or resin with variation in the direction of the cavity walls, and obtained better distribution of stresses in premolars with class I cavities that had a divergent preparation for amalgam and convergent for resin; in premolars with class II cavities, the concentration of stresses was higher with resin than with amalgam and coincided with contact points, and greater effects were found in restorations than in dentary tissue [19]. The success or failure of a restoration does not depend on the conformation of the cavity walls and the distribution of stresses on the tooth or on the type of photopolymerization lamp used in the elasticity module of each material. The study indicates that BT has less fatigue along the tooth and bone relative to the tooth restored with NT and increased fatigue compared to conducted a study that analyzed clinical performance over 1 year of 109 class II restorations in 54 patients performed with a high viscosity glass ionomer [14,20], bulk composite resin (Filtek Bulk Rear Restorer) and a micro hybrid composite resin placed with the incremental technique. It was shown that there were no changes in restoration in terms of retention, color, anatomy, marginal discoloration, point of contact, marginal adaptation, secondary cavities, postoperative sensitivity and surface texture, indicating that the resins performed better in class II cavities compared to the glass isomer. These findings coincide with those of Colak et al. 2017 [18,21], where no change at 6/12 months was found in terms of postoperative sensitivity, anatomy, retention and secondary cavities. There were changes in terms of color, discoloration and marginal adaptation, but these did not represent significant differences between the two materials. Jafari mentioned that the physical and mechanical properties of bulk resin are similar to or greater than those of a conventional resin. Peñafiel et al. [21] subjected conventional bulk resins to a thermocycle treatment for 24 hours at 55°C, confirming that bulk resin resulted in less stress than conventional resin [22,23]. Bulk resin was created to minimize the problems that conventional resin possesses, with mechanical properties designed to better dissipate tension such as the incorporation of more reactive photoreactive molecules, polymer particles and fiberglass particles. In addition, monomers modulate the polymerization reaction, relieving stress [24,25].

Filler placement prevents the incorporation of bubbles, contamination between layers, penetration of dye into the gingival floor, and preserves the critical area of class II restorations, which can be explained by the increased contraction of bulk resin, for which detachments and microfiltration are more frequent, especially when the margins are not in the enamel. This is consistent with the present study, where the areas of the BT safety factor compared to the HT and the NT had more areas prone to fracture along the tooth and bone, including the bulk restoration area proximal box and occlusal box [26,27]. The stress and contraction of the restoration depend on the number of walls to which it is attached, the presence of fewer surfaces, and less stress. Assessing the marginal sealing of four filler compounds, SDR, SonicFill, Tetric Evoceram BF, and Filtek BF, it was shown that fluid bulk resins had better marginal adaptation than regular resins [28].

For cavities that are not bigger than 2 mm wide mesio-distal × 4 mm wide buco-lingual and 5 mm deep occluso-cervical, and taking into account the wear limits required to perform a direct restoration of a class II cavity, a bezel is recommended to finish the preparation and improve the marginal quality; with the finite element software, it was complex to adjust details as small as the bezel, and the angles were rounded, so the beveled finishes were similar [29]. Proximal box preparations have excellent success rates, all resins are ductile and fragile materials, and bulk and nanocomposite resins have acceptable stress resistance; furthermore, resin has more elasticity and has greater resistance to fracture [30].In the study, bulk and nanocomposite resins [31] had different modules of elasticity, and similarities were found in the maximum fatigue values between the two resins. Stress points were visualized in critical areas such as marginal sealing and the interface between tooth resin walls, which could result in adhesive failure. Despite not being included in the restorative area, the cervical third or so-called tooth neck is also a critical area, and the chewing forces are concentrated in this place, producing rupture of the enamel and dentin. Von Misses failure results indicate that during BT chewing, and in NT, the stress peaks are located in the neck. The large numerical difference is above 0 and below 4, which is the approved safety range to prevent fracture or deformation. If the chewing forces were not properly distributed along the tooth and bone, the restorations absorbed most of the impact, and the increased overload resulted in marginal sealing failures and microfractures in the tooth-restoration interface area. EF is considered an easy and economical way to evaluate the mechanical behavior of complex structures. Limitations are considered, such as the omission of some structures of finite element models. For example, the models of teeth do not have periodontal ligaments. Clinically, the chewing loads pass from the crown, through the root, transfer to the periodontal ligament, and finally reach the bone. The lack of physiological mobility of the teeth in this study negatively influences the outcome of AEF [32]. 

HT* Healthy premolar tooth BT** premolar tooth restored with bulk resin NT** premolar tooth restored with nanocomposite resin.

Acknowledgment

The authors would like to express their special thanks to UDLA (Universidad de las Americas).

Conflict of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper. The authors declare that they have contributed significantly to preparation of the manuscript and that all authors are in agreement with the content of the manuscript.

Funding

All funds used to support this work were allocated by the UDLA. 

1. Bourdiol, P. et al. “Masticatory adaptation to occlusal changes.” Frontiers in Physiology, vol. 11, 2020, pp. 1–17.

2. Sterenborg, A. et al. “Impact of tooth wear on masticatory performance.” Journal of Dentistry, vol. 76, 2018, pp. 98–101.

3. Corral, C. et al. “Revisión del estado actual de resinas compuestas bulk-fill.” Revista Facultad de Odontología Universidad de Antioquia, vol. 27, no. 1, 2015, pp. 177–196.

4. Costa, L. et al. “Microleakage of ‘bulk-fill’ composite resin for class II restorations pretreated with CO₂ laser in deciduous molars: An in vitro study.” Journal of Lasers in Medical Sciences, vol. 10, no. 4, 2019, pp. 304–309.

5. García, L. et al. “In vitro evaluation of microleakage in class II composite restorations: High-viscosity bulk-fill vs conventional composites.” Dental Materials Journal, vol. 38, no. 5, 2019, pp. 721–727.

6. Akman, H. and G. Tosun. “Clinical evaluation of bulk-fill resins and glass ionomer restorative materials: A one-year follow-up randomized clinical trial in children.” Nigerian Journal of Clinical Practice, vol. 23, no. 4, 2020, pp. 489–497.

7. Alqudaihi, F. S. et al. “Comparison of internal adaptation of bulk-fill and increment-fill resin composite materials.” Operative Dentistry, vol. 44, no. 1, 2019, pp. E44.

8. Ali, A. et al. “Simultaneous evaluation of creep deformation and recovery of bulk-fill dental composites immersed in food-simulating liquids.” Materials, vol. 11, no. 7, 2018, pp. 1180.

9. Ausiello, C. et al. “Mechanical behavior of bulk direct composite versus block composite and lithium disilicate indirect class II restorations by CAD-FEM modeling.” Dental Materials, vol. 33, no. 6, 2017, pp. 690–701.

10. Lagotcha, R. et al. “Analysis of base monomer elution from three flowable bulk-fill composite resins using high performance liquid chromatography (HPLC).” Medical Science Monitor, vol. 24, 2018, pp. 46.

11. Jyothi, M. et al. “Microtensile bond strength of bulk-fill restorative composites to dentin.” Journal of Clinical and Experimental Dentistry, vol. 9, no. 8, 2017, pp. e1023–e1028.

12. Trivedi, S. “Finite element analysis: A boon to dentistry.” Journal of Oral Biology and Craniofacial Research, vol. 4, no. 3, 2014, pp. 200–203.

13. Guimarães, G. F. et al. “Minimization of polymerization shrinkage effects on composite resins by the control of irradiance during the photoactivation process.” Journal of Applied Oral Science, vol. 26, 2018, pp. e20170528.

14. Bayraktar, Y. et al. “One-year clinical evaluation of different types of bulk-fill composites.” Journal of Investigative and Clinical Dentistry, vol. 8, no. 2, 2017, pp. e12210.

15. Borgia, E. and R. Baron. “Quality and survival of direct light-activated composite resin restorations in posterior teeth: A five- to twenty-year retrospective longitudinal study.” Journal of Prosthodontics, vol. 28, no. 1, 2019, pp. e195–e203.

16. Brunet, J. et al. “Correlation between disclusion guides and the presence of abfractions.” Avances en Odontoestomatología, vol. 32, no. 3, 2016, pp. 145–151.

17. Cabrera, F. and B. Acurio. “Comparative evaluation of compressive strength of conventional resins versus bulk-fill composites.” Odontología Vital, vol. 2, no. 2, 2017, pp. 69–77.

18. Colak, H. et al. “A prospective, randomized, double-blind clinical trial of one nano-hybrid and one high-viscosity bulk-fill composite restorative system in class II cavities: Twelve-month results.” Nigerian Journal of Clinical Practice, vol. 20, no. 7, 2017, pp. 822–831.

19. Dejak, B. and A. Młotkowski. “Comparison of stresses in molar teeth restored with inlays and direct restorations, including polymerization shrinkage of composite resin and tooth loading during mastication.” Dental Materials, vol. 31, no. 3, 2015, pp. e77–e87.

20. Derchi, G. et al. “Stiffness effect of using polywave or monowave LED units for photo-curing different bulk-fill composites.” Dental Materials Journal, vol. 37, no. 5, 2018, pp. 709–716.

21. Dong, A. et al. “Cure cycle optimization of rapidly cured out-of-autoclave composites.” Materials, vol. 11, no. 3, 2018, pp. 421.

22. Ilie, N. “Impact of light transmittance mode on polymerization kinetics in bulk-fill resin-based composites.” Journal of Dentistry, vol. 63, 2017, pp. 51–59.

23. Jafari, T. et al. “Evaluation of cavity size, kind, and filling technique of composite shrinkage by finite element analysis.” Dental Research Journal, vol. 15, no. 1, 2018, pp. 33–39.

24. Peñafiel, M. et al. “Comparison of the resistance of dihybrid, hybrid, and bulk-fill resins to compression force.” Recimundo, vol. 3, 2019, pp. 585–595.

25. Vargas, O. et al. “Bulk-fill resin restorations: A review.” Revista Latinoamericana de Ortodoncia y Odontopediatría, 2020, pp. 1–7.

26. Kuramochi, G. and M. Del Sol. “Method to generation of finite element models of teeth.” International Journal of Morphology, vol. 31, no. 3, 2013, pp. 997–1002.

27. Nika, S. et al. “Real-time temperature monitoring during light-curing of experimental composites.” Acta Stomatologica Croatica, vol. 52, no. 2, 2018, pp. 87–96.

28. Okoye, L. et al. “Pattern of malocclusion and caries experience in unrepaired cleft lip and palate patients in Enugu.” Nigerian Journal of Clinical Practice, vol. 23, 2020, pp. 59–64.

29. Cedillo, J. et al. “Marginal adaptation analysis of posterior bulk-fill resin restorations applied in increments: A SEM-FE study.” Rodyb, vol. 8, no. 3, 2019, pp. 22–28.

30. Srivastava, B. et al. “Comparative evaluation of various temperature changes on stress distribution in class ii mesial-occlusal-distal preparations restored with different restorative materials: A finite element analysis.” International Journal of Clinical Pediatric Dentistry, vol. 11, no. 3, 2018, pp. 167–170.

31. Van Ende, A. et al. “Bulk-fill composites: A review of the current literature.” Journal of Adhesive Dentistry, vol. 19, no. 2, 2017, pp. 95–109.

32. Keulemans, F. et al. “Three-dimensional finite element analysis of anterior two-unit cantilever resin-bonded fixed dental prostheses.” The Scientific World Journal, 2015, pp. 1–10.