Biomechanical Analysis of a Custom-Made Mouthguard Reinforced With Different Elastic Modulus Laminates During a Simulated Maxillofacial Trauma

Abstract

Background/Aims: There is a lack of data regarding the influence of different laminates for mouthguard reinforcement in the mechanical response during an impact in the orofacial region. The aim of this study was to verify the influence of the laminate framework on the stresses and strains of the anterior teeth and displacement of ethylene-vinyl acetate (EVA) custom-made mouthguards during a simulated impact. The null hypotheses was that the different laminates reinforcement would present the similar effect in maxillary structures, regardless the elastic modulus. Methods: A finite element model of human maxillary central incisors with an antagonist contact was used. A linear quasistatic analysis was used to simulate the force exerted during an impact. A total of 5 different layers were simulated inside the mouthguard at the labial portion according to the Elastic Modulus 1 MPa (Extremely flexible), 9 GPa (Low modulus reinforcement), 18 GPa (Without reinforcement), 50 GPa (Flexible alloy), 100 GPa (Titanium alloy) and 200 GPa (Hard material). The results were evaluated by means of Maximum Principal Stress (in the tooth and bone), Microstrain (periodontal ligament) and Displacement (mouthguard) criteria. Results: The elastic modulus of the material inside the MG influenced the stress distribution on the enamel buccal face. However, it did not affect the bone tissue stress, periodontal ligament strain or root dentin tissue stress. Conclusion: The use of reinforcement inside the custom-made mouthguard can modify the stress generated in the enamel buccal surface without improvement to the root dentin, periodontal ligament or bone tissue.

1. Introduction

Several studies have stated that the risks of dental and maxillofacial traumas can be significantly reduced with the use of mouthguards. [1,2] Mouthguards (MG) are intraoral devices used by athletes and amateurs practicing physical activity with a risk of physical contact. [1,2,3] This device aims to reduce the stresses generated in the tooth and surrounding structures found in the regions of the face susceptible to injuries during a trauma. [4] The use of mouthguards assists in reducing dental trauma, even when the impact is not directly exerted on the teeth. [3] The main teeth which can be affected by facial trauma are the upper central incisors due to their 3D position in the maxilla and the presence of a single root. [5]

The biomechanical response of mouthguards can be influenced by its thickness, [6] its design, [6] by the impact [7] and the presence of antagonist dentition. [8] The most suitable mouthguards are custom-made, being individualized for each patient. [6] In addition to being able to contact all the faces of the maxillary teeth, this type of MG is generally made of Ethylene-vinyl acetate (EVA) which guarantees adequate physical properties and enables it to act as a buffer for the impact. [2,9]

However, even with the use of this device during an impact to the face, the stresses generated in the rigid structures are still considerable. [4] Thus, several studies have been developed to improve the MG mechanical response through inserting other materials inside the mouthguard structure, such as: the use of laminates in sorbothane, [10] acrylic resin, [11] silica mesh, [4] titanium [12] and even air. [13] However, the literature is inconclusive regarding the best method to reinforce a custom-made mouthguard, as both rigid and flexible materials are indicated to improve the mechanical response of this device.

In vitro studies which evaluate the performance of mouthguards generally use pendulum devices in which a jaw is simulated in metal or resin and receives a solid body impact, then a strain gauge can check the displacement of the assembly. [14,15] However, other studies have used a numerical analysis which enables visualizing the regions with the highest stress concentration during the impact, thus the dental structure can be analyzed individually, discretizing the results in the dentin, enamel and bone. [4,6,8,16]

Therefore, the objective of the present study was to evaluate the influence of the stiffness of the reinforcement material inserted in a custom-made mouthguard through the finite element method. The null hypothesis is that there will be no difference for the stresses generated on the dental surface, regardless of the reinforcement material inside the MG.

2. Methods

2.1. Finite Element Analysis (FEA) Pre-Processing

This study is a theoretical computer simulation that was performed using a linear finite element impact analysis (FEA) simulating impact on a human maxillary central incisor model involved with different custom-made mouthguards and antagonist contact. A two-dimensional (2D) model of maxillary and mandibular human central incisors in occlusal contact (maximal intercuspal position) was modeled based on a previous study. [8] The model was composed of enamel tissue (E = 80 GPa, n = 0.3 17), dentin tissue (E = 18 GPa, n = 0.318), periodontal ligament E = 50 MPa, n = 0.3, cortical bone E = 13.7 GPa, n = 0.3, cancellous bone E = 1.4 GPa, n = 0.3 and soft tissues E = 1.8 MPa, n = 0.3. The model was created simulating a patient using a 3 mm custom-made mouthguard. Reproducibility is one of the main advantages of bi-dimensional analysis due to the modeling process facility. Thus, the model picture presented in the literature was exported in BMP (Bitmap) and used as the background in a modeling software (Rhinoceros). [8] As the scale was not present, the model was adjusted to present 21 mm for the upper central incisor 19 and the other structures were modeled proportionally based on the stereolithographic skull model from São Paulo State University (Unesp), São José dos Campos.

After modeling refinement, a different software was used to simulate the trauma and calculate the results. Therefore, the OBJ (Object file) was exported to the Computer Aided Engineering software (ANSYS 19R1, Ansys Inc, Houston, TX, USA). Next, a bi-dimensional analysis was selected for the coordinate creation and an insignificant thickness was assumed. A geometric model was generated and the element meshes were created based on a convergence test (10%) using a tetrahedral element with hard behavior and soft transition. The model presented a total of 36665 elements and 275164 nodes. Ideal contact was used between all the interfaces, not allowing the separation, and a quasi-static impact analysis was subsequently performed. The boundary conditions were defined with a rigid impact object (steel sphere) hitting the tooth surface with 500 N (Figure 1). [6] The model displacement was constrained at the nodes on the base of the maxillary and mandibular bone in X and Y directions. All materials were considered linear, isotropic and homogeneous.

Different elastic moduli were simulated for the laminate framework properties inside the mouthguard’s labial portion (Figure 1). The group descriptions and the laminate materials’ elastic modulus are summarized in

Table 1. The Laminate Materials’ Elastic Modulus (in GPa) Simulated in this Study, the Maximum Values of Maximum Principal Stress in MPa (in the enamel, dentin and bone) and Microstrain (in the Periodontal Ligament).

Simulated Laminate Material	Elastic Modulus (GPa)	Enamel	Dentin	Cortical Bone	Periodontal Ligament
Extremely Flexible	0.0001	144	125	92	651
Low modulus reinforcement	10	130	126	92	652
No Reinforcement	18	126	126	92	652
Flexible Alloy	50	125	126	92	652
Titanium Alloy	100	123	126	92	653
Hard Material	200	117	126	92	655

. [17,18,19]

2.2. FEA Solution

The Maximum Principal Stress evidencing the tensile stress concentration areas was used to identify critical areas for fracture in the tooth and bone. Next, the microstrain was recorded for the periodontal ligament. The results in the teeth and in the bone were then plotted in colorimetric stress maps, whereas the nodal stress peaks for the periodontal ligament and MG were recorded as a linear graph for a visual comparison.

The tensile stress peaks on the internal surfaces on the enamel and dentin tissue were exported in spreadsheets, according to the element number corresponding to the numerical calculation. [20] The 30 highest values were selected for each structure of all 6 groups, totaling 360 values for tensile stress in MPa. The data were analyzed by descriptive statistics (mean and standard deviation), one-way ANOVA for each studied factor, followed by Tukey’s test for differences between groups. All tests were considered significant at 5% due to the correspondence of the mesh convergence test, and, were performed in the Minitab statistic software (Minitab 18, State College, Pennsylvania, USA).

3. Results

Maximum Principal Stress distribution in the models are shown in Figure 2 and Figure 3. The stresses can be visualized using a linear color scale in which blue indicates the lowest stress values and red the highest stress values. The model with the more flexible material inside the mouthguard had the highest stress values in the enamel and dentin, mainly at the buccal surface near to the impact region. A smaller difference in the stress values and concentration in the enamel and dentin structure was found between the models with more rigid frameworks, but it is still notable that the more rigid materials reduced the stress concentration for the tooth models.

For the bone tissue (Figure 3), the stress concentration area was the buccal region of alveolar bone, near the apex of the upper central incisor, regardless of the laminate material inside the mouthguard. The strain values in the periodontal ligament are shown in Figure 4 without a visible difference between the groups, showing that there is no possible increased damage in its structure using any reinforcement.

The displacement pattern for the Mouthguard was only different for the most flexible laminate, indicating that the compliance of this structure is different from the other simulations (Figure 5).

One-way ANOVA revealed that for the enamel, the reinforcement was significant (F = 2.46; p = 0.035) for the Maximum Principal Stress peaks, different for the dentin tissue (p = 0.999). Tukey test revealed that 0.001 GPa (138^A; elastic modulus of the reinforcement showed significantly increased mean stress values in the enamel compared to 9 GPa (128)^B, 18 GPa (126)^B, 50 GPa (125)^B, 100 GPa (124)^B and 200 GPa (121)^B. One-way ANOVA also revealed that the reinforcement elastic modulus did not affected the Displacement (F = 0.99; p = 0.426) and Microstrain (F = 0.43; p = 0.828) results.

4. Discussion

The present study demonstrated that the elastic modulus of the laminate material used inside the mouthguard is able to modify the stresses generated on the buccal surface of the tooth, denying the null hypothesis. The results showed that if the inner layer of the mouthguard presents a rigid material, part of the generated stress in the impacted area is reduced, preventing the dental surface. Figure 3 shows that there is no difference for stresses generated in the alveolar bone, especially in the region close to the apex of the central incisor in the buccal alveolar bone. This region of stress concentration in the bone tissue is similar to the fractured areas reported in the literature in cases where avulsion of the dental element occurred. [21]

The strain for the periodontal ligament was identical for all groups, demonstrating that this structure is not affected by the use of different mouthguards. In addition, its mechanical response does not depend on the reinforcement used inside the mouthguard. Although it is reported that the use of mouthguards decreases the elastic energy generated in the periodontal ligament during an impact, [22] the same does not seem to occur for different mouthguards with reinforcement inside. This can be explained because a large part of the mouthguard is identical for all groups, with the internal and external part being composed entirely of EVA. The MG displacement for all groups is very similar, concentrated in the buccal region. This corroborates previous studies demonstrating that this area moves more due to the impact. Previous reports have demonstrated that the displacement of the mouthguard can be mitigated with the use of an adequate occlusion 6 and the antagonistic teeth contacting the lower surface of the MG. [6,8] As this situation was the same for all groups in the present study, it is believed that the similar displacement can be explained.

In observing Figure 2, it is possible to see that enamel concentrated different amounts of tensile stresses. Unlike other studies, this failure criterion was selected to highlight possible areas of tooth fractures, [23] and not just a general result of the resulting stresses. It is clearly possible to observe that the mouthguard with extremely flexible reinforcement (1 MPa) presents the worst result with the buccal enamel totally involved in larger areas of stress concentration.

As the stiffness of the laminate reinforcement increases, the stress magnitude decreases, with a visible reduction in the red area on the enamel buccal surface. Although trauma in the enamel region can often cause cracks which do not grow and just need to be preserved, [24] it is evident that this region is very affected and that the use of a rigid laminate could positively modify the mechanical response generated in that area. However, fractures between the crown and root do not seem to be possible to be avoided using a reinforcement inside the MG. This can be justified because the groups showed very similar stress in the root dentin tissue region. The fractures which separate the crown from the root can be considered as the most complicated, [25] and therefore other methods of improving the biomechanical response of the MG should be investigated in order to avoid them, such as the design of the device and the thermoplastic material used for its individualization.

A previous in vitro study tested different materials inserted in the mouthguard: polyethylene terephthalate glycol modified, nylon mesh and air space. [9] The authors found that all mouthguards showed significantly better protection qualities than the situation of not using a mouthguard. Small differences can be evidenced as the present study only simulated the condition of a patient using the MG during the impact.

In a different way, another research group [12] compared the use of 1 mm laminate titanium reinforcement versus a conventional mouthguard design in EVA. The authors concluded that there was no statistically significant difference between the groups in terms of any energy transmission or absorption. Both groups were simulated in the present study, and the stress results are displayed in Figure 2C and Figure 2E, respectively. It is possible to observe very few differences between both designs for all structures, regardless of the enamel tissue. This could explain why the authors could not find any difference between any of the group.

However, not only rigid reinforcements are not able to induce different mechanical response in comparison with a conventional design. A previous study [4] used a 3D-FEA to compare the use of a silica-nylon mesh and the conventional design without reinforcement. The authors did not find any difference for the stress generated in the roots or in the bone tissue. The same behavior can be found in the present study in comparing Figure 2 and Figure 3, letters b and c. The figures show a resin laminate reinforcement and the conventional group without a visible difference for the dentin and the bone tissue, respectively.

As clinical implication, the dental surgeon should plan the mouthguard manufacture with rigid laminates reinforcements, and, explain to their patients the advantages of wearing this device during contact activities. However, it is important to note that even with the mouthguard in position, some stress can be calculated in the central incisor during impact and traumatic events can still culminate in injuries.

Another investigation [14] reported that some reinforcements could present fracture during impact. This is especially concerning for the high elastic modulus groups, when brittle materials could be used. Thus, a mouthguard with an access envelope which enables replacing the laminate reinforcement could be an interesting design. Even though this study is a theoretical computer simulation, it is bidimensional, it does not consider adjacent teeth and also does not enable visualizing all the stresses which involve a 3D structure, such as the human tooth. [26] In addition, more complex 3D models can be used to demonstrate results in other areas and a more realistic view of the stress maps. However, the results presented herein are valid for a first response to reject the hypothesis.

5. Conclusions

With the limitations inherent to this methodology, it is possible to conclude that: The use of a laminate reinforcement inside a custom-made mouthguard can modify the stress generated in the enamel buccal surface. However, it did not show improvement for the root dentin, periodontal ligament or bone tissue.