*4.2. Main Results*

Almost all studies in the field of sports dental medicine pay attention to the need to use mouthguards to prevent dentofacial injuries during sport activities with a high and moderate level of injury [17,25,26]. The effectiveness of mouthguards has been proven in practice. The choice of mouthguards is wide [26]: from standard to individual, from single-layer to multilayer, from ordinary to specialized, etc. Standard and thermoplastic designs of protective mouthguards have a number of disadvantages [18]:


The most effective protective dental constructions are individual mouthguards [18,27]. Sousa et al. [27] consider different designs of mouthguards, highlighting individual mouthguards as more effective, including multilayer ones. The classic single-layer mouthguard design reduces the maximum stress intensity in the teeth by a maximum of 54.4% [14]. The multilayer individual EVA mouthguard reduces the maximum stress intensity in teeth by 72.2%, as shown in this work. The efficiency of the multilayer mouthguard is more than 17% higher than that of the classic design. The use of a multilayer mouthguard from relatively soft material changed the stresses distribution nature in the dentition elements (Figures 3 and 4b). In this case, the maximum stress level was observed in the tooth neck. The stress in the teeth decreased in general but increased in the tooth neck by 2.2–2.3 times.

Rationalization of mouthguard designs has been ongoing. The work is aimed at a number of factors [28,29]: materials, production methods, and geometry, etc. The assumption is considered that by introducing an intermediate layer made of a harder material into a structure, it is possible to achieve a better effect in protecting the teeth [16,29,30]. The intermediate interlayer of A-silicone in mouthguards made it possible to maximally reduce the stress intensity of the teeth by 87.6%, as shown in this work. Nevertheless, the insertion of additional elements or layers into the structure of the mouthguards is not always effective. For example, the mouthguard with an interlayer of a silica-nylon mesh did not show an improvement in the mechanical reaction of teeth [28]. Mouthguards with A-silicone interlayer reduced tooth stress significantly more than adaptive and multilayer EVA mouthguards, the study showed. At the same time, it was established that the interlayer geometry significantly affects the performance of the biomechanical unit. The maximum stress concentration in the upper tooth was observed, with an interlayer thickness of 22.8–27.1% of the total mouthguard thickness, but not in the teeth closing zone. The geometry of the A-silicone interlayer of the mouthguard affects the deformation of the elements of the dentition. An improperly chosen interlayer shape can lead to a stress concentration in the tooth, with further cracking.

Another feature of this work is the use of a nonlinear model of EVA material behavior. Kerr et al. [17] revealed that when choosing and analyzing the operation of mouthguards, it is necessary to consider the physical and mechanical properties of the materials from which they are made. One of the most common materials in mouthguards is the EVA polymer [28]. Currently, there is a significant amount of research devoted to the analysis of the properties of EVA from different manufacturers [29,30]. The behavior EVA model is nonlinear and reflects elastoplastic deformation. Many works consider the EVA material as elastic, for example, Lokhov et al. [16]. Consideration of EVA in terms of elastic deformation distorts the research results. The many effects and patterns were made noticeable due to the nonlinear description of the EVA operation within this study.

### *4.3. The Mouthguard Thickness Analysis 4.3. The Mouthguard Thickness Analysis*

*Materials* **2021**, *14*, x FOR PEER REVIEW 10 of 12

Standard thickness of EVA mouthguards is 3–4 mm. This work considered a multilayer EVA kappa with a thickness of 7 mm. Of interest is the comparative analysis of the operation of EVA mouthguards of standard thickness and mouthguards with a layer of A-silicone. The stresses in the teeth in contact with EVA mouthguards of 3 and 4 mm thickness of a load of up to 600 N are shown in Figure 7. Standard thickness of EVA mouthguards is 3–4 mm. This work considered a multilayer EVA kappa with a thickness of 7 mm. Of interest is the comparative analysis of the operation of EVA mouthguards of standard thickness and mouthguards with a layer of A-silicone. The stresses in the teeth in contact with EVA mouthguards of 3 and 4 mm thickness of a load of up to 600 N are shown in Figure 7.

Another feature of this work is the use of a nonlinear model of EVA material behavior. Kerr et al. [17] revealed that when choosing and analyzing the operation of mouthguards, it is necessary to consider the physical and mechanical properties of the materials from which they are made. One of the most common materials in mouthguards is the EVA polymer [28]. Currently, there is a significant amount of research devoted to the analysis of the properties of EVA from different manufacturers [29,30]. The behavior EVA model is nonlinear and reflects elastoplastic deformation. Many works consider the EVA material as elastic, for example, Lokhov et al. [16]. Consideration of EVA in terms of elastic deformation distorts the research results. The many effects and patterns were made noticeable due to the nonlinear description of the EVA operation within this study.

**Figure 7.** Dependence of maximal stress intensity on indentation force without (**a**) and with (**b**,**c**) a mouthguard (for teeth of the upper (**b**) and lower (**c**) teeth dentition): 1—case b of 4 mm thickness; 2—case b of 3 mm thickness; 3—case c-A; 4 case c-B; 5—case c-C. **Figure 7.** Dependence of maximal stress intensity on indentation force without (**a**) and with (**b**,**c**) a mouthguard (for teeth of the upper (**b**) and lower (**c**) teeth dentition): 1—case b of 4 mm thickness; 2—case b of 3 mm thickness; 3—case c-A; 4—case c-B; 5—case c-C.

It was found that with a standard thickness, the EVA aligner reduces the intensity of stresses in the teeth to a level close to the aligners with a layer of A-silicone. The maximum intensity of stresses in a mouthguard made of EVA with a thickness of 3–4 mm were observed in the neck of the tooth. It was found that with a standard thickness, the EVA aligner reduces the intensity of stresses in the teeth to a level close to the aligners with a layer of A-silicone. The maximum intensity of stresses in a mouthguard made of EVA with a thickness of 3–4 mm were observed in the neck of the tooth.

At loads less than 250–350 N, EVA splints perform better than multilayer splints with an A-silicone interlayer. Under heavy loads, the mouthguard with an interlayer adjusted to the geometry of the elements of the dentition (case c-C) reduces tooth stress better than all other aligners reviewed.

The geometry of the A-silicone interlayer has a significant effect on the deformation distributions.
