# The Effectiveness of Dental Protection and the Material Arrangement in Custom-Made Mouthguards
## 4 Discussion
Hence, compared to this one, in the CMG model the displacement was 5.6% lower, in the FMG model it was 17.6% lower, and in the HMG model it was 42.4% lower, as can be verified in Table 5 and Figure 6. The average values of the magnitude of bone displacement were about 2.8%, 15.9%, and 35.5% lower in the CMG, FMG, and HMG models, respectively, to that of the NMG model. Nevertheless, the average values of the magnitude of tissue displacement increased by about 66.7%, 158.5%, and 187.8% in the CMG, FMG, and HMG models, respectively, to that of the NMG model. Hence, these results, and those presented in Table 10, seem to indicate that improving the mouthguard flexibility can be responsible for the increase of soft tissue injuries. Nevertheless, when dental injuries are restricted to the soft tissues, such as lacerations, abrasions, and contusions, they create wounds that usually heal without major complications [29]. The stresses and stretches over the mouthguard reveal its ability to shock absorption, which depends in the material shock absorption ability. The EVA and the EVA foam materials show good energy absorption capabilities [1, 8, 9, 14, 28] and, therefore, all the three mouthguards are well positioned relatively to this parameter. Nevertheless, it was the HMG model that presented higher values of first principal stretches and lower values of third principal stretches, i.e., it shows greater elongation and compression, due to the flexibility of its outer layer, compared to FMG and CMG models. but it was also the HMG model that showed higher stress in the loading region. Hence, these results show that air inclusion in EVA mouthguard materials may change the impact characteristics and performance of this polymer [14, 15]. The von Mises stresses over teeth showed critical regions in the roots of central incisors in all models, but the higher values appear in the NMG model. The results of Table 9 show that, compared to NMG, the wearing of a compact EVA mouthguard (CMG) allowed the reduction of the von Mises stresses in 14.5%, whereas wearing the EVA/EVA Foam/EVA mouthguard (FMG) reduces the von Mises stresses in 22.6% and, finally, wearing an EVA/hollow/EVA mouthguard (HMG) reduces the von Mises stresses in 62.5%. In any of the cases, the maximum values of effective stress detected do not exceed the elasticity limit of the tooth and, therefore, it is concluded that there was no fracture of the teeth. For the bone tissue, the stress concentration area was at the crestal level of the alveolar bone in the buccal side, regardless of the design or material used in the mouthguard. Some of the published finite element studies do show a different critical area in the maxilla bone [16]. This behavior can be justified with the presence or not of the periodontal ligament (PDL). In fact, as shown by [PERSON] et al. [30] and by [PERSON] et al. [26], the deterioration of the PDL mechanical characteristics might alter the biomechanical response of the tooth. Nevertheless, the present study corroborated with all of these previous statements, showing a stress magnitude decrease associated with the wearing of MGs. Moreover, as showed by [PERSON] et al., wearing an MG in a deteriorated PDL reduces the stress magnitude at a smaller level than in the sound PDL model. Hence, if the PDL were included in all geometrical and numerical models [18], it would be expected that those results related to MG use would be amplified. Some limitations of this study are related to the geometrical and mechanical properties of the maxilla bone and of the teeth. As is well known, maxilla bone has variability in cortical and cancellous mechanical properties [31], and the tooth structure is more complex [32, 33] than the one used in the present work. Nevertheless, because this is a comparative study and the realistic mechanical properties of all components will remain in the linear elasticity range, it is expected that these simplification assumptions will affect all models similarly. Hence, the comparison results should remain almost unchanged.