# Ergonomic Sports Mouthguards: A Narrative Literature Review and Future Perspectives
## 2 Narrative Literature Review Search Strategy
### Reinforcements in Mouthguards  
As previously mentioned, the use of mouthguards in sports activities is of utmost importance for preventing orofacial injuries. However, even with the use of mouthguards, instances of trauma can still occur [36; 37; 38]. This phenomenon arises when the impact force exceeds the protective capacity of the mouthguard, potentially leading to more pronounced injuries, especially in cases of restored or endodontically treated teeth, as these teeth possess altered strength and resilience when compared to intact teeth. Furthermore, in situations involving dental implants, the peri-implant tissues become susceptible to secondary injuries due to the high elastic modulus of the dental substitute [37].  
Therefore, numerous studies have been conducted to explore new methods aimed at enhancing the impact absorption capability (effectiveness) of conventional mouthguards against traumatic impacts on anterior teeth [12; 14; 15]. These studies examined whether the incorporation of intermediate reinforcement layers would enhance the protective response of these devices against impacts, thereby reducing the impact effect on adjacent structures [12]. This entailed designing mouthguards with air cavities and the addition of Sorbothane, metallic wires, sponges, and acrylic inserts [12; 14; 15], as well as using materials like polyamides, polyethylene terephthalate glycol (PETG), and polyurethane (TPU) [8].  
Reinforcements in EVA layers are feasible due to their long-term adhesive capacity between layers [19; 20], market availability, and ease of manipulation [16; 19], at ideal temperatures of 80-120 \({}^{\circ}\)C [8]. It was observed that EVA mouthguards with a 4 mm air cavity reduced force transmission by 32% compared to conventional mouthguards of the same thickness [12]. Mouthguards with a silicone intermediate layer demonstrated the ability to reduce stress transmission in dental structures by approximately 15-25% when compared to conventional laminated mouthguards [37]. Analysis of mouthguards with sponge inserts between EVA layers demonstrated a 49% increase in impact absorption compared to the control group without reinforcement [35]. Promising results with acrylic reinforcements have also been observed [12]. However, the use of rigid materials does not offer as much clinical safety [35].  
Nylon fibers modified with silica are an example of materials that can be used for reinforcing mouthguards during the fabrication process of the device [6; 7]. Nylon fibers possess polar amides that enable adjacent chains to form hydrogen bonds, which enhances their crystallinity, strength, and durability. The effectiveness of reinforcements using these fibers depends on their structural design and orientation, as well as the applied force and their interaction with the material to be reinforced for load transfer [39]. Moreover, one of the most successful methods to enhance the mechanical properties of acrylic resin is the incorporation of silica nanoparticles, which have significant effects on both mechanical and thermal aspects. Based on this, studies have been conducted where this structure was incorporated into polymers like acrylic resin and bisacrylic to fabricate temporary fixed prosthetic structures [39] and mucosa-supported and implant-supported complete arch dentures [39; 40; 41], resulting in improvements in the strength of the tested structures. With these positive results, a proposal was made by our research group to increase the absorption of impacts using this fiber as a reinforcement in customized mouthguards; this proposal is under development and an example is shown in Figure 4 below.