# A Novel Method for the Mechanical Testing of Sports Mouthguards
## 3 New Method for Testing Sports Mouthguards
### Methods  
#### 3.1.1 Materials and Specimen Design  
The investigated mouthguards are made from ethylene-vinyl acetate (EVA). The EVA material is bought as 3 mm thick, 120 mm sheets from Keystone Industries. This material is widely used for custom-fitted mouthguards. The EVA material has a mean Shore A hardness of 82, which was obtained using different studies ([13; 14; 15; 16; 17; 18]). Five specimens were produced to be tested at five different impact energies based on the observations of the field study. The thickness of the mouthguards is directly related to the energy experienced and inversely related to the transmitted forces; however, there is a trade-off in the user comfort \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline  & \multirow{2}{*}{**Distance**} & \multicolumn{8}{c}{**Players**} & \multirow{2}{*}{**Total**} \\ \cline{2-2} \cline{5-10}  & & **1** & **2** & **3** & **4** & **5** & **6** & **7** & **8** & \\ \hline Mean & 3.2 m & 30.29 & 27.28 & 30.55 & 27.21 & 27.14 & 25.53 & 30.35 & 29.86 & 28.49 \\ speed & 4.8 m & 23.15 & 27.23 & 23.77 & 20.73 & 27.90 & 20.70 & 25.64 & 24.93 & 24.21 \\ \hline Standard & 3.2 m & 6.25 & 7.09 & 6.67 & 6.60 & 7.04 & 6.76 & 6.06 & 6.96 & 6.89 \\ deviation & 4.8 m & 4.97 & 5.56 & 5.16 & 5.27 & 5.76 & 5.11 & 6.01 & 3.00 & 5.71 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Mean and standard deviation (ms\({}^{-1}\)) of each data set including the total values across the total number of people. Figure 2: Demonstration of (**a**) beeswarm plot showing all shots from 3.2 m and (**b**) distribution of obtained ball speeds taken from 3.2 and 4.8 m distance to the board and the corresponding impact energies. as the thickness increases. [PERSON] et al. found that, for EVA mouthguards, increasing the thickness resulted in greater energy dissipation and reductions in the transmitted forces; however, the improvements lessened when increasing the thickness past 4 mm [19]. For this work, the thickness of the mouthguard models was designed to be 5 mm. This was to closely emulate the best performing thickness of 4 mm found by [PERSON], but is slightly thicker due to the nature of the impact tests being conducted at up to 100 J; much higher than the 4.4 J used in [PERSON]'s study. A simplified mouthguard based on a real mouth scan was designed as shown in Figure 2(a). The original scan was taken from a healthy adult and used to ensure that the dimensionality of the simplified model was representative enough of a set of teeth. To fix the mouthguard in a way that resembles how it would appear in the user's mouth, it was necessary to include a material that mimics the role of the teeth, of which there are multiple possibilities; therefore, materials that have similar mechanical properties to tooth enamel were investigated. [PERSON] et al. studied the mechanical properties of teeth and found the results shown in Table 2[20]. A material that [PERSON] et al. found to have similar properties to that of enamel is aluminium, which is a metal with low brittleness, but with similar hardness and Young's modulus to that of enamel. Although aluminium does not represent perfectly how the teeth will respond, the mechanical properties are similar enough for the study to quantify the effect of the impacts that are analysed. Hence, the specimens were mounted for each test on the mouthguard fixture made from aluminium as shown in Figure 2(a),b. The mouthguards were manufactured using five layers of 3 mm thick EVA sheets. Figure 2(a) represents the dimensions of the mouthguard. Each layer was heated and as it was added on top, it fuses to the layer beneath. The layer was then allowed to cool and harden before another sheet was added on top. Although the mouthguards were designed with the same dimensions, due to variations in the manufacturing process, they were not identical. Table 3 provides the measured mean thickness of the manufactured mouthguards. Figure 2(a) indicates with the black dots the different thickness measurement locations.