Mouthguard Thermoforming Method to Decrease Palatal Thickness While Maintaining Labial and Buccal Thickness

Wearing a mouthguard reduces the risk of sports-related injuries, but a more comfortable design is required in order to increase the wearing rate. The aim of this study was to investigate a thermoforming method that decreases palatal thickness while maintaining labial and buccal thickness. Mouthguards were fabricated from an ethylene-vinyl acetate sheet (thickness: 4.0 mm) by using a vacuum forming machine. Four working models were prepared: 1) the anterior height was 25-mm and the posterior height was 20-mm (model A), 2) model A with the palate trimmed (model B), 3) heights 5 mm greater than model A (model C), and 4) model C with the palate trimmed (model D). The two forming conditions were as follows: 1) The sheet was formed when it sagged 15 mm below the level of the sheet frame at the top of the post under ordinary use (control); 2) The sheet frame at the top of the post was lowered and the sheet covered the model when it sagged by 15 mm. The rear side of the model was pushed to move the model forward 20 mm, and then the sheet was formed (MP). Differences in mouthguard thickness due to forming conditions and model forms were analyzed by two-way analysis of variance and Bonferroni’s multiple comparison tests. Difference in forming conditions was similar for all model forms; for the MP, the thickness of the incisal edge, labial surface, cusp and buccal surface were greater, and the palatal surface was thinner than the control. On the labial and buccal surface, the thickness difference due to the model form was observed only for the MP, and models A and B were thicker than models C and D. The palatal thickness tended to be thin in the models with the trimmed palate. This study suggested that the labial and buccal thickness of the mouthguard can be maintained, and the palatal thickness can be decreased by using the model with the palate trimmed with the forming method in which the model position is moved forward immediately before the vacuum formation.


Introduction
Wearing a mouthguard decreases the risk of sports-related injuries, and the material and thickness of mouthguards have substantial effects on effectiveness and safety [1] [2] [3] [4] [5]. Custom-made mouthguards are very comfortable and safe compared with commercial products [4] [6]. Flasking and injection molding are superb techniques for producing mouthguards with appropriate thickness. In particular, injection molding is a practical technique that produces mouthguards that are very comfortable and provide excellent protection [6]. Thermoforming has the advantage of having a production process that is simple with a short turn-around time. Forming machines are now available with semi-automatic functions, heating time or temperature programs according to sheet thickness, and a device capable of making indentations of the occlusal surface [7] [8] [9]. These developments have made thermoforming mouthguard fabrication even easier.
It is crucial to provide a thickness of 3 -4 mm on the labial and buccal sides of the mouthguard to ensure the proper distribution of force and counter the stress and strain generated during impact [10] [11] [12] [13]. However, a 35% -60% decrease in post-formation thickness during thermoforming is large, so it is difficult to obtain sufficient thickness with a single layer [14] [15] [16] [17] [18]. To address this, several thermoforming methods have been investigated, such as controlling the heating temperature of the sheet or the sheet shape [19] [20] [21] [22] [23]. These methods can provide athletes with a thicker single-layer mouthguard than conventional methods. However, few reports have examined the palatal thickness of the mouthguard, even though it affects the tongue space [24] [25].
It is important to avoid a situation in which athletes refuse to wear mouthguards because of discomfort [24], thereby decreasing the incidence of sports-related orofacial damage. However, comfort and fit are the main reasons behind the low rate of athletes wearing mouthguards [26] [27] [28]. In a study of elite water polo players, decreasing the length of the palatal margin of the mouthguard decreased interference with basic oral functions, such as conversation, swallowing, and respiration, and increased overall satisfaction [24]. Decreasing the palatal margin was also confirmed to not affect tooth deflection, mouthguard retention, or protection ability [24] [29] [30] [31]. This indicates that the external form of the mouthguard improves player satisfaction. The improved comfort during speaking, breathing, and swallowing was explained by the greater space allowed for the tongue [24].
Thus, the comfort of mouthguards is affected by their external form and thickness.
The aim of this study was to investigate a method for fabricating mouthguards of sufficient thickness and comfort that resist external forces. That is, the mouth-  Co., Tokyo, Japan), and a center trimmer (No. 528, Dentpia Co., Nagoya, Japan).    For all measurements, the differences in mouthguard thickness due to forming conditions and model forms were analyzed using statistical analysis software (IBM SPSS 24.0 SPSS Japan Inc., Tokyo, Japan). The Shapiro-Wilk test for normality of distribution and Levene's test for homogeneity of variance were also used. Each measurement exhibited normality and equal dispersion; accordingly, analysis was performed by two-way analysis of variance and Bonferroni's multiple comparison tests. All analytical methods were carried out with a significance level of 5% and a detection power of 80%, and a difference was considered significant when both were satisfied. Table 1 shows the results of two-way analysis of variance for the thickness of the mouthguard after formation. At all measurement points, the main effects of the forming condition and model form were significant, and their interaction was also significant. Based on the results, simple main effect tests were performed prior to multiple comparisons among levels. Table 2  The results of this study suggest that the labial and buccal thickness can be made thicker and the palatal thickness thinner than with the conventional method by adjusting the forming method and model form during thermoforming.

Figures 4(A)-(F) and
The MP condition ensures the labial thickness is approximately 1.5 times that of the conventional method [34]. In addition, the palatal thickness decreases because the sheet elongated by the model movement is formed against the palate side of the model [25].    The MP conditions gave thicker mouthguards, except for the anterior palate and the posterior palate, and showed the same trends as reported previously [25]. This study was examined whether palatal trimming and MP conditions do not affect the labial and buccal thickness and lead to a thinner palatal side. First, it was determined that the effectiveness of palatal trimming by comparing models C and D.
Second, it was considered whether the difference between models C and D was similar to the difference between models A and B. Third, it was investigated There was no significant difference in thickness between models C and D, other than in the palate under both forming conditions. The anterior palate was thinner for model D than for model C. The palate for model C had a smooth impression surface, whereas for model D, the sheet was pressed into a corner formed by the impression surface and the trimmed surface. The effect of the model form on the thickness of the molar palate was less than that on the thickness of the anterior palate. The height of the molars was lower and they had fewer edges than the anterior part, and there was less morphological difference was caused by palatal trimming.
There was no significant difference between models A and B at any of the measurement points under both forming conditions. The palatal thickness tended to be thinner in model B than in model A, but the difference was smaller than that between models C and D. These two comparisons showed that for athletes with a deep palate, palate trimming of the model does not significantly affect the mouthguard thickness, whereas for athletes with a shallow palate, it makes the mouthguard palatal side thinner.
Under the MP conditions, there was no significant difference between models B and D in the thickness of the anterior and molar palates. The height of the model with the palate trimmed did not affect the palatal thickness of the mouthguard.
However, the labial and buccal thickness were affected by the model height, and model D was thinner than model B. Under the MP conditions, model D was thinner than model A on the palate side, and on the labial and buccal sides. Therefore, the hypothesis that it would be more effective to increase the height of the model and trim the palate side did not hold. However, trimming the palate would not adversely affect the absorption and dispersion of impact forces because there was no significant difference in thickness except for the palate side between models A and B, or between models C and D.

Conclusion
This study suggested that the labial and buccal thickness of the mouthguard can be maintained, and the palatal thickness can be decreased by using the model with the palate trimmed with the forming method in which the model position is moved forward immediately before the vacuum formation. This method is expected to contribute to improving athletes' comfort when wearing a mouthguard.

Funding
This study was supported by Nippon Dental University Intramural Research Fund.