Effect of thermal shrinkage during thermoforming on the thickness of fabricated mouthguards: Part 2 pressure formation.

Background: The aim of this study was to examine the effect of thermal shrinkage, which occurs during thermoforming of ethylene vinyl acetate (EVA) sheets on the thickness of mouthguards fabricated by pressure formation.

Materials And Methods: Mouthguards were fabricated from 4.0-mm-thick EVA sheets by utilizing a pressure-forming machine. Two molding conditions were compared: The sheets were placed in the thermoforming machine with the sheet extrusion direction either vertical or parallel to the model's center line. The working model was trimmed to the height of 20 mm at the cutting edge of the maxillary central incisor and 15 mm at the mesiobuccal cusp of the maxillary first molar. The sheet was pressed against the working model for 2 min where the center of the softened sheet sagged 15 mm lower than the clamp. After fabrication, the thickness of mouthguard sheets was determined for the incisal (incisal edge and labial surface) and molar (cusp and buccal surface) portions, and dimensional measurements were made. Differences in molded mouthguard thickness with the sheet orientation of extruded sheets were analyzed by Mann-Whitney U-test.

Result: In comparison with the parallel axis orientation, the sheets in vertical orientation with the model's centerline yielded significantly higher thickness measurements at the incisal edge, labial surface, and the cusp (P < 0.01, respectively).

Conclusion: The results suggest that the EVA sheet produced by extrusion molding in vertical axis orientation with the model's centerline can effectively reduce loss of thickness in mouthguards after pressure formation.