Background/Aims: Despite the use of conventional mouthguards, preventable sports-related dental injuries continue to occur. The authors have developed a twolayered ethylene polyvinyl acetate (EVA) mouthguard with a hard polyethylene terephthalate (PET) insert and a buffer space (H&SMG). However, adapting the PET onto the EVA layer requires skill. A light-cured Splint Resin (SRLC) and a glass fiber net (NET) reinforcement appear to resolve this issue. The aim of this study was to investigate whether SRLC with NET could replace PET and find a more practical application for NET. Materials and Methods: A pendulum impact testing machine and a dental model with strain gages were used. Six types of mouthguards were made: one with two laminated EVA blanks (LAM-MG), a three-layer type with a PET insert and an intermediate space (PET-H&SMG), a H&SMG with SRLC insert (LC-H&SMG), and three other types with differential NET-SRLC reinforcement; NET on the outer surface of SRLC, NET on the inner surface of SRLC, and NET on both the outer and inner surfaces. Five mouthguards of each type were fabricated and tested ten times with impact distances of 15 and 30 cm. Forty more impacts were applied to all H&SMGs to confirm the durability of the hard inner layer. Results: All H&SMGs showed significant strain reduction compared to the LAM-MG. PET-H&SMG and the four types of LC-H&SMG exhibited an equally slight strain (approximately 95% shock absorbing ability) in all conditions. During the test against the smaller impact, all H&SMGs showed no cracks. When tested against the stronger impact, only the LC-H&SMG with the reinforced inner surface, the double NETreinforced LC-H&SMG, and the PET-H&SMG remained intact. Conclusion: The NET-reinforced SRLC can replace PET as an intermediate mouthguard material. The NET application, at least on the internal surface, is indispensable for the LC-H&SMG reinforcement.

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