Statement of problem. The comfort and effectiveness of athletic mouth guards are believed to depend on their degree of fit to oral tissues. Vacuum-forming machines are simpler and less expensive than pressure-forming machines. However, it is thought that vacuum-formed mouth guards often do not exhibit adequate adaptation. Purpose. The purpose of this study was to evaluate the effects of cast residual moisture and temperature on the fit of athletic mouth guards made with a vacuum-forming machine. Material and methods. A metal master model simulating the cross section of the maxillary molar region was used to form 20 working plaster casts. The casts represented 4 (5 specimens each) conditions before the forming of the mouth guard specimens: storing in a wet environment at room temperature and storing in a dry environment at room temperature, 5 degrees C, and 40 degrees C. Mouth guard specimens were fabricated with ethylene vinyl acetate sheets (3.8-mm thick) with the use of a vacuum-forming machine. Testcasts were created by pouring hand-mixed type III dental stone into each of the mouth guard specimens. The differences in the sagittal cross-sectional heights at the line angle area of the test casts and the working casts were compared. This was achieved by superimposing their 3-dimensional images scanned by a laser scanner. The air permeability was also measured for the hand-mixed stone casts under wet and dry conditions, the vacuum-mixed stone cast, and the high-strength stone specimen. This was achieved by measuring the volume of transmitted air passing through the specimen in the testing tube. One-way analysis of variance with the Scheffe post hoc test (P <.05) was applied to determine the conditions of the working cast required to achieve the best fit. Results. With regard to the fit of the mouth guard specimens to the working cast, those with dry and heated working casts showed a significantly better fit than those with wet working casts (P<.05). A significantly larger volume of transmitted air was found in the dry stone specimen (P<.05) followed by the dry high-strength stone cast and then the wet stone cast. Conclusion. Within the limitations of this study, residual moisture in the working cast was the most critical factor in determining the fit of the mouth guard made by vacuum-forming machines. The best fit was achieved when the working cast was thoroughly dried and its surface temperature was elevated.
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