PubMed · 5278706
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P R Barbell. 1971. A neglected responsibility.. https://pubmed.ncbi.nlm.nih.gov/5278706/
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Clinical problems associated with the polymerization shrinkage of dental composite restorations include tooth sensitivity, crown fracture, discoloration, recurrent decay, and loss of restoration. Our goal was to determine whether these complications could be attributed to the transient stresses developed during contraction. Thus, a finite element model was used to calculate the transitory deformations and composite-tooth interface stresses produced during the shrinkage of a chemical-cured ideal Class-V composite restoration. It was found that the interface stress peaks moved with the polymerization front, and that in some instances, their intermediate magnitudes were higher than the final, fully cured, values. Therefore, the results indicate that clinical failure may be related to these transitory changes during the polymerization process.
Thermoforming of polymethyl methacrylate sheet is used to produce a number of different types of dental appliances. The purpose of this study was to determine the dimensional accuracy of thermoformed polymethyl methacrylate specimens. Five blanks of the acrylic resin were thermoformed on stone casts prepared from a silicone mold of a brass master die. The distances between index marks were measured both on the cast and on the thermoformed blanks with an optical comparator. Measurements on the blanks were made again 24 hours after processing and then 1 week, 1 month, and 3 months after immersion in water. Linear shrinkage of less than 1% (range 0.37% to 0.52%) was observed 24 hours after removal of the blanks from the cast. Immersion of the thermoformed specimens in water resulted in an increase in measured dimensions, but after 3 months' immersion these increases were still less than those of the cast (range 0.07% to 0.18%). It was concluded that it is possible to thermoform Perspex polymethyl methacrylate accurately.