A Comparative Study on the Effect of Two Different Investing Mediums on the Movement of Artificial Teeth during the Fabrication of Complete Denture: An in vitro Study

Acrylic processing of waxed-up dentures is considered to be a crucial and technique-sensitive procedure. Polymethylmethacrylate (PMMA) has become the most commonly used material for denture bases since its introduction in 1937. The advantages of PMMA include excellent esthetic properties, adequate strength, low water sorption, low solubility, lack of toxicity, facility of repair, and construction by a simple molding and processing technique.1 Since, their introduction in dentistry, a certain lack of dimensional accuracy has been accepted as one of the disadvantages of complete denture construction, resulting from the unavoidable denture base shrinkage during acrylic resin polymerization.1,2 Polymerization shrinkage affects the position of the teeth on maxillary and mandibular dentures, and thus the final occlusion of the dentures. Even after investing a lot of time and clinical skill a definitive movement of teeth during and after processing has long been observed which lead to occlusal discrepancies and disturb the harmonious occlusal scheme achieved earlier. Shifting of teeth results in an opening of vertical dimension3 and this is indicated by lifting of the incisal guide pin,4 when the casts are remounted on an articulator. Purpose: Acrylic processing of waxed-up dentures is considered to be a crucial and technique-sensitive procedure. Even after investing a lot of time and clinical skill a definitive movement of teeth during and after processing has long been observed which lead to occlusal discrepancies and disturb the harmonious occlusal scheme achieved earlier. Shifting of the teeth can occur as a result of the investing procedures and investing materials. The objective of this study was to evaluate and compare the movement of artificial teeth in two-dimensions of space (anteroposterior and mediolateral) during processing of the waxed-up dentures using two different gypsum materials for investing. Methods: Two groups of 15 waxed-up maxillary dentures were processed using two different investing mediums (a) dental plaster and (b) combination of dental stone core and plaster mold. The artificial teeth movement was measured in the mediolateral and the anteroposterior directions between predetermined reference points before and after processing with the aid of a traveling microscope. The findings were statistically analyzed using parametric t-tests and ANOVA F-test. Results: Artificial teeth movement in both the anteroposterior and mediolateral direction was less with the use of investment combination of dental stone core and plaster mold when compared to the conventional method of investing with dental plaster. Conclusion: The dental stone core method is superior in view of the fact that it produces significantly less artificial teeth movement than the conventional method. However, neither of the investing methods is successful in completely preventing artificial teeth movement.