Abstract
The aim of this study was to evaluate the influence of mixing method and the empiric water/powder ratio on the dimensional change of two types of dental stones (type III and type IV) at different setting times: baseline, loss of brightness, initial and final setting (1/4 and 1pound Gilmore´s needle, respectively) and, after 48 hours at 37°C. Five mixing methods were used: Gc – manufacturer´s recommendation, G1 – weighed powder/ empiric water volume and powder on water, G2 - weighed powder/ empiric water volume and water on powder, G3 – empiric powder volume, graduated water and powder on water, G4 - empiric powder volume, graduated water and water on powder. The linear expansion was measured according to ISO 6873. The expansion values (x10-2mm) were analyzed by Two-way ANOVA and Tukey HSD test (?=0.05). For type III, the results showed significance for both factors and for interaction; for type IV, only the time were significant (p<0.05). Limited by methodology, it could be concluded that both dental stones expanded more than the recommended by ISO. The expansion of type III was dependent on the mixing method, and once correct consistency is reached with type IV, there is no influence of mixing method on dimensional stability.KeywordsGypsum; Dental stones; Expansion behavior.
Highlights
D espite the digital technology for virtual cast construction, dental stones are still widely used in Dentistry for different applications in private offices and in laboratories for cast construction, and for mounting casts in articulator. [1,2,3] Its popularity can be explained by its low price, ease of handling and ability to produce reliable results. [4]
In 2000, Ferreira et al determined the magnitude of the expansion of some types of dental gypsums
[10] The results indicated that accuracy was higher for the special type of gypsum, justifying its use for casting and, when high mechanical strength is required
Summary
Dental stones handling characteristics have been studied aiming to reduce the setting expansion, providing higher accuracy in casts. Manufacturers recommend the weighing of powder, to measure the water in a test tube and, to pour the powder on water If possible, this should be performed in a vacuum mixer machine to decrease air incorporation in the mass. In daily practice professionals use to mix gypsum under tap water, adding water and/or gypsum powder repeatedly in the attempt to reach the wright consistency, which in many cases is not obtained. This empiric agglutination leads to flaws in the mass and to dimensional alteration of the cast. This empiric agglutination leads to flaws in the mass and to dimensional alteration of the cast. [5]
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