Effect of mixing method on the porosity of encapsulated glass ionomer cement

Objective: To observe cement specimens mixed by various methods non-destructively using microfocus computerized tomography (micro-CT) and to evaluate the effect of mixing method on porosity. Method: Five glass ionomer cements were used: two were hand-mixed and three were encapsulated. The latter were mixed either by shaking or rotating. Fifteen cylindrical specimens were prepared for each material by each mixing method and stored in distilled water at 37 °C until testing. The specimens were observed and analyzed using micro-CT. 201 sliced images were obtained horizontally along the length of the specimens. Three-dimensional reconstructions were made and the sizes, numbers and total volume ratio of bubbles in the specimens were calculated. Results: Mixing method had a significant effect on porosity (P<0.05). For the luting/lining cement, mechanical mixing produced a significantly higher porosity (P<0.05). For the restorative cement, there were only small differences in porosity between specimens mixed by the two methods. More large bubbles were detected in specimens mixed mechanically than by hand for the fluid material. This suggested that for low-viscosity materials bubbles may combine until a certain viscosity is reached during setting. Significance: The strength of glass ionomer cement is affected by incorporated porosity and this depends on the mixing method. For low-viscosity cements, hand-mixing is favored in order to reduce porosity and increase strength, but this is not generally applicable to high-viscosity cements.