Effects of setting under air pressure on the number of surface pores and irregularities of dental investment materials

Abstract

Surface roughness and irregularities are important properties of dental investment materials that can affect the fit of a restoration. Whether setting under air pressure affects the surface irregularities of gypsum-bonded and phosphate-bonded investment materials is unknown. The purpose of this study was to investigate the effect of air pressure on the pore size and surface irregularities of investment materials immediately after pouring. Three dental investments, 1 gypsum-bonded investment and 2 phosphate-bonded investments, were investigated. They were vacuum mixed according to the manufacturers' recommendations, then poured into a ringless casting system. The prepared specimens were divided into 2 groups: 1 bench setting and the other placed in a pressure pot at 172 kPa. After 45 minutes of setting, the rings were removed and the investments were cut at a right angle to the long axis with a diamond disk. The surfaces of the investments were steam cleaned, dried with an air spray, and observed with a stereomicroscope. A profilometer was used to evaluate the surface roughness (μm) of the castings. The number of surface pores was counted for 8 specimens from each group and the means and standard deviations were reported. Two-way ANOVA was used to compare the data. Specimens that set under atmospheric air pressure had a significantly higher number of pores than specimens that set under increased pressure (P<.05). No statistically significant differences for surface roughness were found (P=.078). Also, no significant difference was observed among the 3 different types of materials tested (P>.05). Specimens set under positive pressure in a pressure chamber presented fewer surface bubbles than specimens set under atmospheric pressure. Positive pressure is effective and, therefore, is recommended for both gypsum-bonded and phosphate-bonded investment materials.