title: Effects of setting under air pressure on the number of surface pores and irregularities of dental investment materials.
abstract: BACKGROUND
  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.
  
  
  OBJECTIVE
  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.
  
  
  METHODS
  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.
  
  
  RESULTS
  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).
  
  
  CONCLUSIONS
  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.
authors.affiliation: Clinical Assistant Professor of Dentistry, Division of Restorative Sciences, Herman Ostrow School of Dentistry, University of Southern California, Los Angeles, Calif.
authors.forename: Anita
authors.initials: A
authors.lastname: Tourah
authors.affiliation: Postdoctoral Research Fellow, Center for Craniofacial Molecular Biology (CCMB), Herman Ostrow School of Dentistry, University of Southern California, Los Angeles, Calif. Electronic address: moshaver@usc.edu.
authors.forename: Alireza
authors.initials: A
authors.lastname: Moshaverinia
authors.affiliation: Ralph & Jean Bleak Professor of Restorative Dentistry; Director, Implant Dentistry; and Co-Director, Advanced Prosthodontics, Herman Ostrow School of Dentistry, University of Southern California, Los Angeles, Calif.
authors.forename: Winston W
authors.initials: WW
authors.lastname: Chee
mesh_terms: D000399:Air Pressure; D002133:Calcium Sulfate / Q000737:chemistry; D003734:Dental Casting Investment / Q000737:chemistry*; D008422:Materials Testing; D010710:Phosphates / Q000737:chemistry; D016062:Porosity; D011312:Pressure; D013499:Surface Properties; D013997:Time Factors
chemical_list: D003734:Dental Casting Investment; D010710:Phosphates; C118504:ceramigold; C118505:novocast; D002133:Calcium Sulfate