title: A new method for the computer-aided evaluation of three-dimensional changes in gypsum materials.
abstract: OBJECTIVE
  The quantitative evaluation of the time- or process-dependent three-dimensional stability of dental materials is a common question in dentistry. An investigation procedure has been developed based on a CAD-surface model of a prepared upper canine, as well as a high-precision physical copy (metal master die). The specific aim of this study was to test this method's reliability. Additionally, the ability of the developed procedure to determine the three-dimensional stability of resin-reinforced gypsum master casts over time was investigated.
  
  
  METHODS
  Ten duplicate dies of improved dental stone (esthetic-rock, dentona, Germany) were manufactured, and digitized 1, 3, 7, 28 and 56 days after pouring. A three-coordinate optical measuring device was used for the data acquisition. The three-dimensional accuracy of stone dies was determined by comparing the digitized data of the stone dies made from the metal master die to its CAD-surface model (Surfacer) Version 9.0. Imageware Inc., Ann Arbor Michigan, USA). To assess the procedure, test surfaces were created from the digitized data and compared with a reference.
  
  
  RESULTS
  The mean deviation between the digitized point cloud and the test surface was less than 3 microm. During the 56 day examination period no significant three-dimensional changes in dimensional stability were found.
  
  
  CONCLUSIONS
  The procedure for the quantitative three-dimensional evaluation was shown to be suitable. Best-fit registration enabled a reliable alignment of the point cloud to the CAD-surface model. Alteration of three-dimensional accuracy over 6 weeks was insignificant and without clinical relevance.
authors.affiliation: Department of Prosthetic Dentistry, Dental School, University Hospital Carl Gustav Carus, Dresden University of Technology, Fetscherstrasse 74, 01307, Dresden, Germany. ralph.luthardt@mailbox.tu-dresden.de
authors.forename: Ralph G
authors.initials: RG
authors.lastname: Luthardt
authors.forename: Peter
authors.initials: P
authors.lastname: Kühmstedt
authors.forename: Michael H
authors.initials: MH
authors.lastname: Walter
mesh_terms: D002133:Calcium Sulfate; D003198:Computer Simulation; D017076:Computer-Aided Design; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000379:methods*; D007180:Incisor; D003765:Models, Dental; D015203:Reproducibility of Results; D013997:Time Factors
chemical_list: D002133:Calcium Sulfate