A new method for the computer-aided evaluation of three-dimensional changes in gypsum materials

Objectives. 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 methodʹs 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 μm. During the 56 day examination period no significant three-dimensional changes in dimensional stability were found. Significance. 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.