Xeroradiographic determination of effective setting expansion of a cristobalite investment

Previous studies of setting expansion of gypsum-bonded investments have not examined how wax patterns expand with time. The purpose of this study was to provide a method of measuring wax patterns during the setting reaction. Ten MOD inlay wax patterns were invested individually in plastic casting rings and imaged at specific intervals on a stage on which both the rings and xeroradiographic cassettes were fixed. Distances between the axiogingival proximal walls, axiopulpal proximal walls, and axial walls were measured at each interval. Reproducibility of measurements of xeroradiographic images was about 0.3%. After setting, the wax was burned out, and the rings were cooled to room temperature and cast in a Bi-Sn alloy that did not shrink upon solidification. The castings were measured and the means compared with the means of xeroradiographic measurements. There was no significant difference between the expansion means of the axiogingival or axiopulpal dimensions measured by casting or xeroradiographically (p greater than 0.05, two-tailed t test). Significant differences (p less than 0.05) were seen between means of the axial expansion measured by castings and xeroradiographically, suggesting the formation of a void at the proviral walls of the wax patterns. All horizontal expansion occurred between two and 22 min from the start of mix.