Delayed linear expansion of improved dental stone.

Statement Of Problems: Specification 25 of the American Dental Association (ADA) states that final setting expansion measurements of improved dental stone are made 2 hours after mixing. In a limited pilot study on selected type IV and V dental stones, measurements showed delayed linear expansion or early contraction continuing up to 120 hours.

Purpose: The objectives of this in vitro study were to measure the linear setting expansion of 6 ADA type IV and V dental stones, to compare their expansion at 2 hours relative to ADA Specification 25, and to characterize expansion changes up to 120 hours.

Material And Methods: The improved dental stones tested in this study were Die-Keen, Jade Stone, Vel-Mix, ResinRock, Fuji Rock, and Silky Rock. Mixing was performed in accordance with a standardized technique and the manufacturers' instructions. The mixes were poured into an electronic expansion-measuring device, and setting expansion values were continuously measured and recorded for 120 hours. The collected data were subjected to a 2-way repeated-measures multivariate analysis of variance at alpha=.05. Planned comparisons between the 2-hour experimental and 2-hour manufacturer expansion values were made at the 95% confidence interval.

Results: All stones showed higher mean linear expansion values at 120 hours than at 2 hours. Expansion was essentially complete at 96 hours for all stones except ResinRock. Die-Keen exhibited the highest total mean setting expansion (0.35% +/- 0.013%), and Silky Rock exhibited the lowest mean linear expansion (0.14% +/- 0.008%) at 120 hours. Multivariate analysis of variance revealed significant interactions between stone and time (P<.0001). Mean expansion values at 2 hours were comparable to those stated by the manufacturer except for Die-Keen and Jade Stone.

Conclusion: Within the parameters of this study, all dental stones tested exhibited continued expansion while setting under clinical conditions. The amount of expansion varied significantly among the materials, with 22% to 71% of the expansion occurring after 2 hours. Die-Keen exhibited the highest total mean expansion but had the least amount of expansion after 2 hours; both Die-Keen and Silky Rock remained stable after 72 hours.