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.