An evaluation of the time-dependent dimensional stability of eleven elastomeric impression materials.

The dimensional stability of three polysulfides, one condensation-cured silicone, six addition-cured silicones, and one polyether, was evaluated when poured immediately and after storage for 1, 4, and 24 hours. Dimensional stability was determined by measuring the amount of gap width developed by a master coping when seated on the die that had been poured in the impression. The greatest accuracy occurred when the impressions were poured immediately. All the addition-cured silicone materials exhibited excellent dimensional stability for all storage times. Delayed pouring of artificial stone in the impressions made in these materials should result in very little change in die accuracy. The condensation-cured silicone material Polytrans has good accuracy if poured immediately. Delayed pouring results in a rapid loss of accuracy. The polysulfides have good accuracy only if poured immediately, with the exception of Omniflex, which can be stored up to 4 hours with only a minimum loss of accuracy. The polyether material Impregum expanded during storage. Since the dimensional change of the impression during storage is a characteristic of the material, a reduction in its bulk should lead to a desired amount of dimensional change. To achieve this desirable state, custom trays that will provide for the minimum recommended bulk of material should be fabricated. The complex geometry of the cavity preparation makes any prediction of the desirability of impression material expansion or contraction difficult, because a change in impression material dimensions may be beneficial to the fit in one portion of the casting but have an extremely adverse effect in another.(ABSTRACT TRUNCATED AT 250 WORDS)