[Accuracy of post impressions using metal models]

The purpose of this study is to investigate how differences in post shape, storage time and condition after removal affect impression accuracy with respect to dimension and shape. The following results were obtained for various elastic impression materials which were tested using a metal model with five different post shapes. 1. Alginate, polysulfide (regular type) and agar-agar showed substantial deformation due to undercut in post shape. Polysulfide (injection type) and polyether showed slight deformation due to undercut. Silicone (both condensation and addition types) showed very little deformation regardless of post shape. 2. As for elapse of time after removal, dimensions of all impressions decreased in varying degrees depending on type of materials. 3. When impressions were placed horizontally, increased deformation was observed as time elapsed with both the polysulfide and the alginate. Increased deformation was not observed with elapse of time for other impression materials. While the above results show that accurate impressions with little deformation at time of and after removal are obtainable depending on the types of materials, nothing currently available fully satisfies the demand for the type of impression materials that would slightly decrease in dimensions with no deformation for best results. This finding indicates the need to develop new impression materials to better satisfy the demand.