How addition silicone wash thickness effects accuracy and dimensional stability?

Materials and methods: impressions of a stainless steel master model analogue to prepared tooth with 6 degrees taper, sub gingival margins and sulcus of 1x0.2mm (n=135) were made with either: Express regular set(3M), Express fast set(3M) , Aquasil regular Dentsply) impression materials. The impressions were taken via the 2-step technique. Each group was subdivided into 3 groups, according to spacer thickness of 0.5, 1, 1.5 mm. Spacer thicknesses were kept through omnivac plastic sheets .Direct measurements from the impressions of the prepared tooth (mid buccal-lingual and mid mesio-distal) as well as planar distance between 2 distal dots on the model were taken after 2,24,48,72 hours by toolmaker microscope (Mitutoyo, Japan) and compared to the same distances in the master model. Statistical analysis was done by 2 way ANOVA

Results: The differences between the materials and spacers were significant (p<0.01). The change was also time-dependent (p<0.02). Express fast showed the highest deviation from the model (14-64µ) followed by Express regular (4-40µ) and Aquasil(8-22 µ). 1.5mm wash thickness showed the highest positive deviation from the master model (25-52 µ) followed by 1mm thickness (21-48µ). The 0.5mm wash thickness showed negative deviation (-2-12µ).

Conclusions: There is no significant difference in accuracy between spacers of 1 and 1.5mm. The use of 0.5mm spacer produced a small negative deviation. Pouring of the impressions may be postponed up to 72 hours.