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Background: Amongst the silicone impression materials, the addition silicones are superior in accuracy and dimensional stability. These days a wide range of condensation silicones are marketed with claims of equally good results as addition silicones.

Aims and objectives: To evaluate and compare the time dependent dimensional accuracy and stability of addition silicone and condensation silicone impression materials.

Materials and methods: Two addition-cured silicones (Reprosil, Dentsply/ Caulk, USA and Express XT, 3M ESPE, Germany) and two condensation-cured silicones (Speedex, Coltene Whaledent, USA and Zetaplus, Zhermack, Italy) were evaluated. A stainless steel master die simulating a three unit fixed partial denture preparation was fabricated. For each brand of the impression material, 6 impressions were made and cast poured immediately. The impressions were repeatedly poured at 1 h, 24 h, 48 h and 16 8 h. Interpreparation dimension and occlusogingival dimension were measured in the recovered stone dies (Type IV, Kalrock) using a travelling microscope calibrated to an accuracy of 0.001 cm.

Results: i) the addition silicone impression materials were superior in accuracy and dimensional stability in comparison to condensation silicone; ii) at 1 hour of pour, all the materials tested seemed to have comparable accuracy and dimensional stability; iii) the gypsum dies produced from the successive pours of all the tested impression materials were generally larger in interpreparation distance and smaller in occlusogingival dimension than the stainless steel master die, with the addition silicone demonstrating the smallest change.

Conclusion: The addition silicones have better dimensional accuracy and stability than condensation silicones. An impression made from condensation silicone should be poured as soon as possible.

Accuracy, Addition silicone, Condensation silicone, Impression materials