A comparison of the accuracy and dimensional stability between two silicon impression materials (Speedex and Irasil) Dr. M. Sabouhi* - Dr. M. Dakhilalian** - Dr. N. Nehchiri*** *- Assistant Professor of Fixed Prosthodontics Dept. - Faculty of Dentistry – Isfahan University of Medical Sciences. **- Member of Fixed Prosthodontics Dept. - Faculty of Dentistry – Isfahan University of Medical Sciences. ***- Dentist. Background and Aim: Condensation silicons are more commonly used in fixed Prosthodontics than other elastomers. The aim of this study was to compare the accuracy and dimensional stability of two condensation silicons: Speedex and Irasil. Materials and Methods: A metal master model containing two abutments representing two prepared teeth with definite dimensions was prepared and a metal tray was made. 40 impressions were taken (20 of each impression material) and 10 of them were poured with die stone (Prevest) after 30 minutes and the other 10 were poured after 120 minutes, according to the manufacturer’s instructions. Using a steriomicroscope M6c-10 and Digital camera moticam-480 the following dimensions were measured three times on the metal and stone models: (1) x = distance between the center of the abutments (2) y1 = height of the abutment with an undercut (3) y2 = height of the abutment without an undercut (4) z1= width of the abutment with an undercut, and (5) z2 = width of the abutment without an undercut. The difference between the dimensions of the main and stone models was assessed. Results: Multivariance and T-tests were used for statistical analysis. There was no significant difference in dimensional stability between groups 1 and 4 (Speedex, 30' and Speedex, 120') and between groups 2 and 3 (Irasil, 30'-120'). A significant difference was observed between Speedex and Irasil in both time periods (30 and 120 minutes). The difference between the die stone and the master model was significant in mesiodistal (x) and buccolingual (z1) dimensions for both materials. A significant difference was also found between Irasil and the main model in occlusogingival (y1) and buccolingual (z2) dimensions in both time periods, but Speedex failed to show a significant difference in the same dimensions and time periods. In the y2 dimension in contrast to Irasil, Speedex demonstrated a significant decrease of dimensional stability. Conclusion: Timing did not affect the dimensions of the two impression materials. The mesiodistal dimensions (x) of both Speedex and Irasil increased on the casts. A significant difference was found in dimensional stability between Speedex and Irasil impression materials. Key words: Dimensional change - Dimensional stability - Condensation silicones, Dimensional accuracy | 
