Making accurate impressions is critical for restorative dentistry.  New impression materials and a new class of impression materials have been developed.  Since tear strength of an impression material is critical in obtaining an accurate impression when the margin is subgingival, this study compares them. Objective: This study measured and compared the tear strength of one polyether, one hybrid and 7 addition silicone impression materials. Methods: A mold was constructed which would make .4mm thick tear strength specimens.   The mold was placed into an incubator and maintained at 98.6⁰F.  Each of the impression materials were mixed following the manufacturers directions and inserted into the mold which was returned to the incubator for the setting time of the impression material being tested.  At the manufacturers recommended setting time the specimens were removed and placed into the Instron (model 1144).  The impression material specimens were loaded in tension until failure using a .5mm/min crosshead speed.  The tear strength of each impression material was calculated.  A one-factor ANOVA and Tukey B post-hoc analysis evaluated the data for intergroup differences. Significance was set at .05. Results:  Tear strength= MPa (n=5)

Material	Classification	Tear Strength
Impregum soft quick	Polyether	1.3±.1
Fusion	Hybrid	1.1±.06
Aquasil XLV	Addition silicone	3.5±.09
Affinis	Addition silicone	1.9±.1
Virtual	Addition silicone	2.2±.1
Exafast	Addition silicone	1.2±.1
Sultan	Addition silicone	1.7±.1
Splash	Addition silicone	2.4±.1
Precision	Addition silicone	1.7±.2

Mean tear strengths varied significantly between materials (p>.05). As a group addition silicone impression materials had significantly greater tear but significant variations occurred within that material group.   Conclusions:  Impression material selection should be partially determined by tear strength.