Torsional resistance of an endodontic restorative system is an important consideration for a long-term restoration. However, currently no studies have established the amount of torsional resistance necessary for a tooth reconstruction: no cut-off or threshold limit values for success or failure are known in the literature. This in vitro study determined the torsional resistance of the natural human teeth by investigating a possible correlation between tooth dimensions and their resistance to torsional forces. This study also compares the torsional resistance of post-core components which make up a restoration from previously published studies. Three groups of 10 specimens per group were studied. A total of 30 recently extracted human single rooted (incisors) teeth were used. All teeth were measured at the cementoenamel junction (CEJ), buccolingually (B/L) and mesiodistally (M/D) with the use of an electronic digital caliper. The groupings were selected according to teeth dimension: group 1 B/L=5 mm and M/D=3 to 4 mm, group 2 B/L=6 mm and M/D=4 to 5 mm, and group 3 B/L=7 to 8 mm and M/D=5 to 6 mm. The extracted teeth were mounted in acrylic resin blocks with the coronal portion of the tooth remaining above the acrylic block. Test specimens were placed into a special jig with a custom vise grip that was tightened around the coronal portion of the tooth. Torsional force was measured in the clockwise direction using a Lebow 1102-200 torque apparatus (with a maximum torque value of 200 inch X ounce) and the force was recorded with a Hewlett Packard 7915B X-Y recorder. The torsional resistance values (inch X ounce) were: group 1; 33.97 ± 9.20 (B/L=5 mm, M/D=3 –4 mm), group 2; 37.67 ± 17.68 (B/L=6 mm, M/D=4-5 mm) and group 3; 76.15 ± 33.71 (B/L=7-8 mm, M/D=5-6 mm). A one-way ANOVA and SNK (Student-Newman-Keuls) revealed that group 3 had significantly greater resistance to torsional loading than groups 1 and 2 (P< 0.0004). Groups 1 and 2 were statistically similar to each other. The tooth dimensions have an effect on torsional resistance of the tooth. The torsional resistance of teeth with B/L=7-8 mm and M/D=5-6 mm are significantly greater than the torsional resistance of teeth with B/L=5-6 mm and M/D=3-5 mm. The results of this study may be used as a threshold for assessing the suitability of an endodontic restorative component such as, a endodontic post, core, bonding agent and crown reconstruction. The torsional results in this study were in the same range as previous studies concerning in vitro torsional properties of post-core components. The torsional data obtained in this study can be used to predict a restorative components clinical behavior.

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