Effects of designs of Class 2 preparations on resistance of teeth to fracture.

This study was designed to evaluate the effect of isthmus width on the strength of the remaining tooth structure in maxillary first premolars restored with class 2 silver amalgam restorations. One hundred ten sound maxillary first premolars freshly extracted as a part of orthodontic treatment were collected. The teeth were then divided into 10 groups of 11 teeth each. Ideal class 2 mesio-occlusal, disto-occlusal and mesio-occlusodistal cavities with different isthmus widths i.e. one half the intercuspal distance, one third the intercuspal distance and one fourth the intercuspal distance and one fourth the intercuspal distance were prepared. Compressive forces were applied using a Universal Testing Machine and load to the point of fracture was determined. Intact teeth produced the best tooth fracture resistance i.e. 105.4 MPa, followed by preparations with isthmus width of one fourth, one third and one half intercuspal distance in the order.