Effect of grooves on resistance form of Class 2 amalgams with wide occlusal preparations

This study evaluated in vitro the necessity of retention grooves in approximal boxes in class 2 preparations with faciolingually wide occlusal extensions (occlusal isthmus width 1.8 mm). Thirty-six class 2 mesio-occlusal cavities were prepared in sound human maxillary premolar teeth and divided into three groups of 12. In one group, the preparations had no retention grooves. In another group, retention grooves 0.3-0.5 mm deep were prepared at the axioproximal line angles and extended from the gingival floor occlusally to just gingival to the occlusal dentinoenamel junction. In the third group, a 1 mm-long groove was prepared extending from the axiopulpal line angle occlusally to just gingival to the occlusal dentinoenamel junction. The marginal ridges of the amalgam were loaded at an angle of 13.5 degrees from vertical in an Instron Testing Machine until the restorations failed. Results indicated no significant difference in load to cause failure in any of the three groups. Restorations without approximal retention grooves possessed as much resistance to failure as those with grooves.