Effect of grooves on resistance/retention form of Class 2 approximal slot amalgam restorations

This study evaluated in vitro the effectiveness of resistance/retention grooves in box-only (approximal slot) class 2 preparations. Forty-eight sound, caries-free maxillary premolars were distributed equally into four groups of 12 teeth based on faciolingual dimensions. Teeth were mounted vertically, and class 2 mesio-occlusal slot preparations were cut in each tooth. Resistance/retention grooves were placed in three of the four groups with a #1/4 round bur to a depth of 0.3-0.5 mm. Teeth were restored with amalgam and positioned 13.5 degrees from vertical; an area was flattened on each amalgam marginal ridge, and the flattened areas were loaded to failure using an Instron with a rectangular flat-ended rod at a cross-head speed of 1 mm/min. Mean load (SD) to failure of the group using "conventional" grooves extending in dentin from the gingival floor occlusally to near the occlusal DEJ was 196N (46N). For long grooves extending from the gingival floor to the occlusal surface, the mean failure load was 169N (58N). Slot restorations with short resistance/retention grooves or points (0.5-1.0 mm) just gingival to the occlusal DEJ had a mean failure load of 132N (44N). Slot restorations with no grooves had a mean failure load of 69N (46N). ANOVA and Student-Newman-Keuls tests were used for analysis. The no-groove group provided significantly less (P < 0.01) resistance than any group with grooves. Approximal slot restorations with "conventional" grooves were significantly more resistant (P < 0.01) than those with short grooves but were not significantly more resistant than those with long grooves.