Effect of an antirotation resistance form on screw loosening for single implant-supported crowns

The gold screw of the single tooth implant-supported restoration has frequently been reported to exhibit the problem of screw loosening.

This in vitro study considered an antirotation resistance form with an increased moment arm length as a strategy to increase the net effect of the retaining screw preload.

Three groups examined included (a) standard abutments (3 mm), (b) modified standard abutments (3 mm) with 4 milled (1 x 1 mm) notches placed equidistant around the periphery, and (c) Estheticone abutments (1 mm). Crown castings of 2% gold-palladium-based alloy were made using plastic and resin patterns molded to fit inside a quarter-inch socket wrench. For testing, each specimen consisted of a 15-mm endosseous threaded implant analog, its abutment (20 N force) and crown casting (10 N force) screwed together and securely clamped in a vise. A clockwise shear (rotational) force was applied using a manual torque driver and the torque recorded to failure of the gold screw or loosening of the abutment screw.

Analysis revealed that the dislodging force for groups (b) and (c) was similar (69.6 and 71.0 N.cm, respectively), whereas group (a) (standard implant external hex) demonstrated that the smallest moment arm required a significantly smaller dislodging force (21.3 N.cm).

These findings suggest that the addition of an antirotation resistance form increased the length of the moment arm, thereby increasing the effect of preload and reducing the problem of screw loosening.