The effect of implant/abutment hexagonal misfit on screw joint stability.

A series of 10 incrementally larger, machined ASTM Grade 23 titanium non-segmented (UCLA type) abutments was loaded off axis with 133 N and cycled at 1150 vertical strokes per minute and 28 counterclockwise rotations per minute to determine screw joint stability. Abutment internal hexagonals ranged from 0.1065 to 0.1110 inches. External hexagonal mean flat-to-flat width was 2.684 mm. Rotational misfit between international and external hexagonals ranged from 1.94 degrees for the smallest abutment to 14.87 degrees for the largest. Screw joint failure ranged from 134,000 to 9.3 million cycles. The tightest matrix/patrix hexagonal screw joint failed at a mean of 6.7 million cycles. This study indicated that there was a direct correlation between hexagonal misfit and screw joint loosening. A rotational misfit of under 2 degrees provided the most stable and predictable screw joint.