THE EFFECT OF IMPLANT ABUTMENT HEXAGONAL MISFIT ON SCREW JOINT STABILITY/

A series of 10 incrementally larger, machined ASTM Grade 23 titanium n onsegmented (UCLA type) abutments was loaded off axis with 133 N and c ycled at 1150 vertical strokes per minute and 28 counterclockwise rela tions per minute to determine screw joint stability. Abutment internal hexagonals ranged from 0.1065 to 0.1110 inches. External hexagonal me an flat-to-flat width was 2.684 mm. Rotational misfit between internal and external hexagonals ranged from 1.94 degrees for the smallest abu tment to 14.87 degrees for the largest. Screw joint failure ranged fro m 134,000 to 9.3 million cycles. The lightest matrix/patrix hexagonal screw joint failed at a mean of 6.7 million cycles. This study indicat ed that there was a direct correlation between hexagonal misfit and sc rew joint loosening. The greater the hexagonal misfit, the greater the probability of screw loosening. A rotational misfit of under 2 degree s provided the most stable and predictable screw joint.