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

External hexagonal implants of known dimensions were assembled with premachined cast abutments and rebroached cast abutments. The abutment screws were tightened to 20 Ncm and 30 Ncm, and the samples were loaded off axis with 133.3 N. Load application was at 1,150 cycles per minute with a sample counterclockwise rotation of 28 cycles per minute. The premachined cast abutments tightened to 20 Ncm failed with a mean of 357,162 cycles (SD = 77,981 cycles). The rebroached cast abutments were cycled to 1 million cycles without failure. At 30 Ncm, the premachined cast abutments failed at a mean of 5.0 million cycles (SD = 2.2 X 10(6) cycles). Two of five rebroached cast abutments failed at 4.3 million and 9.5 million cycles, but the remaining samples showed no evidence of screw loosening after 10 million cycles, at which time the test was terminated. The results indicate a direct correlation between rotational misfit and screw loosening. Screw joints can be made more resistive to screw loosening by the elimination of rotational misfit.