THE EFFECT OF ELIMINATING IMPLANT ABUTMENT ROTATIONAL MISFIT ON SCREWJOINT STABILITY/

External hexagonal implants of known dimensions were assembled with pr emachined cast abutments and rebroached cast abutments. The abutment s crews 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 minut e 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 wer e cycled to 1 million cycles without failure. At 30 Ncm, the premachin ed cast abutments failed at a mean of 5.0 million cycles (SD = 2.2 x 1 0(6) cycles). Two of five rebroached cast abutments failed at 4.3 mill ion and 9.5 million cycles, but the remaining samples showed no eviden ce of screw loosening after 10 million cycles, at which time the test was terminated. The results indicate a direct correlation between rota tional misfit and screw loosening. Screw joints can be made more resis tive to screw loosening by the elimination of rotational misfit.