Fatigue Life Improvement of Nitrogen-Ion–Implanted Pedicle Screws

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Abstract
An experimental study of the fatigue life of pedicle screws and their nitrogen-ion-implanted counterparts was conducted. Nitrogen-ion implantation counterparts were varied in the first part of the study to obtain the best implantation characteristics. Using this optimal parameter for N+-implantation, the performance of two matched groups were compared at two bending moments near the "knee" of the S-N curves. At 3.96 N-m and 5.09 N-m of applied bending moments, the increase in the mean fatigue life for the former was 98% and for the latter 20%. The 98% prolongation was statistically significant at P < 0.005; however, the 20% increase was statistically insignificant (P < 0.1). The implantation depth is about 0.1 .mu. in the near surface of the screw. This appears to be thick enough to inhibit crack initiation at 3.96 N-m, but only marginally at 5.09 N-m. Increasing the implantation depth has the potential of increasing the fatigue life at the higher bending moments.

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