Improved extraction torque of hydroxyapatite-coated pedicle screws

Loosening of the screws is a problem in instrumentation with pedicle screws . Coating with hydroxyapatite improves the holding characteristics for meta l implants, but the possible effects on the anchorage of pedicle screws hav e not been described. In this study, seven patients were operated on with s pinal instrumentation using four stainless steel pedicle screws. Hydroxyapa tite-coated screws were used in either the upper or the lower of the instru mented levels. The insertion torque was measured. In four cases the screws were removed after 10-22 months and the extraction torque was measured. The mean insertion torque was found to be significantly greater in the hydroxy apatite-coated screws (107 Ncm) than in the standard screws (76 Ncm). In th ree cases, the extraction torque for the hydroxyapatite-coated screws excee ded the range for the torque wrench (600 Ncm), while the conventional screw s were loose (< 5 Ncm). In one case, the extraction torque was 475 and 550 Ncm for the coated screws, and 5 and 25 Ncm for the conventional screws. Th e difference in extraction torque was significant. Hydroxyapatite coating w as shown to have improved the purchase of pedicle screws very effectively. By using fully coated screws, as in the present study, extraction was extre mely difficult compared to extraction of conventional stainless steel screw s, which were regularly loose. By reducing the area of the screws that is c oated, it may be possible to achieve an enhanced purchase while extraction will be easier when compared to fully coated screws.