REVISION OF FAILED PEDICLE SCREWS USING HYDROXYAPATITE CEMENT - A BIOMECHANICAL ANALYSIS

Study Design. The biomechanical influence of in situ setting hydroxyap atite cement was examined for use in pedicle screw revision surgery. P ull-out testing of control and pedicle screws augmented with hydroxyap atite cement was performed in human cadaver vertebrae. Objectives. To determine the immediate effect of using hydroxyapatite cement to augme nt revision pedicle screws after failure of the primary pedicle screw fixation. Summary of Background Data, The potential problems associate d with using polymethylmethacrylate to augment revision pedicular inst rumentation have prompted the search for other solutions. The introduc tion of resorbable hydroxyapatite pastes may have provided new biocomp atible solutions for pedicle screw revision. Methods. Ten human cadave r vertebrae were instrumented with 6.0-mm pedicle screws in each pedic le. The screws were loaded to failure in axial tension (pullout). The failed pedicles then were instrumented with 7.0-mm pedicle screws, eit her augmented with hydroxyapatite cement or nonaugmented, which also w ere loaded to failure. Finally, the nonaugmented 7.0-mm screw hole was reinstrumented with a hydroxyapatite cement-augmented, 7.0-mm pedicle screw and loaded to failure. Results. the pull-out strength of the 7. 0-mm, hydroxyapatite cement-augmented screws was 325% (P = 2.9 x 10(-5 )) of that of the 6.0-mm control screws, whereas the strength of the 7 .0-mm nonaugmented screws was only 73% (P = 2.0 x 10(-2)) of that of t he 6.0-mm control screws. The 7.0-mm screws augmented with hydroxyapat ite cement also were able to salvage 7.0-mm pull-out sites to 384% (P = 6.9E-5) of the pullout strength of the 7.0-mm nonaugmented screws. C onclusions. Hydroxyapatite cement may be a mechanically viable alterna tive to polymethyl methacrylate for augmenting revision pedicular inst rumentation and should be considered for future experimental, animal, and clinical testing.