Biomechanics of transfixation in pedicle screw instrumentation

Study Design. The biomechanical role of transfixation in pedicle screw instrumentation was investigated using flexibility tests and finite element analyses. Objective. To assess the stabilizing effect of use and position of transfixators. Summary of Background Data. Transfixation is common in pedicle screw instrumentation, however, its biomechanical role and optimal position are not completely understood. Methods. Specimens underwent nondestructive flexibility tests using a three-dimensional motion analysis system. Tests compared the intact spine with instrumentation with and without transfixators. Rotational angles of the superior vertebra, resulting from the maximum moment of 6.4 Nm, were compared. Three-dimensional finite element models investigated transfixator position. Rotations of the superior vertebra were compared for cases with and without transfixators to determine the position providing the greatest stability. Results. Biomechanical test showed that only axial rotational stability significantly improved with transfixators compared with instrumentation alone. Finite element models predicted improvement in lateral bending and axial rotation with transfixators compared with the case with no transfixator. With one transfixator, the greatest improvement in axial rotation stability occurred with the transfixator at the proximal 1/4 position of the rods, When two transfixators were used, the optimal locations were with one transfixator in the middle and the second at the proximal 1/8 position. Conclusions. Transfixators improved the stabilizing effects of pedicle screw instrumentation. The greatest axial rotation stability was obtained with two transfixators: one in the middle and the other at the proximal iis position of the longitudinal rods.