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 e lement 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. Spe cimens underwent nondestructive flexibility tests using a three-dimens ional motion analysis system. Tests compared the intact spine with ins trumentation with and without transfixators. Rotational angles of the superior vertebra, resulting from the maximum moment of 6.4 Nm, were c ompared. Three-dimensional finite element models investigated transfix ator position. Rotations of the superior vertebra were compared for ca ses 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 c ompared with instrumentation alone. Finite element models predicted im provement in lateral bending and axial rotation with transfixators com pared with the case with no transfixator. With one transfixator, the g reatest improvement in axial rotation stability occurred with the tran sfixator at the proximal 1/4 position of the rods, When two transfixat ors were used, the optimal locations were with one transfixator in the middle and the second at the proximal 1/8 position. Conclusions. Tran sfixators improved the stabilizing effects of pedicle screw instrument ation. The greatest axial rotation stability was obtained with two tra nsfixators: one in the middle and the other at the proximal iis positi on of the longitudinal rods.