Biomechanics of in situ rod contouring of short-segment pedicle screw instrumentation in the thoracolumbar spine.

In situ rod contouring is proposed as a means of three-dimensionally controlling the three columns of the spine. Our goal was to determine the biomechanical effectiveness of the technique for both clinical application and future research. Six cadaver specimens were tested with pedicle screw instrumentation from T12 to L2 in an L1 corpectomy model with all longitudinal ligaments retained across the defect. Contouring moment, anterior longitudinal ligament (ALL) forces, and distraction and lordosis across L1 were monitored. Rods were contoured to 26 degrees, and axial distraction averaged 6.9 mm. Bone/screw-interface failure began during contouring in every case < 10 degrees with no resulting increase in lordosis across L1. High tensile forces generated in the ALL resisted the moment applied by the rod benders and instead caused failure of the weaker bone/screw interface.