Torsional rigidity of scoliosis constructs

Study Design. A biomechanical study of the rigidity of various scoliosis co nstructs instrumented with and without caudal pedicle screw anchors and wit h none, one, or two cross-link devices. Objectives, To determine whether the increased torsional rigidity provided by distal pedicle screw fixation might make cross-linking unnecessary. Summary of Background Data. Pedicle screws and cross-linking devices have b een shown to increase the structural rigidity of spinal constructs. Their r elative contributions to scoliosis construct rigidity has not been determin ed. Methods, "Short" (T2-T11) and "long" (T2-L3) scoliosis constructs were moun ted on an industrially fabricated spine model and rested in a hydraulic tes ting machine. Four different short and four different long constructs were tested: hooks only, hooks with concave side thoracic sublaminar wires, hook s with distal pedicle screw anchors, and hooks, distal pedicle screw anchor s, I and concave thoracic sublaminar wires. There were four iterations for each construct tested: no cross-links, one superior cross-link at T4-T5, on e inferior cross-link at T9-T10, and two cross-links. Torsional rigidity wa s tested by applying a rotational torque at T2. Vertebral body motion was r ecorded with a three-dimensional video analysis system. Results. Constructs with distal pedicle screws were statistically more rigi d in torsion than those with hooks as distal anchors. The additional torsio nal rigidity from one or more cross-links was negligible compared with that provided by pedicle screws. Conclusions. With pedicle screws as distal anchors in scoliosis constructs, cross-linking with one or two devices adds very little additional rotation al stiffness and may be unnecessary in many cases.