THE EFFECTS OF RIGID SPINAL INSTRUMENTATION AND SOLID BONY FUSION ON SPINAL KINEMATICS - A POSTEROLATERAL SPINAL ARTHRODESIS MODEL

Study Design. Spinal kinematics after the implementation of rigid spin al instrumentation or the achievement of a solid fusion was studied us ing a sheep posterolateral spinal arthrodesis model. Objective, To inv estigate the effects of rigid spinal instrumentation or solid fusion o n spinal kinematic parameters. Summary of Background Data. Numerous st udies have attempted to define spinal instability in terms of kinemati cs. Recent in vitro studies have documented the neutral zone, or a mea sure of spinal laxity, as more sensitive to spinal instability than th e range of motion. Methods. Seven skeletally mature sheep underwent :: a single-level posterolateral lumbar arthrodesis using autologous bon e graft augmented with transpedicular screw fixation. The animals were killed 4 months after surgery. The identical surgical procedures were per formed in seven sheep cadaveric spines, which served as acute pos toperative controls. Each functional spinal unit was tested biomechani cally before and after hardware removal. The experimental control grou ps consisted of destabilized spines and spines that underwent transped icular screw fixation alone, whereas the fusion groups included spines that underwent posterolateral fusion alone or posterolateral fusion w ith instrumentation. Results, Rigid instrumentation and solid fusion s ignificantly decreased the neutral zone and range of motion in all tes ting modes. In axial rotation and lateral bending, solid fusion reduce d the range of motion significantly more than transpedicular screw fix ation alone. However, in all testing modes, the neutral zones ,: showe d no statistical difference between transpedicular screw fixation alon e and fusion groups. Conclusions. The range of motion was an equivalen t or better indicator of fixation or fusion stability compared with th e neutral zone. Moreover, the immediate postoperative fixation stabili ty, even if using transpedicular screw fixation, was less than the sta bility present after a solid fusion.