Biomechanical analysis of posterior fixation techniques in a 360 degrees arthrodesis model.

STUDY DESIGN: A biomechanical study to assess the ability of posterior fixation techniques to stabilize a functional spine unit (FSU) after insertion of an anterior load-sharing device.

OBJECTIVE: The objective of this study is to compare various posterior fixation techniques in combination with an anterior load-sharing implant.

SUMMARY OF BACKGROUND DATA: Pedicle screws and translaminar facet screws have been shown to improve the stiffness of an FSU in combination with an anterior load-sharing device. No published studies, to our knowledge, have compared translaminar facet screw fixation versus bilateral and unilateral pedicle screw fixation used with an anterior load-sharing device.

METHODS: Ten cadaveric FSUs were potted using methylmethacrylate and attached to a spine simulator mounted to an MTS Mini-Bionix testing machine. The simulator was configured to control compressive loading, axial torque, flexion, extension, and lateral bending. Each specimen was tested in the intact state and following the application of each of four stabilization techniques: custom cage alone, cage plus translaminar facet screw fixation, cage plus unilateral pedicle screw and plate fixation, and cage plus bilateral pedicle screw and rod fixation with transverse coupling. Compressive stiffness and total range of motion (ROM) between +/-8 Nm of torque were extracted from the raw data.

RESULTS: Each fixation method decreased ROM in torsion, flexion-extension, and lateral bending compared with the intact state. Unilateral pedicle fixation offered less stability than either of the other posterior fixations in all modes of testing except axial loading, where it was equivalent. Translaminar facet screw fixation was equivalent to bilateral pedicle screws in all modes tested.

CONCLUSIONS: Using a load-sharing interbody implant, translaminar facet screws are equivalent to bilateral pedicle screws in resisting motion in all three planes. Translaminar facet screws and bilateral pedicle screws offer greater stabilization in all three planes compared with unilateral pedicle screws and a single plate.