A Comparison of Compressive and Rotational Stiffness in Facet Joint Screws Vs. Pedicle Screws in the Destabilized Lumbar Spine

Facet screws were compared to a pedicle screw-rod construct under axial compression and torsional loading in a destabilized lumbar spine. The mechanical effects of instrumentation located adjacent to the axis of rotation were evaluated. 10 fresh, human, cadaveric spinal functional units from L1 to S1 were tested to cyclic axial loads of 300N and to torsional loads of 5Nm. Comparisons were made between the intact spine, the destabilized spine, facet screw instrumentation and pedicle screw and rod instrumentation for axial compression and torsional loading. There was an increase in stiffness in axial compression with the facet screws greater than the pedicle screws. In torsional loading, the pedicle screws were significantly more stiff than the facet screws. The pedicle screws apply forces at a greater distance from the axis of rotation of the spinal unit and therefore control spinal rotation. Facet screws were technically less difficult to implement, require no x-ray, decrease the risk of neurological damage yet do not control spinal rotation.