An experimental study of porcine lumbar segmental stiffness by the distraction-compression principle using a threaded interbody cage

The objectives of this study were to quantify changes in stiffness and disk height of porcine functional spinal units (FSUs) by installation of a threaded interbody cage and those by gradual resection of the annulus fibrosus. Flexion, extension, bending, and torsion to the FSUs were performed in four sequential stages: stage I, intact FSU; stage II, the FSUs are fitted with a threaded fusion cage; stage III, the FSUs are fitted with a threaded fusion cage with the anterior one third of the annulus fibrosus excised, including excision of the anterior longitudinal ligament; and stage IV, in addition to stage III, the bilateral annulus fibrosus is excised. Segmental stiffness in each loading in the four stages and a change of disk height induced by the instrumentation were measured. After instrumentation, stiffnesses in all loading modes (p < 0.005) and disk height (p = 0.002) increased significantly. The stiffnesses of FSUs fixed by the cage decreased with gradual excision of the annulus fibrosus in flexion, extension, and bending. These results suggest that distraction of the annulus fibrosus and posterior ligamentous structures by installation of the cage increases the soft-tissue tension, resulting in compression to the cage and a stiffer motion segment.