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 thre aded interbody cage and those by gradual resection of the annulus fibrosus. Flexion, extension, bending, and torsion to the FSUs were performed in fou r sequential stages: stage I, intact FSU; stage II, the FSUs are fitted wit h a threaded fusion cage; stage III, the FSUs are fitted with a threaded fu sion cage with the anterior one third of the annulus fibrosus excised, incl uding excision of the anterior longitudinal ligament; and stage IV, in addi tion to stage III, the bilateral annulus fibrosus is excised. Segmental sti ffness in each loading in the four stages and a change of disk height induc ed by the instrumentation were measured. After instrumentation, stiffnesses in all loading modes (p < 0.005) and disk height (p = 0.002) increased sig nificantly. The stiffnesses of FSUs fixed by the cage decreased with gradua l excision of the annulus fibrosus in flexion, extension, and bending. Thes e results suggest that distraction of the annulus fibrosus and posterior li gamentous structures by installation of the cage increases the soft-tissue tension, resulting in compression to the cage and a stiffer motion segment.