An in vitro study of the biomechanical effects of flexible stabilization on the lumbar spine

Study Design. Lumbar motion segments were tested, in vitro to examine biomechanical changed after posterior fixation by a flexible device.

Objectives. To assess changes in load distribution and conformation of vertebral structures after a flexible stabilization. This should provide the foundations for a scientific understanding of the immediate effects of this surgical procedure.

Methods. Hooks were placed over the proximal spinous process and the distal laminas of a motion segment and connected by a polyester braid. Tension applied to the braid then generated a compression of the posterior elements. The force between the articular facets, the displacement of the posterior anulus fibrosus of the intervertebral disc, and the change in the relative position of the adjacent vertebrae were measured as the applied tension was increased.

Results. Facet joint force, disc bulge, and vertebral angulation increased with applied tension until a position of ''locking'' was achieved, apparently when the bony margin of the superior half of the facet joint contacted the inferior pars interarticularis. A tension of between 50 to 100 N in the braid was required for this. Facet joint force was less than 40% of this, and disc bulge was only 0.15 mm. The extension of the motion segment was between 2 degrees and 8 degrees.

Conclusions. The results suggest that if such a system is applied surgically, stabilization is produced by compaction of the bony margins of the facet joints. Only a relatively small proportion of the posteriorly applied toad is carried by the facet joints themselves, and little angulatory change is expected with with minimal disc bulge.