Effect of immobilization and configuration on lumbar adjacent-segment biomechanics.

The motion and facet load changes at the adjacent segment after lumbosacral immobilization (with different angles) were quantified in vitro using canine spines. Changes were examined under flexion, extension, and lateral bending. An increase (extension: 62%; flexion: 85%; left bend: 30%; right bend 26%) in motion at the adjacent segment was found for all motions after immobilization. Adjacent-segment facet load remained constant under any load state or immobilized configuration. For all configurations, the facet contact site impinged in extension, remained unchanged in left bending, and moved superiorly in right bending. This study has shown that after immobilization the facet contact patterns at the adjacent segment changed, load was unchanged, and segmental motion increased when the lumbar spine reproduced the same range of motion. The configuration of the immobilized segments had no effect on these changes.