An Experimental Study of Lumbar Destabilization

Study Design. An investigation of the effects of bone density on lumbar spine stability using destabilizing and restabilizing procedures. Objectives. To measure cadaveric vertebral bone densities using computed tomographic scans and to correlate the measured densities with lumbar spine stability in the intact state and during sequential destabilization and restabilization. Summary of Background Data. The stabilizing effects of lumbar pedicle screw fixation have been widely described. Numerous construct failure mechanisms have been observed, including screw loosening in osteoporosis. Although previous studies have analyzed the effect of bone density on the compression strength of bone similar to that used in interbody fusion and the relationship of pedicle screw pull-out strength to vertebral bone density, a combined study of bone density and construct stability using an interbody bone spacer with pedicle fixation has not been performed. Methods. Bone densities were measured in 20 human cadaveric lumbar spines using computed tomography scans and a hydroxyapatite phantom. After the specimens were mounted in a testing frame, the L4-L5 motion segments were subjected to cyclic axial compression-torsional loads, and axial and rotational intervertebral displacements were monitored. Laminectomy, facetectomy, and pedicle screw-plate fixation were performed sequentially in three specimens. Ten others underwent these procedures with an additional destabilization procedure, discectomy, after facetectomy. Seven others underwent the same sequence as the previous group, followed by the insertion of interbody bone. Cyclic testing was resumed after each procedure. Results. Average bone densities varied widely among the specimens. Average bone densities of the pedicle and of the vertebral body for individual specimens were well-correlated (r = 0.897). Displacements were recorded as a percentage of the intact state before destabilization; average percentages are reported as follows: axial displacements increased after facetectomy(145%) and subsequent discectomy (251%), and rotational displacements increased after facetectomy (295%) and discectomy (390%). Instrumentation without interbody bone resulted in specimens with decreased axial (126%) and rotational (156%) displacements. The addition of interbody bone further decreased axial (111%) and rotational (117%) displacements. The rotational stabilization provided by instrumentation was well-correlated with vertebral bone density (r = 0.804). This correlation was enhanced by the use of interbody bone (r = 0.939). Conclusion. The unstable lumbar spine can be partially stabilized using fixation. Interbody bone provides additional stability. The immediate stability provided by pedicle screws is greater in lumbar vertebrae with higher bone density.