COMPARISON OF IN-VIVO AND IN-VITRO ADJACENT SEGMENT MOTION AFTER LUMBAR FUSION

Study Design. For in vitro studies, there is no basis for choosing a ''load control study'' over a ''displacement control'' study. This study qualitatively compared results from in vitro and in vivo tests, allowing the authors to address the experimental assumptions that in vitro testing contributes to the understanding of the in vivo condition. Objectives. To compare motion changes at segments adjacent to fusions for in vitro and in vivo tests. Summary of Background Data. Investigators have measured the effects of spinal fusions on the adjacent segment in a human cadaver model and found greater adjacent facet joint load after fusions. Others have found significant increases in motion and facet loads at segments adjacent to in vitro lumbosacral and long fusions, when the same range of motion was repeated before and after immobilization of lumbar segments. Methods. L2-L3 motion was measured in vitro by an instrumented spatial linkage under load and displacement control before and after immobilization of segments (L3-L7). In vivo, L2-L3 motion was measured while animals walked on a treadmill. L3-L7 was fused and the L2-L3 motion testing was repeated. The change in in vivo adjacent segment motion was qualitatively compared with the in vitro change under ''load'' and ''displacement'' control. Results. Under ''load'' control, in vitro facet motion did not significantly change after immobilization, whereas under ''displacement'' control, the facet motion significantly increased from 2.2 +/- 0.4 mm to 4.1 +/-0.6 mm. Post-instrumentation, in vivo L2-L3 facet motion increased significantly. This change in vivo related better to the changes seen in the in vitro ''displacement'' control test than to the in vitro ''load'' control test.