Intraoperative measurement of lumbar spinal instability.

To justify lumbar fusion or stabilization, a quantitative assessment and definition of spinal instability are essential. To quantify spinal instability, the tensile stiffness of a motion segment (vertebra-disc-vertebra) was measured with a spinal distractor during spinal decompression surgery. Stiffness was indicated by the relationship between load and displacement between the two adjacent spinous processes where a vertebral spreader was suspended. A load-displacement curve was recorded at each step of surgical decompression and fixation while the motion segment was being distracted at a constant speed. The device used for measuring stiffness of a spinal motion segment is a lumbar spinal spreader with a load strain gauge and a displacement transducer. The stiffness of a spinal motion segment was reduced as disc degeneration developed. Degenerative spondylolisthetic discs showed the least stiffness (lowest, 3.9 N/mm; average, 5.4 N/mm). The stiffness of herniated discs, however, was relatively greater (average, 11.8 N/mm). The stiffness of normal motion segments was greater than affected segments. If the stiffness of a motion segment before decompression was graded as 100, it was reduced to 82% after partial laminectomy and facetectomy and to 65% after discectomy on average. After interbody fusion by iliac bone graft, it increased to 133% and to 184%, after Luque fixation.