A Dynamic Approach to Spinal Instability

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Abstract
Human lumbar functional spinal units (FSUs) were moved throughout their range of motion in sagittal and lateral bending while the dynamics of this movement were computed in vitro. Functional spinal units were tested intact and after subsequent discectomy and unilateral facetectomy. To establish "normal" velocity and acceleration curves during lumbar intersegmental bending in the intact FSU and then evaluate the changes of this dynamic behavior due to surgically induced component instability. In preliminary clinical studies, researchers have provided evidence that dynamic motion measurements may be useful in the assessment of spinal impairment. Human lumbar FSUs moved from extension to flexion, flexion to extension, left to right, and right to left by a pure moment. Range of motion, as well as velocity and acceleration patterns of the main and coupled motions, were evaluated in six degrees of freedom by position changes of attached infrared light-emitting diodes recorded by cameras. Functional spinal units were tested in three surgical conditions (intact, discectomy, and unilateral facetectomy) under two preload conditions (no preload and 400 N preload). Motion of intact FSUs progressed with velocity and acceleration patterns that were relatively independent from motion direction and preload condition. After surgery, however, the dynamic motion became unequal between opposite motion directions (even if range of motion was equal between directions) and more sensitive to preload condition. The results suggest that equilibrium of dynamic motion parameters within a range of motion is an element of segmental stability. From this approach, segmental instability appears to change intersegmental acceleration and velocity patterns as a function of motion direction and load conditions. Whereas dynamic motion patterns in an intact FSU are relatively invariable between reversed motion directions, instability is characterized by a considerable diversity of dynamic motion parameters between reversed motion directions.