A dynamic approach to spinal instability. Part II: Hesitation and giving-way during interspinal motion.

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 determine whether the patterns of small jerks observed during intersegmental motion are sensitive to spinal instability. Small jerks have been observed as hesitation during increasing velocity and as giving way during decreasing velocity in the experiments described in Part I of this study. Human lumbar functional spinal units were moved from extension to flexion, flexion to extension, left to right, and right to left, by a pure moment. Range of motion and 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). Discontinuous accelerations and decelerations (jerks) were computed in these motions and their location in relation to the main angular motion determined. Jerks were observed in almost all motions, in the intact functional spinal units and after surgery. The parameters describing the magnitude of the jerk decreased with increasing component instability. In the sagittal plane, there was a surgical condition by motion direction interaction (P < 0.014) regarding the location of the jerk. Independent from the motion direction, the jerk occurred around the neutral position (in relation to the primary angular motion) in the intact functional spinal units, whereas it shifted from the neutral position toward the beginning of the motion with increasing component instability. The results suggest that a small jerk is a normal component of fast intersegmental motion. The jerk has a certain magnitude and location in an intact functional spinal unit, whereas both of the parameters describing the jerk are sensitive to component instability.