The neutral zone in lumbar joint movements and how it is affected by preload

Abstract

Introduction It is important to understand the mechanics of the lumbar spine, as it has been shown that much low back pain is attributable to mechanical factors. One important aspect of spinal mechanics is the neutral zone, defined as a region of little or no resistance to motion on either side of the neutral position for a motion segment. If the neutral zone is a significant feature of intervertebral joint mechanics then the spinal joints will have little intrinsic stability and rely on muscles to control their movement around the neutral position. This has significant implications for our understanding of how degenerative changes to the spinal joints might destabilise the spine. This study was performed to characterise the size of the neutral zone and the effect of axial preload for different spinal motions. Methods Using a 6 degree-of-freedom (DOF) ABB industrial robot incorporating a 6-DOF JR3 force sensor, six isolated ovine lumbar joint segments were subjected to 5 repetitive movements in 3 directions (6° extension / 15° flexion, +/- 7° lateral bend, +/- 3° axial twist) with 4 different preloads (0, 150, 300, 450N) under 2 conditions (facet joints intact and facets removed). For each direction, the fixed axis about which the joint would rotate with a minimal motion-opposing moment was determined in advance. In accordance with a previous study by this group, the neutral zone was defined as the region where absolute rotational stiffness is less than 0.05 Nm/°. Results When moving from 6° of extension to -15° (flexion) a neutral zone was usually observed starting around 0° and continuing as far as -8 or -9°. The neutral zone was in the same region when moving in the opposite direction, except when the specimen showed a considerable amount of hysteresis, in which case the neutral zone could start as early as -11° or -12° and usually continued to -2°or -3°. Increasing preload usually made the joint stiffer in the regions outside the neutral zone, but did not affect the neutral zone itself. If present without preload, hysteresis usually increased with increasing preload. In lateral bend and axial twist no neutral zone was generally observed. In lateral bend the stiffness gradually increased with rotation, whereas in axial twist the stiffness was usually constant over the range of movement. For all movements, the only effect of facet removal was a constant reduction in stiffness over the whole movement. For lateral bend this meant that the stiffness around 0° usually would drop below the threshold of 0.05Nm/°, hence creating a neutral zone extending over a couple of degrees. Discussion Ovine spinal joints have a region where there is little to no resistance to flexion/extension. This region can be in excess of 10°. This means in their neutral position, the individual spinal joints have virtually no stability and the spine depends on other measures such as muscle activation to maintain stability in the sagittal plane. For lateral bend there is a region of little resistance as well, but it is not nearly as profound as in flexion/extension.