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.

Introduction A very specific group within the 80 percent of the population that suffers from low back pain at some stage in life are young cricket fast bowlers. Amongst them a high occurrence of unilateral L4 pars interarticularis fractures exists, which shows a strong statistical correlation to the presence of a contralateral volumetric increase in the Quadratus Lumborum (QL) muscle. However, there is no clear physical link between these two phenomena. To investigate this relationship, we have combined a mathematical model of the lumbar spine muscles with a finite element model of the fourth lumbar vertebra and analysed the stresses occurring in the L4 vertebra throughout the bowling motion.

Methods A mathematical model of the lumbar spine muscles has been developed previously at QUT. It contains 170 fascicles representing all major muscles in the lumbar region and allows for analysis of the forces and moments on the intervertebral joints caused by these muscles in any given posture. A Finite Element Model (FEM) of an L4 vertebra and intervertebral disc (IVD) was developed based on one created by Theo Smit and obtainable from the Internet through the BEL Repository of the Istituti Ortopedici Rizzoli, Bologna, Italy. Material properties were obtained from literature, while muscle forces, directions and attachment locations in the different postures came from the mathematical model. Six postures occurring in right-handed fast bowling were modelled to determine the differences in stresses between having symmetric and asymmetric QL muscles. The asymmetric condition consisted of a 30% increase in Physiological Cross-Sectional Area (PCSA) on the right side. In all cases it was assumed the left facet joints were ‘locked up’, to create a presumed worst-case scenario for the stress build-up in the pars.

Results It was found that when using muscle activation levels from literature an enlarged right-side QL did not increase the stresses in the left pars noticeably, in fact in some cases it even slightly reduced those stresses. When only the right-side QL muscle was activated, while all other muscles only provided passive muscle force, a 30% PCSA increase of this muscle produced an increase in maximum Von Mises and principal stresses in the left-side pars from typically 30 MPa to 40 MPa but only in the postures close to upright stance. In more extreme postures where the maximum stresses in the pars are higher, the increased PCSA of the right QL only led to small stress increases from typically 125 to 129 MPa.

Discussion Even in the worst-case scenario where only the right-side QL is active and the left-side facet joint is locked up, a PCSA increase of that muscle does not cause a large increase in stresses in postures where the stresses are high. Hence, this study has not demonstrated a clear physical link between asymmetric hypertrophy of QL and pars fractures. It may even suggest the hypertrophy is a response to postural overload attempting to reduce stresses in the pars. To clarify this, an improved FEM of the L3 and L4 vertebrae and IVDs, including all ligaments, is currently being developed. We believe that in the future this combination of models can be used for many more purposes where the influence of posture and musculature on the lumbar spine biomechanics needs investigation.