Postural influence on the neutral zone of the porcine cervical spine under anterior–posterior shear load

Segmental instability, characterized by excessive or aberrant movement of the vertebrae can be assessed quantitatively using mechanical characteristics within a region of minimal resistance called the neutral zone. The diagnosis of instability is often used to decide whether or not to surgically fuse the vertebrae. Alterations in flexion/extension posture cause changes in both contact area and spacing between articulating facets that may lead to changes in the mechanical response of the functional spinal unit (FSU) within the neutral zone. This investigation quantified neutral zone (NZ) length under anterior and posterior shear loading and the influence of posture on the shear NZ characteristics of the vertebral joint. Thirty porcine cervical FSUs (15 C34 and 15 C56) were tested. Endplate area was calculated from measurements of the exposed endplates while facet angles were measured from X-rays taken in the transverse plane. Specimens were exposed to a 300 N compressive preload followed by a test to determine flexion/extension NZ limits. These limits were used as target angles during shear passive tests performed in extended and flexed postures. Displacement rate during shear passive tests was 0.2 mm/s and five cycles of anterior–posterior shear were performed to a target of ±400 N in a randomized order of extended, neutral and flexed postures. Shear NZ length and average stiffness were quantified. Stiffness within the shear NZ was 67 N/mm in the neutral posture. Extended postures produced a 37% (p < 0.0001) increase in shear stiffness within the NZ compared to both flexed and neutral postures. Posture did not influence shear NZ length. Therefore, a true region of zero stiffness does not exist during shear loading with a baseline compressive load. Neutral zone length for the porcine FSU exposed to shear load was not influenced, despite known changes in facet articulation, by changing posture. Average stiffness increased likely as a result of increased contact area and force in extension. The results from this investigation demonstrate that postural deviation of the vertebral joint is not likely a significant confounding factor when assessing segmental stability.