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Introduction: the neutral zone is defined as a region of no or little resistance to motion in the middle of an intervertebral joint’s range of movement. Previous studies have used quasistatic loading regimes that do not model physiological activity1. The aim of the present study was to assess experimentally the existence of the neutral zone of intervertebral joints during spinal motion in flexion/extension, lateral bending and axial rotation during physiological movements simulated using a robotic testing facility. Sheep intervertebral joints were used as they have been shown to exhibit similar mechanical behaviour to human joints2.

Methods: five spines from mature sheep were used. Three specimens were tested from each spine to simulate human l1/2, l3/4 and l4/5 intervertebral joints. The robotic facility enabled the testing regime to be defined for each individual joint based on its geometry. The joints were tested by cycling through the full range of physiological movement in flexion/extension, lateral bending and axial rotation.

Results: a neutral zone was found to exist during dynamic movements only in flexion/extension. The results were equivocal for lateral bending and suggested that a neutral zone does not exist in axial rotation. The zygapophysial joints were shown to be significant in determining the mechanics of the intervertebral joints as their removal increased the neutral zone in all cases. A criterion for defining the size of the neutral zone was proposed.

Conclusions: a neutral zone exists in flexion/extension during dynamic movements of intervertebral joints. This has important implications for the muscular control of the spine consisting of several intrinsically lax joints stacked on one another.

The abstracts were prepared by Dr Robert J. Moore. Correspondence should be addressed to him at The Spine Society of Australia, Institute of Medical and Veterinary Science, The Adelaide Centre for Spinal Research, Frome Road, Adelaide, South Australia 5000


1 Panjabi M. The stabilizing system of the spine. Part ii. Neutral zone and instability hypothesis. Journal of spinal disorders5(4): 390–397, 1992. Google Scholar

2 Wilke H, Kettler A, Claes l. Are sheep spines a valid biomechanical model for human spines?, Spine1997;22(20):2365–2374. Google Scholar