Defining the Neutral Zone of sheep intervertebral joints during dynamic motions: an in vitro study

Objective. To make an experimental assessment of the Neutral Zone of intervertebral joints during dynamic spinal motion in flexion/extension, lateral bending and axial rotation and to develop a criterion for its definition. Design. Dynamic mechanical testing of sheep intervertebral joints with a six-degree of freedom robotic facility under position control. Background. 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 quasi-static loading regimes that do not model physiological activity. This study simulated physiological movements using a robotic testing facility to address this issue. Methods. Five spines from mature sheep were used and three motion segments were tested from each spine. 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 suggested that a Neutral Zone does not exist in lateral bending or 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 new criterion for defining the size of the Neutral Zone during dynamic motion was proposed and its implications for spinal movements in life discussed. Conclusions. A Neutral Zone exists in flexion/extension during dynamic movements of intervertebral joints and is a feature of the natural range of joint motion. This has important implications for the muscular control of the spine consisting of several intrinsically lax joints stacked on one another.