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. RELEVANCE The existence of a Neutral Zone is a feature of the natural range of joint motion and requires complex control of intervertebral joints by the spinal muscles. Defining the biomechanical response throughout the physiological range of motion (RoM) is important in understanding possible injury and rehabilitation mechanisms.

[1]  M J Pearcy,et al.  Instantaneous Axes of Rotation of the Lumbar Intervertebral Joints , 1988, Spine.

[2]  M. Panjabi.  The stabilizing system of the spine. Part I. Function, dysfunction, adaptation, and enhancement. , 1992, Journal of spinal disorders.

[3]  Janet M. S. Skakle,et al.  Sheep, deer and cow intervertebral discs as models for investigating disc replacement , 2002 .

[4]  S. Perren,et al.  Biomechanics in Orthopedics , 1993, Springer Japan.

[5]  H Fujie,et al.  Use of robotics technology to measure friction in animal joints. , 1996, Clinical biomechanics.

[6]  T. Smit.  The use of a quadruped as an in vivo model for the study of the spine – biomechanical considerations , 2002, European Spine Journal.

[7]  A. Silman,et al.  Estimating the Prevalence of Low Back Pain in the General Population: Evidence From the South Manchester Back Pain Survey , 1995, Spine.

[8]  F. Biering-Sørensen,et al.  Low back trouble in a general population of 30-, 40-, 50-, and 60-year-old men and women. Study design, representativeness and basic results. , 1982, Danish medical bulletin.

[9]  Rosemary E. Thompson.  Mechanical effects of degeneration in lumbar intervertebral discs , 2002 .

[10]  L. Claes,et al.  Anatomy of the sheep spine and its comparison to the human spine , 1997, The Anatomical record.

[11]  Glen A. Livesay,et al.  Improvement of Accuracy in a High-Capacity, Six Degree-of-freedom Load Cell: Application to Robotic Testing of Musculoskeletal Joints , 2004, Annals of Biomedical Engineering.

[12]  S L Woo,et al.  Biomechanics of knee ligament healing, repair and reconstruction. , 1997, Journal of biomechanics.

[13]  M M Panjabi,et al.  Role of tectorial membrane in the stability of the upper cervical spine. , 1992, Clinical biomechanics.

[14]  V. Devlin,et al.  Instantaneous Axis of Rotation as a Function of the Three Columns of the Spine , 1992, Spine.

[15]  Kiyoshi Mabuchi,et al.  A New Methodology with an Application of Robotics to Control the Mechanical Environment Around Experimentally Fractured Bone , 1992 .

[16]  L. Claes,et al.  Are Sheep Spines a Valid Biomechanical Model for Human Spines? , 1997, Spine.

[17]  Mack Gardner-Morse,et al.  Measurement of a spinal motion segment stiffness matrix. , 2002, Journal of biomechanics.

[18]  M. Panjabi.  The stabilizing system of the spine. Part II. Neutral zone and instability hypothesis. , 1992, Journal of spinal disorders.

[19]  Manohar M. Panjabi,et al.  Clinical Biomechanics of the Spine , 1978 .

[20]  M. Pearcy.  Stereo radiography of lumbar spine motion. , 1985, Acta orthopaedica Scandinavica. Supplementum.

[21]  S Arai,et al.  The use of robotics technology to study human joint kinematics: a new methodology. , 1993, Journal of biomechanical engineering.

[22]  S. Rolander.  Motion of the lumbar spine with special reference to the stabilizing effect of posterior fusion. An experimental study on autopsy specimens. , 1966, Acta orthopaedica Scandinavica.

[23]  Lars G. Gilbertson,et al.  New methods to study lumbar spine biomechanics: Delineation of in vitro load-displacement characteristics by using a robotic/UFS testing system with hybrid control , 2000 .

[24]  H. Farfan,et al.  The instantaneous center of rotation of the third lumbar intervertebral joint. , 1971, Journal of biomechanics.

[25]  M M Panjabi,et al.  Human Lumbar Vertebrae: Quantitative Three-Dimensional Anatomy , 1992, Spine.