Simple Disc/spine Biomechanics

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Simple Disc/spine Biomechanics

I. Flexion


A. Disc is a closed hydraulic system used for movement and axial loading
B. As the spine bends the superincumbent wt shifts anterior shifting the line of gravity anteriorly
C. As the wt shifts anteriorly it creates a larger moment arm

i. farther the reach larger moment arm

ii. Greater force on Spinal segments


D. The Anterior annulus is loaded by the vertebral segments above and below


E. The nucleus pulposus begins a posterior displacement (80% increase in intradiscal pressure)


F. Potential for annulus mechanical breakdown


i. Mechanical or chemical nocioception occurs
ii. Pain may be local, referred or radicular in nature


G. Disruption of annulus causes nuclear material to become further displaced


II. Extension


A. Disc is a closed hydraulic system used for movement and axial loading


B. As the spine extends the superincumbent wt shifts posterioly


C. As the wt shifts posteriorly the annulus is compressed by the vertebral bodies above and below


i. May experience mechanical pain secondary to compression of posterior nucleus innervations


D. Compression of the posterior annulus by the superincumbent begins anterior displacement of the nucleus


Spine kinematics


I. Fryetts 1st law of spinal kinematics


a. IN a neutral spine position, side bend and rotation occur in the OPPOSITE direction
b. When in neutral spine position and LEFT sidebending occurs in the lumbar spine, the vertebral bodies rotate RIGHT


II. Fryetts 2ND Law of kinematics


a. Outside of a neutral spine position, sidebend and rotation occur in the SAME direction
b. When the spine starts in a flexed or extended position, side bending LEFT results in vertebral body rotation LEFT