FIGURE 7. Diagrammatic sketch of posterior long;tudinalligamentous deficiency. The
rear view drawing shows the narrowing of the ligament that begins at Lt. By the time it
reaches 4 it covers Jess than half the posterior disc margin, The double arrows in the
small sketch show where disc herniation may bulge into the spinal canal.
Of functional and potential pathologic significance is the fact that the
posterior longitudinal ligament is intact throughout the entire length of
the vertebral column until in its caudal approach it reaches the lumbar
region. At the first lumbar level (L,) it begins to narrow progressively so
that upon reaching the last lumbar (L.) first sacral (5,) interspace it has
half of its original width. This ultimate narrow posterior ligamentous
reinforcement contributes to an inherent structural weakness at the
level where there is the greatest static stress and the greatest spinal
movement producing the greatest kinetic strain (L.-S,) (Fig. 7).
Such is the anatomic construction of the anterior portion of the functional
unit for its weight-bearing and shock-absorbing function
A
FICURE 8. Direction of spinal movement is detennined by the planes of the articular
facets.A. The planes are vertical in the lumbar region. thus only anterior· posterior motion
Flexion-extension is possible in this region. Lateral bending and rotation are prevented
The plane of the thoracic facets permits rotation and lateral flexion but denies flexion and
extension. The direction of movement permitted and prevented is indicated for the
individual sections. Plus signs indicate possible motion; minus signs mean motionprevented. B. Details showing facets
FIGURE 9. Facet movement in flexion and hyperextension
anterior portion of the functional unit is one of three "joints" contained
in the total unit. The posterior portion of the functional unit containing
the other two joints of the functional unit. In contrast to the anterior
weight-bearing portion of the functional unit, the posterior portion has
a guiding function
