Intervertebral disc is a structure that is present between two bodies of adjoining vertebrae. Each component of the intervertebral disc has a specific histologic composition and structure that is responsible for a specific function. The vertebral end plates that are in direct contact wiith disc, consist of cortical bone in the periphery, which in adolescence is referred to as the ring apophysis, and compressed cancellous bone in the central disk area, which covers nearly 70% of the disk.
The outer 30% consists of dense cortical margin and is the strongest area of the end plate. The annulus consists of multiple layers or lamellae of collagen fibers arranged in a unique circumferential orientation along the disk periphery.
Each lamella is oriented at a 30 degree angle to the horizontal axis of the disk, and this pattern alternates in successive lamellae. The outer collagen fibers attach to the ring apophysis, and the inner layers attach to the end plate surround the nucleus.
The annular fiber orientation within the lamellae allows for resistance to tension. The obliquity of the lamellae results in tension or relaxation of the fibers in different areas within the disk (anisotropy) and with different forces.
In anterior or posterior translation of a vertebral body and disk along the horizontal axis, all of the fibers of the anulus are stretched in the direction of the applied force.
However, fibers aligned in the direction of the force (every other lamella) will undergo stain while the remaining fibers actually will be brought closer to one another and will relax.
This alternating tension and relaxation with translation and axial rotation maintains the stability of the intervertebral segment.
The nucleus is an incompressible, semifluid composite in the center of the disc space. Under forces of isolated axial compression, the nucleus expands radially toward the inner aspect of the annulus fibers; the annulus fibers also expand circumferentially.
Anterior flexion of sagittal plane rotation of the lumbar spine involves a combination of physical events within the disk that exemplify the complex structure and functional aspects of this components.
As the vertebral body rotates anteriorly, the anterior anulus is compressed. The nucleus also is compressed eccentrically over its anterior aspect, after which it deforms and migrates posteriorly. The posterior annular fibers expand radially the shifting nucleus, and the fibers stretched in line with these forces resist further motion.
As rotation occurs, the weight of the upper body and trunk lead to shear strain forces at the disc and slight translation, which is resisted by the active and passive constraints of the posterior column, may occur.
This interaction of the anterior and posterior lumbar spinal columns is critical for normal physiologic function, load transmission, and kinematics. Lumbar range of motion varies between vertebral levels and individuals.
The instantaneous axis of rotation continually changes throughout the range of motion and typically is located posterior toe the midvertebral body and below the superior end plate of the inferior vertebral body except at L5S1, where it is located posterior to the disc space.
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