The Motion Segment 

The motion segment is the functional unit of the spinal column. The combination of both bony and soft tissue structures forms a motion segment, which is composed of two adjacent vertebral bodies, the facet joint created by their articular processes, the intervertebral disc between them and the associated soft tissue structures already mentioned. The intervertebral disc and the facet joints (one on each side of the midline) allow for motion in flexion, extension, side bending and rotation at the level of the motion segment.

Motion and Biomechanics of the Spine
The lower portion of the spine both bears the most weight and allows the greatest amount of motion. Lumbar spine motion is very complex because of the need for both stability and mobility. The motion segment allows motion within a restrained range and provides stability. Lateral bending occurs mostly in the upper lumbar motion segment. In the lower lumbar spine and lumbo-sacral region, flexion and extension are the primary motions.

The anterior column of the spine, consisting of the stacked vertebral bodies and discs, supports a majority of the body weight in the upright position. Biomechanical studies have shown that the posterior elements (lamina and facet joints) support about one-seventh of the axial load (the vertical load, perpendicular to the ground). When the intervertebral disc is loaded in the sitting or upright position, its two components, the annulus and the disc, behave differently: with lateral bending, the annulus bulges toward the direction of motion while the nucleus slides away from the pressure.

After age 30, the nucleus gradually begins to lose water content and the volume of the nucleus shrinks. The load borne by the annulus subsequently increases and the annulus is subject to weakening and tearing. Resistance to loading is diminished, and motion of the functional spinal unit changes. Disc degeneration is the term used to describe these changes, which as they progress, may result in decreased disc height, irregular bony end plates of the vertebrae and osteophyte (bone spur) formation. Disc degeneration and the accompanying changes in soft tissues and bone can result in increased instability (eg, spondylolisthesis) in some people, and decreased motion in others.

In the lumbar region, the facet joints are positioned to resist rotation and allow flexion and translation. The normal lordotic curvature allows the facet joints in the lumbar spine to bear weight. Disc degeneration and disc space narrowing force the lumbar facet joints to bear increased loads and likely lead to earlier degenerative changes.