Motions of the Spinal Column

Motion of the spinal column is described both globally and at the functional unit or motion segment. The functional unit is comprised of two vertebrae and the joints in between (typically, two zygapophyseal facet joints and one intervertebral disk). Generally, the axis of motion for each unit is in the nucleus pulposus of the intervertebral disk. Because the spine can move from top down or bottom up, motion at a functional unit is defined by what is occurring with the anterior portion of the body of the superior vertebra.

Sagittal plane motion. Motion in the sagittal plane results in flexion (forward bending) or extension (backward bending). With flexion, the anterior portion of the bodies approximate and the spinous processes separate; with extension, the anterior portion of the bodies separate and the spinous processes approximate.

Frontal plane motion. Motion in the frontal plane results in lateral flexion (side bending) to the left or right. With side bending the lateral edges of the vertebral bodies approximate on the side toward which the spine is bending and separate the opposite side.

Transverse plane motion. Motion in the transverse plane results in rotation. Rotation to the right results in relative movement of the body of the superior vertebrae to the right and its spinous process to the left; the opposite occurs with rotation to the left. If movement occurs from the pelvis upward, the motion is still defined by the relative motion of the top vertebra.

Anterior/posterior shear. Shear occurs when the body of the superior vertebra translates forward or backward on the vertebra below.

Lateral shear. Lateral shear occurs when the body of the superior vertebra translates sideways on the vertebra below.
Distraction/compression. Separation or approximation occurs with a longitudinal force, either away from or toward the vertebral bodies.

Motions of the spinal column. Flexion/extension (forward/backward bending). Lateral flexion (side bending). Rotation. Anterior/posterior shear. Lateral shear. Distraction/compression.

Structure and Function of Intervertebral Disks

The intervertebral disk, consisting of the annulus fibrosus and nucleus pulposus, is one component of a three-joint complex between two adjacent vertebrae. The structure of the disk dictates its function.

Annulus fibrosus. The outer portion of the disk is made up of dense layers of collagen fibers and fibrocartilage. The collagen fibers in any one layer are parallel and angled around 60° to 65° to the axis of the spine, with the tilt alternating in successive layers. Because of the orientation of the fibers, tensile strength is provided to the disk by the annulus when the spine is distracted, rotated, or bent. This structure helps restrain the various spinal motions as a complex ligament. The annulus is firmly attached to adjacent vertebrae, and the layers are firmly bound to one another. Fibers of the innermost layers blend with the matrix of the nucleus pulposus. The annulus fibrosus is supported by the anterior and posterior longitudinal ligaments.

Nucleus pulposus. The central portion of the disk is a gelatinous mass that normally is contained within, but whose loosely aligned fibers merge with the inner layer of, the annulus fibrosus. It is located centrally in the disk except in the lumbar spine, where it is situated closer to the posterior border than the anterior border of the annulus. Aggregating proteoglycans, normally in high concentration in a healthy nucleus, have great affinity for water. The resulting fluid mechanics of the confined nucleus functions to distribute pressure evenly throughout the disk and from one vertebral body to the next under loaded conditions. Because of the affinity for water, the nucleus imbibes water when pressure is reduced on the disk and water is squeezed out under compressive loads. These fluid dynamics provide transport for nutrients and help maintain tissue health in the disk.
With flexion (forward bending) of a vertebral segment, the anterior portion of the disk is compressed, and the posterior is distracted. The nucleus pulposus generally does not move in a healthy disk but may have slight distortion with flexion, potentially to redistribute the load through the disk. Asymmetrical loading in flexion results in distortions of the nucleus toward the contralateral posterolateral corner, where the fibers of the annulus are more stretched.

Cartilaginous end-plates. End-plates cover the nucleus pulposus superiorly and inferiorly and lie between the nucleus and vertebral bodies. Each is encircled by the apophyseal ring of the respective vertebral body. The collagen fibers of the inner annulus fibrosus insert into the end-plate and angle centrally, thus encapsulating the nucleus pulposus. Nutrition diffuses from the marrow of the vertebral bodies to the disk via the end-plates.

Intervertebral disk. The annular rings enclose the nucleus pulposus, providing a mechanism for dissipating compressive forces. Orientation of the layers of the annulus provide tensile strength to the disk with motions in various directions.

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