The Functional Spine Unit

Posted on June 6, 2016 | by Dr. Shim

The spine consists of the bony vertebra, the intervertebral disc, and the associated ligaments that hold two or more vertebra together, while still allowing motion. Motion is coordinated between smooth articulations between the vertebra called the facet joints.

This combination of two adjacent vertebra, intervertebral disc, facet joints, and the associated ligaments comes together to form the FUNCTIONAL SPINE UNIT.

The vertebra has three main parts. In the front is the vertebral body. This is the large block of bone that provides structural support, and allow the vertebra to stack on top of each other. The vertebral bodies are attached together with the intervertebral disc, and the anterior and posterior longitudinal ligaments. The large surface area of the vertebral body allow dissipation of the forces experienced by the spine, especially with load activities such as locomotion, and flexion and extension of the spine. In addition to the shock absorption function of the intervertebral disc, there are venous channels in the vertebral filling the vertebral with blood and plasma. The blood and plasma allows compression of the vertebra with greater resistance to fracture. Fluids cannot be compressed, thus giving the vertebral bones some compression protection.

The middle portion of the vertebra includes the ring defined by the pedicles, lamina, and the rear of the vertebral body. The ring allows the nerve structures pass from the brain into the center of the spine, while protecting the nerve structures with the bones of the vertebra. There is also the exit holes for the nerves that come out of the spine into the individual muscles and organs. These exit holes are call the neuro-foramen or intervertebral foramen. These exit holes are defined by the pedicles of the adjacent vertebra, the facet joints ( mostly superior, and medial) and the intervertebral disc. There are two foramen, one on each side of the FSU. The middle portion also includes the transverse process, which are wing like sideward extensions on both sides of the vertebra. The wings provide surface area for attachment of multi-angled stabilizing ligaments between multiple different vertebral bodies.

The back part of the vertebra consists of the lamina, or the boney covering of the spinal sac, the posterior spinous process, and the facet joints, along with associated ligaments and tendons. In the thoracic spine, secondary to the organ protective function of the ribs, there is less motion of the facet joints. The thoracic facet joints have more of a shock absorption function.

The facet joints are oriented to allow flexion/extension and rotation in the cervical and lumbar spine. Each individual facet joint has limited motion, but coupling multiple levels allow much greater flexibility. A competent disc is also necessary to allow the measured, coordinated movement of each FSU.

Interpinous ligaments, and tendons of the posterior muscles, particularly the multifidus muscle provide stability to the spine units.

The normal FSU allows protection of the nerve elements, while providing a structure that allows coordinated motion, and scaffolding for the rest of the muscles, and ligaments of the body.

Unfortunately, the FSU does deteriorate over time, and often times, will present with pain, weakness or stiffness secondary to the lose of FSU integrity. As a Spine Surgeon, the decision often rests on the risks versus benefit of repairing or stabilizing part of the FSU.