The Functional Spine Unit

Hi. I'm Dr. John Shim, and through my online video's, I am sharing how I view situations as a spine surgeon. Before we can go to any specific detail, I think it's important that you understand some basic spine anatomy. As physicians, and scientists, we study the spine for years. To understand some of the concerns, it is very helpful to understand some basic spine anatomy. The spine is a protective scaffolding, that allows the nerve tissues to safely extend from the brain to the limbs and a torso of the body. These nerve tissues are the spinal cord, and the individual nerves that extend from the spinal cord. The spinal cord, and nerves float inside a tough water filled sac called the dural tube. The dural tube is located within the center of this protective spinal bones. Because the spinal cord and nerves float within this water tube, it has protection from some of the twisting and jarring motions experienced by your body. Because of the need for motion, the spine must not only be protected, but must allow movement. The hard nerve encasing bones are connected to ligaments, and tendons. Attached muscles allow coordination of movement. The hard front of the spine bone, called a vertebral body, is sandwiched between large shock absorbers called intervertebral discs. These discs allow motion, and also provide shock absorption. The discs are composed of two main components. The inner soft core is called the nucleus pulposus. It is full of water, and absorbs the forces placed on the large flat vertebral body bones. The outer part of the disc is called the annulus. It is a tough woven cartilage structure that holds the nucleus in place, as well as acts as a ligament that holds that attaches the vertebral bodies together. The intervertebral discs also provides a transition between the boney vertebra, and allows a space and a boney opening for individual nerves to escape from the spine, and travel to it's respective body areas. This hole, or foramen, is an opening between the bone and the disc interface. This spine sandwich the vertebral body, disc, and vertebral body is called the functional spine unit. There is motion initiated by contraction of the attached muscles and tendons. Because of the elastic nature of the vertebral disc, there is also flexion. extension and rotation of the disc. The rear or posteriorly located joints that connect the vertebral together are called the facet joints. Those joints are lined with smooth cartilage, and is designed to move smoothly within a certain arch of movement. As you can see, even the most simplified overview of the functional spine unit anatomy is complex. To further understand how the spine works, the body also needs a way to register and measure how much each ligament, muscle or tendon stretches. These measurements provide feedback, and allow smooth coordination. This means there are small nerves that measure compression, tension, and stretch. These nerves are placed on the various parts of the functional spine unit, and can also signal injury, or register pain. Because of the complexity of the spine, and the natural wear that happens to all our body parts, maybe you can see how the discs can wear out, that ligaments can stretch, and the nerves can get pinched. I hope this video is helpful for your understanding of the spine. Thank You!

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.


  • Oxland TR. Fundamental biomechanics of the spine--What we have learned in the past 25 years and future directions. J Biomech. 2016 Apr 11;49(6):817-832. PubMed PMID: 26706717

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