Alex Simotas, MD

Hospital for Special Surgery
Board Certified Physiatrist
Specializing In Spine & Sports Medicine

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A Basic Anatomy Lesson

The spine is an important part of the body. It is responsible for providing:

  • Support
  • Range of motion
  • Protection of the spinal cord and nerve roots

The spine is made up of many bones called vertebra. The human spine has 33 vertebrae. 24 of these are individual bones.

These 24 vertebrae of the spine are divided into sections called the cervical, the thoracic and the lumbar spine.

  • The cervical spine consists of the top seven vertebrae that form the neck. They are labeled C1 through C7.
  • The thoracic spine consists of the twelve rib-bearing vertebrae. They are labeled T1 through T12.
  • The lumbar spine, or lower back, consists of five separate vertebrae. They are labeled L1 through L5.
  • The lower nine vertebrae are merged together into two bones called the sacrum and coccyx.

 

VERTEBRAE: SPINAL MOTION UNITS

The spine is made up of a front and back (posterior) section. The front section is called the vertebral body. The back, or posterior section is called the vertebral arch.
The spine can be thought of as being a stack of small motion units, consisting of two vertebrae connecting together. They connect through a triangular, or tripod-like joint.
The front part of the tripod that bridges the vertebral bodies is called the disc. The back part of the tripod has two small joints, one on each side. These are called the facet joints.
Vertebrae are fairly consistent in shape, with the exception of the first vertebra (the atlas)

and the second vertebra (the axis). These two vertebrae are shaped differently from the other vertebra because they provide a special combination of stability and mobility of the head on the neck.

A vertebral bone

spinal motion unit

 

THE FLEXIBILITY OF THE SPINE

The spine moves much like a flexible coil. Individual vertebral segments are only able to move 5 to 10 degrees. However, many small movements of individual vertebral segments result in the ability of the spine to be flexible. For example, the ability to bend at the waist is created by the small movements of multiple vertebral segments.

 

THE SPINAL DISC

Between each vertebral body are soft-tissue structures called intervertebral discs. The spine has 23 such discs. Their combined size accounts for about one quarter of the overall length of the spine.

Each disc is complex structure with 2 basic parts: the anulus and the nucleus.

The Anulus Fibrosis

The top and bottom of each vertebral body are called endplates. The endplates are joined together by circular ligament fibers called the anulus. The anulus forms the outer layer of the disc, and it consists of 25-40 bands of very strong fibers anchored in the endplates.

The Nucleus Pulposus
The inner part of the disc is called the nucleus pulposus. It consists of a complex matrix of molecules that attract and trap water. As a result, water makes up 80% of the nucleus pulposus, which fluctuates slightly on a daily basis.

For example, when we are lying down, chemical forces attract water into the discs. When we are upright, compression of the disks forces some water out. This results in a slight fluctuation of our height during the day. We are actually a little taller when we first get up from sleep!

With normal aging, the water content of the disc generally decreases. By our later years, it is often reduced to about 70%. This contributes to the loss of height noted in many middle-aged and older people.

Disc Nutrition
Compression of the spine is needed to sustain nutrition to the spinal discs. Compression forces created by weight bearing and loading of the spine allow essential nutrients and oxygen to diffuse through the endplates.

The Vertebral Arch
The back or posterior part of the spine is called the vertebral arch. To help describe the vertebral arch, visualize the inside of a one-room house. The room itself is the spinal canal. The walls of the room are like two posts called pedicles. The roof is called the lamina and it has a bone tip on top called the spinous process. The spinous process usually protrudes from the surface of the back and can be felt as hard bony lumps through the skin.

On the side of each vertebral arch are two transverse processes. The transverse and spinous processes are connection points for many ligaments and muscles that control stability and motion of the spine. Each arch has a connecting joint with the adjacent arch called the facet joint.

 

FACET JOINTS

Facet Joints are two pads of the arch, one on the right side and one on the left side. These pads meet at the top and bottom across each vertebra to form the facet joints. The pads are called the superior (above) and inferior (below) articular (joint) processes.
Facet joints are similar to hand knuckles. Like the hand knuckle, they have cartilage surfaces and joint capsules that produce synovial fluid; a joint lubricant. These joints provide mechanical stops to limit some spinal motions and to keep the vertebrae in their correct positions.

 

VERTEBRAL CANAL

The vertebral canal is the large opening in the center of each vertebra. The canal extends from the base of the skull to the tailbone. It contains the spinal cord and spinal nerve roots and their supporting blood.

The vertebral canal protects the nervous system while still allowing motion to occur in small degrees between each vertebra.

 

MUSCLES AND LIGAMENTS

Muscles and ligaments are responsible for 95% of the stability of the spine. Muscle groups of the front of the spine include the abdominal musculature and the iliopsoas. Muscles in the back of the spine that support extension of the back include numerous erector spinae muscles, latissimus and gluteal muscles.

Many layers of ligaments envelop the front, back and sides of the spine to help reinforce the stability of the spine. These ligaments bolster the spinal motion segments.