Normal spinal anatomy

Before discussing the confusing aspects of terminology used to describe back problems, it is important to first review the overall terminology used to describe the normal anatomy of the spine.

Vertebrae define sections of the spine
The spine is divided into 4 major sections, typically defined by the number of vertebrae (the round bones that make up the structure of one’s back bone) in each section. Vertebrae are also sometimes called vertebral bodies. (See Figure 1)

1. Cervical spine (neck)—comprised of 7 cervical vertebrae (termed C1 to C7), starting with C1 at the top of the spine and ending with C7 at the bottom of the cervical portion of the spine. Neck problems can cause neck pain and/or pain that radiates down the arms to the hands and fingers.

2. Thoracic spine (upper back)—made up of 12 thoracic vertebrae (known as T1 to T12), which are attached to the rib bones and sternum (breast bone). Because this part of the spine is firmly attached to the ribs and sternum, it is very stable and has fewer problems associated with motion.

3. Lumbar spine (lower back)—typically including 5 vertebrae (known as L1 to L5), which have a great deal of motion and flexibility. Because this section of the spine bears most of the body’s weight and allows for the most motion (which stresses the anatomical structures), this is the area associated with most back problems. Problems in the low back can cause pain that radiates down the legs to the feet.

4. Sacral region (bottom of the spine)—located below the lumbar spine, the sacrum is a series of 5 bony segments fused together (known as S1 to S5) that create a triangular-shaped bone that serves as the base of the spine and makes up part of the pelvis.The segment where the lumbar spine meets the sacral region, L5-S1, is an area that is prone to degenerate and create back problems. Four small bones that extend down from the sacrum make up the coccyx (the tailbone at the very bottom of the spine).

Disorders are common in the lumbar spine and at the top of the sacral region, as this area supports most of the body’s weight which creates stress on the structures in this area. The combination of these two sections of the lower back is often referred to as the “lumbosacral region”.
People with back problems that get better within a few weeks usually have a strained muscle (a pulled muscle) or other soft tissue damage. However, many back problems that don’t get better within a few months are caused by some type of problem with a spinal disc or nerve.

Spinal discs are in between each vertebra
Spinal discs are located in between each of the vertebral bodies along the back bone and may also be referred to as an intervertebral disc, spinal disc, or disk. Each disc is named according to which two vertebral bodies it lies between. For instance:

• The L4-L5 disc in the low back is between the L4 and L5 vertebrae which make up the L4-L5 spinal segment.

• The L5-S1 disc at the bottom of the spine lies between the L5 vertebra and the first bony segment at the top of the sacrum, which is sacral segment 1 (or S1).

The disc and vertebra above and below the disc comprise one segment of the spine – usually called a spinal level or spinal segment. The L4 vertebra and L5 vertebra, along with the disc in between them, make up the L4-L5 segment

Discs are always labeled for the vertebrae that they lie between, and this is consistent throughout the length of the spine—for the cervical, thoracic and lumbar spine. For example, the C1-C2 disc in the neck lies between the first and second vertebrae in the cervical spine, and the T1-T2 disc lies between the first and second vertebrae in the thoracic spine.

However, the same is not true of how the spinal nerves are labeled, and back problems are often described by both the spinal segment and the nerve root that is affected. This is explained in more detail on the next page.