How the back problem relates to patients symptoms can result in real confusion for patients. Spinal anatomy and the location of the back problem is complex and easily leads to intimidation for patients. Even health care professionals who deal with back problems on a regular basis find them complex. Some common questions a patient may have for their doctor are:
- “Is the problem at my L5 level or my L4-5 level?”
- “What does S1 mean?”
- “What is a herniated disc?”
- “What is a nerve root?”
- “How is my back causing the pain shooting down my leg?”
- “Will my back problem leave me paralyzed?”
It is often hard for patients to understand their back problems and how it relates to their symptoms. This is a common and is not anyones fault. A doctor may spend a great deal of time explaining the anatomy of the spine and how a patients symptoms may be caused by an injury or trauma to the back. Back problems are difficult to describe and difficult to understand.
Back problems usually do not involve the spinal cord
Conditions that involve damage to the spinal cord (which can cause paralysis) typically involve trauma or disease in the cervical spine or thoracic spine. This is different from the majority of back problems that cause pain.The spinal cord begins at the base of the brain and runs down through the cervical and thoracic spinal canal. It ends just before the lumbar segments begin (See Figure 1). This means that there is no spinal cord in the low back but there are still nerves.
Some neck problems (such as cervical stenosis) can impinge on the spinal cord and produce symptoms such as difficulty with coordination, it is far more common for back problems to affect the nerve roots rather than the spinal cord.
Nerve roots exit through holes in the bone of the spine at each level. The largest nerve in the body is the sciatic nerve that exits from a bone in the lower lumbar spine. Often back problems affect the nerve roots as they exit the spine. This can cause pain and other symptoms that radiate to the extremities (called a radiculopathy). In the low back, the way the nerve roots exit and branch off from the lumbar spine and sacral region looks like a horse’s tail (cauda equina). They exit to each side between the bones in the spine, and travel down through the low back, across the back of each buttock, down into the lower extremities.
Basic definitions used to describe 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).
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Cervical spine (neck)—comprised of 7 cervical vertebrae (termed C1 to C7), beginning 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.
- Thoracic spine (upper back)—consisting 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.
- Lumbar spine (lower back)—this includes the 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.
- 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. This may result in a herniated disc.
Spinal discs are located between each vertebra
Each of the vertebral bodies (or vertebrae) have a disc (intervertebral disc, spinal disc, or disc) between them along the back bone. Each disc is named according to which two vertebral bodies it lies between. For example:
- 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 vertebra, disc and the vertebrae below the disc comprise one segment of the spine. This is 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. These include 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.
The spinal nerves are labeled differently. Back injuries are often described by both the spinal segment and the nerve root that is affected. Each spinal level nerve exits through holes in the bone of the spine (foramen) on the right side and left side of the spinal column. This occurs from the top to the bottom of the spine.
These nerves are called nerve roots, or radicular nerves. At each level of the spine they branch out and innervate different parts of our body. For example a nerve that exits the cervical spine (cervical nerve root location) traveling down through the arms, hands and fingers can cause pain, numbness and other symptoms. Back problems that affect a lumbar nerve root can radiate through the leg and into the foot causing pain (sciatica).
As mentioned earlier, there is no spinal cord in the lumbar spine. Because of this as well as the spinal canal being fairly spacious in the low back, problems in the lumbosacral region (the lumbar and sacral region of the spine) usually cause nerve root problems, not spinal cord injury. Even a serious condition such as a large disc herniation\or a fracture in the low back are less likely to cause permanent loss of motor function in the legs (or paralysis).
The nerve roots are named for the level of the spine at which they exit. However, nerve roots are not labeled consistently throughout the length of the spine.
- In the cervical spine, the nerve root is named according to the LOWER spinal segment. For example, the nerve at the C5-C6 level is called the C6 nerve root.The root is named this way because it exits the spine where the nerve root passes OVER the C6 pedicle (a piece of bone that is part of the spinal segment).
- In the lumbar spine, the nerve roots are named according to the UPPER segment that the nerve runs between. For example, the nerve root at the L4-L5 level is called the L4 nerve root.The root is named this way because it exits the spine where it passes UNDER the L4 pedicle (a piece of bone that is part of the spinal segment).
The area where the change of name occurs is at the C7-T1 level (Thoracic 1). This means there are 8 cervical nerve roots and only 7 cervical vertebrae. The C8 nerve exits UNDER the C7 vertebrae and OVER the T1 vertebrae. At this point and continuing down through the upper back, lower back and sacral region, the nerve is named for the upper segment of the spine the nerve root runs between (and the pedicle it passes UNDER as it exits the spine).
Two nerve roots at each level
There are two nerves crossing each disc level but only one exits the spine (through the foramen) at that level.
- Exiting nerve root. The nerve root that exits the spine at a particular level is referred to as the “exiting” nerve root.Example: The L4 nerve root exits the spine at the L4-L5 level.
- Traversing nerve root. This is a nerve root that goes across the disc and exits the spine at the next level below. It is called the “traversing” nerve root.Example: The L5 nerve root is the traversing nerve root at the L4-L5 level, and is the exiting nerve root at the L5-S1 level.
When a nerve root is compressed by disc herniation or other cause, it is common to refer both to the intervertebral level (where the disc is) and to the nerve root that is affected. Where the disc herniation or protrusion occurs, will determine if it impinges upon either the exiting nerve root or the traversing nerve root. For example:
- When the traversing nerve root is affected - In the lumbar spine, a weak spot exists in the disc space lying right in front of the traversing nerve root. This means lumbar discs tend to herniate thus impinging on the traversing nerve root. For example, a typical lumbar disc herniation at the L4-L5 level often affects the nerve that traverses the L4-L5 level and exits at the L5 level, called the L5 nerve root.
- When the exiting nerve root is affected - In the cervical spine, the opposite is true, a weakness exists causing the disc to herniate to the side (laterally), rather than toward the back and the side. If the disc material herniates to the side, it would likely compress the exiting nerve root. For example, the C6 nerve root would be affected at the C5-C6 level (because in the neck the exiting nerve root is named for the level below it).
Radiculopathy and sciatica
Another word for the nerve root is “radicular nerve”, and when a herniated disc or prolapsed disc presses on the radicular nerve, this is often referred to as a radiculopathy. Thus, a physician might say that there is herniated disc at the L4-L5 level, creating an L5 radiculopathy or an L4 radiculopathy, depending on where the disc herniation occurs (to the side or to the back of the disc) and which nerve root is affected. The lay term for a radiculopathy in the low back is sciatica.