Anatomy and back pain:

- Vertebral bodies
- Vertebral discs
- Spinal cord and nerve roots
- Muscles

Anatomy and back pain



Spinal anatomy is truly unique in its form and function. It is designed to be incredibly strong, protecting the highly sensitive nerve roots, yet highly flexible, providing for mobility on many different planes.

However, many different structures in the spine are capable of producing back pain, including:

• The large nerve roots that go to the legs and arms may be irritated
• The smaller nerves that innervate the spine may be irritated
• The large paired back muscles (erector spinae) may be strained
• The bones, ligaments or joints themselves may be injured
• The disc space itself can be a source of pain

Therefore, a review of spinal anatomy is important to understand the causes of back pain and evaluate treatment options. This section provides an overview of anatomical structures in the spine, including:

• Vertebral bodies
• Vertebral discs
• Spinal cord and nerve roots
• Muscles

The review includes the three major regions of the spine (Figure 1):

1. Cervical spine (neck)
2. Thoracic spine (upper back)
3. Lumbar spine (lower back)

The sacrum is at the bottom of the spine and lies between the fifth lumbar segment and the coccyx (tailbone). The sacrum is a triangular-shaped bone and consists of five segments (S1 – S5) that are fused together and connect to the pelvis (ilium) and form joints called the sacroiliac joints.

Vertebral bodies

The cervical spine (neck) has seven vertebral bodies (segments). The top two segments are unique:

•  The first cervical segment is a ring that does not have a vertebral body. It     is attached to the second vertebral body, which acts as a post that the
    first vertebral ring rotates around. Most of the rotation in the neck is
    locatedin these top two segments.

•   Like the rest of the spine, the next five vertebral segments have three
    joints at each segment, including one disc in the front and paired facet
    joints in the back.

The thoracic spine (upper back) has twelve vertebral bodies. These structures have very little motion because they are firmly attached to the ribs and sternum (breastbone). Because there is little motion, this region of the spine is not usually a source of pain.

The lumbar spine (lower back) has five vertebral bodies that extend from the lower thoracic spine (chest) to the sacrum (bottom of the spine). The vertebral bodies are stacked on top of each other with a disc in between each one.

All of the vertebral bodies act as a support column to hold up the spine. This column supports about half of the weight of the body, with the other half supported by the muscles.

The vertebral bodies are then attached to a bony arch through which all the nerve roots run. Part of the arch is comprised of the paired facet joints, which in combination with the disc, creates a three joint complex at each vertebral motion segment. The facet joints have cartilage on each surface and a capsule around them. The cartilage can degenerate as one ages, and lead to degenerative arthritis.

The three-joint complex at each vertebral segment allows for motion in flexion, extension, rotation, and lateral bending.

Fifty percent of flexion (bending forward) occurs at the hips, and fifty percent occurs at the lower (lumbar) spine. The motion is divided between the five motion segments in the lumbar spine, although a disproportionate amount of the motion is at L4-L5 (lumbar segment 4 and 5) and L3-L4 (lumbar segment 3 and 4). Consequently, these two segments are the most likely to break down with degeneration. As these segments break down they can become unstable with an excess of motion creating pain. A surgical fusion can help alleviate the pain by stopping the motion.

Vertebral discs

The vertebral disc is an interesting and unique structure. Its primary purpose is to act as a shock absorber. Discs are actually composed of two parts: a tough outer core and a soft inner core and the configuration has been likened to a jelly doughnut (see Figure 1).

At birth, eighty percent of the disc is composed of water. With age, the discs dehydrate and become stiffer. This is a natural aging process, although in some individuals, as the disc degenerates it can become painful. The most likely reason for this is that the degeneration can produce micromotion instability and the inflammatory proteins (the soft inner core of the disc) probably leak out of the disc space and inflame the well innervated structures next to the disc (e.g. nerve roots).

Sometimes a twisting injury damages the disc and starts a cascade of events that leads to degeneration (see Figure 2). The disc itself has very few nerve endings and no blood supply. Without a blood supply the disc does not have a way to repair itself, and pain created by the damaged disc can last for years. In general, as we age there are less inflammatory proteins in the disc space and discogenic pain rarely occurs after 6o years of age.

Spinal cord and nerve roots

The spinal cord comes off the base of the brain, runs throughout the cervical and thoracic spine, and ends at the lower part of the thoracic spine. Therefore, spinal cord damage may accompany trauma or diseases of the cervical or thoracic spine.

The spinal cord does not run through the lumbar spine. After the spinal cord stops in the lower thoracic spine, the nerve roots come off the bottom of the cord like a "horse’s tail" (cauda equina) (see Figure 1).

Therefore, because the lumbar spine has no spinal cord and comprises a large amount of space for the nerve roots, even serious conditions (such as a large disc herniation) are unlikely to cause paraplegia (loss of motor function in the legs).

The nerve roots run through the bony canal, and at each level a pair of nerve roots exits from the spine.

• In the cervical spine, the nerve root is named for the lower segment that it    runs between (e.g. C6 at C5-C6 segment).

• In the lumbar spine, the nerve is named for the upper segment that it runs    between (e.g. L4 at L4-L5 segment)

The nerve passing to the next level runs over a weak spot in the disc space, which is the reason discs tend to herniate (extrude) right under the nerve root and can cause leg pain (radiculopathy or sciatica).

• Cervical disc herniations tend to irritate the nerve exiting at a particular
   level (e.g. C6 at C5-C6)

• Lumbar disc herniations tend to irritate the nerve that lies across a
   particular level (e.g. L5 at L4-L5) (Figure 2)

• Thoracic disc herniations are very rare

Sometimes, a herniated disc will cause only leg/arm pain and not low back/neck pain, and may initially be thought to be a problem with the  leg/arm.

• Arm pain from a cervical disc herniation is usually accompanied by    numbness/tingling and runs to the fingers

• Leg pain from a lumbar disc herniation will usually run below the knee, and    possibly to the foot, and may be accompanied by numbness

The two nerves most commonly pinched are L5 (lumbar 5) and S1 (sacral 1). The L5 nerve supplies the nerves to the muscles that raise the foot and big toe, and consequently, impingement of this nerve may lead to weakness in these muscles. Likewise, S1 impingement can lead to weakness with the large gastronemius muscle in the back of the calf, causing difficulty with foot push off (see Figure 3).

Numbness for L5 runs over the top of the foot and for S1 it runs on the outside of the foot. The S1 nerve root also supplies innervation for the ankle jerk (tap on the achilles tendon and the foot goes down), and a loss of this reflex indicates S1 impingement, although it does not create loss of function.

Most cervical pathology will lead to pinching of either C6 or C7 nerve roots, although sometimes C5 or C8 may be pinched. Depending on which nerve root is pinched, the following symptoms are likely:



• C5 - shoulder pain, deltoid weakness, and possibly a small area of
  numbness in the shoulder. On physical exam, a patient’s biceps reflex may
  be diminished.

• C6 - weakness in the biceps and wrist extensors, and pain/numbness that   runs down the arm to the thumb. On physical exam, the brachioradialis
  reflex (mid-forearm) may be diminished.

• C7 - pain/numbness that runs down the arm to the middle finger. On
   physical exam, the triceps reflex may be diminished.

• C8 - hand dysfunction (this nerve supplies innervation to the small muscles
  of the hand). Pain/numbness can run to the outside of the hand (little
   finger) and impair its reflex.

The nerve consists of one long cell from the back/neck down to the foot/hand, so the nerves tend to heal slowly. They heal from the top down, and depending on how much damage is done at the time the nerve becomes impinged, it may take weeks to months to heal.

Treatment of neural impingement is directed at relieving the pain and then allowing the nerve to heal on its own. Nerves need both inflammation and pressure to be painful, so either relieving the inflammation or the pressure can relieve the pain.

Muscles

The soft tissues around the spine also play a key role in low back pain. The large paired muscles in the low back (erector spinae) help hold up the spine. With inflammation the muscles can spasm and cause low back pain and marked limitation in motion.

An episode of low back pain that lasts for more than two weeks can lead to muscle weakness (since using the muscles hurts, the tendency is to avoid using them). This process leads to disuse atrophy (muscle wasting), and subsequent weakening, which in turn causes more pain because the muscles are less able to help hold up the spine.

Another key structure in back pain is the hamstring muscles in the leg. Patients with tight hamstrings tend to develop low back pain, and those with low back pain tend to develop tight hamstrings.

The theory is that tight hamstrings limit motion in the pelvis, so the motion gets transferred to the bottom lumbar motion segments and increases the stress in the spine. Rehabilitation focuses on strengthening the muscles and stretching the hamstring muscles.

By: Peter F. Ullrich, Jr., MD
September 8, 1999 (Updated July 10, 2001)