and back pain:
- Vertebral bodies
- Vertebral discs
- Spinal cord and nerve roots
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
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 discs
• Spinal cord and nerve roots
review includes the three major regions of the spine
Cervical spine (neck)
2. Thoracic spine (upper back)
3. Lumbar spine (lower back)
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.
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 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.
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
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
• C5 - shoulder pain, deltoid weakness, and possibly
a small area of
numbness in the shoulder. On physical exam, a
patient’s biceps reflex may
• 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
physical exam, the triceps reflex may be
• 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
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
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
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
By: Peter F. Ullrich, Jr., MD
September 8, 1999 (Updated July 10, 2001)