The Normal Structure of the Spine

Spinal column

The spinal column consists of 24 individual bones (vertebra), the sacrum and the coccyx. The sacrum and coccyx are formed from vertebra but these have fused. Most vertebrae share a common basic structure but also have distinct differences. The spinal column has the shape of a double ‘S’. In the neck and lower back, the spine bends slightly backwards (‘lordosis’) and in the thoracic spine and sacrum it bends forwards (‘kyphosis’). When patients are in pain, muscles go often into spasm pulling the neck and lower back straighter. Certain other conditions can cause change in the normal shape. This can alter the way muscles and ligaments have to work causing more pain. The spine is, however, straight looking at it from the front or the back. Scoliosis is a term to describe sideways bending of the spinal column. This is common to a certain degree and patients with mild scoliosis often have no symptoms. In severe cases and in adolescence this should be managed by a specialist.

Common structure

All vertebrae have a vertebral body (corpus). This is connected to the facet joints via the pedicles on each side of the back of the vertebral body. The facet joints are connected to each other at the same level via the lamina. Where the lamina joins a spike arises (spinous process) which can be felt in the back. The body (in the front), the pedicles and facet joints (at each side) and the laminae and spinous process (at the back) form a hole. All these holes lined up from top to bottom form the spinal canal. The spinal cord runs through this canal. At each level, a nerve leaves the spine on each side (spinal nerves). As it leaves it is in close contact with the facet joints and intervertebral discs. The opening where the nerve leaves the spinal canal is called intervertebral foramen. It is in this area where the nerve can get trapped due to a slipped disc or ‘wear and tear’. The facet joints on each side of the vertebral body connect the vertebra to its neighbour above and below. Wear and tear of these joints can cause pain and reduce movement. The space between the sac containing the nerves and spinal cord (dura – see below) is called epidural space. It contains small blood vessels and fat.

Cervical spine (neck)

The vertebrae are relatively small. There are seven in total but the first two are different to all other vertebrae. Most of the movement of the neck is happening in this area. The main movement of bending the head backwards and forwards (flexion and extension) is between the head and the first vertebra (‘Atlas’). Most of the rotation (looking to the side) is between the first and second vertebra (Axis). A fusion involving these structures will cause significant limitations in the range of movement of the neck. On each side of the upper six vertebral bodies in the neck runs an artery (vertebral artery). This artery supplies part of the brain and injury during surgery can be serious. As there are two of them, injury of one will not cause any symptoms but injury to both (which is extremely uncommon in any surgery!) can be life threatening.

Thoracic spine (upper back)

The main difference between the vertebrae in the thoracic spine and all others is the connection to the rib cage. This makes the thoracic spine very stiff. In front of the thoracic spine are the lungs, the heart with its big vessels. An operation to the front of the spine in this region will therefore require opening of the chest (‘thoracotomy’).

Lumbar spine (lower back)

The vertebrae are large as they have to carry a lot of weight. Strong muscles are attached from all sides which help to move the whole trunk of the body and keep it upright when walking. The pedicles (connections between vertebral body and facet joints are also large. Modern surgical fusion techniques often involve the insertion of screws from the back of the spine through these pedicles into the vertebral body. Because of the weight of the upper body resting on the lumbar spine, this part is particularly prone to wear and tear.