Anatomy

What structures are most commonly injured?

The thoracic spine is the section of the spine from the neck to the waist and supports the ribs. It consists of a stack of twelve vertebrae(bones), one on top of the other. The section of the vertebral column below this region is the lumbar spine.

All the thoracic vertebrae are very similar. There is a drum-like vertebral body in the front that takes most of the weight. Just behind that is an arch of bone protecting the nerves. In the thoracic spine, the spinal cord fills most of the space in the spinal canal. The spinal cord is a structure that extends from the base of the brain to the top of the lumbar spine. It runs through the spinal canal of the cervical spine and the thoracic spine. It is a round, cord-like structure about the thickness of your little finger. The spinal cord and the brain are both considered part of the central
nervous system, both contain nerve fibers and nerve cells. The thoracic spinal nerve roots leave the spinal cord within the spinal canal and travel out of the foramen to supply sensation and muscle control to the chest wall. There are two spinal nerves that
leave between each pair of vertebrae, one on each side. If bone fragments from an injury are pushed into the space available for the nerves, the spinal cord may be injured and is unlikely to recover fully.

Sticking out at the back is the spinous process. It comes quite close to the skin. If you run your fingers down between the shoulder blades in the middle of your back (it’s easier on someone else’s back), the hard bumps you can feel are the spinous processes of the individual vertebrae. The transverse processes that jut out from either side of the arch cannot be felt. These “processes” are where the ribs and the muscles that move the spine are attached. Each rib juts outwards, then circles forwards and slightly downwards from the supporting vertebra to form the chest cavity – rather like a barrel. In the front of the chest the ribs join to the breastbone (sternum) through a flexible costal cartilage that allows the ribs to move and rotate as you breathe in and out. The ribs reinforce and splint the thoracic spine. This means that thoracic spine fractures are less common; but it also means that if this area has been fractured a good deal of force was involved and the fracture may be more serious.

The facet joints link one vertebra to the next and allow for some movement. The ribs prevent much movement between the thoracic vertebrae. When you bend forward or bend side-to-side, much of the movement occurs in the lumbar spine. Because the facet joints between adjacent thoracic vertebrae are set quite flat, some rotation can occur in this section of the spine. If you hold the pelvis steady you can twist the trunk and upper body about 20-30 degrees. This rotation occurs as a result of small amounts of rotation between each thoracic vertebrae.

The rest of the movement of the spine takes place between the vertebral bodies. This area is occupied by the intervertebral disc, a ring of tough fibrous tissue with some jelly-like material in the middle. The disc allows some pivoting between the next-door vertebral bodies; but in the thoracic region the amount is limited by the splinting effect of the rib cage. The disc also acts as a shock absorber when the spine is loaded.

Ligaments are the tough inelastic bands of fibrous tissue that pass from one bone to the next. They connect and hold the vertebrae together. The longitudinal ligaments connect the vertebral bodies together. The ligamentum flavum connects the lamina, or posterior arch on the back of each vertebra together. The joint capsule ligaments surround the facet joints. Any of these ligaments can be injured or completely torn if the vertebrae of the spine are fractured or dislocated. Since they do not show on X-ray, injury to ligaments cannot be seen directly. If there is an abnormal gap between the parts of the joint on X-ray, this may suggest that the ligament has been torn.

Muscles also connect vertebra together. Muscles move the spine. They also contract to stop movement, or stabilize, the spine. For example, the muscles stabilize our spine when we walk. Much of the time your backbone holds still, as a support for the upper body. We don’t think about this; it is automatic.

Sometimes when we are suddenly shifted off balance the force of the automatic correction is too great for the bone and a small piece of bone is pulled off the spine. This is called an avulsion fracture. It is also important to remember that a force great enough to break a bone is more than enough to tear muscle. With every bony injury to the spine there is usually an accompanying injury to muscle and possibly to ligaments and nerves as well. This is called a soft tissue injury to distinguish it from the bone (hard tissue) injury.

The neural connection between the brain and the legs is called the spinal cord. It is best thought of as a part of the brain extending down inside the column of the spine. At each level the spinal cord gives off a set of nerve roots that exit the spine passing between the vertebrae.

There are several important points to understand about the spine and fractures:

• The vertebral bodies take most of the weight. If they are broken it was probably caused by an over-load.
• The facet joints restrict rotation. If they are broken the spine was probably over-rotated.
• The arch protects the nerves and spinal cord. If it is broken the risk of injury to the nerves is greater. The nerves are very close to the vertebral body and the intervertebral disc.
• A ring breaks in two places. If the ring of bone around the nerves is broken in one place there is usually another break. Such injuries are unstable. The pieces are likely to drift further apart.
• The transverse processes and the spinous processes are attachments for muscle. If they are broken (and nothing else), it is most likely because of over-pull by muscles. This is called an avulsion fracture.
• The stability of a fracture is a critical concern. If the displacement is likely to get worse the risk of progressive nerve injury is higher.
• The soft tissue component of the injury is important.

When considering the anatomy of the injured spine it is helpful to think of it as having three columns:

• front (anterior)
• middle
• back (posterior)

The anterior column consists of the front half of the vertebral body, the nucleus of the intervertebral disc and the anterior longitudinal ligament.

The middle column includes the back of the vertebral body and the posterior longitudinal ligament.

The posterior column is the ring of bone surrounding the nerves and the facet joints.

Damage to more than one of the columns makes for an unstable injury which is likely to displace further. This means that damage to the middle column is critical and is more serious. It is very unlikely that a fracture would affect the middle column alone. If this region is broken, it is very likely that there is damage in front or behind and the whole spine is unstable.