Causes

How do fractures of the spine commonly happen?

There is really only one cause of fracture: the mechanical load placed on the bone is greater than its breaking strength. When a fracture occurs, either the load is unusually high or the bone is unusually weak.

Abnormal Bone

A very large proportion of fractures of the lumbar spine occur in elderly people through bone that is abnormal in some way. The most common abnormality is osteoporosis. This condition leads to gradual re-absorption of the bone until it too weak to bear normal loads. A compression fracture of the vertebral body is the common result.

In other situations, abnormal deposits replace normal bone. Cancer can spread to the vertebrae causing pain and weakening the structure of the bone. Sometimes a pathological fracture – a fracture through abnormal bone – is the first sign of the disease.

Abnormal load

Even when the bone is normal, a compression fracture of the vertebral body is the most common fracture affecting the lumbar spine. These fractures are called "flexion injuries". They can occur when an accident forces you to bend forward suddenly, imposing a sudden severe load on the front of the vertebral body. The front of the vertebral body is crushed although the back is intact, leaving the vertebral body somewhat wedge shaped. Sometimes this pattern is called a wedge compression fracture.

This type of injury occurs because of a fall in the sitting position or a motor vehicle accident. Falls from a ladder can also cause it. In that case, there may be a fracture of the heel bone as well. In sports, this type of fracture is seen in automobile or motorcycle racing, skydiving, power weight lifting, wrestling, gymnastics, football, horseback riding, and high-speed snow sports.

The backbone is quite flexible and is well protected by muscle. So, a blow on the back seldom causes a major fracture. A heavy blow may fracture the spinous process but is not very likely to break the ring of bone surrounding the nerves.

Major fractures of the lumbar spine are caused by high-energy events such as motor vehicle accidents, falls from a height, or diving into shallow water.

When the main force is forward flexion or axial loading of the spine you may sustain a burst fracture of the vertebral body. The front and back of the vertebral body are affected and there are often multiple fragments. It is common for some of these fragments to be displaced back into the space available for the spinal cord or the nerve roots. Nerve damage is common with this pattern of fracture.

During a burst fracture, both the anterior and middle columns are fractured. It is an unstable fracture pattern and the displacement can get worse.

When the force of the accident results in arching the lumbar spine too far back (hyperextension) the ring of bone around the nerves is likely to be injured. This is also an unstable situation and dangerous for the nerves.

If your spine is very forcefully twisted the facet joints may be broken or dislocated.

Doctors pay a lot of attention to the mechanism of the injury and to the pattern of the fracture because it tells them how the fracture is likely to behave and how great the danger of neurological damage may be.