Fracture of the main body of the bones in the spine is called a vertebral compression fracture. The severity of this type of fracture is based on the condition of the bone after the fracture.

A mild fracture may be a single crack in the bone with mild changes in the shape of the vertebral body. The front half of the vertebra may start to collapse giving it a wedge or pie-shape when viewed from the side on X-ray.

With a moderate vertebral compression fracture, there is the same kind of wedging along with up to 50 percent loss of bone height. Severe fracture has a wedge greater than 30 percent and more than 50 percent loss of bone height.

In some cases the fracture starts from the center and moves out to the edges of the bone. This is called a sunburst fracture. The entire bone can collapse with almost complete loss of vertebral height. When that happens, the joints are compressed together causing pain. Traction may be placed on the nerves that exit the spine at that level also causing pain.

Level of pain isn't always linked with the severity of fracture. A mild fracture can cause severe pain, whereas some severe fractures seem to cause no pain at all. The patient may not even know there is a fracture until an X-ray is taken for something else and shows the old fracture site.