Root Resorption (also called Tooth Resorption) is the shortening of a tooth’s root. If the root gets a little bit short, the tooth may become temporarily loose. However, if the root becomes critically short, the tooth might fall out because it can no longer stay anchored into the jaw bone.
The forces of orthodontic treatment usually cause some amount of inflammatory Root Resorption, which is considered normal and does not effect the overall health or stability of the teeth. However, some people are more prone to serious Root Resorption than others. Unfortunately, orthodontists and dentists have no reliable way to predict who will suffer from serious Root Resorption. Some studies conclude that serious Root Resorption is caused by certain genetic factors, and may run in families.
Root Resorption can be seen on x-rays, so some orthodontists like to take x-rays again at various points during treatment to check on the health of your teeth and their roots. This website contains several scientific articles about Root Resorption.