Trauma to the tooth following a fall or a hard knock can damage the periodontal ligament cells that line the surface of the root. When these protective cells in the periodontal ligament die, the surrounding cells in the bone act on the root, causing the root to resorb (dissolve) away. This results in cavities or defects in the root, and shortening of the root tip. Progressive shortening of the root over the years will result in tooth loss.
See also: Dental implants