All our teeth are held securely in place by their roots in the jawbone. Loss of a single tooth is often due to trauma or tooth decay. When a single tooth is lost, and is not immediately replaced, the surrounding bone will begin to resorb or shrink in the empty space, because it is no longer stimulated by the tooth's roots. With no adjacent support, the neighboring teeth begin to drift toward the area of the missing tooth.
The amount of bone loss and the pattern of drifting will vary depending on the individual and the location of tooth loss. The drifting of one tooth may lead to the drifting of several teeth, resulting in a significant amount of movement.