In man and many other mammals the roots of the teeth close after the teeth have attained a certain size, so that all growth ceases. But in a number of animals, including rodents, walruses, elephants and wild boars, some of the teeth remain open at the roots; new material is added and the teeth continue to grow throughout life.
For such animals as rodents, this continuous growth is usually quite advantageous, since it prevents their teeth from being entirely worn away by their ceaseless gnawing activities.
Occasionally, however, one of the teeth may grow crooked, or the lower jaw may become slightly displaced so that some of the teeth are not used and thus not worn away. But they still continue to grow.
The lower teeth may curve over the snout or head and eventually make it impossible for the animal to open its mouth; or they may actually grow through the upper jaw and penetrate the brain.
I once saw the skull of a rodent, probably a groundhog, in which that condition had developed. One of the lower incisor teeth had pierced the upper jaw, grown through it for about half an inch and emerged continuing to grow for an inch or so before the animal died.