Teeth start to pass through the gums (emerge) during infancy, and again when the primary (baby) teeth are replaced by the permanent teeth.
If a tooth fails to emerge, or emerges only partially, it is considered to be impacted. The most common teeth to become impacted are the wisdom teeth (the third set of molars). They are the last teeth to emerge, usually between the ages of 17 and 21.
An impacted tooth remains stuck in gum tissue or bone for various reasons. It may be that the area is just overcrowded and there's no room for the teeth to emerge. For example, the jaw may be too small to fit the wisdom teeth. Teeth may also become twisted, tilted, or displaced as they try to emerge, resulting in impacted teeth.
Impacted wisdom teeth are very common. They are often painless and cause no apparent trouble. However, some professionals believe an impacted tooth pushes on the next tooth, which pushes the next tooth, eventually causing a misalignment of the bite. A partially emerged tooth can trap food, plaque, and other debris in the soft tissue around it, leading to inflammation and tenderness of the gums and unpleasant mouth odor. This is called pericoronitis.