Teeth start to pass through the gums (emerge) during infancy. This happens again when permanent teeth replace the primary (baby) teeth.

If a tooth does not come in, or emerges only partially, it is considered to be impacted. This most commonly happens with the wisdom teeth (the third set of molars). They are the last teeth to erupt. They usually come in between the ages of 17 and 21.

An impacted tooth remains stuck in gum tissue or bone for various reasons. The area may be overcrowded, leaving 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. This results in impacted teeth.

Impacted wisdom teeth are very common. They are often painless and do not cause problems. However, some professionals believe an impacted tooth pushes on the next tooth, which pushes the next tooth. Eventually, this can cause a misaligned bite. A partially emerged tooth can trap food, plaque, and other debris in the soft tissue around it, which can lead to inflammation and tenderness of the gums and unpleasant mouth odor. This is called pericoronitis. The retained debris may also lead to the decay on the wisdom tooth or the neighboring tooth, or even bone loss.