Third molar teeth (wisdom teeth) are expected to be the last of the teeth to erupt, usually appearing between the ages of 17 and 22. Third molars are commonly removed when their impaction, eruption or presence adversely effects other teeth. They are the most frequently impacted teeth because, as the last of the set of teeth to erupt, they often have limited space available and are obstructed by the other teeth.
Impacted wisdom teeth are categorized by their degree of coverage by bone. If the tooth is completely encased in the bone, it is a bony impaction. If the wisdom tooth has erupted out of the bone but not through the gum tissue, it is called a soft tissue impaction.
Third molars are frequently removed because of the damage they can cause by resting on the roots of other teeth, the crowding of the other teeth caused by pressure from their eruption or infections caused by bacteria invading the site of their partial eruption.