An impacted tooth is a tooth that gets blocked as it is pushing through the gum into your mouth (erupting). Wisdom teeth often are impacted. Wisdom teeth usually begin to come in between the ages of 17 and 21. Dentists call these teeth third molars. An impacted tooth is one that fails to erupt into the dental arch within the expected developmental window. Because impacted teeth do not erupt, they are retained throughout the individual's lifetime unless extracted or exposed surgically. Teeth may become impacted because of adjacent teeth, dense overlying bone, excessive soft tissue or a genetic abnormality. Most often, the cause of impaction is inadequate arch length and space in which to erupt. That is the total length of the alveolar arch is smaller than the tooth arch (the combined mesiodistal width of each tooth). The third molars are frequently impacted because they are the last teeth to erupt in the oral cavity. Mandibular third molars are more commonly impacted than their maxillary counterparts. As a general rule, all impacted teeth must be removed, except canine teeth; canines do not need surgery and may just remain buried and give no further problems..