A wisdom tooth, in humans, is any of the usual four third molars. Wisdom teeth usually appear between the ages of 16 and 25. Most adults have four wisdom teeth, but it is possible to have fewer or more, in which case the extras are called supernumerary teeth. Wisdom teeth commonly affect other teeth as they develop, becoming impacted or “coming in sideways.” They are often extracted when this occurs.
Impacted wisdom teeth (i.e., those that have failed to erupt through the gum line) fall into one of several categories:
- Mesioangular impaction is the most common form (44%), and means the tooth is angled forward, towards the front of the mouth.
- Vertical impaction (38%) occurs when the formed tooth does not erupt fully through the gum line.
- Distoangular impaction (6%) means the tooth is angled backward, towards the rear of the mouth.
- Horizontal impaction (3%) is the least common form, which occurs when the tooth is angled fully 90 degrees sideways, growing into the roots of the second molar.
Typically mesioangular impactions are the most difficult to extract in the maxilla (upper jaw) and easiest to extract in the mandible (lower jaw), while distoangular impactions are the easiest to extract in the maxilla and most difficult to extract in the mandible. Frequently, a fully erupted upper wisdom tooth requires bone removal if the tooth does not yield easily to forceps or elevators. Failure to remove distal or buccal bone while removing one of these teeth can cause the entire maxillary tuberosity to be fractured off, thereby tearing out the floor of the maxillary sinus.
Impacted wisdom teeth may also be categorized on whether they are still completely encased in the jawbone. If it is completely encased in the jawbone, it is a bony impaction. If the wisdom tooth has erupted out of the jawbone but not through the gumline, it is called a soft tissue impaction.
In a small portion of patients, cysts and tumors occur around impacted wisdom teeth, requiring surgical extraction. Estimates of the incidence of cysts around impacted teeth vary from 0.001% to 11%, with a higher incidence in older patients, suggesting that the chance of a cyst or tumor increases the longer an impaction exists. A retrospective review of approximately 10,000 impacted teeth, suggested that the incidence of malignant tumours was 0.02% (2 cases in 9,994 teeth).
The oldest known impacted wisdom tooth belonged to a European woman of the Magdalenian period (18,000–10,000 BC).