Archer defined an impacted third molar or wisdom teeth as 'One which was completely or partly erupted and positioned against another tooth, bone or soft tissue, so that its further eruption was unlikely. Dachi and Howell in their study found that the incidence of patients with at least one impacted tooth was 16.7%. Teeth most often impacted in order of frequency were the maxillary third molars or wisdom teeth, mandibular third molars or wisdom teeth, maxillary canines and mandibular premolars. No sex differences were noted. Bjork and colleagues identified 3 skeletal factors that are separately influencing third molar impaction.
1) Chances of impaction are more if the mandibular length is reduced which is measured from chin point to the condylar head.
2) It is also influenced by the condylar growth. If condylar growth is in vertical direction, which is shown by mandibular base angle the chances of impaction are more.
3) Backward directed eruption of mandibular dentition determined by the degree of alveolar prognathism of lower jaw.
Types of Impaction of Wisdom Teeth or Third Molars
Richardson suggested five categories of impaction.
Type A: The tooth can follow the pattern of an ideally developing third molar, by decreasing its angle to the mandibular plane and becoming more upright, but the up righting may not be enough to allow full eruption.
Type B: The angular developmental position relative to the mandibular plane may remain unchanged.
Type C: The tooth can increase its angulation to the mandibular plane and become more mesially inclined. There is at present no reliable way of predicting which teeth will follow this unfavorable pattern, which sometimes occurs unilaterally and leads to horizontal impaction.
Type D: The tooth can be seen to make favorable changes in angulation, but fail to erupt owing to lack of space. These are so called vertical impactions.
Type E: The tooth can continue to change its angulation beyond the ideal occlusal position, and show distoangular impaction or mesio angular impaction.
How Third molars or Wisdom Teeth Erupt and become Impacted
Differential root elongation might explain differences in eruptive behavior among lower third molars or wisdom teeth. Richardson offered a theoretical explanation for favourable or unfavourable rotational movement. Favorable change in angulation, to a more upright position, seemed to occur in teeth where the mesial root developed ahead of the distal crown surface and root. The typical root configuration showed a mesial root which was curved in a distal direction and was slightly longer than the distal root. Unfavorable mesial tipping, leading to horizontal impaction, seemed to occur when the distal root became the same length, and then longer than the mesial root. Th e distal root on such teeth was seen to appear to have a mesial curvature.
What Influences Availability of Space for Eruption of Wisdom Teeth
Bjork et al measured the distance from the anterior border of the ramus to the second molar and concluded that the bigger the space, the better the chance of eruption. Richardson measured an average of 11.4 mm of growth between the age of 10 and 15 years.
In 1987 Richardson examined the creation of space for third molars or wisdom teeth in 51 patients and found that increased space was obtained from both the mesial movement of the dentition and bone remodeling along the anterior border of the ramus. On an average 2 mm of posterior space was created by bone remodeling.
Space Released by Attrition
In so called primitive dentition, where considerable attrition takes place, the third molars or wisdom teeth erupt to take up the space released. Begg felt that lack of this attrition due to highly refined diets, was a major cause of third molar impaction. Other authors such as Profitt have questioned this hypothesis. Early and extensive interproximal caries could also reduce the size of erupted teeth owing to disappearance of proximal contacts.
Second Molar Extraction
Richardson and Richardson in AJO 93 investigated 63 patients after extraction of lower second molars and found that all the lower third molars or wisdom teeth erupted more or less successfully after an average observation period o f 5.8 years. There was considerable variation in the time taken for eruption, ranging from 3 to 10 years and Richardson noted that it is not possible to predict how long eruption will take.
Bonham Magness in JCO 86 suggests that upper third molars or wisdom teeth have a much more predictable eruption pattern than lower third molars or wisdom teeth. He suggested the extraction of upper second molars in some cases to assist first molar positioning and increase space for upper third molars or wisdom teeth or wisdom Teeth.
Tae-Woo Kim et a l in AJO 2003 confirmed the findings of Faubion and Kaplan that impaction of mandibular third molars or wisdom teeth or wisdom Teeth occurs about twice as often in non extraction patients than in extraction patients. The mechanism may be that premolar extraction therapy is associated with an increase in the amount of mesial move ment of the maxillary and mandibular molars and an increase in the eruption space for the third molars or wisdom teeth or wisdom Teeth. Cephalometric growth studies suggest two important mechanisms for development of the retromolar space in the mandible. Resorption at the anterior border of the ascending ramus and the anterior migration of the posterior teeth during the functional phase of tooth eruption. More than 60% of the patients in the study with a distance of 23 mm or less from the distal of the mandibular second molar to the Ricketts’ Xi point at the end of the active treatment experienced eruption of the mandibular third molars or wisdom teeth or wisdom Teeth. The retromolar space can increase about 2 mm from age 15 to adulthood. They also showed that as many as 60% of the cases with distance from the anterior border of the ramus to the distal of the second molar of 5 mm or less experienced eruption space associated with a high risk of impaction might be smaller than previously suggested.