The incidence of mandibular third molar impactions in different skeletal face types

The cause of mandibular third molar impaction is said to be due to inadequate space between the distal of the second mandibular molar and the anterior border of the ascending ramus of the mandible. The amount of space is determined primarily by facial growth. This study aimed to assess whether different patterns of facial growth lead to a different incidence of mandibular third molar impaction. It was hypothesized that those with predominantly horizontal (brachyfacial) would have lower incidence of mandibular third molar impaction compared with those with a predominantly vertical growth pattern (dolichofacial).

Ninety-eight dental records were sourced from the records of orthodontic patients in the Royal Dental Hospital of Melbourne and all lateral cephalometric radiographs and orthopantomograms were assessed. The degree of impaction was determined by the Pell and Gregory system, and the facial type categorized by the facial axis angle.

The overall rate of mandibular third molar impaction was 58.76 per cent. Those with a facial axis angle >93 (brachyfacials) demonstrated an almost two times lower incidence of mandibular third molar impaction as compared to subjects with a facial axis angle <87 (dolichofacials).

Within the limitations of the study, it was concluded that the greater horizontal facial growth pattern of brachyfacial subjects over dolichofacial subjects provides increased space for full eruption of the mandibular third molars.