Mandibular second molar impaction. Part I: Genetic traits and characteristics

Detection of mandibular second molar (MM2) impaction is imperative for orthodontic diagnosis and treatment. In this study, we examined a possible genetic trait in MM2 impaction in 2 populations and defined distinctive characteristics.

Initial panoramic radiographs of patients of Israeli (n = 3500) and Chinese-American (n = 3000) origin, aged 11 to 15 years, were examined. Twelve distinctive characteristics were compared between the unilateral impacted and the nonimpacted sides.

A total of 120 subjects with MM2 impaction were found (1.8%). The Chinese-American population had a higher prevalence (n = 71, 2.3%) of MM2 impaction compared with the Israeli population (n = 49, 1.4%; P = 0.004). For the subjects with MM2 impaction, the Israelis had significantly (P = 0.039) fewer bilateral impactions (27%) than did the Chinese-Americans (45%). Mesially inclined impacted MM2s were more common (88% and 89%) in the Israeli and Chinese-American populations, respectively. The unilateral impacted side demonstrated reductions in the distance between the mandibular first molar and the ramus (P <0.001), the length of the mesial root of the MM2 (P <0.001), and the height between the MM2 and the mandibular third molar, and increases in the angulations of the MM2 (P <0.001) and the mandibular third molar (P <0.003).

An autosomal genetic trait is present in MM2 impaction with greater penetrance in the Chinese-American population. Within developmental impediments, the deficient mesial root length of the MM2 is the primary impaction factor.