Mandibular second molar impaction. Part 1: Genetic traits and characteristics

Introduction: 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. Methods: 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. Results: A total of 120 subjects with MM2 impaction were found (1.8%). The Chinese- American population had a higher prevalence (n = 1, 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). Conclusions: 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. (Am J Orthod Dentofacial Orthop 2011;140:32-7)