title: Mandibular second molar impaction. Part I: Genetic traits and characteristics.
abstract: BACKGROUND
  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 = 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).
  
  
  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.
authors: Department of Orthodontics, Maurice and Gabriela Goldshleger School of Dental Medicine, Tel Aviv University, Tel Aviv, Israel. shapi@netvision.net.il
authors: Yehoshua
authors: Y
authors: Shapira
authors: Tamar
authors: T
authors: Finkelstein
authors: Nir
authors: N
authors: Shpack
authors: Yon H
authors: YH
authors: Lai
authors: Mladen M
authors: MM
authors: Kuftinec
authors: Alexander
authors: A
authors: Vardimon
mesh_terms: D000293:Adolescent; D001213:Asian / Q000235:genetics*; D002648:Child; D002681:China / Q000208:ethnology; D006801:Humans; D007557:Israel / Q000453:epidemiology; D007585:Jews / Q000235:genetics*; D008334:Mandible; D008963:Molar / Q000000981:diagnostic imaging / Q000473:pathology*; D009518:New York / Q000453:epidemiology; D019683:Penetrance; D015995:Prevalence; D011862:Radiography, Panoramic; D014092:Tooth Root / Q000254:growth & development / Q000473:pathology*; D014095:Tooth, Impacted / Q000000981:diagnostic imaging / Q000453:epidemiology / Q000235:genetics*