Mandibular Permanent Second Molar Impaction -Prevalence and Genetic Trait

Objective: Mandibular permanent second molar (MM2) impaction is relatively rare (0.3%). Early detection of an arrested eruption MM2 is imperative, since early treatment might eliminate its impaction and reduce complicated orthodontic treatment. This study examined the prevalence and the genetic trait of MM2 impaction in Israeli and Chinese-Americans. Materials and Methods: Panoramic radiographs of Israeli (N=3500) and Chinese-American (N=3000) consecutively treated patients, aged 11-15 years, were examined for the presence of impacted MM2. Distinctive parameters were compared between the unilateral impacted and non-impacted sides. Results: 120 patients with MM2 impaction were found (1.8%). The Chinese-Americans presented higher prevalence (N=71, 2.3%) compared to the Israelis (N=49, 1.4%) (P=0.004). Mesially angulated impacted MM2 was most common (88% and 89%) in the Israeli and Chinese-Americans, respectively. Significant differences were found between the impacted and the non-impacted sides in the distance between mandibular first molar (MM1) to the ramus (p<0.001), mesial root length of MM2 (p<0.001), and the difference between the mesial and distal root lengths of MM2 (p<0.001). Each of these parameters was larger on the non-impacted side, except for the angulation of MM2 (p<0.001) which was larger on the impacted side. Conclusions: The prevalence of MM2 impaction (1.8%) was higher compared to previous studies. A genetic trait is present in MM2 impaction, demonstrating two-fold prevalence in the Chinese-Americans compared to the Israelis. Most impactions are mesially angulated. Impaction factors are the shorter mesial root length of MM2 and the smaller space between the MM1 and the anterior ramus rim.