Prediction of Mandibular Third Molar Impaction

Objectives: To identify risk factors for mandibular third molar impaction in adolescent orthodontic patients. Methods: Radiographs made before (T1) and after (T2) treatment and at a minimum of 10 years post-retention (T3) of 134 patients that allowed accurate diagnosis of impaction vs. eruption of mandibular third molars were evaluated. Results: Univariate logistic regression analyses at T1 showed that premolar extraction (P<0.05), an increase of 1mm in mesial molar movement (MM T1/T2) (P<0.05), and an increase of 1mm in eruption space measured from second molar to Xi point (M2/Xi) (P<0.01) reduce the risk of impaction by 63%, 22%, 30% respectively, while an increase of 1 degree in lower third molar angulation relative to the occlusal plane (M3/OP) increases the risk of impaction by 11% (P<0.01). Multivariate logistic regression analyses at T1 concluded that reduced eruption space (P<0.01), signs of forward mandibular growth rotation (P<0.01) and female gender (P<0.05), increased the risk of impaction. However, association tests documented increased frequency of extraction and more mesial molar movement in females among the patients with third molar eruption. Univariate logistic regression analyses at T2 showed that every mm increase in eruption space measured from second molar to anterior border of the ramus (M2/R) as well as to Ricketts' Xi point (M2/Xi) reduced the risk of impaction by 29% and 22%, respectively (P<0.01), while every deg increase in angle M3/OP increased the risk of impaction by 11% (P<0.01). Multivariate logistic regression analyses at T2 concluded that increased eruption space reduced the risk of impaction (P<0.05), while increased mesial angulation of the third molar bud (P<0.01) and signs of forward growth rotation (P<0.05) increased the risk of impaction. Conclusion: Our results may be of clinical significance in borderline extraction cases, choosing the extraction alternative only in situations with a high chance of third molar eruption.