title: Impaction of permanent mandibular second molar: a retrospective study.
abstract: OBJECTIVE
  To determine the prevalence of impacted mandibular second molar (MM2) and the association between MM2 impaction and crowding. The clinical significance of the angle between first and second mandibular molar and of the space between the first mandibular molar (MM1) and the anterior margin of mandibular ramus in MM2 impaction were also evaluated.
  
  
  METHODS
  In this retrospective study , from the dental records of 2,945 caucasian young orthodontics patients , 40 subjects with MM2 impaction were included in a study group (SG) and compared with a control group (CG) of 200 subjects without MM2 impactions. The crowding , the angle of inclination of MM2, the distance between MM1 and mandibular ramus, the canine and molar relationships, and the lower centre line discrepancy were measured. For the statistical analysis , descriptive statistics and t-Student for independent sample groups were used.
  
  
  RESULTS
  The prevalence of impacted MM2 was 1.36%. The independent-Samples t-Test between SG and CG showed: the presence of crowding (P ≤ 0.001), an higher angle values of MM2 inclination (P ≤ 0.001) and a smaller distance between MM1 and the anterior margin of mandibular ramus (P ≤ 0.001) in the SG.
  
  
  CONCLUSIONS
  The impaction of MM2 is a relatively rare occurrence in orthodontic caucasian populations. The crowding, a higher angle values of MM2 inclination and a reduced distance between MM1 and the anterior margin of mandibular ramus, at the time of one third of MM2 root formation (T1), characterize MM2 impaction.
authors.affiliation: Department of Oral and Maxillofacial Sciences, Sapienza University of Rome , Italy . michele.cassetta@uniroma1.it
authors.forename: Michele
authors.initials: M
authors.lastname: Cassetta
authors.forename: Federica
authors.initials: F
authors.lastname: Altieri
authors.forename: Alfonso
authors.initials: A
authors.lastname: Di Mambro
authors.forename: Gabriella
authors.initials: G
authors.lastname: Galluccio
authors.forename: Ersilia
authors.initials: E
authors.lastname: Barbato
mesh_terms: D005260:Female; D006801:Humans; D008297:Male; D008334:Mandible; D008963:Molar; D015995:Prevalence; D012189:Retrospective Studies; D014095:Tooth, Impacted / Q000453:epidemiology*