Third molar impaction--a consequence of late M3 mineralization and early physical maturity.

Abstract

The purpose of the present investigation was to analyse the influence of late M3 mineralization and early physical maturity on normal eruption of upper and lower M3s in both sexes The cases investigated were selected from a larger study of craniofacial growth (Bjork, 1968). It was made a condition, that longitudinal complete yearly records should be available from before puberty until 25 years of age, and all first and second molars should be present throughout the period of investigation. 91 subjects (36 girls and 55 boys) were included in the study. About 75 per cent of them had undergone some kind of orthodontic treatment with or without premolar extractions. A total of 314 M3s were classified in 3 subgroups: Normal Eruption (NE), Vertical Impaction (VI) and Horizontal Impaction (HI), according to their status at the age of 25 years. The age at M3 mineralization stage E (Demirjian et al ., 1973) was determined from the orthopantomograms. Physical maturity was recorded as the age of union of the distal epiphysis in Radius (Ru), as seen on the hand-wrist radiographs. The subgroups VI and HI displayed significantly delayed ages of mineralization of M3 compared to the NE subgroups. The largest and most significant differences were seen in girls. When M3 mineralization age was related to physical maturity, the differences between the NE subgroups and the VI/HI subgroups were accentuated in general. The investigation led to the following conclusions: Late M3 mineralization in combination with early physical maturity is an important aetiological factor in the impaction of M3, where unfavourable space conditions could be the precipitating factor. Clinically, the prediction of the risk of impaction can be improved by taking this into account.