[Some aspects of third molars with regard to the development of malocclusions].
Although the orthodontist is constantly aware of the developing third molar and its possible effects on the dentition during and after orthodontic treatment, the relationship between the third molar and the development of malocclusion had not been resolved. To attempt to clarify some of the problems associated with the third molar, an analysis of the factor in fluencing on the dentition and denture frame was performed. In this study, twenty-one adult cases which had no prothodontic correction of the tooth shape and orthodontic treatment with at least three wisdom teeth were used for evaluation of the denture frame structure and occlusion. The case which impacted third molar (M3 impacted group) showed less than 25 degree mandibular plane angle (FH-MP) without exception, while the case which the third molar erupted (M3 erupted group) were able to divide into high angle (more than 30 degree of FH-MP) and low angle (less than 29 degree of FH-MP) groups. The M3 impacted with low angle group and M3 erupted with low angle group showed relatively normal occlusion and favorable denture frame structure, but M3 erupted with high angle group indicated that the denture frame composition was affected by posterior discrepancy especially the steepness of occlusal plane which might be the over eruption of posterior teeth. The M3 erupted with high angle group also included two cases of severe anterior open-bite with skeletal deformity. These findings suggest that the posterior discrepancy due to existence of third molars influence on the dento-facial-skeletal structure and development of malocclusions.