Objective: The aim of this study was to ascertain whether the lower third molar could contribute to the lower incisor crowding.
Subjects and Methods: Subjects, panoramic radiographs (OPGs), 45-degree cephalometric radiographs (45-CRs) and dental casts of modern Mongolians (mean age was 20.0 years) who had all four third molars, were taken in 2006. Subjects were divided in to two clusters such as Class I normal occlusion (10 females, 4 males) and Class I crowding (12 females, 11 males). On OPGs, the ratio of retromolar space to the width of the crown of the third molar (Ganss ratio; GR) was calculated. On 45-CRs, gonial angle and angulations of lower canines, premolars and molars were measured. On dental casts, the sum of the contact point displacements from anatomic contact point to contact point (Little’s index of irregularity; LII) was calculated.
Results and Discussion: There was significant correlation between the angulation of the third molar and GR in both clusters. On Class I crowding cluster, there was no significant correlation between the LII and angulation of the third molar. However, on the Class I crowding cluster, there were significant correlations between LII and angulations of lower canines, premolars and molars in female and were also significant correlations between LII and gonial angle in male.
Conclusion: The influences of third molars to the discrepancy were limited to the posterior area. It may be considered that the dental compensation and the morphological characters of mandible affect anterior crowding rather than third molars.