The lower incisors crowding, present in the early period of life more or less, is worsening with aging and this situation is considered as a common clinical problem. The etiological factors assumed to affect the late mandibular incisors crowding are stated in the literature as follow; skeletal morphology, growth pattern and late mandibular growth, soft tissue maturation, periodontal forces, anterior component of the occlusal forces, occlusal factors and presence of mandibular third molars. Many clinicians still consider that anterior crowding increases by the mesially directed force resulting from the third molar's eruption. The aim of this study was to investigate the effects of impacted mandibular third molar teeth on the proximal contact tightness on lower posterior segments, mandibular arch perimeters and lower anterior crowding. 60 volunteer dental school students were included in our study in accordance with the selection criteria. The participants were divided into three groups according to presence or absence of their third molar teeth: Bilateral agenesis of lower third molars (Group 1), bilateral impacted lower third molars (Group 2) and bilateral extraction of impacted lower third molars (Group 3). Initial records (T0) included lower study models, cephalometric and panoramic X-rays. Study casts and cephalometric X-rays were taken at 6th month follow up (T2). Interproximal force measurements were carried out at the beginning (T0) and at 6th month follow up (T2) for Groups 1 and 2. Additional interproximal force measurements were performed following the impacted third molar extractions for the Group 3. The data were analyzed by using SPSS (Statistical Package for Social Sciences) for Windows 22.0. There are no significant intergroup differences for interproximal tightness force, arch length, intermolar dimension, inclination and position of incisors parameters at the beginning. However, in Group 3, initial inter-canine dimension and initial irregularity index were significantly less than the other groups. Initial inter-premolar dimension in Group 1 was wider than the others. The interproximal tightness force measurements recorded after extraction and those recorded at the 6th month follow up in Group 3 were found to be significantly lower compared to the initial measurements. There were no significant differences between the initial and 6th month follow up interproximal tightness measurements in Groups 1 and 2. The arch perimeters, irregularity index, inclination and position of incisors and mandibular length were found stable in all groups during the study. There are no significant differences on the position of the lower incisors and on the interproximal tightness forces according to the position of the lower third molars. The inclination of lower incisors is significantly higher on mesioangular and Class C lower third molars. The presence or absence of the lower third molars were found not affecting the initial mandibular arch perimeters, the inclination and position of the lower incisors. However, it was observed that the interproximal tightness values measured from posterior teeth were decreasing immediately after the extraction of the lower third molar teeth, then there was a statistically insignificant increase in these forces during the follow-up period.

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