Third molar eruption patterns and mandibular dental arch crowding

Longitudinal cephalometric and study cast records were analysed for a sample of non-orthodontically treated Caucasians (31 male, 23 female) at two stages of mandibular dental arch development. Stage 1 represented earliest completion of the secondary dentition, with the exception of the mandibular third molars. Stage 2 was approximately 5 years later and was decided on the basis of emergence or, if impacted, at least three quarters root development of the third molars. Parameters at Stage 1 and Stage 2 and the changes between those stages were compared for 3 groups, subdivided for sex and status of third molar development — complete or partial eruption (Group 1), mesioangular impaction (Group 2) and absence (Group 3) of both mandibular third molars.

There were no significant sex differences in either third molar status or dental arch crowding changes between Stage 1 and Stage 2. Overall, it seemed to make little difference to the crowding whether the mandibular third molar erupted completely or partially, remained mesioangularly impacted or was absent. Rather than causing dental arch (incisor) crowding, the third molar appeared to be but one of many complexly interacting factors which affected dental arch crowding.