Class III malocclusion
A class III intermaxillary relationship means that the lower teeth are shifted forward with regard to the upper teeth. The embrasure between the lower canine and the lower first premolar is shifted forward with regard to the upper canine (blue arrows). The center of the lower first molar (mesiobuccal groove) is anterior to the first corner (mesiobuccal cusp) of the upper first molar. The overjet (yellow arrow) is reversed in a way that the upper incisors are behind the lower incisors (anterior crossbite) rather than the opposite as in a normal class I relationship. This case presents, in addition to the anteroposterior discrepancy, a width discrepancy severe enough to have the upper teeth positioned inside the lower teeth (green arrows). This type of intermaxillary relationship is called bilateral posterior crossbite. It is said bilateral because the relationship is in reverse direction both on the left and on the right, but there are situations where the reverse crossbite will be observed on one side only. The underdevelopment of the maxilla in width results in a lack of space and a misalignment of teeth in the dental arch.
Seen from above, the lack of space is obvious and the incisors do not have the necessary space for proper alignment. There is a lack of harmony between the size (or width) of the teeth and the size (or perimeter) of the maxillary and mandibular arches. Therefore, a tooth size jaw size discrepancy is present (arch length deficiency).
Cephalometric profile view
This cephalometric X-ray shows the anterior discrepancy of the lower jaw commonly called “mandibular prognathism”. However, it is important to specify that this patient shows an upper jaw (upper maxilla) displaced posteriorly or “maxillary retrognathia”.
The lower incisors, much more “vertically positioned” than normal, describe an acute angle of 71° with the tangent plane of the lower jaw (yellow arrows) whereas the normal should be 90°. This kind of incisor compensation inclined posteriorly hides the real discrepancy between the maxilla and the mandible (blue line) compared to the visible discrepancy between the upper and lower incisors (green arrow). It also decreases the perimeter of the mandibular arch, which explains the lack of space and the reason why the lower incisor is stuck on the lingual side (see the lower occlusal view above, blue arrow).
All permanent teeth are in the mouth. The discrepancy between the maxilla and the mandible is so important that the upper third molars cannot touch the antagonist lower third molars and they are hyper-erupted, which explains why the upper third molars are not at the same level as the other upper teeth.