Three-dimensional morphometric analysis of mandibular subunits in different sagittal and vertical skeletal patterns

With the application of three-dimensional (3D) radiographic imaging in orthodontic practice, jawbone morphology features that were previously undetectable on two-dimensional (2D) images now have the opportunity to be more thoroughly understood. This study aimed to three-dimensionally assess the mandibular functional skeletal subunits in different skeletal sagittal and vertical patterns.

A total of 281 Chinese adult patients were retrospectively enrolled. Using the 3D reconstructed images from the cone-beam computed tomography (CBCT) scans, the mandibular functional skeletal subunits were evaluated and compared in different sagittal and vertical skeletal classes.

In both genders, almost all mandibular subunit lengths, except for the symphysis unit, showed significant differences across all the sagittal and vertical facial patterns. Skeletal Class III exhibited a significantly longer length of the condyle and body unit, whereas skeletal Class II had only a shorter body unit length. Interestingly, the condylo-coronoid angle was significantly larger in skeletal Class I compared to the other two sagittal groups. The hyperdivergent pattern demonstrated a significantly shorter length of the condyle, coronoid, and angle unit, as well as significantly larger condylo-coronoid, condylo-gonial, and condylo-inferior nerve angles, compared to the other two vertical groups. These findings showed no sexual dimorphism between males and females.

Mandibular hypoplasia in skeletal Class II is primarily attributed to the underdevelopment of the body unit, whereas mandibular hyperplasia in skeletal Class III is due to the overgrowth of all the major subunits. A hyperdivergent facial pattern is characterized by underdevelopment of the ramus subunits but not the body unit.