Bruce Willard Hultgren

Mechanics, growth, and Class II corrections

Growth of the orofacial region is quantitatively described by locating the center of mandibular rotation relative to the cranial base. The center of mandibular rotation is positioned by the ratio of vertical facial growth (AFH/PFH) and the direction of condylar growth. Appliance therapy is associated with changes in the means of both of these parameters. These changes reduce or stop favorable anterior mandibular rotation and redirect the mean condylar growth vector more posteriorly. When appliance therapy is stopped, these parameters return toward their resting values. The mean direction of the condylar growth vector became even more anteriorly directed after treatment than the pretreatment mean value. These data support the hypothesis that orthodontic appliances significantly alter the facial growth pattern and when they are stopped, the growth pattern tends to rebound to or beyond the pretreatment values.

Growth contributions to Class II corrections based on models of mandibular morphology

Various morphologies of human models are modeled with various growth patterns to demonstrate the role of mandibular morphology on growth contributions to Class II corrections. Growth patterns are described by centers of mandibular rotation relative to the cranial base. Centers of rotation are used to determine several parameters of growth generated by a computer programmed to show growth effects. The direction and amount of condylar growth are held constant. With condylar growth constant, various centers of rotation of the mandible reveal that maximum Class II molar correction is present when the condyle is vertically located farthest from the molar. Of lesser importance, Class II corrections are greater when the condyle is anteroposteriorly closest to the molar.