Mandibular Size, Shape, And Orientation Changes During Mixed Dentition Treatment

Objectives: To quantify changes in the size and shape of the mandible during early orthodontic treatment – particularly to quantify the magnitudes of difference in size, shape and orientation associated with differences in Angle Class and Sex.  Methods:   Retrospective treatment records were sampled randomly in the clinical practices of three experienced orthodontists who employ early mixed dentition treatment for a subset of patients. Comparison samples of patients treated during the same time interval using other treatment strategies were also collected, yielding a total sample approximating 500 patients for ongoing analysis. The present study is limited to a random sub-sample of 112 patients who originally presented between the ages of 7.5 and 9.5 years and received two-phase mixed dentition treatment. Data were collected from lateral cephalograms generated at the Start of Phase 1, Start of Phase 2, and End of Phase 2.  Results:   The Table below reports simple statistics for Age at Timepoint, the distances Condyle-Pogonion, Gonion-Menton, and Condyle-Gonion;  Mandibular Plane Angle and Gonial Angle. Means and standard deviations are reported for All Cases and for differences by Sex and AngleClass for each variable at each time point. On average, the duration of each treatment phase was similar for patients of both AngleClasses, as well as for boys and girls. However, there were statistically significant and clinically meaningful differences between boys and girls in the lengths of the three line segments at all time points except for Mandibular Body Length at the Start of Phase 1. Conclusions: There was no evidence in this study to support the idea that mandibular growth changes in Class II patients differ consequentially from those in Class I patients. There was no evidence that the presumed earlier maturation of girls had a statistically significant impact on the timing or duration of treatment in this sample.