The effect of cervical headgear on patients with high or low mandibular plane angles and the "myth" of posterior mandibular rotation.

The purpose of this retrospective survey was to compare the magnitude of posterior mandibular rotation during orthodontic treatment with edgewise appliances and cervical headgear in patients with high or low Frankfort-mandibular plane angles (FMA). The files of a private orthodontic practice were searched, and the records of growing patients with Class II Division 1 malocclusion were selected. These were divided into 2 groups, according to FMA: the hypodivergent (FMA < 22 degrees ) or low-angle group consisted of 29 patients with a median FMA of 19.80 degrees, and the hyperdivergent (FMA > 28 degrees ) or high-angle group consisted of 31 patients with a median FMA of 32.70 degrees. All patients had nonextraction treatment with full edgewise appliances (Roth prescription), including second molars, by the same clinician. Cervical headgear and Class II elastics were used, and several patients also had fixed or removable anterior biteplanes for short periods. Pretreatment and posttreatment cephalometric tracings were superimposed on internal basic structures of the mandible. There was no difference in FMA changes between the 2 groups, nor were there statistically significant differences in changes during treatment, with 2 exceptions: the angle SN-GoGn showed a very small (0.86 degrees ) mean differential change between groups, generated mostly by the counterclockwise mandibular rotation of the low-angle patients. This was verified with structural superimpositions to evaluate vertical changes. In addition, posterior face height increase was significantly greater in the low-angle group. Structural superimposition of the mandible after treatment showed marked counterclockwise rotations in relation to the anterior base of the skull in both groups, with the high-angle group rotating significantly less. On average, growth and treatment resulted in improvements in the high-angle patients but aggravated the problems in the low-angle patients with deep bite malocclusions.