Introduction: Asymmetric headgear is used when different molar distalization forces (MDF) are needed on the right and left sides of the jaw to correct a Class II molar relationship.
Methods: We investigated 2 asymmetric headgear configurations, the outer-bow and the inner-bow, on cervical-pull headgear. In the first configuration, 5 hooks were soldered on 1 side of the outer bow at 10-mm intervals, making this side shorter; in the other, 4 stops (1.5 mm) were added to 1 side of the inner bow, making this side longer. The right and left MDF and the extraoral force (EF) were measured simultaneously with 2 fork transducers and a testing machine, respectively.
Results: A 40-mm difference between the long and short outer bows resulted in a 2.17-fold greater MDF on the long-side molar (7:3 ratio). The 3-4 stop configuration provided the optimal inner-bow arrangement, with stop/no-stop MDF ratios of 7:3 and 10:0, respectively, at 10 N EF. At low-to-medium EF levels, a unilateral MDF developed on the stop side with zero MDF on the no-stop side. The sum of the right and left MDF nearly equaled the EF in the outer-bow asymmetry and was 60% in the inner-bow setting; this suggests strong lateral forces in the latter.
Conclusions: Clinically, for a bilateral unequal Class II relationship, the system of choice is outer-bow asymmetric headgear. For a unilateral Class II relationship with 1 side in a Class I molar relationship (Class II subdivision), inner-bow asymmetric headgear is recommended.