Aim: To analysis the effect of unilateral shortening (Asymmetric Length, AL type) vs. expansion of head-gear outer-bow (Asymmetric Expansion, AE type) in delivering unilateral distal force.
Methods: In 36 situations, composed or different outer-bow shortening, expansion and different neck contour and position, the resultant distal force was analyzed. Applying an analysis described by Haack and Wienstein, the ratio by which the resultant distal force was divided between 2 terminal molars was calculated.
Results: In AL type head-gear a greater portion of force was delivered to contra-lateral terminal while in AE type headgear the ratio of force delivery between two molars could not be higher than 1.04 due to geometrical limitations.
Conclusion: AL type head-gear is a predictable and effective way for asymmetric distal force delivery. The greater force will be received by the contra-lateral terminal to the shorter arm of the outer-bow. AE type is not suggested for distal force delivery due to its geometric limitation and relative ineffectiveness.