Two-couple orthodontic appliance systems: transpalatal arches.

The transpalatal arch (TPA) can be activated to deliver a clinically useful array of forces and couples to move and/or rotate maxillary molars in all three planes of space. Changing the palatal arch form can produce expansion or constriction of intermolar width and activation of the inserts of the TPA will produce couples at the molar sheaths. Activations of the inserts to generate couples is possible in all three planes of space in the form of symmetrical V-bends, asymmetrical V-bends, and step bends. Although the associated equilibrium forces of a couple can be the source of clinical surprises, they can also be harnessed to produce favorable tooth movements.