Activating a 2 x 4 appliance

When the long span of wire in a 2 x 4 orthodontic appliance enters a bracket in a nonparallel manner, it develops a couple and tendency for rotation called a moment. The moment of a couple creates inherent equal and opposite Newtonian equilibrium forces not readily sensed clinically. Moments at successive brackets, producing rotations in opposite directions, create equilibrium forces also in opposite directions which are subtractive from each other. When these equilibrium forces are equal and opposite, they cancel each other out. If the moments produce rotations in the same direction, the equilibrium forces are also in the same direction and are additive. The apparent simplicity of the 2 x 4 appliance conceals the fact that it is a powerful orthodontic tool that uses engineering mechanics in a way not possible with fully bracketed appliances. Clearly the application of engineering mechanics is a distinguishing characteristics of modern orthodontics.