What is the main effect of a couple on a tooth?

A couple refers to the application of two equal and opposite forces that create a rotational effect around a center of resistance. In orthodontics, when a couple is applied to a tooth, it does not produce any net translational movement due to the balanced nature of the forces involved. Instead, the primary effect is pure rotation around the center of resistance of the tooth.

This phenomenon occurs because the two forces in a couple are positioned at a distance from one another, creating torque that rotates the tooth without causing it to move linearly through the arch. The center of resistance acts like a pivot point, allowing the tooth to rotate around it as the forces exert their influence. This effect is crucial in orthodontic tooth movement, as it allows for the precise adjustment of tooth angulation and positioning without altering the overall position of neighboring teeth.

While translational movement or sideways movement could occur under different circumstances or with different force systems, when solely considering the impact of a couple, the predominant effect is that of pure rotation around the center of resistance.