When is the center of rotation and center of resistance coincident?

In orthodontics, understanding the concepts of the center of rotation and the center of resistance is crucial for effective tooth movement. The center of resistance is a point where a force can be applied without producing translational movement, while the center of rotation is the point around which an object rotates.

When rotational movement is produced by a couple, it refers to a situation where equal and opposite forces are applied at two distinct points, creating rotation around a center.

In this scenario, the center of rotation and the center of resistance coincide because the application of forces creates a balanced moment about a central point, allowing for predictable rotational movement without unintended consequences. This condition simplifies the mechanics of tooth movement by ensuring that the center of rotation aligns with the center of resistance, thus allowing for controlled movement of the tooth as intended in orthodontic treatment plans.

In contrast, other options such as controlled tipping, uncontrolled tipping, or translation describe situations where the center of rotation and center of resistance do not coincide, leading to different types of tooth movement that may involve complex mechanics and less predictable outcomes.