When the moment of force (Mf) equals the moment of couple (Mc) in orthodontics, what type of movement is achieved?

When the moment of force (Mf) is equal to the moment of couple (Mc) in orthodontics, the result is pure translation. This condition leads to the force being applied in such a way that the teeth move linearly along the alveolar bone without any angular change in their position. In this scenario, the teeth do not tip or rotate about their axis, since the balance of moments ensures a straight-line trajectory.

Understanding this phenomenon is crucial in orthodontics as different movements are required for various treatment objectives. Pure translation is particularly desirable in situations where the goal is to move the entire tooth without altering the angulation or inclination of the root within the bone, which can help achieve optimal alignment and occlusion.