Centers of rotation with transverse forces: an experimental study.

A new noninvasive method for simultaneously applying a known force system and measuring tooth movement was developed and tested. The instruments proved to be highly accurate and reliable. Using a model of an upper canine with a simulated periodontal ligament, we located the center of rotation and the center of resistance and found that the product of two distances--a, from the center of resistance to the line of application of force (F) and, b, from the center of resistance to the center of rotation (Co)--yields a constant that represents the parallel transverse forces in the same plane. Knowing this constant enables the clinician to determine the appropriate force position for a given center of rotation. We also found that the occlusoapical position of the center of resistance varies somewhat, depending on the transverse direction of loading around the long axis. Differences in values of the constant were found also to depend on the direction of loading in a given plane. Thus a transverse force exerted in a mesiodistal direction at the level of an orthodontic bracket through the long axis produces different effects from those produced when an identical force is applied in a labiolingual direction.