Biomechanical investigation of the hybrid retraction spring.

We succeeded in developing a retraction spring that shows virtually constant retracting forces up to an activation of about 4.5 mm through the use of various non-linear materials. Compared to all other known retraction springs, an actual bodily retraction is possible over a large range for the first time. The clinical application requires no more than one reactivation. The anti-tipping moment is 10 Nmm and is to be considered constant over the entire activation range. This anti-tipping moment produces an extrusive force for the canine and an intrusive force for the molar. This side-effect can be avoided by bending a sweep into the steel portion or compensated by bending a step into the steel portion of this retraction spring. The anti-rotational moment is about 3 to 5 Nmm measured over the entire activation range. In contrast to many other springs, the favorable M/F ratio for the anti-tipping movement allows an actual bodily retraction of canines. Even when the retracting force is no longer active, the moment that moves the root of the canine distally is still acting, so that the spring can also be used for the root movement. The M/F ratio for the anti-rotational movement is between 3 and 5 mm and therefore allows retraction of the canine without causing major distortions. Customary brackets with a .018" or .022" horizontal slot can be used, as the spring is designed for a .018" x .018" vertical slot. Each spring can be used for both the left and the right canines. The steel portion allows second-order and, if desired, third-order bends to be made. The additionally necessary bends have already been described. The hybrid retraction spring can also be applied for en masse retraction of incisors if a cross-tube is used for the anterior area.