Testing force systems and biomechanics--measured tooth movements from differential moment closing loops

Orthodontic tooth movement can be compared to a stimulus-response model, where the stimulus is the applied force system and the response is the resulting tooth movement. Although the principles of mechanics have been applied to orthodontic appliance design, the expression of treatment responses to the force systems is less well known. The purpose of this study was to compare measured tooth movements with the theoretical force system exerted by differential moment closing loops. Sixteen subjects requiring maximum posterior anchorage control were selected to participate in this prospective investigation. T-loop springs designed to deliver a differential moment-to-force ratio to the posterior vs the anterior teeth were used. Initial cephalometric radiographs were taken with special devices attached to the molar and canine teeth to allow precise identification. Immediately after the radiograph, the T-loop archwires were inserted and activated. After an observation period of approximately 90 days, the wires were removed, devices reinserted into the molars and canines, and a second cephalometric radiograph was obtained. Superimposition techniques were used to compare the actual tooth movements. The results showed tooth movements consistent with the prescribed force system. The anterior teeth, as represented by the canines, were retracted an average of 1.73 mm, whereas the posterior anchorage (molars) moved mesially only 0.50 mm. Furthermore, the canine teeth exhibited tipping or translation, and the molars showed mesial root movement. The variability of the treatment response as a function of the stimulus (appliance design), response (biological variation), and measurement technique was described.