Biomechanical design and clinical evaluation of a new canine-retraction spring.

Use of the sectional arch technique facilitates the creation of an optimal force system fulfilling the biomechanical requirements imperative for planned tooth movements. Controlled canine retraction, usually in extraction cases, requires the creation of a biomechanical system to deliver a predetermined force and a relatively constant moment-to-force ratio in order to avoid distal tipping and rotation. The responsive couple delivered to the anchorage unit should be adjusted in such a way that no single tooth is subjected to unwanted side effects and that undesirable changes in the occlusal plane are avoided. On the basis of a series of theoretical considerations described in the present report, a canine-retraction spring was constructed from 0.016 X 0.022 inch stainless steel wire, the principal element being a double ovoid loop 10 mm in height. A "sweep" bend was incorporated to avoid unwanted side effects at the second premolar. Load deflection and moment/force curves were derived experimentally and demonstrate the ability of the spring to generate and maintain biomechanical conditions necessary for optimal canine retraction (that is, load deflection = 45 gm per millimeter of activation, antitip moment/force ratio of approximately 11:1, and antirotation moment/force ratio of approximately 7:1). The clinical applicability of the spring is demonstrated in the present report by the presentation of two treated cases.