A novel and simple design of the closing loop producing optimal force magnitude and moment-to-force ratio for en-masse retraction of anterior teeth

ABSTRACT Purpose: The aim of this study was to clarify the mechanical conditions of loop activation that produce the optimal force system allowing controlled movement of the anterior teeth. Materials and Methods: The closing loop examined in this study was a 10-mm-high teardrop type bent from a 0.017 × 0.025-inch stainless steel archwire. A loop design that can generate a high moment-to-force (M/F) ratio and the optimal force magnitude was investigated by varying the rate of the cross-sectional reduction of the loop apex, the amount of loop activation and the degree of gable bend. Forces and moments acting on closing loops were calculated using structural analysis based on the tangent stiffness method. Results: A force magnitude of 302 g and an M/F ratio higher than 10 (sufficiently high to achieve bodily movement of the anterior teeth) could be produced when the thickness of the cross-section of a 10-mm-high teardrop loop was reduced by half for a distance of 3 mm from the apex and a gable bend of 30° was incorporated into the loop. Conclusions: The optimal force magnitude and M/F ratio for achieving controlled tipping or bodily movement of the anterior teeth can be produced by partially reducing the thickness of the teardrop loop, and by placing a gable bend of 20° to 30° in the 0.018-inch bracket slot system.