T-loop force system with and without vertical step using finite element analysis.

OBJECTIVE: To investigate the effect of vertical steps on a T-loop force system at three interbracket distances (IBDs) and their association with V-bends. MATERIALS AND METHODS: Loop response during simulated loop pulling was determined for 18 T-loop configurations (6-, 9-, and 12-mm IBD with a 2.5-mm canine bracket (CB) end and 0- (plain), 0.5-, or 1-mm vertical step). Loop length-by-height was 8 x 8 or 10 x 10 mm. Horizontal load/deflection, vertical force (Fy), and moment-to-force (M/F) ratios at loop ends were determined for 100-g and 200-g activation by finite element analysis. RESULTS: Plain, 12-mm IBD T-loops showed similar force and moment responses as off-centered V-bends (greater moment close to V-bend) without change in moment direction at the premolar bracket (PB) end; plain, 6-mm IBD T-loop responses were similar to those of centered V-bends (equal, opposing moments at each end). Adding vertical steps to the T-loops raised the M/F ratio at the PB ends enough to produce root movement, while lowering the M/F ratios at the CB ends. Increasing the step bends for shorter IBDs increased Fys and caused rapid changes in M/F ratios. Unlike plain T-loops, increasing activation in stepped T-loops caused substantial variations in M/F ratios and in amount and direction of Fys. CONCLUSIONS: Step bends can dramatically change the force system. Stepped T-loops display combined effects of V-bends and step bends.