M/F ratios of four different closing loops: 3D analysis using the finite element method (FEM).

Background: It has been claimed that the opus loop is capable of delivering a constant and optimum M/F ratio without the need for gable bends.

Objective: To compare the forces, moments and moment/force (M/F) ratios of the opus loop, L-loop, T-loop and vertical helical closing loop (VHC loop) in a segmented arch with the finite element method (FEM).

Methods: The FEM was used to compare 3D models of closing loops in rectangular (0.016 x 0.022 inch) stainless steel wire. The L-, T- and VHC loops were designed with and without preactivation bends. The opus loop had no preactivation bends. The T-Loops were 10 mm in height and 10 mm in length. The horizontal and vertical forces, the moments and the M/F ratios at the alpha and beta ends were recorded at 0, 0. 1, 0.4, 0.7 and 1 mm intervals.

Results: The highest horizontal and vertical forces were produced by the L-loop (with and without preactivation bends) and in most cases the lowest forces were produced by the VHC loop. Loops with preactivation bends produced marked changes in the M/F ratio and loops without preactivation bends low, but relatively constant, M/F ratios over the full range of activation. Of the loops modelled without preactivation bends the opus and T-loop had the highest M/F ratios (7.20 - 7.67 mm at the anterior ends).

Conclusion: Stainless steel opus and T-loops without preactivation bends had constant M/F ratios, but both loops failed to deliver the optimum M/F ratio of 10:1.