Continuous arch and rectangular loops for the correction of consistent and inconsistent load systems in extruded and tipped maxillary second molars

Abstract

The aim of this research was to compare the load systems produced by rectangular loops and continuous arches for the correction of extruded secondmolars with a mesial inclination (inconsistent system) and a distal inclination (consistent system). The maxillary first molar of an acrylic model of a patient, with passive brackets and tubes bonded, was connected to a 3-dimensional load cell of an orthodontic force tester, and the secondmolar was replaced by its respective tube bonded to a second load cell. The second molar tube was moved 2.5mm occlusally and tipped 20° mesially and distally, creating an inconsistent force system and a consistent force system. For each situation, ten 0.017×0.025-in beta-titanium, 8×10-mm rectangular loops were compared with 10 0.014-in nickel-titanium continuous arches. The vertical forces-F(z)-and tipping moments-M(x)-were compared using 4t tests, at 5%. In the inconsistent group, the rectangular loop produced a larger M(x) in both molars: 2.11N.mm in the secondmolar compared with the -0.15N.mm of the continuous arches. On the first molar, the rectangular loops produced -5.58N.mm against -2.08N.mm produced by the continuous arches. The F(z) values produced at the secondmolar with each system were similar, whereas on the first molar they were different; the rectangular loops produced 0.41N, and continuous arches produced 0.53N. In the consistent group, the rectangular loops produced smaller M(x) values at the secondmolar (-3.06N.mm) than did the continuous arch (-4.25N.mm)(P=0.01), as well as a smaller F(z) value (-0.52 vs -0.92N, respectively). At the first molar, the rectangular loops produced smaller M(x) values (-2.32N.mm) than did the continuous arch (-4.18N.mm), as well as a smaller F(z) value (0.59 vs 1.10N). In the inconsistent group, only the rectangular loop produced a system of force that could correct the secondmolar. In the consistent system, both group mechanics produced a system of force compatible with the correction of the secondmolar, but the continuous wire produced larger moments. Both groups showed a tendency for mesial crown tipping of the first molar.