Second molar intrusion: Continuous arch or loop mechanics?

Abstract

Our aim was to compare the load systems produced by rectangular loops and continuous arch mechanics on intrusion of a maxillary secondmolar. An orthodontic appliance was passively bonded from secondmolar to secondmolar on a patient's model fixed to an orthodontic force tester. The second molars were extruded by 3mm, and 2 mechanics systems were compared: beta-titanium rectangular loops and super-elastic nickel-titanium continuous arch. Vertical (Fz) and mesial (Fy) forces, and tipping moments around the buccal axis (Mx) at the tubes were collected at every 0.5mm of deactivation. Repeated measures analysis of variance was used to detect differences between mechanics. The net Mx was also estimated at the center of resistance of each molar. Significant differences were found between the mechanics at the tubes, except for the Mx in the first molar, and more importantly, at the centers of resistance of the molars: Fz forces were greater in the continuous arch than in the rectangular loops, which, on the other hand, produced lower Fy forces than the continuous arch. In the secondmolar, net Mx was low initially and changed with deactivation. In the continuous arch, net Mx decreased from -3.6N.mm of mesial crown tipping to clinically insignificant levels after 2mm of deactivation. In the first molar, net Mx was lower on the rectangular loop than on the continuous arch during the first 2mm of deactivation and became similar in the last millimeter of deactivation. Additionally, the net Mx decreased with deactivation with both mechanics. Both mechanics can intrude the secondmolars. Although the rectangular loop produced less tipping in large deflections, the continuous arch was a better approach when less intrusion was required.