Torque control in lingual orthodontics with lever arm mechanics: a case report

The aim of this report is to illustrate treatment mechanics for torque control in lingual mechanotherapy using a lever arm and transpalatal arch (TPA) tab system during en masse retraction of anterior teeth. An 18-year-old female with bimaxillary dentoalveolar proclination with crowding was treated with a lever arm-TPA tab system. The retraction tabs bent into the TPA placed across the maxillary second molars were used as anchorage. The retraction force on the maxillary anterior teeth was applied using lever arm hooks soldered between the lateral incisors and canines on a lingual mushroom archwire. By applying a retraction force to the lever arm hooks, the maxillary anterior teeth experienced greater palatal root movement as compared to the conventional retraction forces applied at the crown level. The tabs, placed high in the TPA, produced a distal tipping moment on the maxillary second molars, reinforcing their anchorage. The retraction force applied to the long lever arm hooks from the TPA tabs at the level of center of resistance (CRes) of anterior and posterior teeth is advantageous mainly in two aspects. First, it reinforces the anchorage, and second, it favors the palatal root movement of anterior teeth, thus obtaining better control over the torque during en masse retraction.