Influence of arch shape on the transverse effects of transpalatal arches of the Goshgarian type during application of buccal root torque.

Previous studies reveal that the application of buccal root torque to upper molars by means of a transpalatal arch without appropriate adjustments of the arch width can produce, initially, an adverse buccal tipping of the molar crowns. Clinical observation suggests that the amount of this buccal crown tipping depends on the arch form of the palatal bar. In order to determine the influence of the arch width and height, an in vitro experiment was performed on the forces and moments produced by transpalatal arches of the Golshgarian type when symmetrical buccal root torque was applied. The torque moments and expansive lateral forces developed by nine different arch forms of varying height and width were measured and the corresponding moment-to-force ratios were calculated at three different torque levels. It was found that the moment-to-force ratio acting on the upper molars depended mainly on the amount of torque applied and the arch height. This indicates that when low palatal bars are used, the application of buccal root torque of clinically relevant amounts leads to an initial buccal crown tipping, whereas the same amounts of torque, when applied by means of high arches, bring about an initial buccal root tipping. The clinical implications of these findings are discussed.