Differences between two transpalatal arch systems upon first-, second-, and third-order bending activation.

Transpalatal arches are used in passive mode to improve anchor-age and in activated mode to achieve single tooth movement or movement of segments of teeth in first-, second- and third-order. Clinically it seems that the commonly used palatal arches of the Goshgarian type (here in the MIA system) as well as the precision TMA lingual arches of the Burstone system are not equally suitable for all kinds of activations. Using the Orthodontic Measurement and Simulation System (OMSS) the force systems and the efficacy of activated arches of both systems were examined in an experimental study with respect to different malpositions. The following first-, second- and third-order activations were chosen: symmetrical expansion and compression up to 4 mm, symmetrical distal rotation up to 15 degrees, unilateral distal tipping of 15 degrees and symmetrical buccal root torque up to 10 degrees. Attachments of the MIA system (MIA Rotation Lingual Sheaths, 0.072" x 0.036") and of the Burstone system (precision lingual hinge cap, 0.032" x 0.032", unused and a pair after a 4-week introral application period) were measured. Lingual arches made of 0.036" round stainless steel wire (MIA) and 0.032" x 0.032" TMA (Burstone system) were prepared with a height of 18 mm and a width of 30 mm. First-order activation bends (expansion and compression) of the MIA palatal arches caused forces up to 4.4 N compared to 1.8 N of the TMA arches, due to the lower load/deflection rate of the latter. The malpositions were corrected effectively by both systems. Due to the higher stiffness the moments delivered by the MIA palatal arches (39 Nmm) were higher in distal rotation compared to those of the TMA arches (14 Nmm) and the correction was more effective. In second-order activations (tipping) the MIA system delivered no or only small moments because of the curved shape of the attachments. A correction of only 30% was achieved compared to 80% with the Burstone system. In third-order activations, in contrast, the Burstone attachments caused considerable loss of torque. This was obviously due to the strong deformation of the slot by the intraoral loading. If it were possible to improve the dimensional stability of the hinge cap, all corrections carried out with the Burstone TMA system would involve distinctly smaller forces and moments than the MIA system but would still ensure good effectiveness.