[A pseudoelastic NiTi uprighting spring for the molars--its design, biomechanical testing and clinical use]

Uprighting inclined molars is one of the most common problems encountered in pre-restorative orthodontics. To prevent occlusal trauma, an intrusive force has to be applied to the molar in addition to the uprighting moment. Owing to their construction, current mechanical devices for uprighting either to not meet this requirement, or are difficult to adjust when in place. For this reason, an improved uprighting spring is described which utilizes the properties specific to super-elastic (pseudo-elastic) NiTi alloys. The most important property of super-elastic wires is the fact that they produce constant forces or moments within a specific deformation range. In order to utilize this useful property, certain design criteria have to be met. Recent measurements have shown that a super-elastic wire (Sentalloy, 0.016'' x 0.022'') having a length of 10 mm generates a constant moment of 7 to 8 Nmm within a bending angle of 50 degrees to 180 degrees. On the basis of these results, a table that permits the determination of the proper wire length needed to provide a constant moment within a given range of bending angles is proposed. The superelastic uprighting spring described here comprises an NiTi wire segment having a length of 7 mm and a mesial and distal steel wire segment. In the active state, the spring generates an uprighting moment of 8 Nmm and an intrusive force of 0.6 N. Numerical analysis using the finite element program, SOLF/MESY, and biomechanical testing with the orthodontic measuring and simulation system (OMSS) have shown that this force system remains stable throughout the entire uprighting process. The clinical application of the spring is demonstrated in a specific case.