Biomechanical analysis of arch-guided molar distalization when employing superelastic NiTi coil springs.

Two typical treatment situations (distalization of a last molar, distalization of a molar with the guiding arch fixed on the most distal abutment tooth) were simulated experimentally using the Orthodontic Measurement and Simulation System (OMSS). For this purpose an upper first molar was guided along steel arches with differing cross sections (0.016" x 0.022"/0.41 mm x 0.56 mm; 0.017" x 0.025"/0.43 mm x 0.64 mm) using a nickel-titanium (NiTi) compression spring. The 6 NiTi springs investigated differed in their design and in the type of force delivery. The force application that was eccentric with respect to the center of resistance caused a conflict situation between the tube and the guiding arch through rotations of the molar. Although the force level of the springs was almost constant, the orthodontically effective forces measured fluctuated due to frictional losses. With progressing distalization the mean frictional forces increased, reaching values between 50 and 80% of the applied force of the compression springs. Simultaneously fluctuations in the frictional losses increased. Lower frictional losses were determined with a 0.016" x 0.022", than with a 0.017" x 0.025" guiding arch. Fixing the arch on the 3rd molar resulted in higher frictional losses compared to the distalization of a last molar. There were intensive interactions of the compression springs with the guiding arch and the tube. This resulted in extreme frictional losses even in the initial phase of the movement and in a complete stop to the distalizing movement in extreme cases. After removal of the convertible, the freedom of the molar movement could be regulated by the tension of the ligature wire. The friction varied accordingly. The distalization rate was not influenced by the force delivered by the compression spring. Small forces are biologically more favorable and thus should be preferred.