A new application of thermal active nickel titanium alloys: Canine retraction springs

Research objective. To design and fabricate "shape-memorized" canine retraction springs made of Nickel-Titanium (NiTi) alloys for orthodontic canine retraction.; Methods. Two prototypes of springs, the modified Delta-loop and T-loop, were first designed in a simulation computer program Loop RTM (Dr. Halazonetis, Athens, Hellas, Greece). Thirty springs, fifteen of each prototype, were fabricated by an innovative hand-made method with preformed 017 inch x 025 inch 35°C copper NiTi wires (Ormco, Glendora, CA, USA). A force-moment measuring apparatus (OrthoMeasureRTM, Young R&D, Avon, CT, USA) was used to test the horizontal unloading forces and moment to forces ratios of springs during a 7-mm unloading from the interbracket distance of 22 to 15 mm under the ambient temperature of 38 +/- 1°C. The resultant forces and moment to force ratios of the two springs were then compared and analyzed.; Results. (1) The resultant unloading forces of both springs were suitable for canine retraction. Delta-loop springs generated significantly higher unloading forces (150--260 grams) than those produced by T-loop springs (50--210 grams), but a lower force-deflection rate (16.2 grams/mm) than that of T-loop springs (5--210 grams). (2) The moment to force ratios produced from both springs, range from 6.9 to 8.2 mm for Delta-loops and 7.1 to 14.1 mm for T-loops, will mainly retract canines via controlled tipping for Delta-loops, and via controlled tipping and then translation for T-loop springs. The differential moment to force ratios between canines and molars in both springs implied a force system of moderate anchorage control without unwanted vertical forces. (3) The forces and moment to force ratios predicted by the software LoopRTM were 10 to 30 per cent higher than those of the experimental data.; Conclusion. (1) Both thermal active NiTi springs can produce desirable forces, low force-deflection rates, and moderate anchorage control for canine retraction. However, the moment to force ratios generated by both springs were not sufficient enough to produce canine bodily movement along space closure. (2) The forces and moment to force ratios predicted by the software LoopRTM were 10 to 30 per cent higher than those of the experimental data. (3) The advantageous clinical aspect about these springs is what so called "smart springs for dummies". Since all the forces, anti-tipping moment to force ratios and anchorage control are pre-programmed in these shape-memorized NiTi springs, there is no problem such as technique sensitivities for loop activation as in the frictionless mechanics. Moreover, NiTi retraction springs can deliver low declining forces and predictable moment to force ratios over a wide range of activation without loop activation. Therefore, doctors' chair time, patient discomfort, and appointment frequency can be reduced significantly.