Nickel titanium (NiTi) coil springs are a new development in orthodontics, designed to produce light continuous forces. This study compares the force delivery by NiTi open and closed coil springs during unloading (de-activation) to that provided by comparable stainless steel (SS) springs.

Open-coil springs (0.010×0.035 inch) were compressed from their initial length of 15 mm to 6 mm and the forces generated with spring recovery recorded. Closed-coil springs (0.009×0.035 inch) were distracted from their initial length of 3 mm to 9 mm and the force recorded as the spring recovered.

The closed-coil NiTi springs produced light continuous forces of 75–90 g over the distraction range of 6 mm while the open-coil springs produced forces of 55–70 g within the 9 mm compression range. SS springs produced heavier forces, ca. 200 g, for an activation of 1 mm and the generated force increased rapidly as the activation was increased. The findings indicate that NiTi coil springs deliver optimal forces for orthodontic tooth movement over a longer activation range than comparable SS springs.

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Author notes

J.A. von Fraunhofer is Professor and Director of the Laboratory of Molecular and Materials Science at the School of Dentistry, University of Louisville

P.W. Bonds is in private orthodontic practice in Florence, South Carolina

B.E. Johnson is Associate Professor and Chairman of the Department of Orthodontic, Pediatric and Geriatric Dentistry at the School of Dentistry, University of Louisville