Nickel titanium (NiTi) coil springs are a new development in orthodont
ics, designed to produce light continuous forces. This study compares
the force delivery by NiTi open and closed coil springs during unloadi
ng (de-activation) to that provided by comparable stainless steel (SS)
springs. Open-coil springs (0.010x0.035 inch) were compressed from th
eir initial length of 15 mm to 6 mm and the forces generated with spri
ng recovery recorded. Closed-coil springs (0.009x0.035 inch) were dist
racted from their initial length of 3 mm to 9 mm and the force recorde
d 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 comp
ression range. SS springs produced heavier forces, ca. 200 g, for an a
ctivation of 1 mm and the generated force increased rapidly as the act
ivation was increased. The findings indicate that NiTi coil springs de
liver optimal forces for orthodontic tooth movement over a longer acti
vation range than comparable SS springs.