Force levels of 23 nickel-titanium open-coil springs in compression testing

Open-coil springs are commonly used auxiliaries in fixed orthodontic appliance therapy. Space opening for impacted or heavily crowded teeth as well as distalization of molars all require specific force levels. It is the aim of the current study to present an overview of the mechanical properties of currently available nickel titanium (NiTi) closed coil springs.

Twenty-three NiTi open-coil springs were compressed by 25% and 50% of their original length at a controlled temperature of 36°C. Force deflection diagrams were registered using an Instron 3344 (Instron Corp, Wilmington, De). Five samples of each coil spring were measured and evaluated for their mean force as well as their superelastic characteristics.

Almost all coil springs showed a linear behavior in the force deflection diagram. Only a few open-coil springs (GAC light, medium, and heavy [Dentsply GAC, Bohemia, NY] and RMO 12 × 45 [Rocky Mountain Orthodontics, Denver, Colorado]) showed a superelastic behavior with a clear force plateau, also indicated by their high ratio of variance. The results of the tested open-coil springs allow the clinician to choose springs with mean forces between 0.25 N (3M Unitek light; 3M Unitek, St. Paul, Minn) and 1.3 N (GAC heavy) for a compression of 25% and 0.64 N (3M Unitek light) to 2.9 N (OrthoOrganizers 14 × 37 [OrthoOrganizers, Carlsbad, Calif], Dentaurum Rematitan strong [Dentaurum, Ispringen, Germany]) for a compression of 50%.

Superelastic behavior was rarely observed with open-coil springs. The clinician can therefore not rely on the force range indicated without considering the amount of compression of the coil spring.