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.
Material and Methods
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.
Coil springs are widely used in fixed orthodontic treatment. Closed-coil springs and open-coil springs differ not only in design but also in their clinical application. Open-coil springs are designed to deliver an expansion force as indicated in space opening or distalization of molars, whereas the closed-coil springs are intended to deliver compressive forces, for example, as in the retraction of the canines and space-closing mechanics. In addition, superelastic nickel titanium (NiTi) springs must be distinguished from conventional nonsuperelastic springs. Conventional coil springs, which are mostly constructed in stainless steel, display a linear plot in the force deflection graph, whereas superelastic NiTi springs show a typical force plateau. This plateau is due to a structural lattice shift from austenite to martensite in the activation curve and vice versa in the deactivation curve. The deactivation curve is relevant to orthodontics, as it represents the condition encountered clinically. The energy that was stored in the lattice by transforming austenite to stress-induced martensite is continuously released during deactivation and leads to the maintenance of the force level even though the spring is being deactivated. This phenomenon results in the typical force plateau.
Although different authors have investigated the mechanical characteristics of traction springs used, for example, in the retraction of canines, no such investigations exist for open-coil springs. Although, not proved, it is well accepted that various applications of open-coil springs require different force levels. The opening of space for anterior crowding will require a force inferior to the force needed for distalization of molars. Up until now, the clinician has relied on subjective assessment in estimating the force applied by an open-coil spring. It was the aim of the present investigation to give an overview on 23 currently available NiTi open-coil springs.
Material and methods
Five specimens each of 23 NiTi open-coil springs ( Table I ) were subjected to mechanical testing. The experimental setup consisted of a stainless steel cylinder with a hole of 0.8-mm diameter drilled through it. A 0.019-in × 0.025-in stainless steel wire was attached to the force sensor (Instron static load cell 100 N, calibrated according to ISO 7500-1 [Instron Corp., Wilmington, De]). The coil springs were cut to a length of 15 mm and threaded onto the 0.019-in × 0.025-in archwire. The wire was passively inserted into the hole in the cylinder, with the spring lying between the cylinder and the force sensor ( Fig 1 ). With the aid of an Instron 3344 (Instron Corp) the exact length of the springs at a compressive force of 0.05 N was defined. The springs were than compressed with a crosshead speed of 10 mm per minute to 75% and to 50% of the length measured at the above-mentioned force level. The resulting forces were registered using an interface to a personal computer and further analyzed with Microsoft Office Excel (Microsoft, Redmond, Wash). A force-deflection diagram was created for each spring, and force levels 0.5 mm from the start and end point of the activation as well as at the midpoint of the activation were registered. All measurements were performed in a thermal chamber at 36°C. The temperature was controlled by a Julabo FS-18 thermostat (Julabo, Seelbach, Germany) within a range of ±1°C. The force level at half the intended amount of compression (midpoint force) defined the force level reported for each spring type, whereas the forces 0.5 mm from the start and end point of the deactivation curve were used to calculate the ratio of variance by the following formula:
r a t i o o f var i a n c e = m i d p o int f o r c e 0.5 m m s t a r t f o r c e − 0.5 m m e n d f o r c e