Effects of reversing the coiling direction on the force-deflection characteristics of nickel-titanium closed-coil springs

Abstract

Objective: To investigate the effects of reversing the coiling direction of nickeltitanium closed-coil springs (NiTi-CCSs) on the force-deflection characteristics.

Methods: The samples consisted of two commercially available conventional NiTi-CCS groups and two reverse-wound NiTi-CCS groups (Ormco-Conventional vs. Ormco-Reverse; GAC-Conventional vs. GAC-Reverse; n = 20 per group). The reverse-wound NiTi-CCSs were directly made from the corresponding conventional NiTi-CCSs by reversing the coiling direction. Tensile tests were performed for each group in a temperature-controlled acrylic chamber (37 ± 1℃). After measuring the force level, the range of the deactivation force plateau (DFP) and the amount of mechanical hysteresis (MH), statistical analyses were performed.

Results: The Ormco-Reverse group exhibited a significant shift of the DFP end point toward the origin point (2.3 to 0.6 mm), an increase in the force level (1.2 to 1.3 N) and amount of MH (1.0 to 1.5 N) compared to the Ormco-Conventional group (all p < 0.001), which indicated that force could be constantly maintained until the end of the deactivation curve. In contrast, the GAC-Reverse group exhibited a significant shift of the DFP-end point away from the origin point (0.2 to 3.3 mm), a decrease in the force level (1.1 to 0.9 N) and amount of MH (0.6 to 0.4 N) compared to the GAC-Conventional group (all p < 0.001), which may hinder the maintenance of force until the end of the deactivation curve.

Conclusions: The two commercially available NiTi-CCS groups exhibited different patterns of change in the force-deflection characteristics when the coiling direction was reversed.

Keywords: Force-deflection characteristics; Nickel-titanium closed-coil spring; Range of the force plateau; Reversing the coiling direction.

Figure 1. Force-deflection characteristics of a nickel-titanium closed-coil spring. As shown, the deactivation force decreases rapidly (bold solid line) after the end point of the deactivation force plateau (DFP). However, if the end point of the DFP is extended, the amount of deactivation force can be maintained until the spring is almost fully deactivated (bold dashed line).

Figure 2. Procedure used to reverse the coiling direction of nickel-titanium closed-coil springs (NiTi-CCSs). A, Depiction of the reverse coiling procedure. The coiling direction is reversed by changing the order of the coil elements one by one. As a result, the order of “one-white-dot to five-white-dots” is changed into the order of “five-white-dots to one-whitedot.” B, The NiTi-CCS reverse coiling procedure used in this study.

Figure 3. Nickel-titanium closed-coil spring (NiTi-CCS) groups investigated in this study.

Figure 4. Variables used in this study.

A, a: Deactivation force plateau (DFP)-start point (mm); b: DFP-end point (mm); c: DFP-range (mm), i.e., the distance from a to b; B, d: force level at the midpoint (at 7 mm) of the activation curve (N); e: force level at the midpoint (at 7 mm) of the deactivation curve (N); f: amount of mechanical hysteresis (N), i.e., the difference in the force level between d and e.

Figure 5. Comparison of the force-deflection curves between the Ormco-Conventional and Ormco-Reverse groups.

Figure 6. Comparison of the force-deflection curves between the GAC-Conventional and GAC-Reverse groups.