In-vivo force decay of nickel-titanium closed-coil springs

Abstract

Introduction Nickel-titanium (NiTi) closed coil springs are purported to deliver constant forces over extended ranges of activation and working times. In vivo studies supporting this claim are limited. The objective of this study is to evaluate changes in force decay properties of NiTi closed coil springs after clinical use. Methods Pseudoelastic force-deflection curves for 30 NiTi coil springs (used intra-orally) and 15 matched laboratory control springs (simulated intra-oral conditions - artificial saliva, 37°C) were tested pre- and post-retrieval via Dynamic Mechanical Analysis (DMA) and the Instron machine, respectively, to evaluate amount of force loss and hysteresis change following 4, 8, or 12 weeks of working time (n=10 per group). Effect of the oral environment and clinical use on force properties were evaluated by comparing in vivo and in vitro data. Results The springs studied showed a statistically significant decrease in force (~12%) following 4 weeks of clinical use (p<0.01), with a further significant decrease (~7%) from 4–8 weeks (p=0.03) and force levels appearing to remain steady thereafter. Clinical space closure at an average rate of 0.91mm per month was still observed despite this decrease in force. In vivo and in vitro force loss data were not statistically different. Conclusions NiTi closed coil springs do not deliver constant forces when used intra-orally, but they still allow for space closure rates of ~1mm/month.

Force degradation of nickel-titanium closed coil springs: an in vitro

“…This information agrees with other study in which force reductions of 8 to 20% were reported after 28 days comparing stainless steel, CoCrNi and NiTi coil springs (Angolkar et al, 1992). The same was observed in other in vitro and in vivo study in which the coil springs (GAC Sentalloy) lost approximately 12% of the initial strength in 4 weeks, with an additional loss of 7% between 4 and 8 weeks, when the strength levels apparently stabilized (Cox et., 2014). It should be highlighted that these authors found similar results of force degradation, regardless of whether the coil springs were tested in laboratories or used in patients.…”

“…In a systematic review, the authors reported that closing spaces with closed NiTi coil springs is faster than with elastomeric chains, and that the ideal force to close these spaces with coil springs is 150 to 200 grams, similar to the values indicated by the manufacturer on the packages in the present study (Mohammed, et al,2018). The rate of space closure is 0.21mm/week for chain elastics and 0.26mm/week for NiTi springs (Nightingale & Jones, 2003) yet other authors stated that the space closure rate with NiTi springs is around 1mm per month (Cox, et al,2014 ;Geng, et al, 2019). Conversely, the degradation of forces on elastics varies from 37.4% to 71.6% according to the commercial brand and is much greater than the means of NiTi springs (Santos, et al, 2007) .…”

The effect of orthodontic clinical use on the mechanical characteristics of nickel-titanium closed-coil springs

“…The manufacturer of the springs (Dentsply GAC International, Islandia, NY, USA) suggested that the stretch range was ‘up to 15 mm activation’. In a previous study, many of the springs reached the full 12 mm activation range proposed by the manufacturer when stretched to 300 g. 7 These tests were run with a single frequency of 0.1 Hz, which typically yields results equal to traditional mechanical testing.…”

“…The sample size was determined using pilot data from a previous study. 7 Based on the pilot data, a minimum of nine samples (springs) for each time-point and 36 samples in total for four time-points were required to detect a maximum force loss of 30% with 90% power. Ten samples per group were expected to detect the difference.…”

“…19 On deactivation, NiTi springs have an initial non-linear force loss followed by a relatively constant linear region (CLR) of force during the martensitic to austenitic transformation (Figure 1b). 7,8,12,13 This CLR can provide a relatively constant force for tooth movement. Unfortunately, the force loss of CLR has been found to increase over time, similar to stress relaxation in elastomers.…”