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

The Effects of Temperature and Time of Heat Treatment on Thermo-Mechanical Properties of Custom-Made NiTi Orthodontic Closed Coil Springs

Nickel-Titanium (NiTi) springs have been increasingly used in orthodontics; however, no optimum condition of heat treatment has been reported. Therefore, this research was conducted to determine the optimum heat-treatment temperature and duration for the fabrication of NiTi-closed coil springs by investigating their effects on thermo-mechanical properties. As-drawn straight NiTi wires of 0.2 mm diameter were used to fabricate closed coil springs of 0.9 mm lumen diameter. The springs were heat-treated at three different temperatures (400, 450, and 500 °C) with three different durations (20, 40, and 60 min). Electron Probe Micro-Analysis (EPMA) and Differential Scanning Calorimetry (DSC) were used to investigate element composition and thermo-mechanical properties, respectively. Custom-made NiTi closed coil springs composed of 49.41%-Ti and 50.57%-Ni by atomic weight, where their DSC curves of 500 °C presented the obvious endothermic and exothermic peaks, and the austenite finish temperature (A_f) were approximately 25 °C. With increasing temperature, deactivation curves presented decreased plateau slopes generating higher superelastic ratios (SE ratios). At 500 °C, closed coil springs showed superelastic tendency with lower stress hysteresis. The thermo-mechanical properties were significantly influenced by heat-treatment temperature rather than duration. The optimum parameter appeared to be 500 °C for 40 min to produce appropriate force delivery levels, relatively low plateau slope, and lower hysteresis for orthodontic use.

Effect of immersion in hydrochloric acid and sodium hypochlorite and autoclave sterilization on the force characteristics of orthodontic nickel-titanium open coils

BACKGROUND

Nickel-titanium (NiTi) open coils are common tools in fixed orthodontic treatments to apply light and continuous forces. Their favorable mechanical properties and their relatively high cost have prompted many clinicians to decide to reuse them.

AIM

The present study aimed to evaluate the effect of 10% hydrochloric acid (HCl) and 5.25% sodium hypochlorite (NaClO) solution and sterilization, on the unloading force of NiTi open-coils.

MATERIALS AND METHODS

This experimental in vitro study consisted of 160 NiTi open coils from two brands of Highland Metals and Masel were provided. Each group was randomly divided into four subgroups: control, three rounds of immersion, three rounds of autoclave sterilization, and three rounds of immersion and autoclaving. Each round of immersion process included 1 min in 10% HCl, followed by 5 min in 5.25% NaClO. Spring was compressed for 4 mm. Then, the unloading forces of each spring were recorded at 4, 3, 2, and 1 mm of compression, respectively. The data were analyzed with SPSS (P < 0.05), using one-way ANOVA and independent t-test.

RESULTS

One-way ANOVA showed a significant difference in the 2-mm compression of Group 1 and 3-mm compressions of Groups 1 and 2 compared to the control group in the Highland metals brand (P < 0.04, P < 0.014, and P < 0.007, respectively). There was no significant difference between the different compressed lengths and groups in the Masel brand.

CONCLUSION

One-minute immersion in 10% HCL and 5-min immersion in 5.25% NaOCl, followed by autoclave sterilization, even after repeating the process three times, did not significantly change the force properties of NiTi open springs. Therefore, the re-use of these springs with the above-mentioned method as preparation can be recommended.

Evaluation of the force degradation and deformation of the open-closed and open springs of NiTi: An in vitro study

OBJECTIVE

The aim of this study was to evaluate the force degradation and deformation over time of an open-closed and open coil spring.

MATERIAL AND METHODS

40 NiTi springs were divided into 2 groups according to the manufacturer (20 specimens per group): Morelli™ (Sorocaba, SP, Brazil) and Orthopli™ (Philadelphia, PA, USA). Then, they were subdivided into 2 groups according to the type of spring (n=10): open spring and open-closed spring. The springs were submitted to the initial compression test in a Universal Test Machine (Instron) in 43.3% of the initial length and analyzed in 3 points (0.5mm, 3.25mm and 6.5mm). After this, the springs were activated with a 240 gf and those maintained for 4 weeks in artificial saliva in the oven at 37°C, and analyzed by a new compression test with the same initial parameters. Scanning Electron Microscopy/Energy Dispersive X-Ray Spectroscopy (SEM/EDS) analyzed the spring's morphology. Two-way repeated measures Analysis of Variance was applied for each brand and extension of compression. Student t-test with correction of Bonferroni was used to compare open spring vs open-closed springs and pairwise t-test was used to compare initial vs final period. The level of significance was set at 95% in all tests. The most representative images were selected (SEM/EDS).

RESULTS

The Orthopli™ open-closed spring showed a statistically higher deformation (14.52±0.37) in relation to open spring (14.85±0.19) after 4 weeks (P<0.05). No statistical difference was observed between the types of Morelli™ springs (P>0.05). Orthopli™ open-closed spring showed force values statistically higher than the open spring in the initial and final time (P<0.05). Regardless of the type of spring, the initial force was significantly higher than the final force (P<0.05).

CONCLUSIONS

The orthodontist should not rely on the indicated force range without considering the type of spring (open or open-closed), the manufacturer and the amount of compression of the spring.