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

Abstract

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.
Key words: Hydrochloric acid, orthodontics, recycling, sodium hypochlorite