Changes in the Mechanical Properties of Nickel–Titanium Orthodontic Archwires After Clinical Use with Conventional and Self-Ligating Brackets

Background/Objectives: Changes in the mechanical behavior of orthodontic archwires during clinical use are not fully understood, particularly when different bracket systems are employed. Self-ligating (SL) brackets have gained considerable popularity in orthodontic practice in recent years, largely due to claims of improved treatment efficiency and biomechanical performance. Nevertheless, current evidence has not consistently demonstrated statistically significant differences between conventional ligation (CL) brackets and SL systems. The aim of this study was to evaluate changes in the mechanical properties and degradation over time of nickel-titanium (NiTi) archwires after clinical use in orthodontic treatments performed with CL and SL brackets. Methods: A comparative study was conducted using archwires retrieved from orthodontic patients. Round 0.014-inch NiTi wires (GC Orthodontics America Inc., IL, USA) were analyzed. The archwires were used in 60 patients treated with either CL or SL appliances and evaluated at four time points: before clinical use (T0), and after 1 month (T1), 2 months (T2), and 3 months (T3) of intraoral service. Mechanical testing was performed according to ISO 15841:2014 + Amd. 1:2020 using a three-point bending test with a universal testing machine (Z005 Test Control II Universal Testing Machine, Zwick Roell, Kennesaw, GA, USA). The variables analyzed included the mean force delivered by the archwires at deflections of 3 mm (F3), 2 mm (F2), 1 mm (F1), and 0.5 mm (F0.5), as well as the slope of the superelastic plateau at 2 mm, 1 mm, and 0.5 mm. The static and dynamic friction coefficients, as well as the friction forces associated with the wires and the two types of brackets, were determined using a modified MTS-Bionix servo-hydraulic testing machine. The tests were conducted at 37 °C in a saline environment. Results: Both groups showed changes in the superelastic behavior of NiTi archwires. Alterations increased with longer intraoral exposure. In the SL group, significant modifications were already observed after one month of clinical use, with a reduction in the force delivered and a loss of superelastic characteristics. These changes remained relatively stable thereafter, with no statistically significant differences during the following months. In contrast, the CL group showed a progressive reduction in force delivery and superelasticity over time. This is due to the difference in friction between the wire and the CL bracket compared to the SL bracket, which results in greater force transfer for tooth movement. Conclusions: Overall, differences in the mechanical behavior of archwires between CL and SL systems were observed during the initial stages of clinical use. However, these differences diminished over time, and no significant differences were detected after three months. Considering the progressive degradation of mechanical properties, the reuse of archwires that have remained intraorally for more than three months may not be advisable.