Effect of Fluoride Content of Mouthwashes on Superelastic Properties of NiTi Orthodontic Archwires

Corrosion of Fixed Orthodontic Appliances: Causes, Concerns, and Mitigation Strategies

The orthodontic supply market is a prosperous billion-dollar industry, driven by an increasing demand for orthodontic appliances. The supremacy of metallic first-generation biomaterials is evident for manufacturing brackets, archwires, bands, and other components due to their well-recognized chemical inertness, spontaneous passivation, biocompatibility, and favorable mechanical properties combination. However, the oral cavity is the ultimate corrosion-promoting environment for any metallic material. In this work, the general picture of the intraoral degradation of fixed orthodontic appliances is first addressed, from the causes to the harmful effects and their oral clinical implications. Current mitigation strategies are also pointed out, including the alloys’ bulk composition adjustment combined with new and advanced manufacturing processes and/or their surface treatment or coating deposition. The versatile use of thin films and coatings stands out with different deposition technologies: Many in vivo and in vitro efforts have been devoted to oral aging, from monolithic to composite architectures and micro- to nano-scale materials, to meet the best and safest oral practice demands. Unfortunately, literature data suggest that even the existing commercially available protective coatings have drawbacks and are fallible. Further multidisciplinary research is still required to effectively mitigate the corrosion behavior of fixed orthodontic appliances.

Properties of Superelastic Nickel-Titanium Wires after Clinical Use

The aim of the present study was to describe and determine changes in the superelastic properties of NiTi archwires after clinical use and sterilization. Ten archwires from five different manufacturers (GAC, 3M, ODS, GC, FOR) were cut into two segments and evaluated using a three-point bending test in accordance with ISO 14841:2006. The center of each segment was deflected to 3.1 mm and then unloaded to 0 N to obtain a load-deflection curve. Deflection at the end of the plateau and forces at 3, 2, 1 and 0.5 mm on the unloading curve were recorded. Plateau slopes were calculated at 2, 1 and 0.5 mm of deflection. Data obtained were statistically analyzed to determine differences (p < 0.001). Results showed that the degree of superelasticity and exerted forces differed significantly among brand groups. After three months of clinical use, FOR released a greater force for a longer activation period. GC, EURO and FOR archwires seemed to lose their mechanical properties. GC wires released more force than other brand wires after clinical use. Regarding superelasticity after sterilization, GAC, 3M and FOR wires recovered their properties, while EURO archwires lost more.

Effect of Stabilized Martensite on the Long-Term Performance of Superelastic NiTi Endodontic Files

One of the most used rotary files in endodontics is NiTi files due to their superelastic properties. This property means that this instrument has extraordinary flexion that can adapt to large angles inside the tooth canals. However, these files lose their superelasticity and fracture during use. The aim of this work is to determine the cause of fracture of the endodontic rotary files. For this purpose, 30 NiTi F6 SkyTaper^® files (Komet, Germany) were used. Their chemical composition was determined by X-ray microanalysis, and their microstructure was determined by optical microscopy. Successive drillings were carried out with artificial tooth molds at 30, 45, and 70°. These tests were carried out at a temperature of 37 °C with a constant load controlled by a high sensitivity dynamometer of 5.5 N, and every five cycles were lubricated with an aqueous solution of sodium hypochlorite. The cycles to fracture were determined, and the surfaces were observed by scanning electron microscopy. Transformation (austenite to martensite) and retransformation (martensite to austenite) temperatures and enthalpies were determined by Differential Scanning Calorimeter at different endodontic cycles. The results showed an original austenitic phase with a M_s temperature of 15 °C and A_f of 7 °C. Both temperatures increase with endodontic cycling, indicating that martensite forms at higher temperatures, and the temperature must be increased with cycling to retransform it to austenite. This fact indicates the stabilization of martensite with cycling, which is confirmed by the decrease in both transformation and retransformation enthalpies. The martensite is stabilized in the structure due to defects and does not retransform. This stabilized martensite has no superelasticity and, therefore, fractures prematurely. It has been possible to observe the stabilized martensite by studying the fractography, observing that the mechanism is by fatigue. The results showed that the files fracture earlier the greater the angle applied (for the tests at 70° at 280 s, at 45° at 385 s, and at 30° at 1200 s). As the angle increases, there is an increase in mechanical stress, and, therefore, the martensite stabilizes at lower cycles. To destabilize the martensite, a heat treatment can be carried out at 500 °C for 20 min, and the files recovers all its superelasticity.

Thermal Behavior Changes of As-Received and Retrieved Bio-Active® (BA) and TriTanium® (TR) Multiforce Nickel-Titanium Orthodontic Archwires

Multiforce nickel-titanium (NiTi) orthodontic archwires release progressively increasing forces in a front-to-back direction along their length. The properties of NiTi orthodontic archwires depend on the correlation and characteristics of their microstructural phases (austenite, martensite and the intermediate R-phase). From a clinical and manufacturing point of view, the determination of the austenite finish (Af) temperature is of the greatest importance, as in the austenitic phase, the alloy is most stable and exhibits the final workable form. The main purpose of using multiforce orthodontic archwires is to decrease the intensity of the applied forces to the teeth with a small root surface area, such as the lower central incisors, and also provide forces high enough to move the molars. With the optimally dosed forces of multiforce orthodontic archwires in the frontal, premolar and molar segments, the feeling of pain can be reduced. This will contribute to the greater cooperation of the patient, which is of utmost importance to achieve optimal results. The aim of this research was to determine the Af temperature at each segment of as-received and retrieved Bio-Active^® and TriTanium^® archwires with dimensions of 0.016 × 0.022 inches, investigated by the differential scanning calorimetry (DSC) method. A classical Kruskal-Wallis one-way ANOVA test and multi-variance comparison based on the ANOVA test statistic using the Bonferroni corrected Mann-Whitney test for multiple comparisons were used. The incisor, premolar and molar segments have different Af temperatures, and they decrease from the anterior to posterior so that the posterior segment has the lowest Af. Bio-Active^® and TriTanium^® with dimensions of 0.016 × 0.022 inches can be used as first leveling archwires by additional cooling and are not recommended for use on patients with mouth breathing.