Material and methods

We selected for this study a stainless steel type 304 orthodontic archwire (batch number 0540017, TP Orthodontics, LaPorte, Ind). The wires were straight, 180-mm long, and retangular with an 0.018 × 0.025-in (0.45 × 0.625 mm) cross section. A sample specimen was created by using 40 mm of as-received wire and bending a loop stop about halfway in its length. The height of the loop was 3 mm. The sample was reproduced by using Vision software (Microsoft Corporation, Redmond, Wash) and printed as a real-size template for standardizing the shape of 35 specimens ( Fig 1 ). Part of the wire more susceptible to phase transformation was about 1.0 cm from the central loop; the excess length at the edges allowed for clinical adjustments while tying it to a patient’s appliances.

Model of stainless steel sample tied in the orthodontic appliance.

All samples were taken from the same batch of wires and prepared by the same operator (P.P.I) using the same pliers. The 35 specimens were separated in 4 groups (1 control and 3 experimental groups). The control group consisted of 5 specimens of the same size, and equal loop stops and shapes as the experimental ones of as-received wires, having no interaction with the oral environment throughout the trial period. The experimental group consisted of 30 wire specimens that were subjected to the oral environment of 30 patients randomly chosen from the orthodontics clinic of the Universidade Federal do Rio de Janeiro in Brazil. This group was divided into 3 subgroups according to the time of exposure in the mouth: 1, 2, or 3 months. The patients were allocated at random to each subgroup. All participants were undergoing corrective orthodontic treatment, which allowed for specimens to be tied by their edges to the patient’s fixed appliances by using stainless steel 0.010-in (0.24 mm) ligatures and ligature cutters ( Fig 2 ). In all participants, the specimens were placed by the same author (P.P.I.) in the maxillary arch on the right side. This research proposal was approved by the Research Ethics Committee of the Universidade Federal do Rio de Janeiro (number 87/2007). An informed consent form was signed by all 30 patients or their legal representatives. At the end of the clinical period, each experimental specimen was removed with a ligature cutter and tweezers, and placed on a sterile tray with the inner surface that had been turned to the teeth facing up. Before mechanical testing in the laboratory, all experimental and control specimens were placed in an ultrasound cleaner with enzyme detergent Endozyme (DFL, Petrópolis, Rio de Janeiro, Brazil) for 20 minutes. This substance proved ineffective in removing gross debris from the specimens, so the procedure was repeated with a multiuse detergent (Amway, Ada, Mich) for 1.5 hours. After in-vivo clinical time, all samples were subjected to mechanical torque testing.

Orthodontic appliance in a patient’s mouth.

Torque testing is a simple mechanical test. The test consists of twisting a specimen at a defined rate to a prescribed strain or twist, to failure or angle. Figure 3 shows the torque testing device developed by our research team. This apparatus held a specimen by both ends, with 5 mm of wire from the central loop. One end was fixed, and the other was free to rotate, connected to a universal testing machine with a load cell of 50 N (DL10.000, EMIC, Sao José dos Pinhais, Paraná, Brazil). The force required to twist specimen by 15° in torsion was measured. The twist was done at a constant speed of 100 rpm. A 10-cm long rectangular stainless steel wire with a cross section of 0.018 × 0.025 in (0.45 × 0.625 mm) was fixed to the rotating axis, serving as a pointer. A conventional protractor was scanned, printed, and positioned behind the experimental setup; as testing began, the pointer turned, and the test was stopped after a 15° displacement, when force readings were taken in gram-force (gf) units. The output data was force (gf) against twist. But, in principle, the torsion test data are torque-twist curves. To convert the results to torque (gf.mm), the force (gf) readings were multiplied by the radius of the rotating axis (5 mm) and registered in a Microsoft Excel spreadsheet (Microsoft) in gf per millimeter units (gf.mm).

Device for toque testing.

The torsion test simulated the most common clinical situation, when we apply torque in a rectangular orthodontic archwire during the last treatment phase. The orthodontic arch of the last stage has loop or omega in the region of molars. The tested samples were fixed in the region of the molars of the orthodontic appliances. The sample design simulated the region of the archwire, which normally receives torque in the final stage of orthodontic treatment.

In this study, complete data of all parameters, including descriptive statistics and statistical models, were evaluated. The mechanical and magnetic properties results were analyzed with SPSS statistical software (version 13, SPSS, Chicago, Ill). An alpha level of 0.05 was chosen to determine the statistical significance. Post-hoc multiple comparisons were performed by using Bonferroni adjustments for significant tests.

First-derivative ferromagnetic resonance (FMR) spectra were recorded at room temperature and 9.50 GHz by using a Varian E-12 spectrometer with 100-kHz field modulation (Varian, Inc., Santa Clara, Calif). FMR line intensity was computed by double integration of the first derivative absorption spectrum.