Estimation of changes in nickel and chromium content in nickel-titanium and stainless steel orthodontic wires used during orthodontic treatment: An analytical and scanning electron microscopic study.

INTRODUCTION: The biocompatibility of orthodontic dental alloys has been investigated over the past 20 years, but the results have been inconclusive. The study compares standard 3 M Unitek nickel-titanium (NiTi) and stainless steel archwires with locally available JJ orthodontics wires. Scanning electron microscope (SEM) study of surface changes and complexometric titration to study compositional change was performed.
MATERIALS AND METHODS: Ten archwires each of group 1-3 M 0.016" NiTi, group 2-JJ 0.016" NiTi, group 3-3 M 0.019" *0.025" SS and group 4-JJ SS contributed a 10 mm piece of wire for analysis prior to insertion in the patient and 6 weeks post insertion. SEM images were recorded at ×2000, ×4000 and ×6000 magnification. The same samples were subjected to complexiometric titration using ethylenediaminetetraacetic acid to gauge the actual change in the composition. OBSERVATIONS AND
RESULTS: The SEM images of all the archwires showed marked changes with deep scratches and grooves and dark pitting corrosion areas post intraoral use. 3M wires showed an uniform criss-cross pattern in as received wires indicating a coating which was absent after intraoral use. There was a significant release of Nickel and Chromium from both group 3 and 4. Group 2 wires released ions significantly more than group 1 (P = 0.0).
CONCLUSION: Extensive and stringent trials are required before certifying any product to be used in Orthodontics.

Introduction

During fixed orthodontic treatment, various appliances and wires are placed in the patient's mouth for an average duration of about 1–2 years. These materials are mostly made of Stainless Steel or nickel-titanium (NiTi) alloys. The oral environment is conducive to corrosion of dental materials due to constant mechanical, chemical, enzymatic, and bacterial assault. It is possible that metal ions are eluted from orthodontic materials by the caustic action of saliva, electrolytes in food debris, and acids produced by bacteria. Thus, biocompatibility of these materials becomes a major concern. The metallic ions that pose major health concerns are Nickel and Chromium. Nickel is commonly associated with metal induced allergic contact dermatitis, whereas Chromium allergy is estimated to occur in 10% of male subjects and 3% of female subjects. Nickel is transported in the blood by binding with serum and to a lesser degree with Histidine. Ionic Nickel can form metalloproteins by binding with body proteins.[1] The incidence of Nickel allergy is 1% in males and 10% in females. The Nickel excretion rate is 90% of dietary intake, and Chromium is 0.25% of dietary intake.[2] The toxicity can be attributed to its strong oxidizing tendency. Thus, it is important to accurately determine the amount of Nickel and Chromium being leached from orthodontic materials subsequent to their use in the oral environment.[34567]

The purpose of this study was to determine the change in the composition of NiTi and Stainless Steel wires before placement into the subject's mouth in as received condition and after usage for a definite period.

Complexation reactions can be employed in titrations for determining the proportions of individual cations of Nickel and Chromium in the mixture. The chemical reaction that takes place between a metal ion and a molecular entity known as a ligand is called complexation reaction which leads to the formation of an identifiable and quantifiable complex.

In addition, in the present study, the process of corrosion was visualized by scanning electron microscope (SEM).

Materials and Methods

The present study comprises two phases - in vivo and in vitro. The sample comprises 40 as received wires from the manufacturer and 40 retrieved segments after intraoral usage.

The wires were divided into groups as per their composition and dimension. The groups were made as follows:

Group 1: Ten 0.016” NiTi orthodontic preformed archwires of 3 M Unitek company

Group 2: Ten 0.016” NiTi orthodontic preformed archwires of JJ Orthodontics company

Group 3: Ten 0.019” *0.025” stainless steel orthodontic preformed archwires of 3 M Unitek company

Group 4: Ten 0.019” *0.025” stainless steel orthodontic preformed archwires of JJ Orthodontics company.

The retrieved samples of above groups are labeled as Group 1R, Group 2R, Group 3R, and Group 4R respectively.

The present study was conducted in the Department of Orthodontics, Government Dental College, Jaipur, Rajasthan, India. The subjects were selected from the patients reporting to the Department for fixed Orthodontic treatment.

Selection criteria for inclusion of patients in the study were:

No systemic diseases like acidity or gastric reflux

Patient should be a nonsmoker

Patients were instructed not to drink acidulated and carbonated drinks.

Before commencing the study, an informed consent was obtained from all the participating patients and the ethical clearance was obtained from the Institutional Review Board of the Rajasthan University of Health Sciences, Jaipur.

All patients were treated with fixed orthodontic appliance. A piece of 10 mm length was cut from the distal end of all the archwires of all groups, prior to the insertion in subjects, and labeled. The wires were then inserted in the patient's mouth and ligated with stainless steel ligatures. The wires were left in place for 6 weeks. On removal, the wires were cleaned with distilled water and dried. A 10 mm section was again obtained from the retrieved wires from the canine region and labeled. The segments were studied for their initial and post usage composition using complexometric titration method. Prior to chemical analysis, the wires were studied under SEM and images were recorded at ×2000, ×4000 and ×6000 magnification. The SEM study was conducted at University Science Instrumentation Centre, University of Rajasthan, Jaipur, India. The chemical analysis was performed at Maharishi Arvind Institute of Pharmacology, Jaipur, India.

Sample preparation

The sample were prepared as per the standard protocol followed by chemical analysis literature.[8]

The section of the wire was placed in a beaker, 10 ml aquaregia was added using a dropper. Initial reaction of Ni with aquaregia led to the formation of hydrogen gas with effervescence

The acid was allowed to evaporate up to 2–3 ml till a solid residue remained at the bottom. The solution was allowed to cool and then 20 ml Hydrochloric acid (HCl) was added and the beaker reheated till all the liquid evaporated

After cooling of contents, 10 ml HCl was re-added and again evaporated. The beaker was kept in an oven for 1 h at 100°–110°C

Then 20 ml 9 M HCl was added to it. The solution was heated till residue dissolved. The solution was cooled, then quantitatively transferred to a 50 ml volumetric flask, using small portions of 9 M HCl to rinse out the beaker. After the transfer was complete, the solution was diluted to the mark on the flask using 9 M HCl.

Determination of the total nickel and chromium concentration

1.00 ml of the sample solution was pipetted into a clean 250 ml Erlenmeyer flask and diluted with 50 ml of deionized water

3 M NaOH solution was carefully added dropwise while swirling until the analyte solution became colored lightly bluish-green and cloudy

3 M HCl solution was added dropwise while swirling until the cloudiness just disappeared

Buffer (potential of hydrogen [pH] 10) was added dropwise while swirling: After the first few drops, the solution gained a light bluish color, and it became cloudy. Buffer was added till cloudiness just disappeared

A small quantity of Murexide indicator powder was added and swirled to dissolve; the solution then appeared yellow or greenish-yellow

Titration with the ethylenediaminetetraacetic acid (EDTA) solution was done until the color changed to orange, then slowly EDTA was added dropwise until the color changed to violet. This solution was kept for comparison, and the titration was done two more times

Using the EDTA concentration from the standardization, the total concentration of Cr and Ni in the sample was calculated. One of the ions is masked for calculating the true concentration of the other by adding thiosulphate and lowering the pH.

Statistical analysis

The data obtained were subjected to statistical analysis. Mean, standard deviation and standard error were calculated, with the paired t-test used to determine the level of significance and correlation of the amount of nickel and chromium in as received and retrieved wire samples, using the Statistical Package for Social Sciences 12.0 (IBM corporation).

Observations and Results

Scanning electron microscope study results

The SEM images of the groups 1, 2, 3, and 4 wires reveal minor surface defects at ×100 magnification. At ×2000 magnification, deep linear scratches are visible which intensify at ×4000 and ×6000 magnification. The surface of the wire appears to be corrugated and scarred with lines. The images wires after retrieval show dark areas representing the corrosion at ×100 magnification. The pictures at ×2000, ×4000, and ×6000 reveal intense dark patches with scattered debris and fragments chipping off at the wire surface clearly indicating aging in the wire [Figures 1-4]. SEM images of group 3 wires in as received state reveal a smooth surface at ×100 magnification. The images at ×2000, ×4000 and ×6000 magnification reveal linear granular structures appearing like crystal grain boundaries [Figure 5]. Such structures were evenly spread over the entire wire surface. After retrieval images of group 3R, wires reveal a similar picture as the rest of the groups [Figure 6].

Figure 1

Scanning electron microscope view of 0.016” locally available Nickel-Titanium wire (a), view at ×2000 (b), ×4000 (c), ×6000 (d)

Figure 4

Scanning electron microscope view of 0.019” × 0.025” locally available retrieved stainless steel wire (a), view at ×2000 (b), ×4000 (c), ×6000 (d)

Figure 5

Scanning electron microscope view of 0.019” × 0.025” 3 M Unitek stainless steel wire (a), view at ×2000 (b), ×4000 (c), ×6000 (d)

Figure 6

Scanning electron microscope view of 0.019” × 0.025” 3 M Unitek retrieved stainless steel wire (a), view at ×2000 (b), ×4000 (c), ×6000 (d)

Scanning electron microscope view of 0.016” locally available retrieved nickel-titanium wire (a), view at ×2000 (b), ×4000 (c), ×6000 (d)

Scanning electron microscope view of 0.019” × 0.025” locally available stainless steel wire (a), view at ×2000 (b), ×4000 (c), ×6000 (d)

Complexiometric titration results

For group 3, the mean concentration of Nickel was calculated to be 8.816% in as received sample with a standard deviation of 0.006967%.

For group 3R, the mean concentration of Nickel was calculated to be 8.546% in retrieved sample with a standard deviation of 0.08009%.

For group 4, the mean concentration of Nickel was calculated to be 9.846% in as received sample with a standard deviation of 0.001235%.

For group 4R the mean concentration of Nickel was calculated to be 8.582% in the sample with a standard deviation of 0.182% [Graphs 1].

Graph 1

Percentage of Nickel in as received and retrieved 3M SS wire 0.019”*0.025” and JJ Orthodontics SS wire 0.019”*0.025”

This reveals significant loss of Nickel in both groups with P = 0.00. For group 3, the mean concentration of Chromium was calculated to be 19.31% in as received sample with a standard deviation of 0.005397%.

For group 3R, the mean concentration of Chromium was calculated to be 18.77% with a standard deviation of 0.03267%.

For group 4, the mean concentration of Chromium was calculated to be 20.33% in as received sample with a standard deviation of 0.002991%.

For group 4R, the mean concentration of Chromium was calculated to be 19.4% with a standard deviation of 0.09876% [Graph 2].

Graph 2

Percentage of Chromium in as received and retrieved 3M SS wire 0.019”*0.025” and JJ Orthodontics SS wire 0.019”*0.025”

This reveals a significant loss of Chromium in retrieved wires (P = 0.00) in both groups (3 and 4).

For group 1, the mean concentration of Nickel was calculated to be 55.77% in as received sample with a standard deviation of 0.004434%. For group 1R, the mean concentration of Nickel was calculated to be 55.12% with a standard deviation of 0.05083%.

For group 2, the mean concentration of Nickel was calculated to be 55.97% in as received sample with a standard deviation of 0.02892%.

For group 2R, the mean concentration of Nickel was calculated to be 55.05% with a standard deviation of 0.4577% [Graphs 3 and 4].

Graph 3

Percentage of Nickel in as received and retrieved 3M NiTi wire 0.016” and JJ Orthodontics NiTi wire 0.016”

Graph 4

Percentage of Nickel in as received and retrieved 3M NiTi wire 0.016” and JJ Orthodontics NiTi wire 0.016”

This indicates a significant change in the composition of wires from as received to retrieved conditions in both group 1 and group 2 in terms of Nickel composition (P = 0.00).

The mean difference in percentage of Nickel in group 3 and 3R wires is 0.27 with a standard deviation of 0.07513.

The mean difference in percentage of Nickel in group 4 and 4R wires is 1.264 with a standard deviation of 0.1779.

On the application of unpaired t-test, it was found that the release of Nickel is more significant in group 4 wires than group 3 wires (P = 0).

The mean difference in percentage of Chromium in group 3 and 3R wires is 0.546 with a standard deviation of 0.03596.

The mean difference in percentage of Chromium in group 4 and 4R wires is 0.924 with a standard deviation of 0.09709.

On the application of unpaired t-test, it was found that the release of Chromium is more significant in group 4 wires than group 3 wires (P = 0).

The mean difference in percentage of Nickel in group 1 and 1R wires is 0.649 with a standard deviation of 0.04557.

The mean difference in percentage of Nickel in group 2 and 2R wires is 0.913 with a standard deviation of 0.4474.

On the application of unpaired t-test, it was found that the release of Nickel is more significant in group 2 wires than group 1 wires (P = 0).

Discussion

The science of Orthodontics is expanding its horizons by the day not only in terms of mechanics but also in terms of its sibling branches like material science. Oral cavity provides an ideal environment for the study and development of materials as they are exposed to a hostile environment rich in enzymes, microbes, acids, nitrogenous compounds, and ions.[4] These factors over a period of time may influence the composition and mechanical properties of orthodontics materials.

Various techniques have been employed to determine the biosafety of these materials. The in vitro tests though useful are not absolutely accurate as simulating the oral environment extraneously is not possible.

Retrieval analyses (study of material after intraoral use) have gained interest in biomaterials research because of the critical information derived from investigating the performance of the material in the environment in which it was intended to function.[4]

The orthodontic archwires were retrieved from the patients selected for fixed orthodontic treatment. The archwires were tested before insertion into the patients’ mouth and after intraoral use of 6 weeks for their composition. Since the major concern is with metals such as Nickel and Chromium, their concentration was determined using complexometric titration method. The surface of wires was analyzed under SEM (Zeiss EVO 18).

The surface of all the wires in all the groups irrespective of their composition shows certain surface defects in as received state which tend to alter or intensify after intraoral usage. These defects have been attributed to pitting corrosion in manufacturing defects by Theodore Eliades et al.[4] Potentiometric titration studies by Kim et al. have also reinforced the same hypothesis that surface defects due to manufacturing process are sites that are susceptible to corrosion and leaching of nickel in both NiTi and stainless steel wire.[18]

The surface of retrieved wires also shows dark corrosion patches with loose debris and metal fragments which may have worn out under mechanical stresses.

The scratches were seen along the length of the wire in a longitudinal manner as well as running in vertical directions as seen in a study conducted by Daems et al.[19] The wires obtained from 3M Unitek showed a smooth surface at 100× magnification. At higher magnifications, the images indicate an uniform pattern of scattered irregular crossing striations apparently of certain grain boundaries. They might also indicate that the surface of the wire may be coated with an agent which is responsible for such appearance.

However, no sign of striated structures is seen in retrieved wire which also goes in the favor that the wires may be coated, and the coating wears off after intraoral use.

To elaborate on the findings, chemical analysis of wires was done. The concentration of nickel and chromium was detected by dissolution of samples in a series of acid solutions and then chelating the metal ions with a tetravalent complex EDTA that gives a colored reaction. The concentration of ions was calculated volumetrically in moles and then in percentage.[3]

The percentage of nickel in alloys varies from 8% in stainless steel alloys to more than 50% in nickel-titanium alloys. Chromium concentration varies from 14% to 18% in brackets and wires.

The concentration of these elements in blood or saliva can be attributed to various other factors besides leaching out of ions from the orthodontic materials.[8]

The previous retrieval studies indicate the concentration of Nickel and Chromium as ratios to Iron and Titanium in stainless steel and NiTi wires, respectively. For standardization purposes, the Fe content was used as an internal standard in the stainless steel wires, and nickel content was expressed as a nickel/iron ratio. Similarly, titanium content was used as an internal standard in NiTi wires, and the nickel content of the NiTi wires was expressed as a nickel/titanium ratio.[4]

The present study gives the concentration of the pure metal and not as a ratio as the comparison had to be drawn between various brands. It was essential to take individual ion concentrations as Fe as the internal standard in available wires and Titanium in NiTi wires was a suspect.

The percentage of Nickel in as received 0.019” *0.025” 3M and JJ wire is 8.816% and 9.846% respectively and 8.546% and 8.582% in retrieved wires respectively. This implies an already higher concentration of Nickel is present in the JJ Orthodontics wires, and there is a marked leaching of nickel from them when compared with the standard 3M Unitek wires. Similarly, comparing the concentration of Chromium indicates leaching of Chromium is significant in both the samples.

The concentration of nickel in as received 3M Unitek and JJ Orthodontics 0.016” NiTi wires was found to be 55.77% and 55.97%, respectively, and that in retrieved wires to be 55.12% and 55.05% respectively. The loss of nickel from Niti wires was also found to be significant.

The above findings are in contrast to the study done by Eliades et al. who concluded in their study conducted in 2004 that though there was leaching of nickel and chromium it was not statistically significant.[4] They performed SEM and energy-dispersive X-ray microanalysis to assess the ion change in as received and retrieved wires.

The alterations in the oral environment modify the surface characteristics and reactive properties of wires.[7910111213] In the present study, marked release of nickel and chromium is concordant with the results of Kerosuo et al. who detected significant release of Nickel and chromium in fixed appliances and quad helix in synthetic and simulated oral environments during the first few hours.[19]

Although various studies conducted on the amount of nickel and chromium in the saliva of patients have been conducted, but the results have been inconcordant. Agaoglu et al.[15] found significant release of nickel and chromium in the saliva of patients in first 2 months of fixed orthodontic treatment. On the other hand, Kocadereli et al.[16] found no significant release of nickel and chromium in the saliva. Kerosuo et al.[17] also concluded in his study that material from different manufacturers yielded different amount of release of ions.

The current study justifies that nickel and chromium release from orthodontic archwires does take place significantly, though it needs to be correlated to their toxic levels in the human body.[181920]

Extensive and stringent trials should be done before certifying any product to be used in orthodontics. It would not only ensure quality of materials being used but also the biosafety of subjects would be well-guarded.

Conclusion

From the results of the present study, following facts can be concluded:

There is a significant release of nickel and chromium from both 3M Unitek and JJ Orthodontics 0.019” *0.025” stainless steel and 0.016” NiTi wires but it is more in JJ orthodontics wires

The SEM images of all wires before and after intraoral use showed some mechanically induced scratches which may be due to manufacturing process in the as received wires and due to various factors in retrieved wires like manipulation by pliers, mastication stresses etc.