An Evaluation of Corrosion Pits in Esthetic Coated Stainless Steel Orthodontic Archwires in Dry and Wet Environment at Different Intervals ( An In Vitro Study )

Background: The demand for esthetic orthodontic appliances is increasing; so the esthetic orthodontic archwires were introduced. Among them, Teflon and Epoxy coated stainless steel archwires. The amount of force available from the archwire depends on the structural properties and susceptibility to corrosion. All metallic alloys are changed during immersion in artificial saliva, chlorhexidine mouthwash andtoothpaste, but their behaviors differ from one type to another. They corrode at different rates, which lead to decrease the amount of force applied to the teeth. This in vitro study was designed to evaluate the corrosion pits in stainless steel archwires coated with Teflon and with Epoxy in dry and after immersion in artificial saliva, chlorhexidine(0.2%) (Parodontax) and toothpaste media (Sensodyne) for (1, 7 and 28) days intervals. Moreover, this study is intended to compare the corrosion pits for each type of archwires at these different media among all intervals. Materials and Methods: In this study, two hundred forty pieces of orthodontic wires of Teflon (Hubit) coated Stainless steel (120 pieces) and epoxy (Orthotechnology) coated Stainless steel (120 pieces), rectangular in cross section, size (0.019 x 0.025) inch and 15mm in length divided into four groups according to immersion media: (dry environment group, artificial saliva group, chlorhexidine group and toothpaste group). The atomic force microscope was used to measure the corrosion pits for all samples at dry and wet conditions and after different immersion periods. Results: Statistical analysis showed that there was a highly significant increase in the corrosion pits of Teflon coated stainless steel archwires (P ≤ 0.05) in wet environment at 28 days immersion period.The highest corrosion pits were found in the toothpaste medium for the two archwire types at 28 days immersion period. Conclusion: We can conclude that Epoxy coatedstainless steel archwires are indicated to be used above Teflon coated stainless steel archwires in terms of corrosion resistance. IfTeflon coated stainless steel archwires should to be used, they should be change in shorter periods than Epoxy coated stainless steel archwires type


INTRODUCTION
Appearance is one of patients' main concerns during orthodontic treatment so there is a growing demand for esthetic appliances (1,2) but most fixed orthodontic appliance components are metallic and silver in color (3) .This demand has led to the development of orthodontic appliances with acceptable esthetics both for patients and for clinicians (4) .This problem has been partially solved by the introduction of esthetic brackets made of ceramic or composite (5,6) .However, most archwires are still made of metal such as stainless steel and nickel-titanium, a number of alternatives have been explored to create an esthetic arch wire (7,8) .Among these alternatives, coated wires with polymeric materials have been developed (9,10) .Materials used in the coating process are Teflon or epoxy resin.The coating manufactured with a process, which plates the base wire (11) .
The conditions in the mouth are very suitable for the occurrence of corrosion because the oralcavity is warm and wet.The oral environment is particularly ideal for the biodegradation of metal because of its thermal, microbiologic and enzymatic properties (12) .These environmental conditions of the oral cavity might alter the morphological characteristics of archwires (13) .The ideal archwire is the wire that can withstand the extreme conditions of the mouth (14) .
Orthodontic alloys must have excellent resistance to corrosion especially in the oral environment.This corrosion resistance is very important for two reasons first is biocompatibility and second is orthodontic appliance durability (15) Understanding the basic material characteristics becomes essential for selecting wires for use in the treatment.Materials used in dentistry must have specific characteristics such as biological safety, adequate tissue response, and resistance to corrosion because they remain in the oral cavity and subjected to the oral environment's physical properties (chemical and microbiological properties), that stimulate the dissolution of metals (16).
Therefore, the objectives of this study were three.First, to evaluate and compare the effect of Teflon and Epoxy coating material in the corrosion pits of Stainless Steel, second, to evaluate and compare the effect of dry and wet environment, and third, to evaluate the effect of immersion time.

MATERIALS AND METHODS
Total number of (240) pieces of upper-coated stainless steel orthodontic archwires, (120) selected from Ortho Technology Company, (Brazil) and (120) selected from Hubit Company, (Korea) were tested for corrosion.The specimens used in the present study having a rectangular (0.019 × 0.025 inch) cross section and cut in to pieces of (15 mm) length.These pieces of wires divided in to four groups according to the media they immersed in them.
The corrosion pits measurements made at the following time intervals: 1day, 7 days and 28 days.Corrosion pits measurements were obtained by Atomic Force Microscope (AFM; JPK Nanowizard, Nr: H-01-0086, and JPK Image Processing software, version 3.0; JPK Instruments AG, Berlin, Germany) with a non-contact tip coated with silicon (NCLR-20; NanoWorld, Neuchatel, Switzerland), with a constant force of 48 N/mm and resonance frequency of 190 kHz (figure 1).
After preparation, the samples were washed withdistilled water and immersed in 70% ethanol for 4-5 seconds and thenimmersed in acetone (act as a volatile organic solvent) for 8-10 seconds and dried by dryair for one minute.This method of cleaning used to remove all contaminated layerformed on the alloy during storage (21) .
The artificial saliva was replaced every 7 days with a fresh solution to avoid its saturation with the corrosion products (22,23) .
The chlorhexidine solution was used according to the manufacturer instruction and the samples were immersed completely in the solution in the test tubes and covered perfectly by theirs covers, shake by the shaker for 1 minute, then they were removed and washed with distilled water, dried by dry air and re-put in the artificial saliva, then incubated at 37 0 C.This procedure would be repeated 2 times daily for (1day, 7 days, and 28 days) intervals (20) .
The samples of the Toothpaste groups were immersed completely in the paste on a slap for 2minutesAfter the 2 minutes were completed successfully the wires were removed and washed with distilled water, dried by dry air water and reput in the artificial saliva, then incubated at 37 0 C.This procedure would be repeated 3 times daily for (1day, 7 days, and 28 days) intervals (20) .

Statistical Analysis
Data collected analyzed by using relevant software Statistical Package of Social Science (SPSS, Chicago, 21).These data of the corrosion pits for all specimens averaged, and the results analyzed with the following statistics: 1. Descriptive statistics :( mean of corrosion pits and its standard deviation).2. Inferential statistics: {Independent sample ttest, One Way Analysis of Variance (ANOVA) and Least Significant Difference (LSD)}.

RESULTS
The Effect of Coating Material: Dependent sample of t-test showed nonsignificant coating type difference in the corrosion pits in dry environment at different intervals and when the wires were immersed in Chlorhexidine for 1 day and in the Toothpaste for 1 day and 7 days.On the other hand, there was highly significant difference when the wires immersed in the remaining media at different intervals (table 1).

Pedodontics, Orthodontics and Preventive Dentistry 155
The Effect of the Immersion Media: Analysis of variance difference (ANOVA) has demonstrated a non-significant difference among the media at the first day of immersion of the two wire's type but showed a highly significant difference at 28 days.On the other hand, it revealed a highly significant media effect difference of Hubit and a non-significant difference of Orthotechnology at 7 days immersion period (table 2).

The Effect of the Time Intervals:
The time interval has non-significant effect on the corrosion pits at dry environment.Analysis of variance difference (ANOVA) has demonstrated highly significant differences among the different intervals in wet environment except that for Epoxy coated stainless steel at Chlorhexidine (table 3).

DISCUSSION
The Effect of Coating Material The non-significant coating type difference in the corrosion pits in dry environment, in Chlorhexidine for 1 day and in the Toothpaste for 1 day and 7 days may be due to the presence of the protective oxide layer.
On the other hand, the highly significant difference when the wires were immersed in the artificial saliva media at different intervals may be attributed to the fact the Teflon coating layer adds a minimal thickness to the archwires while Epoxy coating dose add a more significant thickness.This result agreed with da Silvaa et al. (24) and Khamees (25) .

The Effect of the Immersion Media
The highly significant difference among the media at 28 days of immersion of the two types of wiremay related to some factor or factors that are able to modify the properties of the wires, such as the pH level, wet condition, thickness of coating, the composition of coating and the temperature.
On the other hand, the result revealed a highly significant media effect difference of Hubit and a non-significant difference of Orthotechnology at 7 days immersion period.The finding of this study disagreed with Neumann et al. (26) study in which they found Teflon coating (Hubit) prevented the corrosion of the wire completely.

The Effect of the Time Intervals
The time interval has no effect on the corrosion pits at dry environment.This may related to the constant factors of the environment at this study.
Thehighly significant differences among the different intervals at wet environment for the Teflon coated stainless steel wires (Hubit) may be due to rapture of the protective layer.This result agree with Al-Najafy (27) who found that the surface roughness of Teflon coated stainless increased with increased immersion time.The finding of this study disagree with Neumann et al. (26) study.
This study also showed non-significant effect of the immersion time for Epoxy coated stainless steel at Chlorhexidine.This may be due to the type, nature and thickness of coating material, which need more time to corrode.

( 1 )
Master Student, Department of Orthodontics, College of Dentistry, University of Baghdad.(2) Assist.Professor, Department of Orthodontics, College of Dentistry, University of Baghdad.