Galvanic Corrosion among Different Combination of Orthodontic Archwires and Stainless Steel Brackets

Introduction

A variety of orthodontic archwires and bracket designs offer the orthodontists a wide choice of different combinations of orthodontic brackets and archwires. The introduction of more flexible wires such as NiTi or TMA wires has increased treatment efficiency and allowed the orthodontist to visit their patients in longer intervals. This entails remaining the wires in the mouth for a correspondingly longer time (1). So; they can be exposed to electrochemical reactions, mechanical forces of mastication, and generalized wear (1,2). All these phenomena are able to accelerate the different types of corrosion processes that can take place in the patient's mouth,and the degradation products from brackets and/or archwires are then released into the oral environment (3). Corrosion is an electrochemical process that arises from 2 concomitant reactions, an oxidation reaction at the anode and a reduction reaction at the cathode (4,5). Based on the conditions of the oral environment, different types of corrosion can occur in the mouth, for instance, crevice corrosion, microbiologically influenced corrosion, and galvanic corrosion.

 Constituents of alloys utilizing as orthodontic appliances are mostly nickel, chromium, cobalt, iron, molybdenum, and titanium. Concerning biocompatibility, nickel stands out among the other elements (6). It has been stated that the ions releasing into oral cavity from orthodontic appliances can cause some oral clinical manifestations such as glositis, metal taste, gingivitis, peeling lips, erythema multiforme, and gingival hypertrophy (7-11). On the other hand, the surface alterations of orthodontic devices due to corrosion, might compromise the appliance’s esthetics, increase friction during sliding, result in wire fractures, and diminish the torque expression of preadjusted appliances (5,6,12-16).

Considering these negative consequences and since an important part of orthodontic devices such as bands, brackets, and archwires are made of metallic alloys, the focus of many studies has been on corrosion in orthodontic appliances (17-20). Most research on corrosion has observed the measurement of ion release, notably nickel ion, ^21-24 or the effect of certain ions, such as fluoride, on corrosion resistance of metals (25-27). Fewer studies have considered galvanic corrosion with various combinations of brackets and wires (1,28-31). Therefore, we aimed to compare the galvanic behavior of different combinations of brackets and archwires that are commonly used in most orthodontic practices.

Materials and Methods

In this study, three types of orthodontic archwires and 80 standard edgewise maxillary central incisor brackets (Equilibrium 2, Dentaurum, Ispringen, Germany) with a slot size of 0.018 inch were used. These three types of archwires were designated as follows: NITI (Dentaurum, Ispringen, Germany), TMA (Dentsply GAC, Calexico, CA, USA), and Stainless steel (Dentaurum, Ispringen, Germany), each with a diameter of 0.016×0.022 inches. The test was composed of four groups, three of which consisted of different wire-bracket couples and the forth was just brackets as a control group. Each group had five samples. All similar specimens were from the same batch number and tested in “as-received” condition from the manufacturers. The wires were cut into 40 mm long specimens. All materials were degreased by swabbing with acetone and placed in an ultrasonic container with distilled water for 10 minutes before testing. Finally they were dried with a hairdryer. Four brackets, then, were connected to each wire by elastic bands made from electrochemically neutral material (American orthodontics, Sheboygan, WI, USA). The bases of all brackets were painted with nail polish to insulate the brackets’ meshes from the corrosive effect of the medium. The samples were immersed into capped containers, including 1.8 cc Fusayama-Meyer artificial saliva (Table 1) and placed inside one incubator (GCA, Precision Scientific) with a temperature of 37±1ºC.

After six weeks, the samples were removed from the incubator and sent to chemical lab for ion absorption test to evaluate the concentration of nickel ion in each medium. The mean and the standard deviation of all four groups calculated and the data were compared together with Kruskal-Wallis non-parametric statistical test, carried out by SPSS 11.0 (SPSS 11.0 Windows, SPSS Inc, Chicago).

Results

As to the obtained data, charted in the Table 2, the highest concentration of released nickel ion belonged to the third group (steel bracket+ steel archwire) and the least was for the fourth group (steel bracket without archwire). Amid these two groups, Bracket-NiTi wire and Bracket-TMA wire groups were in second and third rank, respectively.

As it could be seen, the average of nickel ion concentrations was roughly the same in the groups including NiTi wires and steel wires (Table 2).

However, Based on Kruskal-Wallis test, there were no significant differences in the amount of nickel ion concentration among the four groups (P= 0.319).

Table 1. Fusayama-Meyer artificial saliva solution composition

Table 2. The mean and standard deviation of released nickel ion in experimental groups