In vitro comparison of peracetic acid and autoclave sterilization in the corrosion of orthodontic pliers: a pilot study

ABSTRACT Introduction: The most currently recommended method for sterilization of orthodontic pliers is the autoclave, while peracetic acid has also been shown to be effective in the chemical sterilization process. Objective: This study sought to compare the corrosive effects of peracetic acid and autoclave sterilization process of orthodontic pliers. Methods: Four active tungsten carbide (WC) stainless steel tie-cutting pliers from the manufacturers Quinelato (Rio Claro, SP, Brazil) and ICE (Cajamar, SP, Brazil) were selected. The active ends of the pliers were sectioned, and six active tips were obtained and distributed into the following groups: 1) control group (no sterilization); 2) AC group (two active pliers tips submitted to 100 autoclave sterilization cycles); and 3) AP group (two active pliers tips submitted to 100 cycles of sterilization by immersion in 2% peracetic acid solution for 30 minutes). Results: Chemical analysis using X-ray dispersive energy spectroscopy showed that after autoclave sterilization, only the ICE pliers presented oxidation corrosion (Δ[O] = +24.5%; Δ[Fe] = +5.8%; Δ[WC] = -1.9%). In comparison, following peracetic acid sterilization, both manufacturers ICE (Δ[O] = +1.8%; Δ[Fe] = +18.0%; Δ[WC] = -1.1%) and Quinelato (Δ[O] = +5.3%; Δ[Fe] = -10.4%; Δ[WC] = -15.2%) showed corrosion. The morphological analysis revealed that peracetic acid caused a pitting and localized corrosion in both brands, while the autoclave caused uniform surface corrosion on the ICE pliers. Conclusion: Autoclave application was the sterilization method that generated less corrosive damage to the orthodontic cutting pliers, when compared to the immersion in 2% peracetic acid.


INTRODUCTION
According to biosafety principles, the sterilization of orthodontic tools is essential to prevent cross-infection among patients, mainly due to saliva or blood contamination. Although there are several chemical and physical sterilization methods, the autoclave is the most widely used and recommended for dental use. 1 During the autoclave sterilization process, the materials undergo combined actions of temperature and humidity exposure. The steam moistens the materials, transmit heat, pressure and eliminates microorganisms, bu means of protein coagulation. 2 Nevertheless, repeated cycles of autoclave sterilization can induce corrosion on metallic instruments, 3 and may negatively influence the ability of the instruments subjected to such a process, especially cutting pliers. 4 These damages could lead to a reduction in the duration of the pliers and an increase of cost, due to the need of replacing these orthodontic materials.
Specifically in cutting pliers, the corrosion affects three areas: the hinge, the cutting blade and the blade weld joint. This damage ranges from uniform corrosion (which affects the entire surface area, with less harm) to pitting localized corrosion (that is more destructive and involves the formation of small cavities on the surface). 5 However, both types of corrosion can lead to loss of cutting efficiency. 4 Nunes LKF, Carneiro EL, Palomares NB, Senna L, Sampaio-Filho H, Miguel JAM -In vitro comparison of peracetic acid and autoclave sterilization in the corrosion of orthodontic pliers: a pilot study 5 Although the damage to orthodontic pliers may result from several etiologies, such as improper use and mechanical fatigue, the most frequent cause is autoclave sterilization, due to the combination of heat and humidity. 3 Among the new options for sterilization methods, the peracetic acid stands out, which has been proven effective in the disinfection and sterilization of dental materials. 6,7   Thus, there is a need for new studies that evaluate the corrosion effect of these two different sterilization processes over orthodontic ligature pliers. In this study, therefore, we considered the evaluation of the exposure of the pliers to 2% peracetic acid (chemical) and the traditional autoclave (saturated pressure steam).

SAMPLE
Four orthodontic tie-cutting pliers from two different manufacturers (Lot 16065; Quinelato, Rio Claro/SP, Brazil; and Lot 111604; ICE, Cajamar/SP, Brazil) were selected. All cutting pliers used consisted of stainless steel and active widia tip (tungsten carbide) as factory-made.

PREPARATION OF THE SPECIMENS
The active ends of the four orthodontic cutting pliers were equally sectioned at the plier junction, with a carborundum disc, coupled to a straight and low-rotation engine. Thus, eight active tips were obtained. To create the study specimens, six of the eight active tips were selected and embedded in a self-curing epoxy resin (Aka-Resin Liquid Epoxy; Akasel A/S, Roskilde, Denmark).
Nunes LKF, Carneiro EL, Palomares NB, Senna L, Sampaio-Filho H, Miguel JAM -In vitro comparison of peracetic acid and autoclave sterilization in the corrosion of orthodontic pliers: a pilot study 7 Subsequently, the six specimens were subjected to metallographic preparation in accordance with the American Society for Testing and Materials E 883-86 standard, to remove any porosity or irregularity from the specimens. The specimens were polished using a Politriz machine (APL-4; Arotec, Cotia/SP, Brazil), with the sequence of #400, #600, #1200, #1500, and #2000 granulation sandpaper (Arotec, Cotia/SP, Brazil) and finishing with felt associated with diamond paste.

STERILIZATION METHODS USED IN THE SAMPLE
Three groups were analyzed, as follows: 1) A control group, consisting of two samples of plier tips, which were not submitted to any kind of sterilization.  2) Silver weld, which is composed of an alloy of silver (Ag), copper (Cu), and zinc (Zn), commonly including other metals, such as manganese (Mn) and tin (Sn).
3) Cutting blade, which is mainly composed of tungsten carbide (WC), with a very low content of other components, such as cobalt (Co).   The results of the EDX chemical analysis are presented in Table 1.
In the region of stainless steel, the Quinelato pliers presented a composition consisting of 26.8% Fe, 5.2% Cr, 7% C, and 0.7% Ni.
For the ICE pliers, the percentages of Fe (18.6%) and Cr (3.6%) were lower, and the Ni (2.1%) was higher.
In the silver solder region, all of the constituent chemical elements presented higher percentages in the ICE pliers than the Quinelato ones.       In the silver weld region of the ICE pliers, all of the chemical elements exhibited lower percentages in the autoclave group than in the control group. For the Quinelato pliers, the opposite was detected; lower concentrations in the control group were observed, when compared with the autoclave test group.
In the cutting region, lower WC concentrations were detected in the pliers of both manufacturers submitted to autoclave sterilization, although a more significant degree was found in the Quinelato pliers than in the ICE pliers, suggesting that carbide crystals were released into the medium of WC. In addition, the matrix that surrounds the Quinelato brand WC was completely lost after 100 autoclaving cycles.

DISCUSSION
To minimize, prevent, or reduce the risk of cross-infection between patients within the orthodontic office, standard precautionary measures should be adopted in daily practice. 1, [9][10][11] Many orthodontists neglect to pursue optimal infection control, and end up performing only material disinfection. 12 The reasons for the high percentage of disinfection noncompliance can be attributed to the large volume of patients treated on a day, the shorter duration of care, the cost of care, time taken for the entire sterilization process, and the shortening of the duration of the pliers when subjected to constant autoclave sterilization. 3 In the orthodontic office, orthodontic pliers are the most-used instruments during patient care. 13,14 In the present research, orthodontic tie-cutting pliers were chosen because of the nature of the activity and sensitiveness to damage at the cutting active tip. 5,15 Thus, four orthodontic tie-cutting pliers from two different manufacturers were tested (ICE and Quinelato).
The small sample size was enough to detect differences between the groups, because of the in-vitro setting. Only the active tips of these pliers were evaluated, as the active tip is the most important functional part of the cutting pliers.
Corrosion depends on the material, the corrosive medium and the process conditions to occur. 16  These data can be explained by the percentage differences in the alloys used to manufacture the Quinelato and ICE pliers, as observed in the control group.
As an alternative method to autoclave use, the most widely accepted chemical agent for instrument disinfection is 2% per-  The chemical element C, which was found only in the Quinelato pliers, is added to the pliers' composition to increase the hardness of the material. Ni improves alloy strength at high temperatures and provides ductility and weldability, improving overall strength. Therefore, as the Quinelato pliers presented C and a higher Cr content, it is reasonable that these pliers did not corrode after 100 autoclave sterilization cycles. In a different way, corrosion was detected in the ICE pliers after the same procedure.
The silver solder, which presents in its composition Ag, Cu, Zn, and Mn, allows for the union between the metals that compose the pliers. After 100 cycles in peracetic acid, the ICE pliers presented a reduction of all chemical elements of silver solder, with the loss of these elements occurring to the medium in which they were immersed. However, there was an increase of these components in the Quinelato pliers, together with an increase of O, which indicates that oxidation occurred in this region after sterilization in peracetic acid.
Widia is a cluster of rare metal carbides (mainly tungsten, tantalum, titanium, and molybdenum) with a cobalt or nickel binder, in which the cohesion of the assembly is performed drying, and lack of lubrication -can also lead to corrosion of pliers. 4,19,20 Therefore, future research should also test pliers after clinical use and sterilization protocols for each method evaluated, following the manufacturer's recommendations for disinfection, drying, lubrication, packaging, sterilization, and storage of orthodontic cutting pliers.

CONCLUSION
From the results of this pilot study, it can be concluded that both sterilization methods caused corrosion to the pliers. However, sterilization by immersion in 2% peracetic acid seems to have produced more harmful corrosion when compared with the autoclave method.
Importantly, the development of new research including the evaluation of corrosion after clinical use is essential to expand the knowledge on this subject and the evaluation of other factors related to corrosion.