Evaluation of the Effects of Povidone Iodine and Hydrogen Peroxide Mouthwashes on Orthodontic Archwires: An In Vitro Study

INTRODUCTION

Good oral hygiene is of paramount importance, especially when undergoing fixed orthodontic treatment. Impaired oral hygiene can contribute to the demineralization of enamel and dental caries especially around the brackets. Dental healthcare professionals (DHCP) around the world are suggesting patients seeking dental treatment to follow stringent guidelines like oral and respiratory hygiene measures to control the spread of deadly diseases like coronavirus disease-2019 (COVID-19). In order to satisfy treatment needs and preserve oral health, orthodontists prescribe mouthrinses to their patients to aid in maintaining a healthy oral environment.

An ideal mouthwash should act effectively and rapidly on oral microorganisms, perform its action even at a low concentration, should have no side effects, and be usable without causing any discomfort to the patient.¹ Owing to the current COVID-19 pandemic or otherwise even to reduce plaque accumulation, preprocedural mouthrinses with oxidizing property have been recommended before commencing any orthodontic treatment. The US Centers for Disease Control (CDC) and the American Dental Association (ADA) have identified that 0.2% povidone-iodine (PI) and 1.5% hydrogen peroxide (H₂O₂) are beneficial prior to dental treatment as they reduce the viral load in the oral cavity.² However, to the best of our knowledge, little consideration is paid to the detrimental effects of these mouthwashes on the mechanical properties of orthodontic archwires. Nitinol (NiTi) and stainless steel (SS) alloys are the most widely used alloys in orthodontics. Since the discovery of NiTi wires for use in the field of orthodontics in 1971,³ these wires have been extensively used in orthodontic treatment during leveling and aligning owing to their high elastic limit and low modulus of elasticity.⁴ These SS and NiTi alloys are highly resistant to corrosion due to the formation of an oxide layer that acts as a shield on the surface of the wires. But constant exposure to mouthwashes used for controlling dental caries or periodontal disease, may potentially damage the metallic components of orthodontic wires leading to corrosion and may alter the physical, mechanical, and chemical properties of wires.^5,6 Grimsdottir et al. have demonstrated that metal ions can also be released from metallic orthodontic appliances as a result of corrosion from constant exposure to the oral environment, and this can influence the mechanical properties of the appliance and may also affect the body.⁷

Povidone-iodine is prepared by combining molecular iodine and polyvinylpyrrolidone, which is a solubilizing agent, making it soluble in water. This iodophor is protective against most bacteria, including putative periodontal pathogens, fungi, mycobacteria, and viruses with bactericidal and virucidal activity similar to pure iodine.⁸ Similarly, 1.5% hydrogen peroxide has also been accepted as an effective mouthwash with good virucidal activity as stated by Aggarwal et al.⁹ These mouthrinses are recommended for use by orthodontists prior to any dental procedure as a preprocedural mouthrinse. But, the effect of these two mouthrinses on archwires remains unknown. In order to study these effects, the surface characteristics, such as topography, roughness, and hardness of orthodontic archwires after the use of these mouthwashes have to be determined as they are important determinants of the effectiveness of archwire-guided tooth movement. They also are known to affect the corrosion potential and the esthetics of orthodontic components.¹⁰

The purpose of the current study is to evaluate the effects of the two approved preprocedural mouthrinses, H₂O₂ and PI mouthrinses on the surface characteristics and mechanical properties of NiTi and SS orthodontic archwires.

MATERIALS AND METHODS

The current study was carried out in the Department of Orthodontics and Dentofacial Orthopedics at SRM Dental College and Hospital, Chennai, India for a period of 3 months from June to August 2021. About 1.5% hydrogen peroxide (HYDRO-P®) mouthwash and 0.2% povidone-iodine (Betadine®) mouthwash were used in the present study. The control group comprised distilled water (DW). The wires tested were preformed 0.016” and 0.016 × 0.022” NiTi and SS wires (3M Unitek, USA). About 30 mm segments were cut from the straight portion of the posterior ends of the preformed archwires; 15 wires from each type were taken and divided into four subgroups with a total of 60 wire specimens being tested in the current study (Table 1). The dimensions of the wires were measured using a digital caliper. The diameter of the 0.016” NiTi and SS wires was measured to be 0.41 mm. The width and depth of the 0.016 × 0.22” NiTi and SS wires were measured to be 0.56 and 0.41 mm, respectively.

The modified Fusayama artificial saliva was prepared and used to mimic the oral environment.^11–14 The composition of the modified Fusayama artificial saliva is given in Table 2. In order to simulate the oral environment, the wires in each group were immersed separately in individual glass beakers containing solution with concentrations of 9% of artificial saliva and 91% of mouthwash.¹⁴ The samples were immersed for 90 minutes and incubated at 37°C in an incubator (Thermo Scientific, USA). An exposure for 90 minutes is equivalent to three months of 1-minute daily rinsing with the selected mouthwashes.^15,16 After the stipulated time, samples were removed from their respective mouthrinse solutions and rinsed well with distilled water, and then placed in clean, new containers.

STATISTICAL ANALYSIS

The collected data were tabulated and analyzed using the IBM.SPSS Software v23.0. The continuous variables were described using mean and standard deviation. To find the significant difference between the effects caused by the three groups of test solutions on the archwires, ANOVA was used. Bonferroni post-hoc test was performed to compare the intergroup variations caused by the mouthwashes on the orthodontic wires. In all the statistical tests, a probability value of 0.05 or less was considered statistically significant.

RESULTS

DISCUSSION

Maintaining good oral hygiene has always been considered an indispensable part of successful orthodontic therapy but it is also critical in the unprecedented times of the COVID-19 pandemic. Use of a mouthwash every day, which is effective against a wide spectrum of microorganisms is mandatory. A study done by Kariwa et al. demonstrated the efficacy of PI against the SARS-CoV and proved that gargling with PI mouthrinses can effectively kill the virus in the oropharynx, in addition to its prophylactic effect against the microorganisms causing the common cold and flu.¹⁸ Maren Eggers et al. have evinced that 0.2% povidone iodine (PI) possessed a good virucidal effect.¹⁹ Peng et al. recently have stated that rinsing with strong oxidative agents like 1.5% H₂O₂ could be beneficial because the novel coronavirus is vulnerable to oxidation.²⁰ Though both PI and 1.5% H₂O₂ are considered to be efficacious in reducing the viral count before any orthodontic or dental procedure, dental practitioners must be aware of their inimical effects before prescribing them for patients undergoing fixed orthodontic treatment.

Patients are usually instructed to gargle with a mouthrinse for 1 minute and are advised not to eat or drink at least for 30 minutes after gargling. Though the mouthrinse gets diluted with saliva, an exposure time of 1 minute could be greater as this makes the components of the mouthwash remain in the oral cavity for a long time, altering its pH and can potentially damage intraoral orthodontic appliances, such as archwires, brackets, elastics. Orthodontic archwires, such as NiTi alloy, SS, and beta-titanium alloy are the most commonly used wires in fixed orthodontic therapy. However, titanium-based alloys like nickel–titanium (nitinol) and beta-titanium (TMA) are usually preferred for their low elastic modulus and high resiliency.²¹ Since NiTi wires have a low load-deflection rate, exert light continuous force, and have a high elastic recovery, they are considered suitable during the aligning and leveling stages of orthodontic treatment.²² Garner et al. suggested that SS delivered significantly lesser frictional resistance and possesses higher stiffness than nitinol and TMA, hence recommending its use during sliding mechanics as working archwires.²³

Archwires must be carefully characterized based on their alloys, size, load-deflection ratio, and mechanical properties, such as modulus of elasticity, yield strength, elastic limit, and friction to assist the orthodontist in choosing the appropriate wire during every phase of fixed orthodontic treatment, especially during the aligning phase where most of the wire deflection and tooth movement are taking place.^24,25 This would enable the clinician to achieve a successful treatment outcome and curtail unwanted side effects like root resorption, necrosis, and pain.²⁶ Surface characterization of an archwire alloy is equally essential because of its impact on the mechanical properties as well as the corrosion potential of the alloys.¹⁰

A previous study by Walker and White has proved that NiTi and Copper NiTi wires showed pitting and corrosion on their surface after being treated with fluoride agents which in turn led to the degradation of mechanical properties, such as elastic modulus and yield strength, thus affecting orthodontic tooth movement.¹⁶ Later, Walker et al. investigated the impact of fluoride prophylactic agents on the mechanical properties at loading and unloading as well as the surface characteristics of beta-titanium and SS orthodontic wires, and discovered that the mechanical properties of beta-titanium and SS wires at unloading were greatly decreased (p < 0.05) after exposure to both fluoride agents, which could extend orthodontic treatment. They also found corrosive changes on the surface of the wires after exposure to these agents.¹⁵

During treatment, orthodontic wires are constantly exposed to saliva, which is a complex fluid of dissolved electrolytes with a high chloride content, enzymes, and various organic substances.²⁷ Due to this reason, orthodontic materials are made resistant to corrosion. Literature on the corrosion resistance of nitinol wires has been controversial. While few authors^10,28 have stated that nitinol possesses equal corrosion resistance comparable to SS, few others have demonstrated that nitinol undergoes more corrosion than other alloys.^29,30 However, there are several studies where reduction in corrosion resistance of titanium alloys like TMA and NiTi wires have been observed in acidic environments and also in the presence of fluoride ions.^31–33

In the present study, round and rectangular, SS and NiTi wires of 0.016” and 0.016 × 0.022” dimensions, respectively were examined. Orthodontic treatment is often initiated with the use of smaller dimension archwires (round wires) as the excessive play between the wire and the bracket causes free movement of only the crowns, thus aiding in aligning and leveling, whereas wires with rectangular cross-sections are used at a later stage to provide better control over root movement. The sizes of the rectangular wires are gradually increased in order to bring about better control over tooth movement as they snugly fit into the bracket slots.

In this in vitro study, Fusayama artificial saliva was used to simulate the oral environment since it closely resembles natural saliva.^12–14 After incubating the wire specimens at 37°C for 1.5 hours in 91% mouthwash (PI and H₂O₂) with 9% artificial saliva, the wires were thoroughly rinsed with distilled water and subjected to a three-point bending test which is the standard method for evaluating the mechanical property of orthodontic wires according to the American Dental Association specification no. 32.³⁴

The flexural modulus (E) is the stiffness of a wire, which is directly proportional to the force needed to deflect a wire. It also represents the load-deflection ratio. The greater the E of a wire, the higher the load-deflection ratio and its stiffness. Loading force is the force used to engage the wire into the bracket and the unloading force is the force delivered to the teeth and surrounding tissues during the aligning process.³⁰ Loading and unloading forces of the wires immersed in the three solutions (DW, PI, and H₂O₂) measured at different intervals showed statistically significant difference (p < 0.05). This difference may be attributed to the effects of these solutions on orthodontic wires due to the difference in their concentration and pH. The mean loading force of 0.016” SS and 0.016 × 0.022” SS wires seemed to be greater in the H₂O₂ mouthwash group at all intervals of deflection up to 3 mm. This signifies that the H₂O₂ mouthrinse stiffened the round and rectangular SS wires, thus raising the force required to insert the wire into the bracket slot. This was analogous to the mean values of the flexural modulus of the SS wires. Likewise, there was an increase in the mean values of the loading forces of 0.016” NiTi and 0.016 × 0.022” NiTi wires that were immersed in PI mouthrinse, thus implying that the stiffness of the wires had increased.

In the current study, the mean unloading forces of 0.016” NiTi wires at 0.5 mm were lower than the mean value of the wires in DW. Similarly, for 0.016 × 0.022” SS wires, the mean unloading force in PI was lesser than the corresponding value in DW. These findings suggest that routine use of PI and H₂O₂ mouthwashes may contribute to delayed tooth movement. Ahrari et al. observed a delay in tooth movement and attributed it to a decline in unloading forces of the wires at lower deflections due to fluoride treatment when compared with the control group.³⁵

The current study showed that mouthrinses like 1.5% H₂O₂, and 0.2% PI could influence the mechanical properties and surface characteristics of orthodontic wires within 1.5 hours. But a study by Huang in 2007 showed that minimal concentrations of fluoride-containing pastes, mouthrinses, and gels will not affect the surface topography of nitinol wires, in spite of immersing it in these agents for more than 28 days.³⁶

SS alloys have a passivating layer of chromium oxide which renders them corrosion-resistant. However, this shielding oxide layer can be destroyed by acidic agents making the wire susceptible to corrosion.³⁷ The performance of orthodontic wires can be affected by surface degradation.¹⁰ The scanning electron micrographs were suggestive of qualitative changes on the surface of both SS and NiTi wires on exposure to H₂O₂ and PI mouthrinses. The altered mechanical properties of the archwires, in the present study, may be attributed to the changes to the surface topography of the wires subsequent to treatment with the mouthrinse solutions. The possible mechanism of altered mechanical properties could be by a process known as hydrogen embrittlement.¹⁵ Hydrogen embrittlement of titanium wires is characterized by hydrogen diffusion through interstitial sites, grain boundaries, and cracks and reacting with the lattice atoms to form titanium hydride, forming a body-centered tetragonal structure, which has been considered to affect the mechanical properties.

In the present study, it is evident that surface irregularities in the form of striations, voids, and ridges on nitinol and SS wires immersed in the mouthwashes influenced the mechanical properties of these wires. The 0.016” and 0.016 × 0.022” nitinol wire specimens immersed in PI mouthwash showed a significant increase in the E (p = 0.000) at all intervals of displacement up to 3 mm except at 0.5 mm. In 0.016 × 0.022” SS wires, a significant decrease of E was observed at all intervals (p < 0.05). Immersion of 0.016” and 0.016 × 0.022” SS wires for 1.5 hours in H₂O₂ mouthrinse caused a significant increase in its E with p < 0.05, which apparently increased its stiffness.

Clinically, the surface irregularities on the wires can lead to an increase in friction at the bracket–wire interface, thereby disrupting the effective tooth movement which could extend orthodontic treatment time. The results of the present study suggest that though both the mouthrinse solutions brought about variable surface changes on the wires, the flexural modulus of NiTi wires showed a significant increase when immersed in povidone-iodine, while that of SS wires increased significantly in hydrogen peroxide mouthrinse. This could possibly be due to the difference in composition of the wires which could have been likely targeted by the two mouthwashes at varying rates. Therefore, both the mouthwashes were found to significantly change the mechanical properties of the round and rectangular Nitinol and SS wires. Based on the present study, hydrogen peroxide mouthwash can be recommended in orthodontic practice because it did not show any significant difference in the E of wires at most of the deflection intervals when compared with distilled water. Moreover, it does not cause any adverse reactions to the mucosa, gingiva, or tongue when used at such a low concentration (1.5%) as proved by earlier studies^38,39 and hydrogen peroxide has been suggested to have acceptable taste and no odor when compared with povidone-iodine.

In the present study, the archwires were immersed continuously for 90 minutes in the mouthwashes. To simulate clinical conditions, instead of continuously exposing the wires, repeated shorter exposures to the mouthwashes can be performed. Therefore, a study employing a protocol of cyclic immersion of wires in mouthwashes may be undertaken and also a study of ions leached from orthodontic wires after exposure to hydrogen peroxide and povidone-iodine mouthwashes should be considered in the future as these acidic agents have shown considerable surface deterioration in the present study on SEM analysis.

CONCLUSION

From the current study, it can be concluded that there was a significant increase in the flexural modulus of Nitinol wires in povidone-iodine mouthrinse while there was a significant increase in the flexural modulus of stainless steel wires in hydrogen peroxide mouthwash. SEM analysis showed qualitative surface changes of varying degrees in the form of corrosion, voids, and ridges on the wires after exposure to both the mouthwashes with more corrosion of wires in PI comparatively. Thus, the present study shows that hydrogen peroxide mouthwash can be prescribed to orthodontic patients as a preprocedural rinse since it did not cause significant difference in the E of wires at most of the deflection intervals when compared with distilled water.

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