In Vivo Comparison of Ultimate Tensile Strength of Nickel-Titanium Aligning Archwires Exposed to Fluoridated Mouthwash

Introduction: Orthodontists usually prescribe fluoridated mouthwash to their patients to mitigate risk of dental caries. These mouthwashes may alter the mechanical properties of archwires by dissolving the protective oxide-layer. The aim of the study was to examine the effects of fluoridated mouthwash on ultimate tensile strength of NiTi wires after three months of intra-oral use. Materials & Method: Sixty 0.014” (0.36mm) Nickel-titanium (NiTi) wires (Ormco Corp, Glendora, USA) were divided into three groups: Group A: as-received archwires serving as controls, Group B: intra-oral used wires exposed to placebo mouthwash and Group C: intra-oral used wires exposed to fluoridated mouthwash for a period of 3 months. Each wire was subjected to tensile testing using a table-top tensile testing apparatus. Ultimate tensile strength data was analysed with one-way analysis of variance (ANOVA) and Tukey testing at the 0.05 level of significance. Result: Results showed that amongst tested wires, those exposed to fluoridated mouthwash had least tensile strength (79.45 ± 1.33 N) as compared to wires exposed to placebo mouthwash (99.95 ± 0.91 N) and those in the control group (117.69 ± 0.66 N). The results were statistically significant at p <0.001. Conclusion: Ultimate tensile strength was least for wires exposed to fluoridated mouthwashes for a period of 3 months. Nonfluoridated mouthwashes can be used during the initial phase of leveling and aligning with NiTi wires.


INTRODUCTION
Malocclusion, is a common dental problem in India and has a wide prevalence of 20 to 88 %. 1 Orthodontic mechanotherapy is applied to correct these abnormalities, either by removable or fixed method. Archwires form a cornerstone of such mechanotherapy, by exerting mechanical forces to align maloccluded teeth.
Today's armamentarium of archwires, range from older stainless-steel (SS) wires to newer NiTi (nickel-titanium), CuNiTi (copper nickel-titanium) and TMA (titanium molybdenum alloy) alloy archwires. 2 Among these, NiTi wires have earned a special place due to their inherent properties of superelasticity and shape memory. 3 Corrosion resistance of these wires is supposedly due to formation of a thin, stable titanium oxide layer during intra oral use. 4,5 NiTi wires enable a clinician to apply, light continuous force, over large activation areas with reduced patient discomfort.
Development of dental caries during orthodontic therapy is common, hence orthodontists often recommend prophylactic use of fluoride containing mouthwash to mitigate it. 6,7 There is current evidence to link use of fluoride mouthwashes and increase in the leaching of Ni and Cr ions from NiTi archwires. 8 Fluoridated mouthwashes are commercially available in two strengths, sodium fluoride 0.05 % for daily use and sodium fluoride 0.2% for weekly use, and both are equally effective for prophylactic use. 9 Systemic consumption of fluorides in some endemic areas, through water and even accidental ingestion (common among small children while brushing with fluoride toothpaste) is detrimental to health (skeletal and dental fluorosis). Ni and Cr ion leaching have been Keerthana P, Chitra P,Puneeth SB, Janardhanam LSL : In Vivo Comparison of Ultimate Tensile Strength of Nickel-Titanium Aligning Archwires Exposed to Fluoridated Mouthwash associated with genotoxic and cytotoxic changes in mucosal cells. 10,11 Further, corrosive damage alters the mechanical properties of NiTi archwires by reducing the ultimate tensile strength and increasing NiTi wire fracture rates.
It is widely known that older stainless steel archwires are more prone to corrosion than newer (NiTi) archwires. 6 Many in vitro studies have been carried out on corrosive The purpose of this prospective randomised "in vivo" study was to evaluate the effect of fluoride mouthwash when used for a period of three months on the ultimate tensile strength of nickel-titanium archwires.

MATERIALS AND METHOD
Sixty 0.014-inch NiTi archwires (Ormco Corp, Glendora, USA) were divided into three equal sub-groups of twenty each: (A) As-received wires which served as baseline (B) Intra-oral used wires exposed to placebo mouthwash, (C) Intra-oral used wires exposed to

Fig. 2 Tensile grips
Keerthana P, Chitra P,Puneeth SB, Janardhanam LSL : In Vivo Comparison of Ultimate Tensile Strength of Nickel-Titanium Aligning Archwires Exposed to Fluoridated Mouthwash

Methodology as described by Devaprasad and
Chandrasekaran15 (2012), was followed in this study.
The machine parameters were set to test a scale load of "1000 N", and was operated in a "tensile" mode with a crosshead speed of "5 mm/min" to enable analysis of ultimate tensile strength of each wire. Each wire sample was cut at one end with a standardized span of 20 mm and was placed between the jaws of the machine (Fig 2). The maximum tensile load, which the wire could withstand prior to its failure, was noted ( Fig 3).
The required sample size was estimated using G Power software Ver 3.1.9.2 (Fig 4). The measured effect size (f) at 48%, study power at 80%, and error margin at 5%, was considered for which the total sample size required was 45. So, to consider for the attrition bias, sixty subjects were included in this study. Quantitative analysis of tensile strength was evaluated statistically using one-way analysis of variance (ANOVA), with mouthwash serving as a predictor. Group differences were evaluated using Tukey HSD (honestly significant difference) with a p value < 0.05 considered as the level of statistical significance..    [23][24][25] It is known that titanium (Ti) has a unique property of enhancing corrosion-resistance of orthodontic archwires by forming a protective film of titanium oxide (TiO 2 ). But on surfaces of nitinol archwires, addition of nickel oxide or metallic Ni to TiO 2 makes it more susceptible to degradation. 26 The oral cavity is constantly exposed to saliva, ingestion fluids and foods with different pH (right from alkaline food e.g. fruits to acidic e.g. soft drinks) and showed that ultimate tensile strength of used 0.014" NiTi archwires exposed to fluoridated mouthwash, is lower than that of archwires exposed to placebo mouthwash and as-received archwires. The results

RESULT
were statistically significant (p<0.05). Our results were in line with studies done by Ramalingam et al. 28 and Walker et al., 29 who compared the exposure effects of fluoride rinse and fluoride gels on patients with NiTi wire, 28 and found fluoride agents alter the mechanical properties of the wire 28 and increase the duration of orthodontic treatment. 29 Perinetti et al, 30 in an in vitro study found no significant effects on the fracture resistance of two commercially available NiTi based archwires exposed to fluoridated agents. NiTi wires when used in-vitro are usually unbreakable, 19 but they do break on intra-oral use. In Further, surface evaluation of the exposed archwires can also be done in future studies.

CONCLUSION
• There was a statistically significant reduction in the tensile strength of NiTi archwires after exposure to fluoride mouthwash for a period of 3 months.
• Fluoridated mouthwashes must be used with caution in orthodontic patients who are in the 1st stage of leveling and alignment with NiTi aligning wires.
• Non-fluoridated mouth rinses can be prescribed by orthodontists as an adjunct to oral hygiene regime to orthodontic patients with NiTi aligning archwires.