Effect of fluoride on mechanical properties of NiTi and CuNiTi orthodontic archwires: an in vitro study

ABSTRACT Introduction: After debonding, white spot may appear on the area below the bracket, which is the early clinical sign of carious lesion. There is increased caries risk underneath and adjacent to orthodontic bands and brackets, which call for maximum use of caries preventive procedures using various fluoride application methods. Objective: The aim of the study was to evaluate alterations in the mechanical properties (modulus of elasticity and yield strength) in loading and unloading phases for different orthodontic archwires (nickel-titanium [NiTi] and copper-nickel-titanium [CuNiTi]) when exposed routinely to fluoride prophylactic agents for a predetermined period of time. Methods: Preformed rectangular NiTi and CuNiTi wires were immersed in fluoride solution and artificial saliva (control) for 90 minutes at 37ºC. After immersion, specimens were tested using a 3-point bend test on a universal testing machine. Results: There is a significant reduction in the unloading yield strength when the NiTi and CuNiTi wires were exposed to APF gel. Conclusion: The result suggests that use of topical fluoride agents affect the mechanical properties of the wires, leading to increase in treatment duration. Fluoride prophylactic agents must be used with caution in patients undergoing orthodontic treatment. Injudicious use of these agents may cause corrosive effects on the orthodontic wire surfaces, with alteration in their mechanical properties.


INTRODUCTION
After debonding, white spot may appear on the area below the bracket, which is the early clinical sign of carious lesion. There is increased caries risk underneath and adjacent to orthodontic bands and brackets, which call for caries preventive procedures using fluoride application methods.
Orthodontic tooth movement results from forces produced by the appliances (wires, brackets, elastics, etc) inserted and activated by the clinician. 1 NiTi wires are predominantly used in the early stage of orthodontic treatment.
Orthodontists routinely move teeth by attaching brackets to them and activating archwires within the slots of brackets. During space closure, sliding a wire through the slot of a bracket may produce frictional forces, which opposes the treatment plan.
Slot size, surface characteristics of bracket/archwire and the forces used produce friction during orthodontic tooth movement. According to Watanabe et al,4  Wires containing nickel are used routinely during orthodontic treatment. Oxygen reacts with the surface of all metals to form anoxide surface layer, which protects the metallic surface from corrosion. When friction occurs between the archwires and brackets, the oxide layer dissolves and surface corrosion and pitting take place.
The current study aimed to evaluate changes in the mechanical properties (modulus of elasticity and yield strength), during loading and unloading phases for different orthodontic archwires, when exposed to routinely used fluoride prophylactic agent for a predetermined period of time. Considering the beneficial effects of the prophylactic agents in preventing decalcification of teeth around orthodontic brackets, the objective of this study is to determine whether exposure to these fluoride prophylactic agents causes pitting and corrosion on the surfaces of archwires.

MATERIAL AND METHODS
The materials used for the study included: 1. NiTi and CuNiTi preformed archwires.  The control solution used was deionized water [dH 2 O].
Each wire specimen was 0.42 X 0.62 X 25mm in dimension, cut from the straight portion of the preformed archwires.

METHODS
The control group and experimental groups were established.
Control group: » Ten (n = 10) wire samples consisting of five (n = 5) samples from each wire group, placed in deionized water. The wires were tested in a chamber with the temperature 37 ± 1°C, which is similar to the oral environment. Each wire was loaded to a deflection of 3.1mm and then unloaded to zero deflection at a cross-head speed of 1mm/min.

RESULTS
The distribution of average values for the loading and unloading properties of NiTi wire specimens in deionized water (control) and other experimental fluoride agents is presented in Table 1 and Figure 2A.

DISCUSSION
In this study, evaluation of two types of wires was done to observe the effect of fluoride agents on unloading mechanical properties (yield strength and modulus of elasticity).
Corrosion of titanium-based archwires 2-9 has been reported due to application of topical fluorides, which suggests that mechanical properties of titanium-based alloys may be altered due to fluoride application, mainly because of hydrogen embrittlement. This further may reduce the clinical efficiency, by increasing the resistance during tooth movement, which can lead to anchorage loss during leveling and aligning, and also during retraction phase.
According to Toumelin-Chamela et al, 5 due to hydrogen embrittlement, there is an increase in fracture susceptibility of titanium-based orthodontic wire when kept in fluoride solution.
There is a decrease in alloy's mechanical properties 12,13 due to formation of titanium hydrides, which is reported to form a body centered tetragonal structure 11 . When the NiTi wires were exposed to fluoride for 90 minutes, a decrease in both unloading modulus of elasticity and the unloading yield strength of the wire was found. This is due to the formation of titanium hydride, caused by the hydrogen penetration in the lattice of the NiTi, which alters the lattice's ability to undergo the unloading phase shift from the martensite form to the austenitic form. In a clinical setting, exposure of NiTi wires to fluoride causes reduction of the tooth movement during unloading stage and also affects the spring-back property of these wires. The effects of fluoride on wire also depends upon the pH and fluoride content of the agent.
When NiTi wires were exposed to APF gel, there was a significant decrease in unloading yield strength, but the unloading modulus of elasticity did not significantly decrease. The mechanical properties are more affected, compared to fluoride agents.
When CuNiTi wires were exposed to APF gel there was a significant decrease in unloading yield strength, compared to control samples.
Due to the presence of copper in CuNiTi wires at the alloy/oxide interface, which avoids the hydrogen penetration, that will reduce the formation of titanium hydride in the lattice structure.
In this study, there was significantly higher yield strength in control PreviDent gel, compared to acidulated Phos-Flur gel; and during the unloading phase, the CuNiTi wires showed significant decrease in mechanical properties. Also, in the present study only the unloading phase was studied, therefore there is a scope for the study of effects of fluoride agents on the mechanical properties.

CONCLUSION
In the present study, NiTi wires upon exposure to fluoride agents showed significant decrease in mechanical properties.
Clinically it is the deactivation forces that cause depletion of mechanical properties of titanium-based archwires. Decrease in the unloading yield strength produces adverse effect, due to the spring-back property of these wires.
Previous studies have shown that 90 minutes is equivalent to one minute daily application/rinses for a period of approximately three months. 1 Therefore, in this study the NiTi wires that usually remain in oral environment for more amount of time during leveling and aligning purpose were tested. The fluoride agents that were used are the commonly prescribed oral hygiene agents.
In conclusion, this study suggests that the routinely used fluoride prophylactic agents should be cautiously used in patients during orthodontic treatment because the excess and prolonged use of these agents may cause corrosion of orthodontic wire surfaces along with alteration in mechanical properties, leading to prolonged treatment time.