Introduction

The introduction of ceramic brackets and titanium wires used during orthodontic treatment enabled orthodontists to provide their patients with excellent esthetics and controlled tooth movement during their treatment. However, the various agents used for maintaining good oral hygiene during orthodontic treatment may affect the physical properties of titanium wires, and thus may increase the frictional forces between ceramic brackets and titanium wires. Consequently, this may affect the outcome of orthodontic treatment [1- 4].

Friction is defined as a force that delays or resists the relative motion of two objects in contact, and its direction is tangential to the common interface of the two surfaces [4, 5]. Friction is an important factor in sliding mechanics, where the arch-wire must slide through the bracket slots and tube [5, 6]. High levels of frictional force could result in debonding of the bracket, associated with either a small dental movement or no movement at all. When friction prevents movement of the tooth to which the bracket is attached, friction can reduce the available force by almost 40%, resulting in an anchorage loss [5,7, 8].

Correct selection of orthodontic brackets and wires suitable for each case is the first step for achieving a proper orthodontic movement that can be applied to obtain optimum biologic tissue response [3, 5]. The maintenance of good oral hygiene and caries control is the second step to ensure the success of orthodontic treatment outcome. Fluoride prophylactic agents, such as acidulated phosphate fluorides (APF), have been used extensively for remineralization of white spot lesions around orthodontic brackets. However, the fluoride ions in the prophylactic agents have been reported to cause corrosion and discoloration of titanium and its alloys [9-12], particularly when the passivated wire surfaces break because of mechanical friction between brackets and wires.

To date, there are no combined in-vitro and in-situ studies comparing the effects of fluoride on different orthodontic wires and their frictional resistance. Therefore, the aim of this study was to investigate the effect of topical fluoride on the surface properties of beta-titanium and stainless-steel wires by scanning electron microscope (SEM), and to evaluate the friction resistance of beta-titanium wires and stainless-steel wires to ceramic brackets before/after application of fluoride. The null hypothesis in this study is that the fluoride application will significantly affect the friction resistance and the surface properties of the arch-wires tested in the current study.

Materials and Methods

Two types of orthodontic wires; Stainless-steel alloy TruChrome Resilient archwire (RMO, Inc. Denver, Colorado. USA); and preformed beta-titanium Bendaloy Nickel Free archwire (RMO,Inc. Denver, Colorado. USA) with a cross section of 019 x 0.025 inch were investigated. 50 central incisor ceramic brackets standard edgewise (No built in torque or tip) (Signature III, RMO,Inc. Denver, Colorado. USA) with slots size 0.022 x 0.028-inch were used in this study. (Figure 1) shows summary for the experimental groups.

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In-vitro Study

In this study 20 stainless-steel alloy and 20 beta-titanium archwires were used. The topical fluoride agent used was Acidulated phosphate fluoride (Nupro APF) 1.23% NaF, pH 3.9. (Dentsply International, York, USA). All wire specimens were immersed in fluoride gel for 1 hour [11]. Half of the fluoride treated wires were examined by SEM while the remaining wires from each group were tested for their friction coefficient and then examined by SEM.

In-situ Study

Informed consents were obtained from all the participants of the study and the protocol was approved by the ethical committee of the faculty of Dentistry. Twenty orthodontic patients attending the Department of Orthodontics clinic, Faculty of Dentistry, King AbdulAziz University. wearing full bonded fixed orthodontic appliance were selected according to the following criteria:

All patients were young adult females with permanent dentition ranging from 12-18 years old.

1. All patients had their teeth leveled and aligned.
2. Each patient had a fixed orthodontic appliance with standard edgewise ceramic brackets (RMO, Inc. Denver, Colorado. USA.).
3. Brackets were bonded with no-mix chemical curing bonding resin (Reliance Orthodontics products, Inc).
4. Good oral hygiene instructions were given to all patients.

Patients were randomly allocated to two groups using a computergenerated randomization list (n=10). 10 archwires with a cross sectional dimension 0.019 x 0.025-inch from each type of wire were placed inside the patients` mouths and the fluoride gel (Nupro APF, Dentsply, York, USA) 1.23% NaF, pH 3.9, was applied into the applicator trays. Patients were instructed to expectorate after treatment, not to eat, drink or rinse for 30 minutes.

The stainless-steel and beta-titanium archwires were left passive inside the patient mouth for 3 weeks. Then they were collected with sterile instruments. Half of the fluoride treated wires were examined by SEM while the remaining wires from each type group were tested for their friction coefficient and then examined by SEM.

Frictional force Examination

A total of 50 ivory central incisors were embedded in an acrylic resin block in a metallic mould. 50 Ceramic brackets were bonded on the labial surface of the ivory teeth using a bracket positioner. The universal testing machine (Instron Corporation, Canten Industries, Inc., Florida, USA) was used for measuring the generated frictional force at the bracket-wire interface, by sliding the wire through the bracket slot, under small tangential displacements. The metallic mould was fixed on the universal testing machine. Wires 4 cm long from each group were inserted into the bracket slot.

Elastomeric ligatures (American orthodontics, Sheboygan, USA) were placed over the bracket tie-wings engaging the arch wire. One end of the tested wire was left free, and the other end was attached to the moving upper arm of the universal testing machine with 5-kg load cell.

The tested wire was pulled upward through the bracket slot at a speed of five mm per minute for a minute [3, 13]. The frictional force generated by each wire-bracket assembly was registered in kilograms by the tension load cell. After each test, the mould was removed and a new wire was placed with a new ceramic bracket. Static frictional force was measured as the value of force needed to start wire movement through the bracket slot. This force was measured as the maximal initial moving force on the universal testing machine at one minute intervals of moving wire.

Scanning Electron Microscope Examination

All specimens were examined before and after friction test by SEM (Jeol JSM – 225 – 11- Scanning Microscope, Jeol Ltd., Japan) to determine surface changes on the orthodontic wire after the friction test and the application of fluoride. The wire specimens were examined using the SEM and photomicrographs were taken at a low magnification of x150 and a higher magnification of x500.

Statistical Analysis

The results were recorded and analyzed statistically using paired t-test to compare frictional resistance means of the stainless-steel wires to beta titanium wires in as-received, in-vitro and in-situ experiments (p≤0.05). Dunnett t-test was used to compare the friction coefficient of the control (as-received) wires for each type of wire to their corresponding type of wires after fluoride application either in-vitro or in-situ. All statistical examinations were carried out at a level of significance of (p≤0.05).

Results

Friction Resistance Test

Results revealed that the degree of friction generated at the bracket/archwire interface was affected by the type of archwire (p≤0.05). The mean and standard deviation of frictional resistance of the two archwires are summarized in (Figure 2).

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t-test showed that there was significant difference between the friction resistances of stainless-steel wires attached to ceramic brackets when compared to the friction resistance of beta-titanium wires attached to ceramic brackets in the as-received,in-vitro and the in-situ experiments (Figure 2).

Dunnett t-test showed that there were no significant differences between the frictional resistance means of all wires` in-vitro and in-situ specimens when compared to their as-recieved specimens (p≤0.05) (Figure 2).

Scanning Electron Microscope

The SEM micrographs of the side surfaces of the two types of wires in the three groups are shown in (Figure 3, 4).

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The observed surfaces of the as-received wires showed that stainless- wires exhibited a smooth surface before friction test (Figure 3A). Fine tracks resulting from the draw of stainlesssteel wire after friction were observed (Figure 4A). The as-received betatitanium side surface was rough with many pores, surface defects and corrosion pits produced by the manufacturing process (Figure 3D). After the friction test pits and grooves were observed on the beta-titanium wire surface, along with wear tracks resulting from the friction test (Figure 4D).

The in-vitro application of fluoride exerted detrimental effects on the beta-titanium wires with evident signs of surface corrosion either before or after friction test. Before the friction test fluoride attacked the beta-titanium surface causing numerous grooves and cracks which were associated with generalized corrosion covering the whole surface of the wire (Figure 3E). After the friction test (Figure 4E) generalized roughness and corrosion products together with numerous grooves were observed. However, the stainless-steel wires showed less signs of surface deterioration either before or after the friction test (Figure 3B, 4B).

The application of fluoride in the in-situ experiment affected the surfaces of beta-titanium wires causing the coverage of the wire surfaces with corrosion products (Figure 3F). After friction test beta-titanium wires exhibited peeling of the surface with numerous deep grooves (Figure 4F). The stainless-steel in-situ specimens showed a smooth surface with few pits either before or after the friction test (Figure 3C, 4C).

Discussion

Fluoride application has an important role in the caries control process however; the interaction of fluoride with orthodontic titanium wires should be taken into consideration during the orthodontic treatment. In the current study, fluoride was applied for the first time on the orthodontic wires inside the patients’ mouths and then retrieved to verify the exact effect of fluoride on the beta titanium wires. Fluoride application affected the surface properties of the beta titanium wires; however, it did not affect the friction resistance of the beta-titanium archwires or the stainless-steel arch-wires to ceramic brackets. The null hypothesis adopted in this study was partially accepted.

All wires used in this study were rectangular wires because these types of wires offer control in all three planes [14, 15]. The acidulated phosphate fluoride gel used in the current study has an approximate concentration of 10,000 part per million of fluoride with an acidic pH of 3.9 due to its documented effect on preventing caries. The fluoride agent was applied in-vitro and in-situ to observe its effect on the tested wires [16, 17].

The selected duration of fluoride application in the in-situ study was according to the manufacturer’s instructions. The duration of fluoride application in the in-vitro study was 60 minutes which corresponds to 21 days of daily tooth brushing which is equivalent to average follow up period during orthodontic treatment [11]

In the in-situ experiment all patients had their teeth leveled and aligned with the wires placed with no force applied on them to avoid any frictional forces exerted from the brackets.

Beta-titanium arch-wires recorded significantly high coefficient of friction when compared to stainless-steel wires in the as-received, in-vitro and in-situ experiments. These results may be attributed to the rough surfaces observed in the beta-titanium wires by SEM in the current study.

This agrees with many studies documenting the direct relation between the increase of surface roughness and the increase of friction resistance [6, 13, 18-21].

The observed beta-titanium rough surface for the as-received wires was probably due to the complex manufacturing process of the beta-titanium wires [22].

On the other hand, stainless-steel as-received wires exhibited a smooth surface with the existence of very few scratches and minute pits which may be due to the polishing of the wire during its manufacturing.

The SEM pictures of the two types of wires after one hour of topical fluoride immersion (invitro experiment) showed the different responses of the wire surfaces to this challenge due to the difference in corrosion behavior between the two wires. Stainlesssteel wires corrosion resistance is ascribed mainly to the presence of Cr2O3 in the passive film formed on its surface. On the other hand, the corrosion behavior of the titanium containing alloys depends on an oxide film composed of mainly titanium oxide on the surface, which spontaneously covers the surface of titanium and its alloys in the presence of oxygen.

It is speculated that in the in-vitro current experiment stainlesssteel wires exhibited the least changes on its surface, due to the stability of the protective Cr2O3 layer formed on its surface [23]. However, the beta-titanium wires exhibited a deteriorating surface when exposed to the acidulated fluoride topical agent which suggests that the titanium oxide protective layer was destroyed due to the attack of the fluoride ions in addition to the acidic medium of the topical fluoride agent, allowing the process of hydrogen embrittlment to take place [24].

There are some considerations that must be taken when interpreting the results of the in-vitro friction experiment and comparing it to the in-situ experiment results. This may be explained by the fact that when an orthodontic wire is exposed to the oral environment, a non cellular acquired biofilm is rapidly organized on the material surface by spontaneous adsorption of extracellular macromolecules composed of glycoproteins and proteoglycans. The formation of biofilm is followed by a modification of wire surface properties. The outcome of the biofilm adsorption depends on the biological fluid flow rate at the site of contact, the type of interfacial interactions involved, and the attachment strength with substrate. In the oral cavity some precipitation of calcific biofilm and formation of micro particles on the slot wall and wire surfaces may occur. These precipitations might either reduce the coefficient of friction by producing a boundary lubrication effect (i.e. through salivary protein adsorption and plaque accumulation). Alternatively, calcified integuments might increase the surface roughness and friction resistance [25].

However, the in-situ current experiment showed the same deteriorating effects for the surface properties of beta-titanium wires in the in-vitro experiment after fluoride application which suggests that saliva buffering effect on the fluoride applied in-situ was not enough to overcome the negative effect of fluoride on beta-titanium wires [17].

A previous study 26 showed severe deterioration of the Nickel Titanium wires friction resistance to ceramic brackets tested in-vitro after topical fluoride application, while, the in-situ results did not show significant increase in friction resistance of Nickel Titanium wires to ceramic brackets exposed to a similar topical fluoride application challenges. The aforementioned mentioned results and the results obtained from the current experiment confirm the importance of conducting both in-vitro studies and comparing it to the in-situ results.

Further research is needed to investigate the exact biological effect of the released ions from corrosion process of the titanium containing wires on the oral environment.