As there has been a recent increase in orthodontic treatments of adult patients with aesthetic requirements, the use of ceramic brackets and resin brackets is increasing. In order to improve the aesthetics, orthodontic wires as well as brackets with tooth-colored coatings have been developed. However, since the coefficient of friction of aesthetic ceramic brackets is greater than that of the metal brackets, it is necessary to consider the changes in friction between the brackets and orthodontic wires. A large frictional force is needed for managing anchorage and tooth movement in a closed-loop mechanism, while in sliding mechanisms, reduction of the frictional force is needed for preventing anchorage loss and for effective tooth movement.1 Therefore, consideration of the loss of force by frictional forces is necessary for optimal clinical tooth movement. In a study by Kusy and Whitley,2 it was found that 12=60% of the orthodontic force is reduced by the frictional force applied during orthodontic treatment.

Recently, numerous studies have examined the physical properties of aesthetic materials, including the following: the corrosion and the fracture resistance of coated wires by Neumann et al.3; the mechanical properties such as stiffness (evaluated through the three-point bending test), surface roughness, and resistance of the coating of coated wires by Elayyan et al.4,5; and thermo-chemical degeneration of coated wires by Bandeira et al.6 However, there are insufficient studies on the evaluation of frictional changes as a result of wire coating.

Jang et al.7 compared the frictional forces of rhodium-coated wires (Hubit Co., Ltd., Uiwang, Korea) and non-coated wires (Tomy Inc., Futaba, Japan) using 0.016-inch nickel titanium (NiTi) wires, 0.016 × 0.022-inch NiTi wires, and self-ligation brackets. The angles between the brackets and wires were set at 0°, 3°, 6°, and 9°. When the angles between the brackets and the wires were 3° using only a 0.016 × 0.022-inch NiTi wire and 6° and 9° using all 2 types of wires, the frictional forces of the coated wires were significantly higher than those of the uncoated wires. The higher frictional forces were the cause of the destruction and separation of the surface coating material on the tension side of the wires as the angle between the brackets and the wires increased.

This in vitro study aimed to compare the frictional forces of polymer- and rhodium-coated wires with those of uncoated wires in various angulations between the brackets and the wires, using self-ligation brackets (Clippy-C®, Tomy Inc.) and round and rectangular wires.

Frictional force is the force that resists the relative motion of surfaces sliding against each other. One type of frictional force is the static frictional force that is present until movement starts, and the other is the kinetic frictional force that appears during movement. There is controversy concerning which frictional force is more significant in clinical orthodontic treatment. In the study by Drescher et al.,8 as the static friction and kinetic friction occurred at nearly the same time owing to the low crosshead speed of orthodontic tooth movement, distinguishing between the static and kinetic frictional forces was difficult. Frank and Nikolai9 used only the maximum kinetic frictional force because it was difficult to reproduce the kinetic frictional force in the oral cavity. In a study by Burrow,1 it was found that the static frictional force was more appropriate than the kinetic frictional force as orthodontic tooth movement is not continuous. On the other hand, Kusy and Whitley10 measured both static and kinetic frictional forces; Tselepis et al.11 used only the kinetic fictional force after the maximum static frictional force was reached because there was a constant alternating between the static and the kinetic frictional forces as the tooth intermittently slid and bound along the arch wire during orthodontic movements. In this study, the maximum static frictional force and the kinetic frictional force were measured because we believe that frictional values may vary for a given bracket-arch wire combination as the tooth moves along the arch wire in a tipping and uprighting fashion.11

The maximum static frictional forces and the kinetic frictional forces of the coated wires were equal to or higher than those of the uncoated wires, and there was a difference in the degree of change depending on the type of coating. The R group had the greatest frictional forces and the control group had the smallest frictional forces because the surfaces of the wires had different levels of roughness owing to the coating materials. The wires of the P group had an Ag coating for the white color and a Parylene membrane coating on the outer surface of the Ag layer. Parylene coatings had been used in many industries before being used for orthodontic wires. A parylene coating is generally known to be very robust, durable, and waterproof, have excellent chemical resistance, and exhibit less discoloration. It also has a great impact on the aesthetics because of its excellent light permeability and its non-toxic nature. Above all, the smooth membrane of a Parylene coating can reduce the roughness of the surface, and this is why the P group had lower frictional forces than the R group in our study. A rhodium coating is used on orthodontic wires because it is white, more aesthetic than other metals, and chemically stable because rhodium is a precious metal; it also has excellent wear resistance. However, the high surface roughness of rhodium produces greater frictional forces. In this study, the differences in frictional forces according to the type of coating were due to the direct influence of the polymer and rhodium in the coating. But we must consider the surface roughness as a characteristic of the material itself, and we need to take into account that corrosion, creep and relaxation, and the manufacturing processes (polishing, heating treatment, etc.) can affect the change in the resistance (corrosion, creep, and relaxation).9

We have to consider the changes in the thickness of the wire that result from the differences in the aesthetically superior materials that are used to coat the wire. Wires in both P and R groups had additional coatings on ready-made non-coated 0.016-inch NiTi wires and 0.017 × 0.025-inch SS wires, so fine increases in thickness appeared. In a study by Iijima et al.,12 the coating thickness in the R group was 10 µm, and so it did not affect the mechanical properties. DANY BMT, which manufactures the wires in the P group, explained that the changes in the wire thickness resulting from the additional coating of two polymer layers are less than 5 µm unilaterally and 10 µm bilaterally (Figure 7). However, the wires with polymer coating were thicker than the other two wires, as measured by digital vernier calipers that can measure lengths as small as 0.01 mm. The thickness of the polymer-coated 0.016-inch NiTi wires was 0.41 mm; the thicknesses of the uncoated wires and rhodium-coated wires were 0.37 mm; the thickness of the polymer-coated 0.017 × 0.025-inch SS wires was 0.42 × 0.62 mm; the thickness of the other two wires was 0.040 × 0.61 mm. Even if the wires were the same size like the 0.016-inch and 0.017 × 0.025-inch wires, different thicknesses, depending on the coating materials, could change the frictional forces by affecting the binding between the brackets and the wires. Consequently, we would need additional studies to elucidate the clinical significance of the changes in frictional force according to different coating thicknesses.

Changes in the frictional force according to the angle between the brackets and the wires are considered an important issue because in most cases, the orthodontic force acts at a slight angle rather than parallel to the brackets and wires during tooth movement. In a study by Kusy and Whitley,13 three components were found to affect frictional forces when the orthodontic wire slid. They were the static and the kinetic frictional forces, binding in the contact between the wires and the bracket's slot corner, and notching, which is a permanent deformation of the wires. In addition, the researchers suggested that the critical contact angle was 3.7°, which increased the frictional force rapidly as the brackets and wire were binding when a 0.016-inch wire was used with a 0.022-inch bracket slot. To evaluate the frictional force that resulted when the angle between the brackets and wires was changed to 0°, 5°, and 10° in this study, the angles used included critical contact angles.

Depending on the difference in angle between the brackets and wires, the maximum static frictional force and kinetic frictional force increased significantly in all cases as the angulations between the brackets and wires increased. The frictional resistance occurred at the bracket slot base when the angulation between the bracket and wire was calibrated at 0°. However, as the second-order angulations increased, frictional resistance occurred on the vertical planes of the bracket slot, as well as on its base. This additional vertical resistance increased the frictional forces. The frictional resistance increased rapidly when the angulation between the bracket and the wire was set at 5° because the critical contact angle was 3.7°. The frictional force also increased rapidly as the bracket and wire were binding if a 0.016-inch wire with a 0.022-inch bracket slot was used.13

The order of the groups regarding the values of the maximum static frictional force and kinetic frictional force was different when the angle was 10° between the brackets and the 0.016-inch NiTi wires. The maximum static frictional force was as follows: control group < R group = P group. The order for the kinetic frictional force was as follows: P group = control group < R group. The maximum static frictional force in the P group was greater than that of the R group, although the difference was not statistically significant. It is supposed that the degree of binding altered as the stiffness of the 0.016-inch NiTi wire changed according to the type of coating. It is believed that because the 0.017 × 0.025-inch SS wires was thick and less elastic, they exhibited different results compared to the 0.016-inch NiTi wires.

Using SEM, the different appearances of the surfaces of uncoated and coated wires could be observed. Surface roughness is the main factor that determines the frictional force, and our results showed that the R group had the roughest surface and the largest frictional force. It is considered that the frictional force was affected by the pattern and degree of coating damage during the friction test as well as by the surface roughness. Clinical studies show that coating damage can easily occur during tooth brushing or intake of food in the oral cavity, thus increasing the frictional force.

Coating materials continue to be developed to increase the aesthetics of orthodontic devices. The frictional forces of coated wires are equal to or higher than those of uncoated wires. However, the conditions in the oral cavity are very different from laboratory conditions because of the various tissues involved in oral functions, such as chewing, swallowing, and speaking, as well as the oral tissues that are in contact with the orthodontic appliance. Additional research on friction using conditions similar to those in the oral cavity will be needed to produce findings that have a clinically meaningful impact. In addition, to make the proper choice of coated wires, increased understanding of many factors such as abrasion resistance, discoloration, and compatibility with the teeth in the oral cavity, as well as frictional forces, is needed.