The resistance to sliding of orthodontic brackets has three major components: Friction, binding, and notching of the archwire. ¹ In situ ations where the contact angle θ is below the critical value, the only component existing is friction, because binding ² , ³ and notching do not take place at these situations.

During orthodontic treatment, teeth can be moved either by closing loop mechanics, which minimizes the frictional forces (FFs) or by sliding mechanics, in which FFs are considerable. There are two types of friction. Static friction is the smallest force needed to initiate the motion. Kinetic friction is the force needed to resist the sliding motion of one solid object over another at a constant speed. ⁴ Kusy and Whitley ¹ and Articolo andKusy ⁵ showed that 12%–60% of the applied force may have been lost due to frictional resistance.

Different factors can influence frictional resistance: Wire and bracket materials, wire section, active torque at the wirebracket interface, the surface condition of brackets and archwires, force and type of ligation, saliva, lubrication, interbracket distance and the angle between wire and brackets. ² , ⁶ , ⁷ , ⁸

The appearance of the orthodontic appliances has always been of great importance for orthodontic patients, especially the adult ones. Ceramic brackets were basically introduced to meet these esthetic demands, but several problems came up in practice, including brittleness of ceramics which may cause fracture of the tie wings or brackets, iatrogenic damage to enamel on debonding, enamel attrition of the opposing tooth and the most important of them all, high levels of FFs during sliding mechanics. ⁶ Different studies reported that the FFs of ceramic brackets were higher than stainless steel (SS) ones. ⁹ , ¹⁰ To reduce these unwanted side-effects, ceramic brackets with smoother slots, such as metal insert ceramic (MIC) brackets, were introduced. Although the FFs in MIC brackets were lower than conventional ceramic brackets, ¹¹ , ¹² they were still significantly higher than SS brackets. ⁹

Along with the developments occurring in esthetic brackets, tooth-colored archwires were also introduced. Zufall and Kusy ¹³ reported that the coated composite archwires increased the friction and binding. Ion implanted coated beta-titanium archwires were reported to have frictional resistance comparable to SS wires. ¹⁴ , ¹⁵ The most common method for coating the archwires is using teflon or epoxy resin. There is some controversy about their effect on friction, but all the previous studies agree that these coatings are “undurable.” ¹⁶ , ¹⁷

As silica-insert ceramic (SIC) brackets and coated archwires have been introduced in recent years, the purpose of this study was to evaluate the frictional behavior of this bracket in combination with coated archwires in the wet state.

In this in vitro study, two types of maxillary canine 0.022 inch slot brackets with Roth prescription were tested: Eighty SIC brackets (GAC Int., Bohemia, New York, USA) and forty SS brackets (GAC Int., Bohemia, New York, USA). The tested archwires (GAC Int., Bohemia, New York, USA) were of two types: 80 conventional SS and 40 Teflon-coated (TC) SS. The archwires had four different dimensions: 0.016, 0.018, 0.016 × 0.022 and 0.018 × 0.025 inch. The archwires were cut into 4 cm sections. Before the tests, the diameter of the archwires was measured by a digital gauge. Scanning electron microscopy (SEM, Model S360, Oxford, England) was used to evaluate the surface morphology of the SS and TC archwires before and after the tests.

All the brackets were bonded onto a 2.5 cm diameter tubes filled with self-cure acrylic (Meliodent, Bayer Co., Germany), using a two-paste instantaneous glue.

Friction was measured with a universal testing machine (Model Z250, Zwick, Ulm, Germany) at a room temperature of 25°C. Special jigs were designed for the testing machine. The lower jig enclasped the tubes. It had a line scribed in the midline to act as a guide for reproducible bracket position. The archwire segments were fixed in the jig connected to the upper jaw. A total number of 120 bracket archwire segments were evaluated. Each bracket was tested only once, and each archwire segment was drawn through only one bracket to eliminate the effect of wear.

To better simulation of the clinical conditions, elastomeric modules (Ortho Technology Inc., Tampa, Florida, USA) were used to ligate the archwires in the brackets. The same person placed all the elastomeric modules, and it was done immediately before each test to avoid the force degradation effect. To standardize this procedure, an elastomeric module placer was used to assimilate the amount of elastomerics extension. Before the evaluation, three drops of artificial saliva (Fusayama–Meyere formula) were applied at the bracket-archwire interface using a 10 mL syringe.

The 10 N load cell was calibrated between 0 and 10 N, and the archwires were drawn through the brackets as the crosshead moved superiorly at a rate of 2.5 mm/min. The frictional values were then transmitted to a computer hard disk and analyzed with testXpert ^®software (Zwick, Ulm, Germany). The data were recorded on an XY recorder. The X axis showed bracket movement in millimeters. The Y axis recorded the FF between the bracket and the archwire in cN. Initial peak of movement represented static friction. Kinetic friction was calculated by averaging 5 recordings 10 s apart on the Y axis after the initial peak. Descriptive statistics, including mean, standard deviation, minimum and maximum values were calculated for each bracket-archwire combination. To ensure the normal distribution of the data, Kolmogorov–Smirnov test was used. The results were examined by using one-way analysis of variance. For the post hoc test, the Tukey test was used. The level of significance for all tests was set at P < 0.05. All statistical analyses were performed with SPSS software (version 11.5, SPSS, Chicago, IL, USA).

The descriptive statistics of static and kinetic FFs for each bracket-archwire combination are shown in Table 1. Two-by-two comparison between the groups was done by Tukey post hoc test and is presented in Table 2. The highest levels of static and kinetic FFs were observed in SS brackets and 0.018 × 0.025 inch SS archwires. The lowest levels were seen in SIC brackets and 0.016 inch TC wires.{Table 1}{Table 2}

The groups with SIC brackets and SS wires had the lowest amounts of FFs. The groups with SS brackets and SS wires had the highest levels.

Increasing the size of the archwires increased the levels of static and kinetic FFs. Generally speaking, rectangular wires showed higher amounts of FFs than round ones. In SIC brackets, the TC archwires had significantly greater levels of FFs (P< 0.05), compared to SS wires.

In all groups with SS archwires, the static FFs were higher than kinetic FFs. However in groups containing TF archwires, extremely small differences were observed between static and kinetic FFs. Even in one group (SIC brackets and 0.016 × 0.022 inch TC wires), the kinetic friction was higher than static friction.

SEM examination of SS archwires in SIC brackets showed a slight increase in the amounts of facets and wears Figure 1a and Figure 1b. In contrast to SS wires, the TC ones showed a significant amount of coating deterioration and indentation Figure 1c and Figure 1d. The coating in some parts was delaminated or missed. Figure 1

The scanning electron microscopy microphotographs of (a) the stainless steel wire before and (b) after the friction test, (c) the Teflon-coated wire before and (d) after the friction test.

Considering the rising demand for esthetic brackets, the use of ceramic brackets for anterior teeth has increased. However, previous studies have demonstrated higher FFs of ceramic brackets compared to SS braces. ¹⁸ , ¹⁹ , ²⁰ Despite the emphasis it now receives in the marketing of brackets, some believed that friction is not the major component of “resistance to sliding” in the clinic. ¹⁹ They stated that binding of the wire against the corners of the bracket is much more important than previously thought. However, in the present study, we evaluated the static and kinetic “FFs” of the SIC brackets. It does not rule out the influence of binding or notching in increased “resistance to sliding.”

The tested archwires were all steel and divided into two groups of TC and conventional SS. The SS brackets were used as the control.

Table 1demonstrates that an increase in the size of the archwires may cause an increase in the frictional levels. Several studies so far, have investigated the effect of archwire size on the amount of friction generated. All of them, in accordance with our study, reported that larger wires would increase FFs. ²¹ , ²² , ²³ , ²⁴

The current study showed that SIC brackets have lower FFs, even lower than SS brackets. To the best of our knowledge, very few studies have investigated the friction of SIC brackets. Cha et al. ²⁵ reported that silica layer and rounded edges of the SIC brackets lowered the FFs. These brackets had minimal frictional resistance among ceramic brackets, comparable to the conventional SS brackets. Doshi and Bhad-Patil ²⁶ investigated ceramic brackets with gold-palladium slots and concluded that these brackets have a smoother surface and lower friction than SS brackets and seem to be a good alternative to SS brackets in space closure with sliding mechanics.

Till date, the body of the literature considered the SS brackets as the gold standard of friction that has the lowest FFs. However, our study showed that the SIC have lower static and kinetic FFs than SS brackets. Two reasons may be hypothesized for this finding: (1) as we tried to follow clinical conditions, artificial saliva may cause the SS wires to show an adhesive behavior, as reported by Kusy et al. ²⁷ This adhesion could be responsible for the increased FFs during the tests. (2) With the help of the modern technologies, we are now capable of manufacturing new brackets that have lower friction than SS brackets. It seems that the default notion that SS brackets have the lowest levels of FFs will be challenged even more in future.

There has been some controversy on whether static or kinetic friction is more important during orthodontic therapy, ²⁸ but the majority of the studies reported the kinetic one to be more important, because sliding of a tooth or bracket on an archwire is not a continuous or constant motion. ²⁹ In fact, due to the dynamic condition of the oral cavity, it is relatively a series of short steps. ³⁰ In previous studies, the static FFs were greater than kinetic FFs, ³¹ , ³² although this difference was not always significant. ³³ In the current study, we found the same thing to be true about the conventional SS wires. But with TC wires, the difference between static and kinetic FFs became considerably narrow, and even in one of the test groups (SIC brackets and 0.016 × 0.022 inch TC wires), the kinetic FFs were greater than static ones. We found this unusual behavior of increasing kinetic FFs with time to be present in about half of the TC wires Figure 2a. One possible reason for this finding might be the indentation of TC wires when sliding in SIC brackets. Figure 2

Frictional diagrams of (a) three Teflon-coated wires pulling through silica-insert ceramic brackets, (b) a 0.018 inch stainless steel archwire pulling through a stainless steel bracket, (c) a 0.018 inch stainless steel archwire pulling through a silica-insert ceramic bracket and (d) a 0.018 inch Teflon-coated archwire pulling through a silica-insert ceramic bracket.

Based on our findings, the current in vitro study illustrated that as follows:

Static and kinetic FFs in SIC brackets were significantly lower than SS brackets

Acknowledgments

The authors would like to thank vice-chancellor of research of Mashhad University of Medical Sciences for financial support of this project.

The results mentioned in this manuscript are derived from a Doctoral thesis orthodontic (No: 438).

Financial support and sponsorship

Mashhad University of Medical Sciences for financial support of this project.

Conflicts of interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or nonfinancial in this article.