Friction Force

The friction test was performed using acrylic plates (area of 6.0 × 6.0 cm and thickness of 0.4 cm).^18,19 Four brackets were bonded to each base with conventional composite resin (Opallis, FGM, Joinville, Brazil) and, before photocuring, the brackets were perpendicularly aligned to the bases using a 0.021 × 0.025-inch stainless steel (SS) wire to obtain 0° of angulation and inclination in the friction test device²⁰ (Figure 2). All wires were cleaned with 70% alcohol before each friction test to eliminate any debris or oiliness that could interfere with the results.¹⁹

View Figure

• Download as Powerpoint
• Download Figure

The second premolar brackets from the three groups were tested using an Instron universal testing machine (model 3342, Instron Corp, Canton, Mass) simulating 5-mm sliding on 6-cm segments of 0.019 × 0.025-inch SS wires²¹ (Figure 3). The speed of the tests was 5 mm/min, and the force levels were registered by a 10N load cell.^11,22 The ligation system was standardized using 200 g of force with a device adapted to the universal machine, as previously reported.²⁰ This device provided a constant 100 g of force on each side (mesial and distal) of the bracket (Figure 4). The tests were performed in a dry environment, and each bracket and wire was used only once.²³

View Figure

• Download as Powerpoint
• Download Figure

View Figure

• Download as Powerpoint
• Download Figure

Two friction forces were considered during tests: static friction (maximum initial force before the sliding movement) and kinetic friction (mean friction force measurements between the second and third millimeter of sliding). The first and the last 2 mm were disregarded.^11,22

Superficial Roughness

Two samples of first premolar brackets from each group were selected and directly examined by SEM (Aspex Express, Fei Europe, Eindhoven, the Netherlands). Frontal and lateral images at 25× and 100× magnifications were captured, respectively.¹⁶

Statistical Analyses

Normal distribution of the variables was checked and confirmed through Kolmogorov-Smirnov tests. Analysis of variance followed by Tukey tests was used to evaluate intergroup differences regarding static and kinetic friction. Statistical tests were performed with Statistica software (Statistica for Windows, version 7.0, StatSoft, Tulsa, Okla), with a level of significance set at P < .05.

Orthodontic Treatment Characteristics

Previous studies^5,6 confirmed differences regarding surface roughness on orthodontic brackets of different brands, models, and materials. Thus, the same brand and bracket material (Kirium-Abzil) was used in this study. In addition, the patients were treated by the same experienced orthodontist, and the wire sequence during the leveling and alignment phase was also standardized as follows: 0.014, 0.016, 0.018, and 0.016 × 0.022-inch and 0.019 × 0.025-inch nickel-titanium superelastic archwires, using elastic or metallic ligation. During this phase, only two brackets from two patients experienced adhesive failures.

The prophylaxis methodology performed in groups 1 and 2 was based on the recommendation of other studies^12,17 that compared different angulations, distances, and blasting times. These factors were controlled during the prophylactic procedure to maintain the bracket integrity and retention.^12,16

Removal of the brackets for subsequent in vitro evaluation was performed with a thin cutting plier, which has previously been reported as an effective method to prevent bracket distortion.^18,26 In general, every possible effort to standardize the in vivo period was performed to prevent interference with the in vitro tests.

In Vitro Test

After removal, the brackets were tested after a few hours. Differently from previous studies,^11,22,27 which performed bracket cleaning after removal, the friction force was tested in a dry environment without cleaning, to preserve the bracket conditions.²³

A device associated with a 0.021 × 0.025-inch SS wire was used to avoid any angulation and inclination effect produced by the brackets, thereby preventing friction originating from binding and notching effects that could interfere in the results²² (Figure 2). The friction test was performed using a single 0.019 × 0.025-inch SS wire for each bracket tested, with both tested only once to prevent any possible damage to the sliding structures, increasing surface roughness and consequently the coefficient of friction^6,28 (Figure 3B). Even though care was taken to avoid any failure, one bracket from the control group failed during the in vitro analysis and was discarded.

Another variable controlled during the in vitro tests was the intensity of the ligation force applied between the bracket and wire. Instead of using an elastic or metallic ligation force, which are unstable²⁹ or operator dependent,³⁰ a device capable of applying an exact ligation force of 200 g was developed for the present study, as previously suggested (Figure 4).²⁰

Prophylaxis and Friction Force

Accumulation of calcified and noncalcified plaque within the bracket slot is one of the main factors responsible for increasing the friction force during orthodontic therapy.^22,31 Thus, satisfactory control of oral hygiene is important because plaque accumulation not only decreases the effectiveness of the sliding mechanics by increasing the friction force but also promotes higher cariogenic potential.³² Although many hygiene protocols could be investigated, only prophylactic methods were compared in this study since they are the most effective and common procedures clinically performed in orthodontics.^12,33

The groups treated with both prophylaxis protocols exhibited effective hygiene control, resulting in significantly smaller friction values when compared with the group without prophylaxis (Table 1). Similar results were found in previous investigations^14,15; however, this study followed a standardized clinical protocol and in vitro methodology to reduce the incorporation of possible biases. Nevertheless, it should be considered that the impact of prophylaxis on the friction force is still controversial. Some studies showed that the prophylactic procedure with sodium bicarbonate may compromise the bracket surface and increase the amount of friction during sliding mechanics.^16,17

Even though prophylaxis may cause changes in the surface of the bracket slots, this study demonstrated that, clinically, the effect of the procedure was positive regarding friction, considering that the presence of debris and plaque promoted a significantly greater increase in the friction force (group 3) than the possible damage caused by prophylaxis (groups 1 and 2). Another concern that should be considered is the abrasiveness of sodium bicarbonate on the enamel. However, this damage can be reduced by applying the prophylactic blast carefully only in the bracket slots.

In this study, the brackets were evaluated after leveling and alignment since, after this phase, greater sliding mechanics could be expected. However, further research should be performed with similar methodology to confirm these findings and to better understand this multifactorial phenomenon.²⁵

In summary, available evidence suggests that professional oral hygiene instructions only are not sufficient to prevent the accumulation of debris and bacterial plaque inside the bracket slot.¹³ The SEM images from group 3 confirm these findings and emphasize of the importance of prophylactic cleansing (Figures 5 and 6). Orthodontists should consider the positive clinical effects that prophylaxis can produce during sliding mechanics, characterized by a decrease in the static and kinetic friction forces.

It must be considered that in vivo evaluation of resistance to sliding is not plausible so far. Although this study showed significant effects of the two prophylaxis protocols on friction force reduction, the main concern is related to the impact that this friction decrease would have on treatment efficiency and whether it would be considered clinically significant or not. Future clinical studies should be performed to answer this matter.