Objectives:

To evaluate the frictional resistance of ceramic and metal brackets using rectangular stainless steel orthodontic wires before and after using sodium bicarbonate air abrasive polishing and to evaluate the surface micromorphology of these brackets by means of scanning electron microscopy.

Materials and Methods:

Two commercial brands of metal brackets and two commercial brands of ceramic brackets were evaluated. The specimens were divided into eight groups (n  =  10) according to brackets and the application or not of sodium bicarbonate airborne particle abrasion for 10 seconds. A device adapted to a universal testing machine was used to simulate the movement of retraction in sliding mechanics, measuring the traction force needed to slide 10 mm of the wire over the test specimen brackets. The test speed was 5 mm/min. The data were analyzed by two-way analysis of variance (ANOVA) and Tukey test.

Results:

There was higher frictional resistance after airborne particle abrasion irrespective of the type of bracket (P < .05). One of the ceramic brackets showed higher frictional resistance before and after airborne application than the other metal and ceramic brackets. The micromorphologic analysis showed that airborne particle abrasion caused alterations on the metal bracket surfaces.

Conclusions:

It may be concluded that it is not recommended to apply airborne particle abrasion on the slots of ceramic or metal brackets.

Friction is defined as a force that retards or resists movement related to two objects in contact. In sliding mechanics, friction is determined by the type of arch, type of bracket, and type of ligature.1 

The growing esthetic demand, especially by adults who began seeking orthodontic treatments, culminated in the first ceramic brackets being introduced in orthodontics at the end of 1986. This esthetic alternative, an explicit attempt to eliminate the use of stainless steel brackets, has been developed with the use of new technologies in esthetic bracket manufacturing.2 

Although orthodontic treatment provides the functional and esthetic correction of teeth, one of its inconvenient aspects is that the brackets (whether they are made of metal or ceramics) and other accessories allow a greater accumulation of residues and consequently, the formation of dental plaque, especially around the brackets. Professional prophylaxis must be performed to correct the patient's deficient tooth brushing, particularly with the use of sodium bicarbonate abrasion, a system that releases controlled jets of air, water, and sodium bicarbonate particles. It was introduced at the end of the 1970s as a fast and easy mechanism to remove dental plaque.3 

The effectiveness of sodium bicarbonate airborne particle abrasion and the rubber cup has been shown to be more efficient for removing dental plaque in addition to promoting less operator fatigue due to reduced working time.4,5 

With regard to alterations to the dental substrate, it has been shown that the use of sodium bicarbonate airborne particle abrasion does not cause surface alterations in healthy enamel, but it does affect and change the micromorphology of dentin and cementum.6 Therefore, the use of the sodium bicarbonate airborne particle abrasion is an efficient and safe method for removing dental plaque from healthy enamel, but its use on exposed dentin and cementum must be avoided.37 On the other hand, the surface micromorphology of brackets after jet application has not yet been established.

Thus, it is important for dentists to know the effects of sodium bicarbonate airborne particle abrasion on orthodontic brackets during prophylactic procedures since the micromorphologic alterations in brackets could significantly affect sliding mechanics. Therefore, the aim of this study was to evaluate in vitro the frictional resistance offered by ceramic and metal brackets, using rectangular stainless steel orthodontic wires before and after the use of the sodium bicarbonate airborne particle abrasion and surface micromorphology of the brackets before and after applying sodium bicarbonate airborne particle abrasion, by means of scanning electron microscopy.

Experimental Design

The factors being studied in this experiment were:

  • Types of brackets, at four experimental levels, two metal (bracket 1 and 2) and two ceramic (bracket 3 and 4).

  • Sodium bicarbonate airborne particle abrasion, at two levels, one experimental (presence of jet application) and one control (absence of jet application).

The experimental units consisted of brackets that received rectangular stainless steel wires in the slots submitted to sliding in a universal testing machine. The response variable was “frictional resistance” measured in kgf and quantitatively assessed.

Fabrication of Test Specimens and Sodium Bicarbonate Airborne Particle Abrasion

Eighty acrylic resin cylinders were fabricated using a PVC tube, 3.5 cm high and 0.5 inches wide, as a mold. The tubes were filled with self-curing acrylic resin (Jet, Clássico, São Paulo, Brazil). With the purpose of differentiating the experimental groups, different colors of resins were used (green, red, blue, black, colorless, yellow, purple, and pink), obtaining eight groups with 10 cylinders of each color (n  =  10), totaling 80 cylinders. The different colors were used to differentiate the metal or ceramic brackets of different commercial brands and those that received sodium bicarbonate airborne particle abrasion from the ones that did not. The brackets were fixed to the cylinders using a cyanoacrylate adhesive.

The brackets used in the experiment are shown in Table 1. Maxillary right first premolar brackets were used: Roth prescription, 0.022 × 0.028-inch slot, with torque of −7° and angulation of 0°. Twenty units of brackets of each commercial brand were randomly distributed between the control (absence of sodium bicarbonate airborne particle abrasion) and experimental groups (presence of sodium bicarbonate airborne particle abrasion), totaling 10 brackets in each group (n  =  10).

Table 1

Type, Chemical Composition, and Lot Number of the Brackets Used in the Experiment

Type, Chemical Composition, and Lot Number of the Brackets Used in the Experiment
Type, Chemical Composition, and Lot Number of the Brackets Used in the Experiment

The sodium bicarbonate airborne abrasion was performed with a sodium bicarbonate appliance using sodium bicarbonate particles 4 µm in diameter. The sodium bicarbonate was inserted in the appliance up to 50% of the total reservoir capacity, replacing the power when it reached 20%. The airborne abrasion was applied perpendicularly to the brackets at a distance of 5 mm for 10 seconds with a 2.3 bar pressure.

Frictional Resistance Tests

For the friction test, rectangular stainless steel orthodontic wires measuring 0.019 × 0.025 inches (GAC, Matão, SP, Brazil) were used, cut into 10-cm long segments, and connected by elastic ligatures (Morelli, Sorocaba, SP, Brazil) in order to remain in contact with the wire and brackets. The upper extremity of each segment of the wire was folded by a device coupled to the load cell to keep the wire parallel to the slot track and to hook it onto the testing machine.

To support the cylinders with the brackets, a piece of glass measuring 10 × 10 cm was fabricated, with a 0.5-inch perforation into which the acrylic resin cylinders fitted perfectly. This glass square was filled with type III special plaster (Quimidrol, Joinville, Santa Catarina, Brazil).

The test specimens were submitted to the tensile test in the mechanical testing machine (EMIC DL-2000, São José dos Pinhais, Paraná, Brazil). Figure 1 shows the device and the bracket/wire positioned in the universal testing machine. A maximum load of 5 kgf was used under dry conditions. Tensile force needed to slide 10 mm of the wire over the test specimen brackets for 2 minutes, at a speed of 5 mm per minute, was measured, and the maximum tensile force value obtained during the range of motion of each bracket was also measured. The data were obtained in kgf and transformed into Newtons.

Figure 1

Device and the bracket/wire positioned for the friction resistance tests.

Figure 1

Device and the bracket/wire positioned for the friction resistance tests.

Close modal

Assessment of Surface Micromorphology of Brackets by Scanning Electron Microscopy

The surface micromorphology of two samples of each type of metal and ceramic bracket was examined by scanning electron microscopy (EVO 40, Zeiss) before and after the jet application at the Department of Mechanical Engineering of the Federal University of Espírito Santo. The nonconductive metaloceramic brackets were sputter coated with gold using a sputtering machine (EMITECH K500X, Bad Schwalbach, Germany). The metal brackets were directly analyzed by scanning electron microscopy without previous gold coating. Images of surface micromorphology at 60× and 1000× magnification were captured in the bracket slot regions.

Statistical Analysis

Exploratory analysis of the data indicated the need for transformation into square root so that the data would meet the presuppositions of the analysis of variance (ANOVA). After transformation, the data were submitted to two-way ANOVA in a factorial design (four types of brackets × two types of treatments) and Tukey test was applied, considering a level of significance of 5%. All analyses were performed with the statistical program SAS (SAS Institute Inc, Cary, NC, Release 9.1, 2003).

This manuscript was authorized by the Committee on Ethics of the Dental School and Institute and Research Center São Leopoldo Mandic.

According to Table 2, it was observed that mean resistance was higher in the group that received airborne particle abrasion, regardless of the type of bracket (P ≤ .05). The ceramic bracket 3 showed a higher mean resistance than the others, both in the control group and after airborne particle abrasion (P ≤ .05).

Table 2

Mean Resistance in Newtons (Standard Deviation) According to Treatmenta

Mean Resistance in Newtons (Standard Deviation) According to Treatmenta
Mean Resistance in Newtons (Standard Deviation) According to Treatmenta

Surface micromorphologies of the brackets before and after jet application are shown in Figures 2 to 17. It was found that the use of sodium bicarbonate airborne particle abrasion changed the surfaces of the metal brackets, resulting in an irregular bracket surface visualized at 1000× magnification (Figures 4, 5, 8, and 9). For the ceramic brackets, there was no surface alteration both at 60× and 1000× magnification.

Figure 2

Micromorphology of metal bracket 1 before airborne particle abrasion at 60× magnification.

Figure 2

Micromorphology of metal bracket 1 before airborne particle abrasion at 60× magnification.

Close modal
Figure 3

Micromorphology of metal bracket 1 after airborne particle abrasion at 60× magnification.

Figure 3

Micromorphology of metal bracket 1 after airborne particle abrasion at 60× magnification.

Close modal
Figure 4

Micromorphology of slot of metal bracket 1 before airborne particle abrasion at 1000× magnification.

Figure 4

Micromorphology of slot of metal bracket 1 before airborne particle abrasion at 1000× magnification.

Close modal
Figure 5

Micromorphology of slot of metal bracket 1 after airborne particle abrasion at 1000× magnification.

Figure 5

Micromorphology of slot of metal bracket 1 after airborne particle abrasion at 1000× magnification.

Close modal
Figure 6

Micromorphology of metal bracket 2 before airborne particle abrasion at 60× magnification.

Figure 6

Micromorphology of metal bracket 2 before airborne particle abrasion at 60× magnification.

Close modal
Figure 7

Micromorphology of metal bracket 2 after airborne particle abrasion at 60× magnification.

Figure 7

Micromorphology of metal bracket 2 after airborne particle abrasion at 60× magnification.

Close modal
Figure 8

Micromorphology of slot of metal bracket 2 before airborne particle abrasion at 1000× magnification.

Figure 8

Micromorphology of slot of metal bracket 2 before airborne particle abrasion at 1000× magnification.

Close modal
Figure 9

Micromorphology of slot of metal bracket 2 after airborne particle abrasion at 1000× magnification.

Figure 9

Micromorphology of slot of metal bracket 2 after airborne particle abrasion at 1000× magnification.

Close modal
Figure 10

Micromorphology of ceramic bracket 3 before airborne particle abrasion at 60× magnification.

Figure 10

Micromorphology of ceramic bracket 3 before airborne particle abrasion at 60× magnification.

Close modal
Figure 11

Micromorphology of ceramic bracket 3 after airborne particle abrasion at 60× magnification.

Figure 11

Micromorphology of ceramic bracket 3 after airborne particle abrasion at 60× magnification.

Close modal
Figure 12

Micromorphology of slot of ceramic bracket 3 before airborne particle abrasion at 1000× magnification.

Figure 12

Micromorphology of slot of ceramic bracket 3 before airborne particle abrasion at 1000× magnification.

Close modal
Figure 13

Micromorphology of slot of ceramic bracket 3 after airborne particle abrasion at 1000× magnification.

Figure 13

Micromorphology of slot of ceramic bracket 3 after airborne particle abrasion at 1000× magnification.

Close modal
Figure 14

Micromorphology of ceramic bracket 4 before airborne particle abrasion at 60× magnification.

Figure 14

Micromorphology of ceramic bracket 4 before airborne particle abrasion at 60× magnification.

Close modal
Figure 15

Micromorphology of ceramic bracket 4 after airborne particle abrasion at 60× magnification.

Figure 15

Micromorphology of ceramic bracket 4 after airborne particle abrasion at 60× magnification.

Close modal
Figure 16

Micromorphology of slot of ceramic bracket 4 before airborne particle abrasion at 1000× magnification.

Figure 16

Micromorphology of slot of ceramic bracket 4 before airborne particle abrasion at 1000× magnification.

Close modal
Figure 17

Micromorphology of slot of ceramic bracket 4 after airborne particle abrasion at 1000× magnification.

Figure 17

Micromorphology of slot of ceramic bracket 4 after airborne particle abrasion at 1000× magnification.

Close modal

Considering the esthetic advantages of ceramic brackets in comparison with stainless steel accessories, their introduction made orthodontic treatments more attractive, especially to adult patients.8 With the development of ceramic brackets, it was possible to use a material with improved optical and structural characteristics with an innovative design, without compromising function in the movement of teeth, durability, and less pigmentation when compared with the polycarbonate/resin brackets. In specific cases in which the esthetic factor is relevant and when patients show some type of allergy to nickel, or when light forces are applied, the replacement of metal brackets for ceramic brackets may be required.9 

The influence of the bracket material with regard to friction caused by the wire during sliding mechanics has been assessed, and it was found that ceramic brackets show higher friction than those made of stainless steel, not only due to the type of material, but particularly due to the irregularities on ceramic bracket surfaces.1012 However, the results of the present study partially corroborate the literature because the ceramic bracket 4 was shown to be similar to the stainless steel bracket. It must be taken into consideration that the morphology of the tested brackets differed. As observed in the microscopic images at 60× magnification (Figures 2, 4, 6, and 8), the bracket slot presented different areas of contact with different wires; the stainless steel brackets presented smaller areas of contact—corresponding to “points”—in comparison with the ceramic types that showed the total slot area in contact with the wire.

In the present study, polycrystalline ceramic brackets were used. The polycrystalline ceramic, or polycrystalline alumina brackets, are made of aluminum oxide crystals fused at high temperatures, which allows several brackets to be molded simultaneously. With regard to the metal brackets, the 302 austenitic stainless steel brackets were used, which presented greater surface smoothness than the esthetic ceramic brackets. This difference in surface smoothness may be a possible explanation for the difference found between the frictional resistance of the ceramic and stainless steel brackets.13,14 However, the ceramic bracket 4 did not differ from the metal brackets both before and after the jet application. Kusy and Whitley15 reported that, in general, the surface roughness of ceramic brackets is similar to that of a block of concrete in comparison with stainless steel brackets, which has a porous, irregular, and polyhedral surface, and this was observed in the scanning electron microscopy images in the present study. The ceramic bracket 3 showed higher friction even before the jet application because it presented a more irregular surface (Figure 13), retaining a larger amount of sodium bicarbonate particles after the airborne particle abrasion and therefore, increasing friction.

In addition to friction being influenced by the composition of the bracket, the thickness of the wire used must be taken into consideration. It was observed that the greater the wire thickness, the greater will be the friction, for a thicker wire almost completely fills the bracket slot, reducing free space between them.10 In this study, stainless steel wire measuring 0.019 × 0.025 inches was used, which was also used in sliding mechanics in order to preserve the anterior teeth in relation to torque, so that while the anterior teeth are being moved, their inclinations will be maintained.10 

The surface micromorphology analysis of brackets by scanning electron microscopy showed that the greatest alterations in the metal brackets occurred due to the removal of the polished surface area, leaving it rougher as a result of the action of bicarbonate particles 4 µm in diameter. Due to the pressure used (2.3 bar) and the particle sizes, erosion areas were seen on the bracket surfaces. In dental enamel, Boyde6 showed that there are no alterations caused by the bicarbonate airborne particle abrasion, but its use in dentin and cementum must be avoided because it causes superficial erosion. On the other hand, the sodium bicarbonate airborne particle abrasion did not cause surface alterations on ceramic brackets because the ceramic material hardness was greater than that of the metal material.

Although there were micromorphologic alterations on the surface of the metal brackets and no alteration on the ceramic brackets, frictional resistance was higher for one type of ceramic bracket. This may have occurred due to the characteristics of the metal, which has a low friction coefficient and allows good surface finishing.15 This may also be explained by the difference in material between the ceramic brackets and the stainless steel wire that must slide over the surface of this bracket. For this reason, manufacturers started producing ceramic brackets with metal slots (stainless steel) to reduce friction considerably.16 Furthermore, the surfaces of the metal brackets were very smooth, even after the sodium bicarbonate airborne particle abrasion, whereas the surface of the ceramic brackets were porous, as it can be observed in microscopic images, allowing the latter to retain bicarbonate particle residues in the slots of these brackets.

Considering the clinical applicability of using the sodium bicarbonate airborne particle abrasion in orthodontic patients, it is a routine prophylactic procedure and it was found that there is an increase in friction between the bracket slot and the wire, irrespective of the type of bracket used (metal or ceramic). Therefore, it must be considered that jet application on the bracket slot should be avoided, and if it is done, abundant washing with water must be performed to remove the residues, which occurs mainly in the ceramic brackets due to their greater surface irregularity.

  • Based on the results obtained, it may be affirmed that the ceramic bracket 3 showed a higher friction coefficient in comparison with the other brackets, which did not differ among themselves.

  • Regardless of the type of bracket tested, mean resistance was higher in the group that received sodium bicarbonate airborne particle abrasion.

  • The micromorphologic analysis showed that the airborne particle abrasion caused greater changes on the surface of the metal brackets, but even after the airborne particle abrasion, the surfaces of these brackets were more polished than the ceramic brackets.

  • The application of airborne particle abrasion on the slots of ceramic or metal brackets is not recommended.

We would like to thank The Angle Orthodontist reviewers for the opportunity to improve the quality of the manuscript for publication.

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