Effects of third-order torque on frictional force of self-ligating brackets
To investigate the effects of third-order torque on frictional properties of self-ligating brackets (SLBs). Three SLBs (two passive and one active) and three archwires (0.016 × 0.022-inch nickel-titanium, and 0.017 × 0.025-inch and 0.019 × 0.025-inch stainless steel) were used. Static friction was measured by drawing archwires though bracket slots with four torque levels (0°, 10°, 20°, 30°), using a mechanical testing machine (n = 10). A conventional stainless-steel bracket was used for comparison. Results were subjected to Kruskal-Wallis and Mann-Whitney U-tests. Contact between the bracket and wire was studied using a scanning electron microscope. In most bracket-wire combinations, increasing the torque produced a significant increase in static friction. Most SLB-wire combinations at all torques produced less friction than that from the conventional bracket. Active-type SLB-wire combinations showed higher friction than that from passive-type SLB-wire combinations in most conditions. When increasing the torque, more contact between the wall of a bracket slot and the edge of a wire was observed for all bracket types. Increasing torque when using SLBs causes an increase in friction, since contact between the bracket slot wall and the wire edge becomes greater; the design of brackets influences static friction.ABSTRACT
Objective:
Materials and Methods:
Results:
Conclusions:
INTRODUCTION
Frictional force (resistance to sliding) between the bracket and the wire (archwire) during tooth movement is one of the primary issues in clinical orthodontics, and if the frictional force can be decreased, the efficiency of tooth movement can be improved.1 Friction during clinical tooth movement depends on the size and shape of the wire,2 the bracket type,3–6 the bracket and wire materials,7 the angulation of the wire relative to the bracket,8 the type of ligation,2 whether the environment is wet or dry,9 and whether a surface coating is present.10–12
The origin of the self-ligating bracket (SLB) concept can be traced back to the Russell Lock edgewise attachment described by Stolzenberg.13–15 Since the SPEED appliance was introduced by Hansen in the early 1980s,16 various SLBs, such as the Damon (Ormco, Glendora, Calif) and In-Ovation (DENTSPLY GAC, Bohemia, NY), have been introduced commercially and acquired popularity.15,17 The basic advantages of SLBs involve the elimination of certain utilities or materials such as elastomeric modules, along with the process or tools associated with their application, and the slide or clip opening-closing system of SLBs increases chair-side efficiency due to elimination of the ligaturing process.5,6 Other advantages claimed by bracket manufacturers, such as increased patient comfort, improved oral hygiene, superior patient cooperation and acceptance, less chair time, shorter treatment time, and enhanced expansion of arches, have interested orthodontists. However, there is no evidence for some of these claimed advantages.6 Although previous in vitro studies reported that SLBs had lower friction,18,19 randomized clinical trials investigating the efficiency of orthodontic treatment using SLBs have shown inconsistent results.5,6 In various treatment stages, the contact state between the bracket and wire should largely influence the frictional properties of a multibracket system, and few research groups have studied the effects of second-order angulation on frictional properties of SLBs.9,19 Only the study by Chung et al.20 has described the effects of third-order torque on frictional properties of SLBs, and limited information is currently available on this issue. It is important to understand the frictional characteristics of the SLBs in detail to demonstrate their clinical performance, and systematic in vitro studies may be required.
The purpose of this in vitro study was to investigate the frictional properties in SLBs with different designs (two passive types and one active type) under different third-order bracket torques. The contact state between the bracket with different designs and the wire was studied using a scanning electron microscope (SEM). We hypothesized that design of the bracket and the torque values do not affect friction.
MATERIALS AND METHODS
This study used three different SLBs, two passive types (Damon Q, Ormco; SmartClip, 3M Unitek, Monrovia, Calif) and one active type (In-Ovation R, DENTSPLY GAC International), three wires (nickel-titanium with 0.016 × 0.022-inch cross-section dimensions [Nitinol Super-Elastic, 3M Unitek] and stainless steel [Stainless Steel, 3M Unitek] with 0.017 × 0.025-inch and 0.019 × 0.025-inch cross-section dimensions). A conventional stainless-steel bracket (Mini Uni-Twin, 3M Unitek) was used for comparison (Table 1; Figure 1). Considering that different third-order torque values affect friction, the use of brackets with 0° built-in torque eliminates the influence of original torque, and such brackets were chosen for the present study, although their type (teeth) was different (Damon Q for mandibular canine, SmartClip for maxillary canine, In-Ovation R for maxillary canine, Uni-Twin for standard canine). All four bracket products were clinically popular 0.022-inch types.
Citation: The Angle Orthodontist 84, 6; 10.2319/111913-845.1
Friction Test
The static frictional force generated with each bracket-wire combination was measured under dry conditions and at room temperature (25°C) using a custom-fabricated friction-testing device attached to a universal testing machine (EZ Test, Shimadzu, Kyoto, Japan), as shown in Figure 2.12 Each bracket was bonded to a stainless-steel plate that could provide four different bracket torques (0°, 10°, 20°, and 30°) and 0° angulation with the aid of a bracket-mounting device (Figure 2) that employed a nonfiller adhesive resin (Superbond, Sun Medical, Shiga, Japan). The stainless-steel plate with the bracket was attached to the friction-testing device, and a 5-cm segment of wire obtained from the posterior straight portions of nickel-titanium or stainless-steel wires was then fixed to each SLB or ligated to the conventional bracket using a 0.010-inch-diameter ligature wire (Preformed Ligature Wire, Ormco) or an elastomeric module (AlastiK Easy-to-Tie Ligatures, 3M Unitek), except that the SLBs were tested in a closed position. The upper end of the wire was fixed with a grip that was attached to the load cell, and the lower end of the wire was fixed to a 150-g weight. The distance from the grip to the center of the bracket slot was adjusted to 16.5 mm. Each wire was drawn through the bracket slot at a cross-head speed of 10 mm/min for a distance of 5 mm. The static frictional force was determined from load-displacement curves.1 The sample size for each bracket-wire combination was 10.
Citation: The Angle Orthodontist 84, 6; 10.2319/111913-845.1
Observation of Bracket-Wire Combination Specimens by SEM
To observe the cross-section views of the wire-bracket contacts, each combination with a different bracket torque was first encapsulated in epoxy resin (EpoFix, Struers, Copenhagen, Denmark). After 24 hours, the encapsulated specimens were cut mesiodistally with a slow-speed water-cooled diamond saw (Isomet, Buehler, Lake Bluff, Ill) and polished using silicon carbide abrasive paper and a diamond suspension (particle sizes of 3 µm and 1 µm, respectively) and final slurry of 0.05 µm alumina particles. All specimens were sputter coated with pure gold and examined by an SEM (S-3500N, Hitachi, Tokyo, Japan) operating at 20 kV.
Statistical Analysis
A statistical analysis was performed using SPSS Statistics (version 20.0J for Windows, IBM, Armonk, NY). The static frictional force values were not homogenous (Levene test), and a Kruskal-Wallis test was applied to determine whether a significant difference existed among the bracket-wire combinations. The Mann-Whitney U-test was then used for two independent bracket-wire combinations, and the Bonferroni correction was applied (P < .0050 or P < .0083).
RESULTS
Tables 2 and 3 and Figures 3 and 4 show the frictional forces measured at four different bracket torques (0°, 10°, 20°, and 30°) for the three SLBs (Damon Q, SmartClip, and In-Ovation R) and the conventional bracket (Mini Uni-Twin) ligated by two different methods (ligature wire [L] or elastomeric module [E]), with the nickel-titanium wire (0.016 × 0.022 inch) and the stainless-steel wires (0.017 × 0.025 inch and 0.019 × 0.025 inch). In most bracket-wire combinations, increasing the torque from 0° to 10°, 20°, or 30° produced an increase in static frictional force (Table 2). All SLB–nickel-titanium wire combinations for all torque values produced less frictional force than that from the conventional bracket (Table 3). The active SLB–nickel-titanium wire combinations showed higher static friction than the passive SLB–nickel-titanium wire combinations, except at 0° of torque. The active SLB–stainless-steel wire combinations showed higher static friction than the passive SLB–stainless-steel wire combinations, except at 30° of torque for the 0.019 × 0.025-inch stainless-steel wire. Some Mini Uni-Twin (L)–wire combinations showed higher static friction than Mini Uni-Twin (E)–wire combinations.
Citation: The Angle Orthodontist 84, 6; 10.2319/111913-845.1
Citation: The Angle Orthodontist 84, 6; 10.2319/111913-845.1
The SEM photomicrographs are shown in Figures 5 to 8, for a representative specimen of each cross-sectioned bracket-wire combination. Each SLB showed a different appearance for the contact area. In comparing passive SLBs, the SmartClip bracket-wire combination specimens exhibited relatively less contact between the bracket slots and wires compared with the corresponding Damon Q bracket specimens. The clips of the In-Ovation R bracket (active SLB) contacted the wire surfaces for all wire sizes and torque conditions. When there was an increase in torque or wire size, greater contact of the bracket slot wall and the wire edge was observed for all bracket types.
Citation: The Angle Orthodontist 84, 6; 10.2319/111913-845.1
Citation: The Angle Orthodontist 84, 6; 10.2319/111913-845.1
Citation: The Angle Orthodontist 84, 6; 10.2319/111913-845.1
Citation: The Angle Orthodontist 84, 6; 10.2319/111913-845.1
DISCUSSION
To investigate the effects of third-order torque of some popular SLBs, this study examined the static frictional forces and obtained SEM images of the cross-sectioned SLB-wire combinations to reveal the contact areas. It has been found that the bracket design is a highly important factor for friction. The present observation that increasing the torque had a highly significant influence on the static friction in most bracket–stainless-steel wire combinations suggests that tipping of teeth in labiolingual or buccolingual directions is an important factor influencing the efficiency of initial orthodontic alignment using SLBs. The SLB–nickel-titanium wire combinations for all torque values produced less frictional force than that from the conventional bracket, suggesting that the tooth alignment capability using nickel-titanium wire with SLBs might be superior to use of conventional stainless-steel brackets.
Except for combinations with relatively large wire size (0.019 × 0.025-inch stainless-steel), each SLB–stainless-steel wire combination at all torque conditions (0°, 10°, 20°, and 30°) showed less frictional force than that found with conventional bracket-wire combinations. This observation is in agreement with previous studies.18–20 Thus, the present in vitro results suggest that the SLBs produce less frictional force during anterior retraction. In addition, the use of heavy (stainless-steel) archwires may not be preferred during anterior retraction with sliding mechanics, and sufficient alignment using a nickel-titanium archwire with large cross-section dimensions is required before the anterior retraction stage.
The classification of SLBs into active and passive types has been described by two previous articles.15,17 Both articles note that active SLBs usually have a sliding spring clip, which encroaches on the slot from the labial aspect and reduces the slot size in the horizontal dimension, potentially placing an active force on the archwire. In contrast, passive SLBs have a slide that opens and closes vertically, thereby creating a passive labial surface that lacks the ability to encroach upon the slot and store force by deflection of a metal clip.15 A previous study21 showed that passive SLBs exhibit lower friction than active SLBs, and it was suggested that the wire-binding effect of active SLBs might be higher than passive SLBs. Similarly, the present study showed that passive SLBs have less static friction than active SLBs, and SEM observations of cross-sectioned bracket-wire specimens showed that passive SLBs have relatively loose contact between bracket slots and wires compared with active SLBs. An additional consideration is that the sliding spring clip of an active SLB encroaches on the slot, facilitating contact between the slot and edge of the wire. However, this study showed that the Damon Q bracket (passive SLB) with 30° torque and 0.019 × 0.025-inch stainless-steel wire produced higher friction than the In-Ovation R bracket (active SLB). The reason for this inconsistency may be distortion of the clip, which requires further study for verification.
In the present study, all wires did not show any permanent deformation after the friction test, even when 30° torque was imposed on the 0.019 × 0.025-inch stainless-steel heavy wire. The reason for this is that a relatively long distance from the grip to the bracket was used (16.5 mm) for the friction testing, and this value is greater than the distance between adjacent brackets bonded to a patient's teeth under clinical conditions.
CONCLUSIONS
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Increasing the third-order torque experienced by SLBs produces an increase in static friction.
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The SLBs produce less static friction than conventional brackets, and passive SLBs produce less friction than active SLBs.
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The extent of contact between the bracket and edge of the wire may be influential for static friction.
Photographs of three self-ligating brackets and the conventional bracket used in the present study. (a) Damon Q. (b) SmartClip. (c) In-Ovation R. (d) Mini Uni-Twin.
Friction testing system (a, b) Custom-fabricated frictional testing device attached to universal testing machine. (c, d) Bracket-mounting device. (e) Stainless-steel plate that could apply third-order torque to the bracket. A, grip; B, bracket-wire combination specimen; C, stainless-steel plate; D, weight (150 g); E, angle measurement device.
Static frictional forces measured at four different bracket torques for the three self-ligating brackets and the conventional bracket with the nickel-titanium wire and the stainless-steel wires.
Static frictional forces for the three self-ligating brackets and the conventional bracket with the nickel-titanium wire and the stainless-steel wires. DQ indicates Damon Q; SC, SmartClip; IO, In-Ovation R; MU(L), Mini Uni-Twin ligated with ligature wire; MU(E), Mini Uni-Twin ligated with elastomeric module.
Scanning electron microscope photographs of cross-sectioned Damon Q bracket and each wire combination. (a–d) 0.016 × 0.022-inch nickel-titanium wire. (e–h) 0.017 × 0.025-inch stainless-steel wire. (i–l) 0.019 × 0.025-inch stainless-steel wire. (a, e, i) Wire torque at 0°. (b, f, j) Wire torque at 10°. (c, g, k) Wire torque at 20°. (d, h, l) Wire torque at 30°.
Scanning electron microscope photographs of cross-sectioned SmartClip bracket and each wire combination. (a–d) 0.016 × 0.022-inch nickel-titanium wire. (e–h) 0.017 × 0.025-inch stainless steel wire. (i–l) 0.019 × 0.025-inch stainless-steel wire. (a, e, i) Wire torque at 0°. (b, f, j) Wire torque at 10°. (c, g, k) Wire torque at 20°. (d, h, l) Wire torque at 30°.
Scanning electron microscope photographs of cross-sectioned In-Ovation R bracket and each wire combination. (a–d) 0.016 × 0.022-inch nickel-titanium wire. (e–h) 0.017 × 0.025-inch stainless-steel wire. (i–l) 0.019 × 0.025-inch stainless-steel wire. (a, e, i) Wire torque at 0°. (b, f, j) Wire torque at 10°. (c, g, k) Wire torque at 20°. (d, h, l) Wire torque at 30°.
Scanning electron microscope photographs of cross-sectioned Mini Uni-Twin bracket and each wire combination. (a–d) 0.016 × 0.022-inch nickel-titanium wire. (e–h) 0.017 × 0.025-inch stainless-steel wire. (i–l) 0.019 × 0.025-inch stainless-steel wire. (a, e, i) Wire torque at 0°. (b, f, j) Wire torque at 10°. (c, g, k) Wire torque at 20°. (d, h, l) Wire torque at 30°.
Contributor Notes