INTRODUCTION

Self-ligating brackets (SLBs) are known to show lower frictional force (FF) than conventional brackets.1–16 Conventional brackets have three slot walls (gingival horizontal wall, occlusal horizontal wall, and vertical wall), while SLBs have an additional facial wall (sliding door/clip in passive type or clip in active type) that conventional brackets do not have. Metal or elastomeric ligatures play a role in conventional brackets that is similar to that played by the facial wall in SLBs.

SLBs can be divided into two groups: active (ASLB) and passive (PSLB) types. The major difference between ASLBs and PSLBs is the function and structure of the facial wall. ASLBs such as In-Ovation-R and In-Ovation-C (GAC International, Bohemia, NY) have an active clip and short gingival horizontal wall (0.0195 inch) compared to the conventional occlusal horizontal wall (0.0285 inch).1 PSLBs exhibit structural differences. Damon-3Mx and Damon-Q (SDS Ormco, Glendora, Calif) incorporate a passive sliding door as a facial wall. However, SmartClip-SL3 and Clarity-SL (3M Unitek, Monrovia, Calif) have a unique clip structure without a solid facial wall.

In friction studies, study design can be categorized according to the number and alignment of the brackets tested. In previous studies,2–13,17–19 five or fewer brackets were used to measure FF, and these brackets were aligned in a straight line rather than following the arch curvature. Only a few studies14,15,20,21 examined entire dentitions aligned according to the arch curvature. Recently, Kim et al.16 introduced a custom-designed typodont system that could include the whole dentition, align the dentition according to the arch curvature, and simulate malocclusion status. Therefore, for clinical relevance, the frictional values should be measured to check the ease with which the archwire is able to slide through the brackets in the whole dentition with the arch curvature.

In several studies7,8,17,18 that assessed FFs generated between brackets and wires, the displacement direction of a single bracket was usually vertical (occluso-gingival) in relation to the direction of wire movement. However, malocclusion is a matter of three-dimensional (3D) displacement of individual teeth. When one tooth or bracket is displaced vertically, friction is generated between the gingival/occlusal walls, corners of the bracket slots, and wire. If one tooth or bracket is displaced horizontally to the labial or the lingual, friction is generated between the vertical/facial walls, corners of the bracket slots, and wire. Therefore, it is necessary to demonstrate that the frictional properties resulting from vertical displacement (VD) show the same characteristics as those resulting from horizontal displacement (HD). But there are no previous studies that compare these two kinds of frictional properties. Therefore, the purpose of this study was to compare the frictional properties according to VD and HD of teeth and the bracket types during the initial leveling/alignment stage.

MATERIALS AND METHODS

In this study, the custom-designed typodont system used in Kim et al.16 was modified and upgraded using a stereolithographic technique. Using computed tomography data, 3D virtual tooth models with root and periodontal ligament (PDL) spaces were designed to emulate a stress-absorbing mechanism (Figure 1) and were fabricated into 3D structures using the Viper™ Pro SLA® System (3D Systems Corporation, Valencia, Calif). The PDL space was filled with Imprint™ II Garant™ Light Body Vinyl Polysiloxane Impression Material (3M ESPE, Seefeld, Germany), which effectively reproduces the mobility of human teeth (Figure 1). In this study, Periotest (Siemens AG, Bensheim, Germany) was used to test the mobility of the typodont teeth. The average Periotest value of the typodont teeth (6.9 ± 0.7, n  =  28) was similar to the results obtained from previous studies of natural human teeth (−0.6 to +13.6).22,23

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The metal frame holding the tooth structures could be moved in occluso-gingival (up and down) and labio-lingual (forward and backward) directions from the ideal position to a maximum of 5 mm in displacement to produce arbitrary displacement of each tooth. At zero position, all teeth were aligned to the ideal position according to the ovoid arch form (OrthoForm III-Ovoid, reference no. 701-723, 3M Unitek) (Figure 2).

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The SLBs tested in this study were as follows: (1) two ASLBs: In-Ovation-R and In-Ovation-C, and (2) four PSLBs: Damon-3Mx, Damon-Q, SmartClip-SL3, and Clarity-SL. As a control group, two conventional brackets, Clarity (3M Unitek) and Mini-Diamond (Ormco), were selected. All brackets tested had a .022-inch slot. As a typical archwire for the initial leveling/alignment stage, 0.014-inch austenitic nickel-titanium (A-NiTi) and copper nickel-titanium (Cu-NiTi) archwires (Ormco) were selected.

To simulate malocclusion, standardized displacements of the teeth were produced when all brackets were coupled with A-NiTi or Cu-NiTi wire. The upper canines on the right and left sides were displaced in the gingival direction, while the upper lateral incisors on both sides were displaced in the lingual direction by 0, 1, 2, and 3 mm (Figure 2).

In this study, selections of the bracket-archwire combinations were made randomly. Once each combination was selected, the brackets were bonded in clinically appropriate positions according to the facial axis point using Transbond XT (3M Unitek) on stereolithographically fabricated typodont teeth. For the conventional brackets, after ligation with the elastic modules (Unistick Ligatures, American Orthodontics, Sheboygan, Wisc), a 3-minute waiting period allowed a reproducible amount of stress relaxation to occur. The typodont was then attached to a custom-made metal plate that was fixed to a mechanical testing machine (model 4466, Instron, Canton, Mass). A custom-designed adaptor gripped one distal end of the archwire, which was extruded from the upper second molar tubes.

Each combination was tested five times with different wires of the same type. Two and a half millimeters of wire was drawn through brackets and tubes at a crosshead speed of 0.5 mm per minute in the dry state and at room temperature. Both the static FF (SFF) and kinetic FF (KFF) were calculated by the same method used in Kim et al.16 A total of 560 tests were conducted.

Descriptive statistics, including means and standard deviations, were calculated for each combination of brackets, type of wire alloy, type of malocclusion, and amount of displacement. A one-way analysis of variance test (ANOVA) was used to evaluate the effects of the variables on FF. If the assumption that the variances were equal was broken by the Levene test, the Welch variance weighted ANOVA test was used. The Duncan test was applied as a post hoc test. The level of significance for all of the tests was set at P < .05.

RESULTS

Conventional brackets showed significantly higher levels of SFF and KFF for each alloy type, type of malocclusion, and amount of displacement when compared with SLBs (P < .001; Tables 1 and 2).

Table 1 Static and Kinetic Frictional Force (cN) According to Combinations of Bracket, Wire Alloy Type, and Vertical Displacement of the Upper Canines^a

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Table 2 Static and Kinetic Frictional Force (cN) According to Combinations of Bracket, Wire Alloy Type, and Lingual Displacement of the Upper Lateral Incisors^a

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Gingival Displacement of the Upper Canines by 3 mm

In the combinations of SLBs and A-NiTi, Clarity-SL produced significantly lower SFF and KFF levels (P < .001). Damon-3Mx produced significantly higher SFF (P < .01) and In-Ovation-R higher KFF (P < .001) (Table 3; Figure 3A,B).

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Table 3 Multiple Comparisons According to Combinations of Self-Ligating Bracket, Wire Alloy Type, and Type of Malocclusion^a

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In SLBs combined with Cu-NiTi, Clarity-SL, Damon-Q, and Damon-3Mx produced significantly lower SFF (P < .01) and Clarity-SL lower KFF levels (P < .001). However, In-Ovation-C produced significantly higher SFF and KFF levels (P < .001).

Lingual Displacement of the Upper Lateral Incisors by 3 mm

In SLBs combined with A-NiTi, Damon-Q, In-Ovation-C, and Damon-3Mx produced significantly lower SFF (P < .01) and In-Ovation-C significantly lower KFF levels (P < .001). However, Clarity-SL produced significantly higher SFF and KFF levels (P < .001) (Table 3; Figure 3C,D).

In SLBs combined with Cu-NiTi, Damon-Q and Damon-3Mx produced significantly lower SFF (P < .01) and Damon-Q lower KFF levels (P < .001). However, Clarity-SL produced significantly higher SFF and KFF levels (P < .001).

In summary, there were significant differences in FF for each combination of brackets, type of wire alloy, type of malocclusion, and amount of displacement (Tables 1 through 3). Clarity-SL combined with A-NiTi and Cu-NiTi, Damon-3Mx combined with A-NiTi, and In-Ovation-C combined with Cu-NiTi showed differences in frictional properties between VD and HD (Figure 4).

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DISCUSSION

In most of the frictional studies9–11,17,18 using the conventional brackets, the brackets were either aligned straight or one bracket was displaced vertically in relation to the direction of wire movement. There were few friction studies about HD because ligature materials holding a wire, rather than the bracket itself, play a major role in terms of friction. Since SLBs have an innate facial wall or clip and do not need ligatures, the FF generated by either VD or HD is produced mainly from the SLB itself. Therefore, it is necessary to investigate the frictional properties in both HD and VD in SLBs.

Despite their similar structures, we detected a significant difference in frictional properties between In-Ovation-R and In-Ovation-C (Tables 1 and 2). Voudouris et al.12 reported that In-Ovation-C demonstrated lower fictional force than In-Ovation-R when tested with stainless-steel wires. In this study, when these two SLBs were combined with A-NiTi, our findings showed similar tendencies compared with those of Voudouris et al.12 In-Ovation-C produced less FF in both VD and HD (by 3 mm) than In-Ovation-R (P < .001; Figures 3 and 4A,B). Voudouris et al.12 explained that compared with In-Ovation-R, the Cr-Co clips within In-Ovation-C showed more freedom within the bracket slot and that the reduction in the curved shape of the In-Ovation-C clip may result in lower seating forces and lower friction.

When combined with Cu-NiTi, In-Ovation-C produced a higher FF than In-Ovation-R in VD of 3 mm (P < .001; Figures 3 and 4C,D). The reason why the Cu-Ni-Ti wire generated higher friction than the A-NiTi wire may be related to the differences in surface chemistry and chemical affinity between A-Ni-Ti and Cu-Ni-Ti archwires.24 Further studies are needed to investigate the reasons for this result. In addition, In-Ovation-C in VD by 3 mm produced approximately 1.7 times more FF than in HD by 3 mm. This was the highest FF that we detected in SLBs combined with Cu-NiTi (Figure 4C,D). Therefore, it can be stated that different frictional properties existed between VD and HD in In-Ovation-C combined with Cu-NiTi.

When Damon-3Mx and Damon-Q were combined with Cu-NiTi, a lower FF was produced compared to other combinations of SLBs and archwires (Figure 4C,D). The manufacturer recommended that Cu-NiTi be used during the leveling/alignment phase with Damon bracket series.25 This combination would be reasonable based on our findings from this study. In addition, Damon-Q with A-NiTi also exhibited good frictional properties compared to other SLBs with A-NiTi (Figure 4A,B).

Damon-3Mx with A-NiTi demonstrated different results. Damon-3Mx belonged to the lowest SFF group in HD by 3 mm but produced the highest SFF in VD by 3 mm among SLBs (Table 3; Figure 4A). SFF of Damon-3Mx with A-NiTi was highly increased from 2 mm to 3 mm in VD (Figure 3A).

Thorstenson and Kusy13,19 reported that for second-order angulations greater than the critical angle, binding increasingly contributed to resistance to sliding. In this case, it could be stated that binding forces increased in VD by 3 mm in combination with Damon-3Mx and A-NiTi. Therefore, it can be stated that different frictional properties existed between VD and HD in Damon-3Mx combined with A-NiTi.

As with In-Ovation-R and In-Ovation-C, there were differences in frictional properties between SmartClip-SL3 and Clarity-SL, despite their similar structure. When Clarity-SL was combined with A-NiTi, the lowest FF was produced in VD by 3 mm (Table 3; Figure 4A,B). However, this combination produced the greatest friction in HD by 3 mm among all SLBs (Table 3; Figure 4A,B). FF in HD by 3 mm was more than two times FF in VD by 3 mm (Tables 1 and 2; Figure 4A,B). Clarity-SL with Cu-NiTi also showed similar results when compared with A-NiTi (Figure 4C,D). However, there were no major discrepancies between VD and HD in SmartClip-SL3 (Figure 4).

If the upper lateral incisor is displaced lingually, friction will be generated between the vertical walls, corners of bracket slots, clips, and a wire (Figure 5). Therefore, clips of SmartClip-SL3 and Clarity-SL influence FF. Differences in friction between SmartClip-SL3 and Clarity-SL might be due to the differences in the clip force and shape between them. The manufacturer announced that the SmartClip-SL3 has more reduced clip force than the Clarity-SL and previous versions of SmartClip.26 In addition, there were differences in bracket shape that influenced FF. SmartClip-SL3 and Clarity-SL commonly have vertical slot-wall extensions (VSEs) to hold clips (Figure 6). As VSE increased, the distance between the bracket margins decreased in both the mesial and distal sides of the horizontally displaced bracket (Figure 6). Eventually, FF increased.27 Since the VSEs of Clarity-SL are longer than those of SmartClip-SL3 (Figure 6), they would contribute to an increase in FF of Clarity-SL in HD.

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From the clinical aspects, for the initial leveling/alignment with A-NiTi, Damon-Q and In-Ovation-C are recommended as a result of their lower FFs in both VD and HD, compared with the other combinations of SLBs and A-NiTi (Figures 3 and 4A,B). If the initial leveling/alignment is performed using Cu-NiTi, Damon-Q and Damon-3Mx are recommended for their lower FFs, compared to the other combinations of SLBs and archwires, all other factors being equal (Figures 3 and 4C,D).

The use of a stereolithographically made typodont system, which can simulate malocclusion status and the periodontal ligament space, still has some limitations. Further studies regarding rotation, mesiodistal tipping, and labiolingual inclination of individual teeth are needed.

CONCLUSIONS

• It is necessary to develop SLBs with low friction in both VD and HD of teeth since the frictional properties of SLBs would be different between VD and HD of teeth.