Background
In fixed appliance therapy, torque moments are transferred through the bracket and are important for the final position of the tooth. Studies have analyzed only the torque-relevant bracket deformation in single bracket-archwire combinations with respect to various archwire rotations, although segmental torque is often used clinically.
Methods
In this study, experiments were conducted in an experimental setup with 2-bracket segments with 0.018-in and 0.022-in conventional brackets with commonly used rectangular archwires and ligatures to measure the torque moment and the relevant tie-wings deformation for 6, 7, and 8 mm interbracket distances (IBD).
Results
The torque moment in both 0.018-in and 0.022-in brackets increased as the archwire dimension and rotation increased. The torque moment with stainless steel ligation was significantly larger than that of the elastic ligature for all the archwires tested in both brackets. Each segmental bracket-archwire-ligature-IBD combination tested had shown a significant difference between the distal and mesial tie-wing deformations for varying torque.
Conclusions
We conclude that in both 0.018-in and 0.022-in brackets, the behavior of the 2-bracket segment-archwire-ligature combination with varying IBD was strongly influenced by the individual factors and also in combination.
Graphical abstract
Highlights
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A new orthodontic torque simulator was used to measure the segmental torque relevant to bracket deformation.
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Brackets were analyzed for various archwire-ligature-interbracket distance combinations for different archwire rotations.
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Distal tie-wings deformation was higher than mesial tie-wings deformation.
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Elastic ligatures lead to more tie-wing deformation than stainless steel ligatures.
In fixed appliance therapy, the final tooth position is refined by the torquing of rectangular archwires. There are factors involving the brackets (slot size, type, and materials), archwires (geometry, dimensions, and materials), and ligatures (materials) available, which may result in torque variation. So, it is necessary to understand comprehensively the bracket deformation relevant to these factors alone and in combination.
Vieira et al reviewed the efficiency of the 0.018-in or 0.022-in bracket and concluded that there is no difference between the 2 systems. Archambault et al found that the torque expression in stainless steel (SS) brackets is significantly influenced by the bracket type, slot, and archwire sizes. Another review by Al-Thomali et al concluded that ligated brackets demonstrated higher torque expression than self-ligating brackets. There were inconsistent conclusions reported on the influence of bracket type on torque transmission. ,,,,,
Bracket deformation relevant to torquing forces was measured by various methods. Such deformation in self-ligating brackets is shown by the digital image correlation technique. Magesh et al measured the slot deformation experimentally for conventional single bracket–archwire combinations. A finite element analysis by Harikrishnan and Magesh showed that bracket deformation was more in the slot wall region than in the tie-wings. Lacoursiere et al showed torque relevant elastic and plastic tie-wing deformation in a preadjusted 0.022-in bracket with 0.019 × 0.025-in archwire by an optical image correlation technique. Melenka et al showed a difference in the tie-wing deformation between Damon Q and Speed brackets through an orthodontic torque simulator (OTS), signifying the role of bracket geometry. Sundar et al reported the tie-wing deformation in conventional single bracket–archwire-ligature combinations experimentally. Thus, literature shows only single-bracket deformation relevant to torque, whereas clinically, segmental torque is often required.
During segmental torquing, interbracket distance (IBD) plays a crucial role. Thushar et al compared the torque generated in Damon Q and Smartclip brackets by finite element analysis, which showed reduced torque moment on increased IBD. There is no experimental study simulating the real-time torque application in segmental brackets involving all the relevant factors. Hence, in our simulated experimental study, the torque-relevant tie-wing deformation in conventional 0.018-in and 0.022-in SS brackets in a segmental bracket-archwire-ligature-IBD combinations for various archwire rotations were analyzed.
Material and methods
Conventional SS orthodontic brackets (maxillary central incisors)—0.018-in (S1) and 0.022-in (S2)—(Leone; Firenze, Italy) were used. The 0.018-in brackets were torqued with 0.016 × 0.022-in (W1), 0.017 × 0.025-in (W2), and 0.018 × 0.025-in (W3) SS straight rectangular archwires (G&H Orthodontics; Franklin, Ind). The 0.022-in brackets were torqued with 0.018 × 0.025-in (W4), 0.019 × 0.025-in (W5), and 0.021 × 0.025-in (W6) SS straight rectangular archwires. Orthodontic ligatures, SS wire (L1) (0.012-in; G&H Orthodontics) and elastic modules (L2) (0.009-in; Phyx Dental; Ontario, Canada) were used. Both brackets were analyzed for different IBDs—6 mm (I1), 7 mm (I2), and 8 mm (I3) in between them. In this study, combinations of S1 with W1, W2, and W3, and S2 with W4, W5, and W6, with L1 and L2 were experimented across I1, I2, and I3, amounting to 36 experiments. Each experiment was repeated with 8 specimens, leading to 288 trials. Each trial included 8 archwire rotations (0° to 35° in increments of 5°), totaling 2304 measurements performed by the same researcher to minimize repeatability errors. The OTS was modified with bidirectional image capturing to assess the segmental torque–relevant bracket deformation in the bracket-archwire-ligature-IBD combinations across different archwire rotations.
The OTS consists of 4 sub-systems. First, a segmental bracket-archwire-ligature combination assembly, which is held using special fixtures for IBD adjustment. The bracket slots were aligned with the archwire axis to provide rotation through frictionless bearings. Second, a strain gauge–based torque transducer (TP-2KCE; Kyowa, Japan) to measure the torque induced in the fixture assembly was connected to record the torque moments. Third, a digital image capturing system with a complementary metal oxide semiconductor–based camera (VT_EX500CS; Contras Tech, Hangzhou, China) with a resolution of 2592 × 1944, an optical size of 1/2.5-in, a pixel size of 2.2 μm × 2.2 μm, a sensitivity of 550 nm, a dynamic range of 71 dB, and a maximum frame of 4 fps was used to measure the bracket slot width during archwire rotations. Lastly, a stepper motor–controlled drive system was included to twist the archwire. These sub-systems were connected to a personal computer for processing the data. A comprehensive view of the experimental setup is shown in Figure 1 .
The comprehensive experimental setup used to measure the simulated segmental torque moment and the tie-wing deformations in the bracket-archwire-ligature-IBD combinations.
A magnified view of the bracket-archwire-ligature assembly at neutral position (0°) with 8 mm IBD with marked tie-wings is shown in Figure 2 . The slot width at neutral position was measured from the digital images captured by the camera located above the bracket-archwire assembly.
The representative magnified view of 0.018-in brackets with 0.016 × 0.022-in archwire at neutral position (0°) with an SS ligature and an 8-mm IBD.
The digital images of the tie-wings were captured for every archwire rotation. The slot width was measured between the 2 straight lines formed between the lower border of upper tie-wings (T1 and T2) and the upper border of lower tie-wings (T3 and T4). This distance was measured from the images through Image J software (NIH, Bethesda, Md), in which 36.5 horizontal pixels represented 1 mm. The slot width was measured at the distal and mesial tie-wings of the brackets for every rotation. The distal tie-wing deformation (Ddistal) was calculated by subtracting the slot width of distal tie-wings (T2 and T4) at neutral position (0°) and the mesial tie-wing deformation (Dmesial) between the mesial tie-wings (T1 and T3) after every rotation. Each trial had 4-slot width measurements from the captured images.
Statistical analysis
The data were statistically analyzed using the SPSS software (version 31.0.1.0[49]; IBM Corp, Armonk, NY). The Shapiro-Wilk test was performed for each set of experimental trials to check the data for normality. The descriptive statistics, including mean and standard deviation, were calculated for the dependent variables, torque, and distal and mesial tie-wing deformations. For analyzing repeated measures, multivariate analysis of variance (RM-MANOVA) was performed to determine whether these factors (archwire size, ligature, IBD, and the repeated measurement variable-archwire rotation) produced statistically significant effects on the dependent variables. The post-hoc test and the Tukey honest significant difference test were conducted for the main effects of the factors. Pairwise comparisons for repeated measurements, design factors, and between-groups factors and their interactions were performed using estimated marginal means. Bonferroni adjustment was applied to control for multiple comparisons, and adjusted P values <0.05 were considered statistically significant.
Results
For both 0.018-in and 0.022-in bracket combinations, the RM-MANOVA results showed that all 4 main factors had significant multivariate effect across all tests (Wilks lambda [Λ], Pillai trace, Hotelling trace, and Roy largest root), with P <0.001 in every case. For consistency and clarity, the Wilks lambda values were extracted for interpretation.
The representative results (archwire rotations 15° and 25°) on torque (Newton millimeter) experienced and the deformations of tie-wings in the brackets-archwire-ligature-IBD combinations of 0.018-in brackets are tabulated in Table I . The RM-MANOVA output for the tests from the 0.018-in brackets is tabulated in Table II .
Table I
The torque moment and the distal and mesial tie-wings deformation in the 0.018-in SS bracket with 0.016 × 0.022-in (W1), 0.017 × 0.025-in (W2), and 0.018 × 0.025-in (W3) SS straight archwires ligated with SS wire and elastic ligatures for 15° and 25° archwire rotations and IBD of 6 mm (I1), 7 mm (I2), and 8 mm (I3)
| Archwire | Ligature | IBD (mm) | Angle of rotation (°) | Torque (Nmm) | Tie-wing deformation (μm) | ||||
|---|---|---|---|---|---|---|---|---|---|
| Mean | SD | Distal (Ddistal) | Mesial (Dmesial) | ||||||
| Mean | SD | Mean | SD | ||||||
| 0.016″ × 0.022″ | SS | 6 | 15 | 10.30 | 0.71 | 8.00 | 0.30 | 2.00 | 0.40 |
| 6 | 25 | 27.24 | 1.02 | 16.00 | 0.30 | 5.00 | 0.50 | ||
| 7 | 15 | 7.37 | 0.27 | 7.00 | 0.40 | 2.00 | 0.30 | ||
| 7 | 25 | 21.26 | 0.80 | 14.00 | 0.70 | 4.00 | 0.60 | ||
| 8 | 15 | 6.70 | 0.52 | 6.00 | 0.10 | 1.00 | 0.30 | ||
| 8 | 25 | 19.61 | 0.76 | 13.00 | 1.10 | 4.00 | 0.30 | ||
| Elastic | 6 | 15 | 8.13 | 0.47 | 8.00 | 0.50 | 5.00 | 0.30 | |
| 6 | 25 | 22.70 | 0.72 | 16.00 | 0.60 | 9.00 | 0.40 | ||
| 7 | 15 | 6.97 | 0.41 | 8.00 | 0.50 | 3.00 | 0.50 | ||
| 7 | 25 | 19.67 | 0.87 | 16.00 | 0.70 | 7.00 | 0.60 | ||
| 8 | 15 | 6.36 | 0.29 | 4.00 | 0.40 | 3.00 | 0.20 | ||
| 8 | 25 | 16.41 | 0.58 | 12.00 | 0.70 | 7.00 | 0.60 | ||
| 0.017″ × 0.025″ | SS | 6 | 15 | 18.47 | 1.17 | 9.00 | 0.40 | 3.00 | 0.40 |
| 6 | 25 | 41.65 | 0.93 | 17.00 | 0.60 | 7.00 | 0.80 | ||
| 7 | 15 | 13.02 | 0.84 | 9.00 | 0.50 | 3.00 | 0.60 | ||
| 7 | 25 | 35.01 | 1.31 | 17.00 | 1.00 | 7.00 | 0.80 | ||
| 8 | 15 | 10.66 | 0.70 | 8.00 | 0.20 | 3.00 | 0.10 | ||
| 8 | 25 | 29.82 | 0.62 | 15.00 | 0.20 | 6.00 | 0.20 | ||
| Elastic | 6 | 15 | 15.95 | 0.71 | 10.00 | 0.80 | 5.00 | 0.60 | |
| 6 | 25 | 36.63 | 0.75 | 18.00 | 1.10 | 10.00 | 0.70 | ||
| 7 | 15 | 12.38 | 0.49 | 9.00 | 0.70 | 3.00 | 0.50 | ||
| 7 | 25 | 31.67 | 1.05 | 17.00 | 0.90 | 8.00 | 0.80 | ||
| 8 | 15 | 9.58 | 0.52 | 5.00 | 0.20 | 3.00 | 0.20 | ||
| 8 | 25 | 25.69 | 0.73 | 14.00 | 0.30 | 7.00 | 0.30 | ||
| 0.018″ × 0.025″ | SS | 6 | 15 | 27.90 | 0.68 | 11.00 | 0.70 | 4.00 | 0.60 |
| 6 | 25 | 58.89 | 0.88 | 17.00 | 0.90 | 8.00 | 0.70 | ||
| 7 | 15 | 23.83 | 0.34 | 9.00 | 0.30 | 1.00 | 0.70 | ||
| 7 | 25 | 54.81 | 0.49 | 16.00 | 0.60 | 6.00 | 0.80 | ||
| 8 | 15 | 18.80 | 0.01 | 6.00 | 0.10 | 1.00 | 0.30 | ||
| 8 | 25 | 48.64 | 0.02 | 15.00 | 1.10 | 4.00 | 0.30 | ||
| Elastic | 6 | 15 | 19.96 | 0.51 | 12.00 | 0.60 | 3.00 | 0.80 | |
| 6 | 25 | 53.25 | 1.33 | 22.00 | 0.90 | 9.00 | 0.40 | ||
| 7 | 15 | 17.12 | 0.62 | 11.00 | 1.00 | 3.00 | 0.30 | ||
| 7 | 25 | 50.29 | 1.07 | 21.00 | 1.40 | 7.00 | 0.40 | ||
| 8 | 15 | 13.62 | 1.50 | 7.00 | 0.40 | 3.00 | 0.20 | ||
| 8 | 25 | 43.35 | 0.71 | 15.00 | 0.70 | 7.00 | 0.60 | ||
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