Evaluation of Frictional Forces Generated between Three Different Ligation Methods with Four Different Sizes of Orthodontic Archwires: An In vitro Study.

AIM: This in vitro study was done to evaluate and compare the frictional forces produced by a passive self-ligating bracket (SLB) and two types of elastomeric ligatures (unconventional elastomeric ligatures [UELs] and conventional elastomeric ligatures [CELs]) on a conventional bracket used with four types of wires. MATERIALS: In this in vitro study, 8 numbers of conventional preadjusted edgewise appliance stainless steel brackets and 4 numbers of passive SLBs were tested. Four types of archwires and two types of ligatures were tested. Brackets were divided into three groups - Group I, Group II, and Group III. All the three groups were further divided into four subgroups based on different archwires.
METHODS: Custom-made mounting jig was specially constructed for this study - upper member and lower member. After the samples were mounted, traction test was conducted using universal testing machine and readings were tabulated for all specimens. The variables in each group were subjected to one-way analysis of variance.
RESULTS: All the three groups were tested for its total friction, static friction, and kinetic friction. SLBs showed a static mean little lower and had kinetic mean little greater than conventional bracket with unconventional module.
CONCLUSION: UELs on conventional brackets and SLBs are able to produce lower frictional force when compared with CELs on conventional brackets. Copyright:

INTRODUCTION

High frictional force that occurs between the guiding archwire and the bracket gives a negative treatment outcome. By controlling the friction at the bracket/archwire/ligature interfaces, lower levels of force can be applied during orthodontic treatment to obtain an optimal biological response for effective orthodontic tooth movement. Tweeny and Hughes in 1961 defined friction as the resistance to motion, when it is attempted to slide one surface over another with which it is in contact.

There are many factors that influence frictional forces during orthodontic tooth movement.[1] The method of archwire ligation seemed to be an important factor in friction generation, yet there are only a few studies to confirm the above fact. Elastomeric modules are economical and can be applied fast and are comfortable. Reduction of frictional force can be obtained by stretching the elastomeric ligature by double diameter; elastomeric ligature can be placed in conventional figure of “0” pattern or figure of “8” pattern.[2]

Self-ligating brackets (SLBs) were introduced in the mid-1930s. SLBs showed very less friction, but it varied depending on the type of self-ligating system used – active or passive. Passive SLBs offer more freedom for teeth to move to their natural position even though they are still interconnected because the archwire is never tightly engaged with the bracket slot.[3]

This in vitro study was done to evaluate and compare the frictional forces produced by a passive SLB and two types of elastomeric ligatures (unconventional elastomeric ligatures [UELs] and conventional elastomeric ligatures [CELs]) on a conventional bracket used with four types of wires.

MATERIALS

Two types of upper premolar brackets were used: [Figure 1] (1) conventional preadjusted edgewise appliance stainless steel (SS) bracket (Gemini, 3M Unitek) 8 numbers and (2) passive SLBs (SmartClip, 3M Unitek) 4 numbers. Four types of archwires are tested: (1) 0.014” nickel–titanium (NiTi) wire, (2) 0.016” NiTi wire, (3) 0.017” × 0.025” SS wire, and (4) 0.019” × 0.025” SS wire. In this study, two types of ligatures are used: [Figure 2] (1) conventional ligature-silver medium (3M Unitek) and (2) unconventional ligature-silver medium (Leone Orthodontic Products, Italy). Instron Universal Testing Machine (LLOYD) L. R-50K was used to test the frictional resistance.

Figure 1

A) Conventional PEA stainless steel upper premolar brackets, B) Passive self-ligating upper premolar brackets – SmartClip 3M Unitek).

Figure 2

A)Conventional modules – silver medium (3M Unitek), B) Unconventional modules – silver medium

METHODS

Brackets were divided into three groups:

Group I: Conventional brackets to be ligated with unconventional modules

Group II: Conventional brackets to be ligated with conventional modules

Group III: Passive SLBs [Figure 1].

All the three groups were further divided into four subgroups. They are

0.014” NiTi

0.016” NiTi

0.017” × 0.025” SS

0.019” × 0.025” SS.

Lower member

Specimen preparation (lower member) [Figure 3]

Custom-made mounting jig was specially constructed for this study. An 8-mm/6-mm thickness steel rod was customized and was cut into four pieces having a length of 75 mm. They were welded together in the shape of “P” with one side open, finished, and polished to have a smooth surface. Two little vertical holes were drilled on the upper and lower part of the “P-” shaped jig for the wire to enter, and two horizontal holes were drilled and threaded for two screws to tighten the wire once entered. This custom-made apparatus will be further called as lower member, to be clamped on the immovable clamp of the universal testing machine.

Upper member

Specimen preparation (upper member) [Figure 4]

Steel rod with dimension of 8-mm/6-mm thickness and 100-mm length was cut, finished, and polished to have a smooth surface. This will be used for welding the bracket for each group and subgroups. This will be further called as upper member, to be clamped on the movable clamp of the universal testing machine.

Care was taken to make the vertical hole in a position so that bracket-fixed upper movable clamp ligated with the wire will be passive.

JIGS mounted on Instron

Specimen preparation [Figure 5]

These brackets of each group were welded on the center of the upper member (4 numbers), such that archwires can slide passively with the bracket. Tests were carried out in Composite Technological Park, Kengeri (Bangalore), by using Instron Universal Testing Machine (LLOYD) L. R-50K-England [Figure 6].

Figure 6

Universal testing machine – Instron (LLYOD-50K)

Evaluation of friction for conventional brackets with unconventional modules (Group I)

The custom-made lower member was clamped tightly to the immovable lower clamp of the universal testing machine. Each upper member for Group I was attached to the upper movable clamp of the universal testing machine, and was tested for friction with four wires (0.014” NiTi, 0.016” NiTi, 0.017” × 0.025” SS, and 0.019” × 0.025” SS). Care was taken so that the archwire/bracket/ligature system passive.

Each four bracket wire and unconventional module combination was tested for ten trials (wire and modules changed for each trial) to minimize the influence of elastic deformation..

Evaluation of friction for conventional brackets with conventional modules (Group II)

The custom-made lower member was clamped tightly to the immovable lower clamp of the universal testing machine. Each upper member for Group II was attached to the upper movable clamp of the universal testing machine, and was tested for friction with four wires (0.014” NiTi, 0.016” NiTi, 0.017” × 0.025” SS, and 0.019” × 0.025” SS). Care was taken so that the archwire/bracket/ligature system passive.

Each four bracket wire and conventional module combination was tested for ten trials.

Evaluation of friction for passive self-ligating brackets (Group III)

The custom-made lower member was clamped tightly to the immovable lower clamp of the universal testing machine. Each upper member for Group III was attached to the upper movable clamp of the universal testing machine, and was tested for friction with four wires (0.014”NiTi, 0.016” NiTi, 0.017” × 0.025” SS, and 0.019” × 0.025” SS). Care was taken so that the archwire and SLB system were passive.

Each passive SLB wire combination was tested for ten trials. After the samples were mounted, traction test was conducted, and readings were tabulated for all specimens.

Traction test

Each traction test was conducted at a speed of 6 mm/min over a distance of 9.5 mm, and the following frictional forces were recorded for static friction and kinetic friction at 5 mm, 7 mm, and 9 mm by a universal testing machine. All measurements were performed under dry condition at temperature 20°C ± 2°C.

RESULTS

All the three groups were tested for its total friction, static friction, and kinetic friction. Each group with its mean, static mean, and kinetic mean for the abovementioned wires was listed in Table 1 (conventional brackets with unconventional modules), Table 2 (conventional brackets with conventional modules), and Table 3 (passive SLBs). The three groups were also compared for homogeneity of means. Another variable to be compared with other means is Kmax. During the traction test of each group and subgroup, maximum force is utilized during kinetic movement in few places. That reading is taken as kinetic maximum (Kmax). The variables in each group were subjected to one-way analysis of variance.

Table 1

Statistical analysis for Group I - conventional brackets with unconventional modules

	Mean	Median	SD	Range	Minimum	Maximum
Static 014 NiTi	0.234	0.05	0.294211	0.7	0	0.7
K5 mm 014 NiTi	0.071	0	0.190523	0.61	0	0.61
K7 mm 014 NiTi	0.121	0	0.231106	0.61	0	0.61
K9 mm 014 NiTi	0.448	0.275	0.61137	1.92	0	1.92
K maximum 014 NiTi	1.38255	1.4695	0.589309	1.667	0.524	2.191
Static 016 NiTi	0.06	0	0.157762	0.5	0	0.5
K5 mm 016 NiTi	0.1228	0	0.323307	1.038	0	1.038
K7 mm 016 NiTi	0.2441	0.02	0.467507	1.213	0	1.213
K9 mm 016 NiTi	0.6501	0.025	0.892251	2.2	0	2.2
K maximum 016 NiTi	1.0151	1.1255	0.906736	2.2	0	2.2
Static 0.17×0.25	0.07157	0	0.148382	0.4657	0	0.4657
K5 mm 0.17×0.25	0.145	0	0.3745	1.2	0	1.2
K7 mm 0.17×0.25	0.14157	0	0.244712	0.7	0	0.7
K9 mm 0.17×0.25	0.265	0.175	0.357499	1.2	0	1.2
K maximum 0.17×0.25	0.74277	0.71385	0.719216	1.943	0	1.943
Static 0.19×0.25	0.369	0.15	0.519133	1.45	0	1.45
K5 mm 0.19×0.25	0.134	0.125	0.154287	0.4	0	0.4
K7 mm 0.19×0.25	0.247	0.07	0.360002	1.13	0	1.13
K9 mm 0.19×0.25	0.44449	0.4	0.425788	1.45	0	1.45
K maximum 0.19×0.25	1.881741	1.9325	0.769729	2.17049	0.68551	2.856

SD: Standard deviation, NiTi: Nickel-titanium

Table 2

Statistical analysis for Group II - conventional brackets with conventional modules

	Mean	Median	SD	Range	Minimum	Maximum
Static 014 NiTi	0.711	0.5	0.690804	2.1	0	2.1
K5 mm 014 NiTi	1.135	1.2	0.349643	0.9	0.7	1.6
K7 mm 014 NiTi	1.467	1.3	0.56549	1.86	0.7	2.56
K9 mm 014 NiTi	0.89	0.9	0.307137	0.9	0.5	1.4
K maximum 014 NiTi	2.1343	2.14	0.642052	1.975	1.109	3.084
Static 016 NiTi	0.33	0.35	0.323351	0.8	0	0.8
K5 mm 016 NiTi	1.0559	0.425	1.272268	2.9	0	2.9
K7 mm 016 NiTi	1.4309	1.5905	1.319933	2.9	0	2.9
K9 mm 016 NiTi	1.4409	1.5905	1.330113	2.9	0	2.9
K maximum 016 NiTi	2.0841	2.307	0.907572	2.919	0	2.919
Static 0.17×0.25	0.5549	0.64	0.455148	1.359	0	1.359
K5 mm 0.17×0.25	0.7634	0.5	0.668933	1.829	0	1.829
K7 mm 0.17×0.25	0.9624	0.85	0.636501	1.829	0	1.829
K9 mm 0.17×0.25	1.0664	0.805	0.732503	2.17	0.2	2.37
K maximum 0.17×0.25	2.2601	2.355	0.644454	1.693	1.346	3.039
Static 0.19×0.25	0.5841	0.51	0.458485	1.761	0.04	1.801
K5 mm 0.19×0.25	0.4783	0.15	0.666005	1.863	0	1.863
K7 mm 0.19×0.25	0.5158	0	0.838875	1.865	0	1.865
K9 mm 0.19×0.25	0.6129	0.1	0.85869	1.865	0	1.865
K maximum 0.19×0.25	2.1168	2.211	0.494665	1.774	1.058	2.832

SD: Standard deviation, NiTi: Nickel-titanium

Table 3

Statistical analysis for Group III - self-ligating brackets

	Mean	Median	SD	Range	Minimum	Maximum
Static 014 NiTi	0.162	0.05	0.334126	1.1	0	1.1
K5 mm 014 NiTi	0.6726	0.6	0.707173	1.66	0	1.66
K7 mm 014 NiTi	0.841	0.995	0.581616	1.66	0	1.66
K9 mm 014 NiTi	1.0586	1.1	0.637584	2	0.1	2.1
K maximum 014 NiTi	1.07963	1.1	0.657381	2.056	0.1	2.156
Static 016 NiTi	0.032	0	0.067462	0.16	0	0.16
K5 mm 016 NiTi	0.226	0	0.365185	0.82	0	0.82
K7 mm 016 NiTi	0.258	0.08	0.349056	0.82	0	0.82
K9 mm 016 NiTi	0.35686	0.16	0.4036	0.9886	0	0.9886
K maximum 016 NiTi	1.37796	1.355	0.821889	2.799	0	2.799
Static 0.17×0.25	0.15	0	0.408928	1.3	0	1.3
K5 mm 0.17×0.25	0.433	0.4	0.325919	1.17	0.03	1.2
K7 mm 0.17×0.25	0.615	0.55	0.312739	1.15	0.2	1.35
K9 mm 0.17×0.25	0.518	0.35	0.589176	1.43	0	1.43
K maximum 0.17×0.25	1.4384	1.55	0.501432	1.734	0.2	1.934
Static 0.19×0.25	0.225	0.3	0.201729	0.5	0	0.5
K5 mm 0.19×0.25	0.245	0.15	0.289108	0.8	0	0.8
K7 mm 0.19×0.25	0.245	0.15	0.289108	0.8	0	0.8
K9 mm 0.19×0.25	0.245	0.15	0.289108	0.8	0	0.8
K maximum 0.19×0.25	1.7318	1.65	0.576188	1.59	0.8	2.39

SD: Standard deviation, NiTi: Nickel-titanium

The mean frictional values for each group are represented in Graphs 1–3. On comparing, conventional brackets with conventional modules showed a higher mean than the other two. SLBs had a mean little greater conventional bracket with unconventional module.

Graph 1

Conventional brackets with unconventional modules

Graph 3

Self-ligating brackets

Conventional brackets with conventional modules

The static friction mean of conventional brackets with unconventional modules is 0.1836, and for conventional brackets with conventional module is 0.5450, 0.1422 for SLBs. On comparing, conventional brackets with conventional module showed a higher static mean than the other two. SLBs showed a static mean little lower than conventional bracket with unconventional module.

The kinetic friction mean of conventional brackets with unconventional module is 0.2519, and for conventional brackets with conventional module is 0.9848, 0.4761 for SLBs. On comparison, conventional brackets with conventional module showed a higher kinetic mean than the other two. SLBs had kinetic mean little greater than conventional bracket with unconventional module.

The Kmax mean of conventional brackets with unconventional module is 1.2368, for conventional brackets with conventional module is 2.1486, and for SLB is 1.4044. On comparison, conventional brackets with conventional module showed a higher kinetic mean than the other two. SLBs had kinetic mean little greater than conventional brackets with unconventional modules.

Descriptive statistics and statistical comparisons of the frictional forces recorded in the different bracket/wire/ligation combinations are reported in Table 4. Table 4 shows the statistical comparison among the three groups: (i) conventional brackets with unconventional modules were compared with conventional brackets with conventional modules, (ii) conventional brackets with unconventional module were compared with passive SLBs, and (iii) conventional brackets with conventional modules were compared with passive SLBs. Asterisk is indicated as significant. (i) and (iii) showed a statistically significant difference in values, whereas (ii) was statistically nonsignificant.

Table 4

Statistical comparison for all groups

	1 versus 2	1 versus 2	1 versus 3	1 versus 3	2 versus 3	2 versus 3
Static 014 NiTi	0.018085	S	0.710829	NS	0.041517	S
K5 mm 014 NiTi	0.084933	NS	0.000592	S	0.047603	S
K7 mm 014 NiTi	0.013631	S	0.011203	S	0.934615	NS
K9 mm 014 NiTi	0.052503	NS	0.902512	NS	0.040474	S
K maximum 014 NiTi	0.802608	NS	0.749979	NS	0.94511	NS
Static 016 NiTi	0.043836	S	0.018586	S	7.25E-05	S
K5 mm 016 NiTi	0.00037	S	0.722597	NS	0.00098	S
K7 mm 016 NiTi	0.004892	S	0.397105	NS	0.00051	S
K9 mm 016 NiTi	0.249916	NS	0.027021	S	0.001514	S
K maximum 016 NiTi	0.997856	NS	0.774527	NS	0.772475	NS
Static 0.17×0.25	0.002629	S	0.005842	S	0.754904	NS
K5 mm 0.17×0.25	0.098984	NS	0.685626	NS	0.043458	S
K7 mm 0.17×0.25	0.008872	S	0.476278	NS	0.045795	S
K9 mm 0.17×0.25	0.043921	S	0.152744	NS	0.526763	NS
K maximum 0.17×0.25	0.748998	NS	0.297538	NS	0.466281	NS
Static 0.19×0.25	0.717308	NS	0.009571	S	0.022507	S
K5 mm 0.19×0.25	0.000173	S	0.075298	NS	0.020566	S
K7 mm 0.19×0.25	0.01904	S	0.523816	NS	0.003994	S
K9 mm 0.19×0.25	0.048461	S	0.264245	NS	0.003354	S
K maximum 0.19×0.25	0.203853	NS	0.401226	NS	0.656816	NS

NiTi: Nickel-titanium, NS: Not significant, S: Significant

DISCUSSION

Most investigations[456] have concluded that elastomeric modules significantly increase resistance to sliding compared with SS ligatures, especially when the SS ligatures are tied loosely. Thus, SLBs[78910] were considered to reduce friction. When the UELs are tied to conventional brackets, the ligature is completely passive. Previous in vitro studies[1112] showed that the UELs can reduce friction compared to CELs during leveling and aligning orthodontic tooth movement.

The results of the present study indicate that both SLB and UEL on CB produced significantly lower frictional forces compared with CEL on CB when coupled with 0.014” NiTi wire, 0.016” NiTi wire, 0.017” × 0.025” SS wire, and 0.019” × 0.025” SS wire. These results are in agreement with those that of previous studies[1314] which found that passive SLBs generated less frictional forces than conventional ligatures on CBs. The differences between SLB and CEL on CB are significant in the current study and are very similar to those reported by Paolo Gandini et al.[15] and Thomas et al.[16]

A previous in vitro study[13] by Lorenzo Franchi and Tiziano Bacetti compared the frictional forces generated by UELs and CELs during leveling and aligning phases with 0.014” superelastic NiTi wire and 0.019” × 0.025” SS wire. The results indicated that, when a slight amount of tooth alignment is needed (1.5 mm), the differences in the performance of UEL and CEL were minimal, but these differences become extremely significant when correction of misalignment of more than 3 mm is attempted. A null amount of force for alignment was produced by CEL, thus they came to a conclusion that the amount of force generated with UEL during the aligning phase of orthodontic tooth movement was significantly greater than that produced with CEL.

The results of the present study corroborate the results of the previous studies by Paolo Gandini[15] et al. and Baccetti et al.[11] who reported significantly lower frictional values for CB with UEL compared with CB with CEL.

Based on the result, it is concluded that UELs are able to produce significantly lower levels of frictional forces than CEL when applied on conventional brackets, and they produce less friction almost similar to SLBs. Thus, UEL may represent a valid alternative to passive SLBs for low frictionless mechanics.

The clinical interpretation of these experimental data, however, requires further considerations that modulate the findings. Minimal adjustments at the bracket/wire/ligature system may significantly change frictional resistance because of physiologic oral functions as well as the oral tissues or food contacting the orthodontic appliance.

CONCLUSION

UELs on conventional brackets and SLBs are able to produce lower frictional force when compared with CELs on conventional brackets when coupled with 0.014”, 0.016” NiTi wire and 0.017” × 0.025”, 0.019” × 0.025” SS wire

UELs may represent a valid alternative to passive SLBs for low friction biomechanics

Friction and low-friction mechanics can be applied simultaneously on the same archwire by using CELs and UELs only in particular segments, i.e., UELs can be used in the posterior segments to reduce friction, whereas CELs are used in the anterior segment.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.