Comparison of Friction Resistance Between Different Bracket Systems: An In Vitro Study

Abstract

Introduction

The efficiency of tooth movement associated with orthodontic sliding mechanics can be compromised by friction between archwire and tube or bracket slot. Orthodontic brackets and archwires are important constituents of an orthodontic appliance system. Friction is an uncontrolled variable of particular interest when continuous arch techniques are used to align and move teeth by way of sliding a tube or bracket along an arch.

Aim and Objectives

Comparison of frictional resistance between different bracket systems of 3M and MO brands in combination with different wires of 3M, MO, and G&H brands. Evaluation of frictional resistance between two different types of bracket systems and different types of wire combinations.

Materials and Methods

Two types of bracket systems were used: metal bracket system and ceramic bracket system (3M and MO brands). Four different orthodontic wire alloys were used: stainless steel (SS), TMA, NiTi, and CuNiTi (3M, MO, and G&H brands). A rectangular block made up of self-cure acrylic resin was designed to hold the bracket during the mechanical test. Each bracket base was glued to an acrylic block with cyanoacrylate glue. The wire segment was inserted into the bracket and was ligated passively with SS ligature wire. The bracket–wire combination was submitted to mechanical tests with a universal testing machine. One end of the acrylic block was mounted to the lower grip of the universal testing machine. The free end of the archwire was fixed to the upper grip of the universal testing machine. The friction which was generated between the wire and bracket was measured under dry conditions and at room temperature. The test wire was pulled through the bracket at a speed of 0.5 cm/min. The load cell registered a maximum force level. This data was stored in the computer and was submitted to statistical analysis, and parameter tests were applied.

Results

The highest mean friction resistance of 5.998 N was generated by group B2-E2 (ceramic bracket – MO + TMA archwire – MO). The lowest mean friction resistance of 2.552 N was generated by group A1-C1 (metal bracket – 3M + SS wire – 3M). There was a statistically significant difference between the friction generated by the metal bracket on SS archwire and the ceramic bracket on TMA archwire regardless of 3M or MO brand material (p value < .0001) and metal bracket (p = .0004).

Introduction

Friction between the archwire and the tube or bracket slot can undermine the efficiency of tooth movement in orthodontic sliding mechanics. Friction is an uncontrolled variable of particular interest when continuous arch techniques are used to align and move teeth by way of sliding a tube or bracket along an arch. ¹

The enhanced utilization of sliding mechanics, after the development of pre-adjusted Edgewise systems, has led to increased scrutiny regarding the frictional forces between the bracket and archwire and their role in resisting tooth movement. ²

In fixed orthodontic therapy, teeth can be moved by using sliding or loop mechanics. In sliding mechanics, the rate of friction is much higher and may lead to anchor loss. There are two types of friction: (a) kinetic (dynamic)—which occurs during the motion and (b) static—which prevents the motion. ³

Friction is defined as the force that opposes the movement of an object when it slides along the surface of another. ³ In the field of orthodontics, numerous studies have investigated the factors affecting frictional resistance between brackets and archwires, utilizing experimental testing models that incorporated both single and multiple brackets. The results of these investigations indicate that several significant factors are essential in influencing the levels of friction. These factors encompass the materials of the brackets and wires, the surface conditions of both the archwires and bracket slots, the cross-sectional area of the wire, the torque at the interface between the wire and bracket, the nature and intensity of ligation, the utilization of self-ligating brackets, the spacing between brackets, and the influence of saliva and oral functions. ⁴

Static friction (SF) represents the minimum force required to initiate the movement of one solid surface across another. In contrast, kinetic friction (KF) is the force necessary to maintain the sliding motion of one solid object over another at a uniform speed, acting as the resistance to motion.^{5, 6}

During the sliding process, biological tissues react, and tooth movement takes place only when the applied force surpasses the friction present at the bracket–wire interface. Increased level of frictional force results in debonding of the bracket with either small or no dental movement at all. Friction can reduce the available force, resulting in anchorage loss. Although various studies were done regarding friction between brackets and wires, enough evidence is not available with respect to newer brackets and archwires available commercially. ⁷

This study is to evaluate frictional resistance generated between two different types of brackets: metal and polycrystalline ceramic brackets (3M and MO brands) and four different types of wires: SS, nickel-titanium (NiTi), CuNiTi, and TMA (3M, MO and G&H brands) combinations, to compare each group with each other, and to determine which bracket and wire combination produces the highest friction resistance and how it affects orthodontic tooth movement and its outcome.

Materials and Methods

This in vitro study was carried out in the Department of Orthodontics and Dentofacial Orthopedics to evaluate frictional resistance generated between two different types of brackets and four different types of wire combinations, to compare each group with each other, and to determine which bracket and wire combination produces the highest friction resistance and how it affects orthodontic tooth movement and its outcome. This was an in vitro study. we have obtained institutional ethical committee approval (CODS/IEC/137/2022).

All brackets were 0.022″ in dimension and MBT bracket prescription.

Sr. No.	Bracket System	Company	Group
1.	Metal bracket	Victory series, 3M Unitek	A1
2.	Metal bracket	Micro-LP low profile, Modern Orthodontics	A2
3.	Ceramic bracket (polycrystalline)	Clarity advanced, 3M Unitek	B1
4.	Ceramic bracket (polycrystalline)	Purity, Modern Orthodontics	B2

• Total groups of bracket system: 4

All wires were of 0.019″ × 0.025″ in dimension

Sr. No.	Orthodontic Wire Alloy	Company	Group
1.	Stainless steel (SS)	Permachrome resilient archwire, 3M Unitek	C1
2.	SS	Acti-4s, Modern Orthodontics	C2
3.	NiTi	NiTi super elastic archwire, 3M Unitek	D1
4.	NiTi	Super elastic niti “tinol-1,” Modern Orthodontics	D2
5.	TMA	Beta-iii, 3M Unitek	E1
6.	TMA	Ti-alloy, Modern Orthodontics	E2
7.	CuNiTi	M5 thermal, G&H	F1
8.	CuNiTi	Heat-activated nitinol “tinol-2,” Modern Orthodontics	F2

• Total groups of orthodontic wires (as per wire alloy and company): 8

• Main study groups: 8

1	Group 1: Metal bracket and SS wire (A-C)
2	Group 2: Metal bracket and NiTi wire (A-D)
3	Group 3: Metal bracket and TMA wire (A-E)
4	Group 4: Metal bracket and CuNiTi wire (A-F)
5	Group 5: Ceramic bracket and SS wire (B-C)
6	Group 6: Ceramic bracket and NiTi wire (B-D)
7	Group 7: Ceramic bracket and TMA wire (B-E)
8	Group 8: Ceramic bracket and CuNiTi wire (B-F)

• Repetition of testing of each group was six times. A rectangular block made up of self-cure acrylic resin was designed to hold the bracket during the mechanical test. Each bracket base was glued to the acrylic block with cyanoacrylate glue. The wire segment was inserted into the bracket and was ligated passively with SS ligature wire (Figures 1 to 8). Passive ligation was achieved via lightly tying the wire to the bracket. ⁸ The bracket–wire combination was submitted to mechanical tests with a universal testing machine. One end of the acrylic block was mounted to the lower grip of the universal testing machine. The free end of the archwire was fixed to the upper grip of the universal testing machine (Figure 9). The friction which was generated between the wire and the bracket was measured under dry conditions and at room temperature. The test wire was pulled through the bracket at a speed of 0.5 cm/min. ⁹ The load cell registered a maximum force level. This data was stored in the computer and was submitted to statistical analysis, and parameter tests were applied.

Figure 1.

Source: Taken from Department of Orthodontics, CODS.

Figure 2.

Source: Taken from Department of Orthodontics, CODS.

Figure 3.

Source: Taken from Department of Orthodontics, CODS.

Figure 4.

Source: Taken from Department of Orthodontics, CODS.

Figure 5.

Source: Taken from Department of Orthodontics, CODS.

Figure 6.

Source: Taken from Department of Orthodontics, CODS.

Figure 7.

Source: Taken from Department of Orthodontics, CODS.

Figure 8.

Source: Taken from Department of Orthodontics, CODS.

Figure 9.

Source: Taken from Laboratory of CSIR-CSMCRI, Bhavnagar.

Results

The mean value of friction resistance generated by metal and ceramic brackets on SS, NiTi, TMA, and CuNiTi archwires is shown in Figure 10. The mean frictional resistance generated by different study groups was compared by using Kruskal–Wallis test, shown in Table 1. A pairwise comparison of the mean difference in friction resistance between different groups was done by using Dunn’s multiple comparison test. p Value less than .05 was considered statistically significant.

Figure 10.

Source: Taken from statistical analysis of the study done by Bio-statistician.

Table 1.

Comparison of Mean Friction Resistance (in N) Between Different Study Groups Using Kruskal–Wallis Test.

	Number of Values	Minimum	25% Percentile	Median	75% Percentile	Maximum	Mean	Std. Deviation	Std. Error	Lower 95% CI	Upper 95% CI
A1-C1	6	1.83	1.875	2.73	3.058	3.08	2.552	0.5862	0.2393	1.937	3.167
A2-C1	6	1.97	2.255	2.755	3.125	3.14	2.682	0.4816	0.1966	2.176	3.187
A1-C2	6	2.35	2.358	2.68	3.105	3.33	2.738	0.404	0.1649	2.314	3.162
A2-C2	6	2.08	2.335	2.745	3.038	3.06	2.68	0.4057	0.1656	2.254	3.106
A1-D1	6	2.99	3.328	4.17	4.44	4.77	3.978	0.6535	0.2668	3.292	4.664
A2-D1	6	3.41	3.515	3.995	4.883	4.89	4.12	0.6516	0.266	3.436	4.804
A1-D2	6	3.21	3.698	4.175	4.408	4.79	4.082	0.5241	0.214	3.532	4.632
A2-D2	6	3.33	3.585	3.99	4.575	4.65	4.03	0.5103	0.2083	3.495	4.565
A1-E1	6	3.92	4.198	4.565	4.84	5.23	4.547	0.4371	0.1785	4.088	5.005
A2-E1	6	4.17	4.305	4.61	5	5.33	4.66	0.4229	0.1726	4.216	5.104
A1-E2	6	3.88	4.225	4.43	5.065	5.32	4.563	0.5098	0.2081	4.028	5.098
A2-E2	6	3.96	4.253	4.495	5.17	5.23	4.613	0.4901	0.2001	4.099	5.128
A1-F1	6	2.99	3.335	3.87	4.653	4.87	3.938	0.7092	0.2895	3.194	4.683
A2-F1	6	3.66	3.818	4.11	4.42	4.66	4.125	0.3586	0.1464	3.749	4.501
A1-F2	6	2.99	3.583	4.23	4.688	4.77	4.11	0.6587	0.2689	3.419	4.801
A2-F2	6	2.45	3.275	3.97	4.97	5.24	4.01	1.025	0.4185	2.934	5.086
B1-C1	6	2.77	3.205	3.44	3.77	3.95	3.443	0.4015	0.1639	3.022	3.865
B2-C1	6	2.78	2.975	3.415	3.865	3.97	3.408	0.4826	0.197	2.902	3.915
B1-C2	6	3.01	3.025	3.395	3.625	3.85	3.372	0.326	0.1331	3.03	3.714
B2-C2	6	2.99	3.028	3.255	3.465	3.78	3.28	0.2831	0.1156	2.983	3.577
B1-D1	6	4.09	4.36	4.715	4.853	5.07	4.637	0.3342	0.1364	4.286	4.987
B2-D1	6	4.34	4.573	4.725	5.06	5.21	4.777	0.3042	0.1242	4.457	5.096
B1-D2	6	4.44	4.523	4.7	5.23	5.23	4.808	0.3431	0.1401	4.448	5.168
B2-D2	6	4.57	4.63	4.925	5.098	5.24	4.893	0.2516	0.1027	4.629	5.157
B1-E1	6	4.7	4.708	5.515	6.073	6.53	5.482	0.7071	0.2887	4.74	6.224
B2-E1	6	4.11	5.28	5.985	6.143	6.42	5.703	0.8164	0.3333	4.847	6.56
B1-E2	6	5.76	5.775	5.945	6.08	6.08	5.932	0.1451	0.05924	5.779	6.084
B2-E2	6	5.33	5.668	5.94	6.475	6.55	5.998	0.45	0.1837	5.526	6.471
B1-F1	6	3.67	4.083	4.38	4.508	4.68	4.297	0.3436	0.1403	3.936	4.657
B2-F1	6	3.88	3.895	4.125	4.56	4.56	4.192	0.3127	0.1277	3.864	4.52
B1-F2	6	3.89	4.228	4.525	4.815	5.01	4.507	0.3893	0.1589	4.098	4.915
B2-F2	6	3.99	4.283	4.55	4.755	5.04	4.528	0.3498	0.1428	4.161	4.895

Kruskal–Wallis test
p Value	<.0001
Exact or approximate p value?	Gaussian approximation
p Value summary	***
Do the medians vary signif? (p < .05)	Yes
Number of groups	32
Kruskal–Wallis statistic	141.3

Source: Taken from statistical analysis of the study done by Bio-statistician.

Evaluation of Mean Frictional Resistance

A one-way analysis of variance (ANOVA) test was done to compare the mean frictional resistance generated by the 32 groups.

There was a statistically significant difference between the friction generated by the metal bracket on SS archwire and the ceramic bracket on TMA archwire regardless of 3M or MO brand material (p value < .0001). There was a statistically significant difference between the friction generated by the ceramic bracket on SS and TMA archwire, regardless of 3M or MO brand material (p value < .0001). However, there was a statistically significant difference between the friction generated by SS and TMA archwires with regard to ceramic brackets (p ≤ .0001) and metal brackets (p = .0004).

On comparing the friction resistance generated by metal and ceramic brackets on SS, NiTi, TMA, and CuNiTi archwires, ceramic brackets produced the highest friction resistance with TMA wire either of 3M or MO brand materials.

Discussion

Friction is defined as a force that counteracts the relative motion of two objects that are in direct contact. In orthodontics, it occurs when the archwire, which connects the orthodontic brackets on teeth, encounters resistance as it moves within the bracket slot. This resistance can slow down tooth movement, affect treatment progress, and cause discomfort to the patient. Kusy and Whitley divided resistance to sliding into three components: (a) classical friction, (b) binding, and (c) notching. ¹⁰ Several factors can influence the level of friction in orthodontic treatment: bracket and archwire material, bracket design, ligation, oral hygiene and debris, and patient compliance. ¹¹ When force is applied to move teeth, a portion of that force is dissipated as friction. Orthodontists must carefully balance the forces they apply with the frictional forces present in order to ensure effective treatment. If the friction is too high, it would necessitate applying more force to achieve the desired tooth movement. This can lead to discomfort for the patient and potential damage to the biological structures, including roots and periodontal tissues. ¹¹

Studies, such as the one by Frank and Nikola, ¹² emphasize the importance of considering the bracket and archwire materials in relation to friction. SS archwires tend to have less friction than NiTi archwires when there is no binding. However, the type of archwire can have varying effects depending on the degree of angulation and binding. This highlights the need for careful consideration of archwire selection based on the specific requirements of each patient’s treatment.

The increasing interest in aesthetic treatment alternatives has resulted in the emergence of ceramic brackets. While these brackets offer a more aesthetically pleasing option, they tend to generate higher friction compared to their SS counterparts, primarily due to the increased surface roughness of ceramic brackets. ¹³ Orthodontists must balance aesthetic concerns with the potential impact on treatment efficiency when choosing bracket materials. ¹⁴ The present study shows a statistically significant difference between the friction generated by the metal bracket on SS archwire and the ceramic bracket on TMA archwire regardless of 3M or MO brand material (p value < .0001). The results were in agreement with the study done by Angolkar et al. ¹⁴ In this research, the frictional forces associated with conventional SS brackets, self-ligating SS brackets, conventional ceramic brackets, and ceramic brackets with an SS slot were evaluated. Each bracket was designed with a 0.022-inch slot and was tested alongside archwires of dimensions 0.019 × 0.025 inches, composed of SS, NiTi, and beta-titanium. The method of ligating the archwire also plays a crucial role in friction control, as noted by Pillai et al. ¹⁵ The study examined the influence of ligation methods on the frictional resistance of ceramic brackets, as well as the impact of bracket material on both static and kinetic frictional resistance. The findings indicated that passive self-ligating brackets (PSLB) exhibited considerably lower levels of SF and KF in comparison to active self-ligating brackets (ASLB). Furthermore, no significant differences were observed between the frictional resistance of ceramic and metal PSLBs. Elastomeric ligatures can contribute to friction, especially when first applied, as they exert force to secure the wire in place. To address this issue, self-ligating brackets have evolved as a solution. Self-ligating brackets have mechanisms built into the bracket itself, reducing the need for external ligatures. This can lead to reduced friction, more comfortable treatment for the patient, and potentially shorter treatment times. ¹⁶

The present study compared the frictional resistance generated by a metal bracket (3M and MO company) and ceramic bracket (3M and MO company) with 0.019 × 0.025″ SS, NiTi, TMA, and CuNiTi archwires. In this study, the bracket and archwire setup was tested using a universal testing machine with a crosshead speed of 5 mm/min. The typical rate of tooth movement is approximately 1 mm per month, equating to an average speed of about 2.3 × 10^–5 mm/min. A speed of 5 mm/min was selected for this investigation, as higher speeds did not accurately represent the clinical conditions. The results reveal a statistically significant difference in the frictional resistance among different combinations of brackets and archwires (p < .001). Additionally, a statistically significant difference was observed in the friction generated by SS and TMA archwires concerning ceramic brackets (p < .0001) and metal brackets (p = .0004). The findings of the study indicate a statistically significant difference in the frictional resistance among various combinations of brackets and archwires (p < .001). Specifically, there was a notable difference in the friction produced by SS and TMA archwires when paired with ceramic brackets (p < .0001) and metal brackets (p = .0004). These results are consistent with the research conducted by Ogata et al., ¹⁷ which examined the frictional forces generated by ceramic brackets, ceramic brackets with metal-reinforced slots, and SS brackets in conjunction with SS, NiTi, and TMA orthodontic archwires. The study found that ceramic brackets with metal slots produced significantly lower frictional forces compared to standard ceramic brackets, yet exhibited higher frictional values than SS brackets.^{18, 19}

In their research, Kusy et al. ²⁰ examined the coefficient of friction under dry and wet conditions. They discovered that, irrespective of the slot size, the mean kinetic coefficients of friction were minimized for all SS combinations and maximized for beta-titanium wire combinations in the dry state. When transitioning to the wet state, the kinetic coefficient for the SS combinations increased by as much as 0.05 compared to the dry state. In contrast, the beta-titanium wire combinations experienced a decrease to 50% of their dry state values in wet conditions.

It could be concluded by this study that the generation of friction force mainly depended on bracket and wire material and composition. There was no role of the brand (company) in the generation of friction. Therefore, both 3M and MO are equally standard companies. So, during orthodontic treatment, the proper selection of bracket and wire combination can reduce friction, whether it is 3M brand or MO brand.

Conclusion

The results showed a statistically significant difference in mean friction resistance generated by metal and ceramic brackets on SS and TMA archwires, respectively. The highest mean friction resistance was generated by the ceramic bracket and TMA archwire group. If the friction is too high, it would necessitate applying more force to achieve the desired tooth movement. This can lead to discomfort for the patient and potential damage to the biological structures, including roots and periodontal tissues. The lowest mean friction resistance was generated by the metal bracket and SS archwire group. It could be concluded by the present study that a metal bracket with SS archwire is a much better option for fixed orthodontic treatment than a ceramic bracket.

This study sheds some light on the effect of various bracket archwire combinations on frictional resistance. There are a wide variety of brackets available in the market, like self-ligating SS brackets, monocrystalline ceramic brackets, polycrystalline ceramic brackets, and polycrystalline ceramic brackets with metal slots. Appliance selection is not the only factor that affects friction; other factors should also be taken into consideration.