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| Year : 2012 | Volume
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| Issue : 2 | Page : 203-208 |
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| Evaluation of the effect of bracket and archwire composition on frictional forces in the buccal segments |
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Sajan V Nair1, Ratna Padmanabhan2, P Janardhanam2
1 Department of Orthodontics and Dentofacial Orthopaedics, Sri Sankara Dental College, Varkala, Trivandrum, Kerala, India
2 Department of Orthodontics and Dentofacial Orthopaedics, Meenakshi Ammal Dental College, Chennai, India
Click here for correspondence address and email
| Date of Submission | 20-Apr-2011 |
| Date of Decision | 05-Aug-2011 |
| Date of Acceptance | 11-Feb-2012 |
| Date of Web Publication | 3-Sep-2012 |
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 Abstract | | |
Aim of the study: The aim of this study was to consider friction in buccal segment attachments during overjet reduction by means of sliding mechanics. Friction was compared in Stainless Steel, Titanium and Cobalt Chromium brackets, using Stainless Steel and Beta Titanium wires.
Materials and Methods: This in-vitro study comprised 18 samples (6 x 3 brackets of different materials-stainless steel, titanium and cobalt chromium). Each jig comprised of a buccal segment model of two premolar brackets and a molar tube simulating the upper posterior buccal segment. Each jig was coupled with the wire of 0.019 x 0.025 inch dimension and was tested with a Universal Testing Machine. Ligation was achieved using low friction elastomeric modules (Slide TM , Leone, Italy), a non-conventional elastomeric module.
Results: All three bracket systems showed significantly higher frictional forces for the beta titanium wire than for the stainless steel wire. When coupled with the stainless steel wire, the titanium bracket showed the least friction, followed by stainless steel and cobalt chromium brackets. When coupled with the beta titanium wire, the titanium brackets again showed the least friction; while the difference in frictional levels between the stainless steel and cobalt chromium brackets was not statistically significant. The titanium brackets showed the least friction among all the groups tested for both wires.
Conclusion: Titanium bracket with Beta Titanium wires could and probably should be the alternative metal bracket used in the nickel sensitive patient. Keywords: Buccal segment, cobalt chromium brackets, friction, nickel allergy, titanium brackets
How to cite this article:
Nair SV, Padmanabhan R, Janardhanam P. Evaluation of the effect of bracket and archwire composition on frictional forces in the buccal segments. Indian J Dent Res 2012;23:203-8 |
How to cite this URL:
Nair SV, Padmanabhan R, Janardhanam P. Evaluation of the effect of bracket and archwire composition on frictional forces in the buccal segments. Indian J Dent Res [serial online] 2012 [cited 2023 Mar 22];23:203-8. Available from: https://www.ijdr.in/text.asp?2012/23/2/203/100426 |
| Introduction | |  |
Friction is defined as a force that retards or resists the relative motion of the two objects in contact. The nature of friction in orthodontics is multifactorial, derived from both a multitude of mechanical or biological factors. [1]
Sliding a tooth along an archwire using light forces by means of friction mechanics significantly reduces the unwanted rotations that are often associated with the frictionless or loop mechanics. [2] In friction mechanics, for a tooth to begin movement, the applied force has to overcome the static friction in the system initially. Once movement starts, kinetic friction comes into play between the archwire and the bracket. For optimal tooth movement in sliding mechanics, both static and kinetic friction should be kept to a minimum.
Researchers and manufacturers have constantly been developing newer materials and bracket designs in the quest to minimize friction and thereby increase the efficiency of the appliance.
Nickel sensitivity is another reason for manufacturers to develop brackets made of more biocompatible materials. Nickel hypersensitivity in the general population is estimated to be approximately 10%. Though stainless steel brackets with approximately 6-8% nickel content is generally considered to be safe, increased awareness of Nickel sensitivity and toxicity has led to the development of alternative systems.
Only sporadic reports have shown the relation of arch wire material and bracket type in relation to the frictional resistance. The titanium brackets (Equilibrium® Ti, Dentarum, Germany) and the cobalt chromium brackets (Nu-Edge® , TP Orthodontics, Indianna, USA) are marketed as biocompatible alternatives to the stainless steel brackets, because they contain little or no nickel, with the manufacturers claiming added benefit of lower frictional levels than the industry standard in low friction - stainless steel brackets. The null hypothesis thus can be stated as: there is no difference in the reduction of frictional resistance between the stainless steel, titanium and cobalt chromium brackets used along with the stainless steel and beta titanium wires. The aim of this study was to evaluate the kinetic frictional resistance of two brackets: one of titanium and the other of cobalt chromium; and compare it to the stainless steel brackets, when coupled with stainless steel and beta titanium archwires.
| Materials and Methods | | |
Brackets
The following preadjusted brackets with MBT values and a slot size of 0.022 x 0.028 inch were evaluated in this study:
- Stainless Steel brackets (Victory Series TM , 3M Unitek Corp., Monrovia, Calif)
- Titanium brackets (Equilibrium® Ti, Dentaurum, Inspringen, Germany)
- Cobalt Chromium brackets (Nu-Edge® , TP Orthodontics, Indiana, USA)
Molar tubes
- Stainless Steel tubes (Victory Series TM , 3M Unitek Corp., Monrovia, Calif)
- Titanium tubes (Rematitan® tubes, Dentaurum, Inspringen, Germany)
- Cobalt Chromium tubes (TP Orthodontics TM , Indiana, USA)
Wires evaluated
The following wires with a dimension of 0.019 x 0.025 inch were used in the study:
- Stainless Steel wire (Ortho Technology, Netherlands)
- Beta Titanium wire (Ormco Corp., Orange, CA)
Ligation
Low friction elastomeric ligatures (Slide TM , Leone) were used for the ligation in all cases to minimize any variability.
Universal testing machine-(UTM)
An Autograph AG-IS 50 kN Universal Testing Machine was used for testing the kinetic frictional resistance between the bracket assembly and the wire.
Method
An in-vitro study was planned in the Department of Orthodontics to assess the objectives.
Samples used
18 samples (6 x 3 brackets of different materials-stainless steel, titanium and cobalt chromium). Each jig comprised of a buccal segment model of two premolar brackets and a molar tube simulating the upper posterior buccal segment. Each jig was coupled with a wire of 0.019 x 0.025 inch dimension and was tested with the Universal Testing Machine.
Sampling groups
- Stainless Steel brackets - Bracket I
- Titanium brackets - Bracket II
- Cobalt Chromium brackets - Bracket III
- Stainless Steel wire - Wire I
- Beta Titanium wire - Wire II
Preparation of the clinical set-up
The brackets for this friction study were set up to resemble the anchor segment in a clinical situation, in a non extraction case, consisting of the premolars and the molar. In order to negate the effects of the in-built tip and torque in the brackets, a section of 0.021 x 0.025 inch stainless steel wire was placed in the bracket slots to align the premolar brackets and the molar buccal tubes. The distance between the first and the second premolar brackets was kept at 9 mm, while that between the premolar bracket and the first molar buccal tube was kept at 10 mm, simulating a posterior buccal segment. [3] The interbracket distance was standardized for all the jigs with the help of plastic sleeves [Figure 1]. The brackets were then tightly ligated with stainless steel ligatures, prior to cementing the assembly to an acrylic jig with cyanoacrylate adhesive. The jigs were color coded: white representing the stainless steel, yellow for the titanium brackets and red for the cobalt chromium assembly [Figure 2], [Figure 3] and [Figure 4].
Before testing, the brackets and the straight length wire were degreased with ethanol to remove any residue or debris. [4] Then, the 10 cm long 0.019 x 0.025 inch wire was ligated to the brackets with an un-stretched elastomeric ligature, for the purpose of standardization. A new test jig and wire were paired for each trial to negate any influence of wear on friction. A total of 18 samples were tested.
The UTM was used to achieve a steady state displacement of the archwire relative to the bracket allowing evaluation of the kinetic frictional resistance. The universal joint of the UTM ensured the alignment of the wire as it was drawn through the test assembly [Figure 5]. The crosshead speed was adjusted at 5 mm per minute. For each sample, friction was measured in gram-force at every 1 mm displacement for 10 mm resulting in 10 readings per sample.
Data collection
Ten readings were obtained for each of the eighteen samples, adding up to thirty readings per bracket/arch wire combination and a total of 180 readings for the entire study.
Data analysis and statistics
The following tests were carried out for statistical analysis of the data,
- Student t- test, to compare the values in two independent groups.
- One way ANOVA (Analysis of variance), to compare the values of three or more independent groups
- Tukey Honestly Significant Difference (HSD), to establish the significant groups, if the p- value in the one way ANOVA was found to be significant.
| Results | | |
The results are presented under the following sections:
Comparison of mean force between the stainless steel wire and the beta titanium wire for different brackets. [Table 1]  | Table 1: Comparison of mean force between wire I and wire II for different brackets
Click here to view |
The results show that, for all the three bracket types, the mean frictional force generated by the beta titanium wire was significantly higher than that by the stainless steel wire.
Comparison of mean force among different brackets for the stainless steel wire [Table 2]
The one way ANOVA test, comparing the frictional forces between all three bracket types coupled with the stainlessssteel archwire, yielded a p-value less than 0.0001 showing that it was highly significant. Since the p-value obtained through the one way ANOVA was significant, a Tukey HSD procedure was carried out to find out the significant groups.
The comparison of mean frictional force in the cobalt chromium brackets with that of the stainless steel brackets proved to be insignificant with a P-value of 0.019.
Comparison between the stainless steel bracket and the titanium bracket resulted in the P-value being less than 0.0001, showing that the mean force in the stainless steel bracket was significantly higher than the mean frictional force in the titanium bracket when the brackets were coupled with the stainless steel wire.
A similar result was obtained when comparing the cobalt chromium brackets with titanium brackets with the former having significantly higher mean frictional force.
Comparison of mean force among different brackets for the beta titanium wire [Table 3]
As the results of the one way ANOVA test done to compare the frictional forces generated by the stainless steel, titanium and the cobalt chromium brackets, when they were coupled with the beta titanium wire was found to be significant (P-value <0.0001), a Tukey HSD procedure was also carried out to find the significant groups.
The mean force for the cobalt chromium bracket (82.66 ± 4.28) was significantly higher than the mean frictional force levels in the titanium bracket, 61.99 ± 4.99, with the P-value being less than 0.0001.
Similarly, the mean frictional force generated in the stainless steel bracket, 78.20 ± 11.44, was significantly higher than that in the titanium brackets when these were coupled with the beta titanium wires. The P-value was less than 0.0001.
However, the result of the Tukey HSD procedure between the stainless steel and the cobalt chromium bracket yielded a P-value of 0.066, which was not statistically significant.
| Discussion | | |
The efficiencies of orthodontic couples, as stated by Kusy and Whitley, [5] range from 40% to 88% with the loss of applied force due to friction estimated to be around 12% to 60%. Lesser the frictional levels, greater would be the efficiency of the orthodontic appliance.
The rectangular stainless steel which were used in this study are considered most suitable for friction mechanics owing to their resiliency, stiffness, corrosion resistance and low cost. Beta titanium wires were introduced as an alternative to stainless steel with the advantages over the latter being that, it exerts lower force, has adequate springback, good formability and true weldability. Beta titanium wires are probably the only alternative to the stainless steel wires in a nickel sensitive patient. But when it comes to friction, studies have shown that the beta titanium produces the greatest friction among all archwire materials. [6],[7] The reason for this has been attributed to the increased roughness of the wire and the tendency for galling and fretting. [6],[7] Beta titanium was chosen for this study to evaluate whether the newer bracket materials, especially titanium, produced any different result with it.
The method of ligation of the archwire to the bracket has shown to affect the friction. Loosely tied stainless steel ligatures have been shown to have the least friction, while conventional elastomeric modules contribute to an increase in the friction. However for this study, in order to reduce the inconsistency of tying the stainless steel ligatures at different levels of tension, unconventional low friction elastomeric modules were used. The design of these particular modules result in the creation of a passive ligation around the slot, turning the three walled slot into a 4 walled box, similar to a passive self-ligating bracket. Being made of a special medical polyurethane mix reduces the friction further. Baccetti et al, [8] showed that, these non conventional passive elastomeric ligatures were able to produce significantly lower levels of friction when compared to the conventional elastomeric ligatures. Gandini et al, [9] considered them a valid alternative to passive self-ligating brackets.
Ireland et al, [10] showed that, there was no significant difference in the friction levels when the crosshead speed of the UTM was varied from 0.5 mm per minute to 50 mm per minute. For this study, the frictional force was recorded by drawing the rectangular wire through the attachments at a speed of 5 mm per minute. This speed was selected for the experimentation because it was felt that the higher speeds did not represent the clinical situation.
The results indicated that, the Titanium brackets exhibited the least friction among the three bracket types that were tested. This was in accordance with a study by Kusy et al, [11] where it was shown that, Titanium brackets had relatively lower frictional levels than the stainless steel brackets. Another study by Kusy et al, [12] found that, the coefficients of friction of titanium brackets were comparable to the stainless steel brackets in both, active and passive configuration and regardless of whether it was under dry or wet conditions. The titanium brackets generated the lowest frictional values when coupled with both, the stainless steel as well as the beta titanium archwires. Kapur et al, [13] also showed that, with higher dimension rectangular wires, similar to those used in this study, the frictional levels generated by the titanium brackets were considerably lower than that by the stainless steel brackets. The wire dimension used in this study was a 0.019 x0.025 inch archwire.
Though the titanium brackets have a much rougher surface texture than that of the stainless steel brackets, as revealed by scanning electron microscope studies, [7] and so would be expected to have greater coefficients of friction than the stainless steel brackets, our results prove otherwise. The archwire slides on the titanium brackets on a passivated layer of the carbon, oxygen, titanium and nitrogen, similar to the passivated layer of chromium and oxygen on the stainless steel brackets. [7] The manufacturers also claimed that, the slot of the bracket was put through a special surface treatment. Therefore, the surface chemistry may have been the reason for the reduced frictional resistance of the titanium brackets.
The cobalt chromium brackets showed significantly higher values of kinetic friction than the titanium brackets for both, the stainless steel and beta titanium archwires. The frictional levels were also statistically significant when compared to the stainless steel brackets coupled with the stainless steel archwires. However, there was no statistical significance when compared to the stainless steel brackets coupled with the beta titanium wire. This was in accordance with a study done by Moore et al.[14] Another study by Redlich et al, [15] of various reduced friction brackets also showed that, Nu-Edge® brackets produced relatively low friction levels. The manufacturer claimed that, the increased hardness of cobalt chromium over stainless steel aids in reduction of friction. And as the Nu-Edge® brackets are cast and not milled or machined, the result is a smoother finish of the slot.
Of the two archwires evaluated, the results show that, the beta titanium wires were associated with the greater friction when compared to the stainless steel archwire for all the bracket-archwire couples tested. This was in accordance with the results seen in studies by Angolkar [16] and Kusy et al.[6] The increased friction generated by beta titanium has been attributed to the its increased surface roughness. But, laser spectroscopy studies [6] showed that, while nickel titanium wire had greater surface roughness than the beta titanium, the former produced comparatively lesser friction. They attributed this to the tendency of beta titanium to cold weld to the stainless steel brackets resulting in a repeated stick-slip movement between the archwire and the bracket. Titanium has a strong tendency to undergo adhesive wear or gall, owing to its chemical reactivity and the ease with which it forms alloys, resulting in the formation of minute junctions which has to be broken down for further movement to occur. This phenomenon becomes more pronounced with the increased titanium content in the alloy. [7] This study sought to examine the probability of any uncharacteristic change in friction of the beta titanium wire over the stainless steel wire when coupled with the titanium brackets. However, as the results showed, the increase in the kinetic frictional values when the beta titanium was used with the titanium brackets was only along the lines of increase similar to a situation when it was used with the stainless steel and the cobalt chromium brackets. The surface oxide layer on the titanium bracket could be responsible for this by preventing any galling. It can be hypothesized that unless there is a breach in the surface oxide layer, which could occur in an active configuration, there would not be any interaction between the beta titanium wire and the bracket material that could cause a substantial increase in the friction levels.
The oral cavity presents another major factor, which could influence the way different archwire- bracket couples react. Saliva has been purported to act as a lubricant by some authors, and as an adhesive by others. [17],[18],[19],[20] Kusy and Whitley [21] have stated both, dry and wet states exist in the oral cavity, the former when the contacts between the archwire and brackets squeeze out the boundary layer of saliva, and the latter when the appliance is bathed from the salivary ducts. They found that, saliva would behave as a lubricant or an adhesive depending on the material of the archwire-bracket couple [22] rather than the viscosity of the saliva. [23] In an experiment conducted by the same authors [24] on different archwire materials coupled with stainless steel brackets, they found that for stainless steel, nickel titanium and beta titanium wires, the saliva acted as an adhesive, increasing the resistance to sliding when compared to dry state; while for cobalt chromium wires, there was no difference under the two conditions indicating that these wires are not as readily wetted as the other alloys. These findings may also have implications when using brackets made of different materials as in our study.
As with any in-vitro model, this study does not replicate the numerous variables that come into play in vivo, which can alter the friction levels either way. We considered only the kinetic friction, produced when the archwire was drawn through a buccal segment model of brackets and tubes in a passive configuration. Other factors such as the slot dimension, torque expression and friction in an active configuration must be considered before a judgment is made on the superiority of one bracket over the other. Further studies on these aspects would enhance the orthodontist's understanding and ability to obtain outstanding treatment results. The other limitation would include the fact that only one type of ligation technique was considered. Further research would be necessary to evaluate the friction with various other ligation techniques.
| Summary and Conclusion | | |
Titanium brackets coupled with the Stainless Steel wires will serve as best choice for friction mechanics producing the least frictional resistance.
Between the Titanium and the Cobalt-Chromium brackets, which are bio-compatible alternatives to the Stainless Steel brackets, Titanium brackets with Beta Titanium wires would be the choice for a Nickel sensitive patient.
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Correspondence Address:
Sajan V Nair
Department of Orthodontics and Dentofacial Orthopaedics, Sri Sankara Dental College, Varkala, Trivandrum, Kerala
India
 Source of Support: None, Conflict of Interest: None  | Check |
DOI: 10.4103/0970-9290.100426
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2], [Table 3] |
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