|Year : 2011 | Volume
| Issue : 1 | Page : 90-94
|Comparative evaluation of frictional characteristics of coated low friction ligatures - Super Slick Ties™ with conventional uncoated ligatures
Deepu Leander1, Jyothindra K Kumar2
1 Government Dental College, Thumba, Trivandrum, India
2 Material Characterization Division, INSTEF, VSSC, Thumba, Trivandrum, India
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|Date of Submission||07-Dec-2009|
|Date of Decision||19-May-2010|
|Date of Acceptance||10-Nov-2010|
|Date of Web Publication||25-Apr-2011|
| Abstract|| |
Background: Elastomeric ligatures have constituted a versatile method of securing the archwire to bracket slot, though self-ligating brackets have been a recent innovation. Coating elastomeric ligatures with a hydrophobic polymeric substance has been suggested as a methodology in reducing friction at the archwire-bracket interface and in repelling salivary adherends. A new polymeric coated ligature tie - Super Slick Ties™ (SST), manufactured using Metafasix technology, has been marketed by TP laboratories for potential reduction in treatment time.
Aim: The basic in vitro design is to compare the frictional characteristics of the coated ligatures with those of uncoated ligatures in four different archwires, namely, stainless steel, NiTi, TMA, and Timolium.
Materials and Methods: Four archwires used are stainless steel archwires, TMA archwires, Timolium, NiTi archwires, and two types of elastomeric ligatures (the coated and uncoated ligatures) were used. The wires used were of 0.019 × 0.025 dimension. The evaluation of friction between the brackets and the archwire was carried out as per the test protocol described by Tidy. The values for kinetic friction were obtained and tabulated. Mean and standard deviation were calculated. Paired Student's "t" test was performed to analyze the significance of difference between means.
Results: The results indicate a significant difference in friction produced when coated and conventional uncoated ligatures were used.
Conclusions: SST produced lower levels of friction (11%) for all archwire materials when compared to conventional uncoated ligatures (Dispense-A-Stix) and both conventional uncoated ligatures and coated ligatures gave a rank order of coefficient of kinetic friction (μkf) among archwires, with stainless steel archwires exhibiting the least and TMA TM showing the highest.
Keywords: Coated ligatures, frictional resistance, metafasix technology
|How to cite this article:|
Leander D, Kumar JK. Comparative evaluation of frictional characteristics of coated low friction ligatures - Super Slick Ties™ with conventional uncoated ligatures. Indian J Dent Res 2011;22:90-4
From the advent of mechanotherapy in orthodontics, the motive force for moving teeth was generated by the archwire, and for proper transmission of the force to the bracket, it is necessary to hold it securely in the bracket slot. Ligatures have constituted the method of securement, in the vast majority of cases, though self-ligating brackets have been a recent innovation. Stainless steel ligatures have been the time-tested method of ligation and continue to be widely used. Elastomeric modules were introduced for their ease of application.
|How to cite this URL:|
Leander D, Kumar JK. Comparative evaluation of frictional characteristics of coated low friction ligatures - Super Slick Ties™ with conventional uncoated ligatures. Indian J Dent Res [serial online] 2011 [cited 2019 Oct 15];22:90-4. Available from: http://www.ijdr.in/text.asp?2011/22/1/90/80004
Elastomeric chains, which are nothing but elastic modules, as a chain, were introduced to the dental profession in the 1960s and have become an integral part of many orthodontic practices. Structural geometry of these polymers consists of primary bonds and weak secondary bonds. 
The type of ligation employed to secure archwire to brackets can account for a part of frictional resistance occurring during sliding mechanics. One of the problems encountered with elastomeric modules is that they can act as a potential host for microbial accumulation. Current scientific thinking warrants that materials used in oral environment should be a poor biohost. Coating elastomeric ligatures with a hydrophobic polymeric substance has been suggested as a methodology in eliminating friction at the archwire-bracket interface and in repelling salivary adherends. They also provide good retention, tensile strength and better elastic properties. If this is indeed true, then canine retraction and other tooth movements could be facilitated with greater ease, at lower force levels and without taxing the anchorage.
A new polymeric coated ligature tie - Super Slick Ties (SST™), manufactured using Metafasix technology, has been marketed by TP Laboratories for potential reduction in treatment time. Being a recent introduction, the literature is scant, and hence this study was undertaken to assess whether there is reduction in frictional resistance while using SST when compared to Dispense-A-Stix™ in four different archwire/bracket combinations, namely, stainless steel™, NiTi™, TMA™ and Timolium™ [Figure 1], [Figure 2], [Figure 3] and [Figure 4].
| Materials and Methods|| |
The study was conducted in the Department of Orthodontics, Government Dental College, Trivandrum. The in vitro tests were conducted at the Material Characterization Division, INSTEF, VSSC, Thumba, Trivandrum. The basic study design is to compare the frictional characteristics of the coated ligatures (SST) with uncoated ligatures (Dispense-A-Stix), manufactured by the same company (TP Orthodontics, Inc., LaPorte, IN, USA), using four different archwire materials, i.e.,
The evaluation of friction between the brackets and the archwire was carried out as per the test protocol described by Tidy.  It consisted of a simulated half arch fixed appliance with archwire ligated in position. Four edgewise brackets having a slot dimension of 0.022 × 0.028 with zero torque and zero angulations were bonded onto a rigid Perspex sheet at 8 mm intervals. A space of 16 mm was left at the center for sliding the canine bracket to simulate canine retraction [Figure 5].
- Equire stainless steel archwires (Dentaurum, Ispringen,Germany),
- TMA® archwires (Ormco Corp., Glendora, CA, USA),
- Timolium (TP Orthodontics) and
- NiTi (Ormco Corp., Glendora, CA, USA) [Figure 1], [Figure 2], [Figure 3] and [Figure 4].
|Figures 5: Perspex sheet with Brackets; Instron Universal testing machine|
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The movable canine bracket was soldered with a 12-mm power arm from which weights of 50 g/100 g were hung to represent the single equivalent force acting at the center of resistance of the tooth root.
All tests were conducted in wet condition with an Instron universal testing machine. The movable bracket was suspended from the load cell of the testing machine, while the Perspex sheet was mounted on the crosshead below. The full-scale load was set at 10 N with a crosshead speed of 10 mm/minute [Figure 6]. The test closely simulates the clinical retraction of a canine and was used in this study. This can only be taken as a means of comparing the frictional characteristics of different alloy archwires in similar testing conditions because this does not replicate the exact intraoral environment.
|Figures 6: Perspex sheet with Brackets; Instron Universal testing machine|
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At the start of each test, a trial run was performed with no load on the power arm to check whether there was any binding between the archwire and bracket. Then, a 50 g followed by 100 g weight was suspended from the power arm and the load needed to move the bracket across the central span in the apparatus was recorded separately. Five representative readings were taken for each alloy wire and the type of ligature used. The load cell reading represents the clinical force of retraction that would be applied to canine, part of which would be critical friction while the rest would be the translation force on the tooth. The difference between the load cell reading and load on the power arm represents frictional resistance [Figure 5]. The coefficient of friction of the archwire-bracket interface can be calculated by the formula:
P = frictional resistance,
F = equivalent force acting at a distance,
W = bracket slot width,
h = 12 mm, and
μ = coefficient of friction.
The values for kinetic friction were obtained and tabulated. Mean and standard deviation were calculated. Paired Student's "t" test was performed to analyze the significance of difference between means.
The values obtained on testing the coefficient of kinetic friction of different archwires when used along with SST and Dispense-A-Stix with two equivalent weights were calculated and tabulated.
The statistical analysis was performed using a statistical package, SPSS version 7.5, for MS windows [Table 1] and [Table 2].
|Table 1: Mean, SD and paired 't' test values for group I and II ligatures (Load 50 gms)|
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|Table 2: Mean, SD and paired 't' test values for group I and II ligatures (Load 100 gms)|
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Statistical analysis to evaluate the significance of the difference between the mean coefficients of kinetic friction (μkf ) of SST against the conventional Dispense-A-Stix ligatures in four wire types with two weights was done. The analysis was carried out by applying the paired "t" test [Table 1].
| Results|| |
While comparing the SST with conventional uncoated Dispense-A-Stix, it was found that coated ligatures had a lower friction with both the titanium alloys and stainless steel wires than conventional uncoated ligature, and statistical analyses reveal that great majority of these comparisons are significant [Table 1].
For 50 g load, coated ligatures gave a mean and standard deviation of 0.342 ± 0.0239 for stainless steel wires, while uncoated ligatures gave a mean and standard deviation of 0.382 0.0047 which was least when compared to titanium alloy wires. For 50 g load, coated ligatures gave a value of 0.856 ± 0.01517 for TMA wires, while uncoated ligatures gave a mean and standard deviation of 0.954 ± 0.0054 which was the highest among the titanium alloys. For 50 g load, coated ligatures gave a value of 0.718 ± 0.00477 for NiTi wires, while uncoated ligatures gave a value of 0.840 ± 0.000. For 50 g load, coated ligatures gave a mean and standard deviation of 0.720 ± 0.000 for Timolium wires, while uncoated ligatures gave a value of 0.916 ± 0.0547 [Table 1].
For 100 g load, coated ligatures gave a value of 0.236 ± 0.0894 for stainless steel wires, while uncoated ligatures gave a mean and standard deviation of 0.268 ± 0.0447 which was the least when compared to titanium alloy wires. For 100 g load, coated ligatures gave a mean and standard deviation of 0.738 ± 0.0447 for TMA wires, while uncoated ligatures gave a mean and standard deviation of 0.850 ± 0.000 which was the highest among the titanium alloys. For 100 g load, coated ligatures gave a mean and standard deviation of 0.420 ± 0.000 for NiTi wires, while uncoated ligatures gave a mean and standard deviation of 0.560 ± 0.000. For 100 g load, coated ligatures gave mean and standard deviation of 0.448 0.00472 for Timolium wires, while uncoated ligatures gave a value of 0.590 ± 0.000 [Table 2].
Among the wires used, stainless steel archwires showed the least amount of friction, μ= 0.342 and 0.236 for 50 and 100 g, respectively. TMA wires gave a higher value for coefficient of kinetic friction, μ= 0.856 and 0.738 for 50 and 100 g, respectively, with NiTi and Timolium coming in between [Figure 7].
|Figure 7: Graph comparing the coefficient of friction of Super Slick Ties and Dispense-A-Stix with the four different arch wires|
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| Discussion|| |
Elastomers and their myriad applications in orthodontic mechanotherapy is one area that has grown exponentially in the last two decades. Elastomeric chains, ligatures, threads and other such applications are commonplace in orthodontics today. Elastomeric ligatures are a ubiquitous entity in today's practice and facilitate the rapid positioning of the archwire in the slot, and in some cases, to a certain degree, help in achieving tooth rotation.
The force exerted by elastomeric ligatures is usually in excess of what is required to achieve the intended function. The excess binding force impedes movement of the bracket along the archwire by increasing friction, thereby extending clinical treatment times. This situation is further exacerbated by the fact that polyurethane ligatures exhibit a high degree of surface "physical adhesion" to other binding surfaces as compared to metal ligatures.
Frictional resistance is one of the critical factors that determine the efficiency of orthodontic tooth movement, especially when sliding mechanics are adopted. In sliding mechanics, an archwire that is slightly smaller than the slot width is inserted into the bracket slot. All the applied retraction force will contribute to the tooth movement if no friction exists.  However, this situation does not occur in clinical applications because some force by the ligation will hinder the movement of the archwire.
Kusy et al. had outlined that the purpose of the ligature during sliding is to retain the archwire within each bracket's slot, not to press the archwire into the bracket. There is loss of control that is inherent with the absence of knowing the ligation force and therefore the frictional force. The solution relies on the premise that short-term forces should be resisted by an elastic, high-strength material and long-term forces should be accommodated by stress relaxation characteristics of an appropriately designed ligature material. Admittedly, no known material today fulfills the requirement, including the polyethylene ligatures.
Kusy et al. had shown that an ideal ligature should have optimum stress relaxing characteristics so that normal force due to ligation decays rapidly with time, thereby reducing the frictional force and consequently the frictional couple.
Newer elastomeric ligatures like SST, a low friction ligature, are the latest entries into the market. By reducing the friction between the ligature and the archwire, it is possible to eliminate the "drag force" while still allowing the ligature to rotate teeth and "keep" the archwire. The ligatures are subjected to a hydrophobic coating using Metafasix technology, which changes the elastomeric surface characteristics, rendering it slippery on contact with water or saliva.
The cause of frictional resistance between archwire and brackets is multifactorial, and influencing variables include archwire size, archwire material, mode of ligation bracket width, and angulations of the wire to the bracket. 
Devanathan,  in his technical paper on the performance study of SST, claims a one-fourth reduction in friction when compared to uncoated ligatures. The 11% reduction in friction exhibited by SST revealed by the present study is in agreement with the above conclusion.
Schumacher et al.,  in their study to determine the loss in orthodontic force caused by friction between archwires and ligature, observed that friction caused by elastomeric chain is significantly lesser than friction caused by steel ligature. Riley et al. also showed in their studies that steel ligatures produce greater friction than elastomers. Most likely, the lower friction values were the result of the coated ligatures possessing a lower coefficient of friction than the uncoated ligature.
However, Frank and Nikolai et al. compared frictional resistances between elastomeric and steel ligations of 225 g of force and found no differences between elastomeric chains and stainless steel ligatures. On the contrary, investigations by Shivapuja and Berger  have shown that higher frictional resistance occurs with elastomeric ligatures when compared with self-ligating brackets.
Krishnan et al.,  in their evaluation of mechanical properties and surface characteristics of archwire alloys, have reaffirmed that stainless steel with high values for strength, low friction, and an almost smooth surface continues to be the mainstay archwire in orthodontic mechanotherapy. TMA appears to be kinder to tissues by generating a low, consistent force, when compared with other two alloys for load deflection characteristics. Friction at archwire-bracket interface appears to be higher when TMA wires are used, whereas Timolium has smooth surface, reduced friction, low modulus, and better strength.
Nishio et al. have pointed out that the ligation between bracket and wire is another variable that could influence the frictional force level. Authors are unanimous in reporting that the force used through stainless steel ligature is subjective, varying according to the orthodontist. On the other hand, elastomeric ligature loses elasticity with time and can alter the frictional force values.
According to Drescher et al.,  the high values of frictional force in TMA wire are due to its inherent surface roughness. Garner et al. supported this with electron photomicrographs illustrating the variance in surfaces of stainless steel, NiTi, TMA and beta-titanium, among which the smoothest surface was stainless steel. The low coefficient of friction obtained with SST can be attributed to the hydrophobic coating incorporated on the elastomer surface using Metafasix technology. Further clinical studies are required to substantiate these results in vivo.
Bortoly et al. evaluated the sliding resistance of esthetic ligature in an in vitro study and pointed that frictional forces generated by esthetic elastomeric ligatures under simulated oral environments are not stable and are more related to tensile force than to surface characteristics of the ligatures. Teflon-coated and stainless steel ligatures showed the lowest initial frictional forces, but there was no difference in friction of stainless steel and post-stretched elastomeric ligatures.
| Conclusions|| |
- The coated ligatures had produced minor attrition coefficient when compared to conventional uncoated ligatures [Table 1] and [Table 2].
- Both conventional uncoated ligatures, Dispense-A-Stix and SST, gave a rank order of coefficient of kinetic friction (μkf ) among archwires, with stainless steel archwires exhibiting the least and TMA showing the highest. The order is as follows:
Stainless steel < NiTi < Timolium < TMA
| Acknowledgment|| |
The authors are grateful to the personnel at Material Characterization Division, INSTEF, VSSC, Thumba, Trivandrum.
| References|| |
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Government Dental College, Thumba, Trivandrum
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]
[Table 1], [Table 2]
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