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\title{In Vitro Comparison of Friction Generated by Various Models of Self-Ligating and Conventional Brackets While Performing Retraction with Sliding Mechanics}
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             \author[1]{Mario Cappellette  Jr}

             \author[2]{Luciano Nogueira de Almeida  Campos}

             \author[3]{Andre  Besen}

             \affil[1]{  Universidade Federal de Sao PauloUNIFESP/EPM, Sao Paulo, Brazil}

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\date{\small \em Received: 9 December 2016 Accepted: 3 January 2017 Published: 15 January 2017}

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\begin{abstract}
        


In vitro studies suggest that certain variables such as friction coefficient, archwire size and force decay affect the effectiveness of sliding mechanics. To maximize the efficiency of sliding mechanics one should seek to control these variables.Objective: This in vitro study aimed to compare frictional forces in several models of self-ligating brackets, conventional systems, as well as different ways to tie the wire to the brackets during a simulation of sliding mechanics using 0.019"X0.025" stainless steel wire. Material and Methods:The study evaluated the levels of dynamic and static friction in sixdifferent types of brackets andthree different ligation systems were used with conventional brackets: elastomeric modules, unconventional elastomeric ligature low friction system, and 0.20mm stainless steel-ligature.

\end{abstract}


\keywords{friction; in vitro; orthodontic brackets; selfligating brackets. sliding mechanics.}

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\let\tabcellsep& 	 	 		 
\section[{I. Background}]{I. Background}\par
remolar extraction is a common treatment option in orthodontics. Space closure can then be achieved with sliding mechanics, which consists in pulling or pushing a tooth along a straight archwire using an appropriate system of forces to produce a sustained movement. Elastomeric materials or springs are often employed to produce this force. In vitro studies \hyperref[b0]{1} suggest that certain variables such as friction coefficient, archwire size and force decay impair the effectiveness of sliding mechanics. Other factors that affect friction include saliva, material and wire size, and angulation between bracket, wire and ligation system 2 . To maximize the efficiency of sliding mechanics one should seek to control these variables \hyperref[b0]{1} . In orthodontic movement, friction (static or dynamic) results from the interaction of an archwire with the walls of the bracket slots or the ligatures \hyperref[b3]{3} . Moreover, the forces generated at the bracket/wire interface may hinder the achievement of optimal force levels in the supporting tissues. Therefore, a decrease in this response is likely to benefit the response of hard and soft tissues. Frictional force is classified into static and kinetic. Static friction is the smallest force needed to start a movement between solid objects at rest and the kinetic friction force resists the sliding motion of a solid object against another at a constant speed \hyperref[b3]{3} . It has been reported that 50\% of the force applied to slide a tooth is used just to eliminate friction.\par
With the increase in the use of self-ligating brackets \hyperref[b4]{4} , many studies have been conducted using self-ligating brackets and reported advantages including increased patient comfort, improved oral hygiene, less chair time, anchorage conservation, and reduction of the friction \hyperref[b5]{[5]}\hyperref[b6]{[6]}\hyperref[b7]{[7]} . Although reports of reduced friction are one of the advantages of self-ligating brackets when compared with conventional brackets \hyperref[b8]{8,}\hyperref[b11]{9} , this issue is still controversial. The term self-ligation in orthodontics implies that the bracket has the ability to engage itself to the archwire by a mechanical device (clip) built into the bracket to close off the slot \hyperref[b12]{10} and the clip could be active when the ligation clip exerts a pressure on the arch wire or passive when the clip transforms the slot to a tube.\par
This in vitro study aimed to compare static and dynamic friction frictional forces in self-ligating brackets, conventional systems with different methods to tie the wire to the brackets during a simulation of sliding mechanics devised by Bennett \& McLaughlin 11 using 0.019"X0.025" stainless steel (NiCr) wire. 
\section[{II. Methods}]{II. Methods}\par
Six different types of brackets -0.022 x 0.027 -in slots, were selected both self-ligating and conventional appliances: Gemini (3M Unitek ® Monrovia, California, USA), SmartClip (3M Unitek ® Monrovia, California, USA), Empower (American Orthodontics ® ), Quick (Forestadent ® ), In-Ovation (GAC ® ), Vision LP. (Table \hyperref[tab_0]{1})\par
Three different ligation systems were used with conventional brackets, i.e., conventional elastomeric modules (EMs) manufactured by Morelli ® unconventional elastomeric modules (Slide by Leone ® Italy) and 0.10-in ligatingstainless-steel ligature also manufactured and marketed by Morelli ® .\par
The tests were conducted using 0.019"x0.025" (Morelli ® ) steel wire on all brackets or ligation systems. Five observations were carried out for each bracketsligation system combination. To eliminate the influence of wear, a wire sample was drawn only once through a brackets-ligation system combination and news brackets, ligation and wire were used in each test run.This generated a trial with 200 brackets and 40 tests readings were taken for the study. Altogether, there were eight separate groups of brackets and ligation systems (Table \hyperref[tab_1]{2}). 
\section[{a) Friction assessment device}]{a) Friction assessment device}\par
To evaluate the friction levels a device \hyperref[b14]{12} was created specifically designed for this purpose. It was adapted to an EMIC DL2000 machine to simulate retraction movements commonly used in orthodontic sliding mechanics at a constant speed of 10 mm/min (Fig  {\ref 1}). The device consisted of a stainless-steel base fixed with screws, and cylindrical rods each with a cavity where each bracket was bonded. This set of grouped rods simulates a group of teeth.\par
The brackets were attached to a bonding guide with 0.10-in steel ligatures (Morelli). This guide consisted of a stainless-steel plate with a thickness of 0.019-in where the brackets were placed. Once positioned at the same distance, height and with the same buccolingual relationship, which neutralized any expression of torque or tip preadjusted in the brackets, the latter were bonded to the cylinders with Transbond XT (3M Unitek ® ) adhesive and light-cured for 20 seconds. The brackets were all aligned and leveled so as to avert any factors that might generate friction and thereby impair the accuracy of the data \hyperref[b14]{12} and the effect of different forms of ligation could be isolated with greater precision \hyperref[b15]{13,}\hyperref[b16]{14} . (Fig.  {\ref 2 a, b, c}) 
\section[{b) Statistical analysis}]{b) Statistical analysis}\par
Statistical analysis of all data collected in this research was initially performed descriptively by calculating some summary measures such as mean, median, minimum, maximum and standard deviation values. Additionally, one-dimensional scatter diagram charts were built \hyperref[b17]{15} .\par
The Kruskal-Wallis test was employed as inferential analysis in order to compare static and dynamic friction between the eight types of brackets \hyperref[b18]{16} .\par
A significance level of ? = 5\% was applied to all conclusions reached through inferential analyses.\par
The data were entered spreadsheets in Excel 2010 for Windows software for proper information storage. The statistical analyses were performed with R software version 2.15.2. 
\section[{III. Results}]{III. Results}\par
The sample in this study consisted of 40 specimens, 5 each of 8 different types of brackets (Gemini/EMs, Gemini/Ligature, Gemini Leone, Empower, Vision, Quick, GAC and SmartClip).\par
Static and dynamic friction was measured for each of the specimens (see details in Table \hyperref[tab_0]{1} and Graphs 1 and 2).\par
Gemini/EMs brackets showed a mean static friction of 5.86N, ranging from 5.31 to 6.70N, with a standard deviation of 0.59N. Mean dynamic friction was 5.12N, ranging from 4.80 to 5.50N, with a standard deviation of 0.29N.\par
Gemini/Ligature brackets showed a mean static friction of 3.27N, ranging from 2.58 to 4.38N, with a standard deviation of 0.73N. Mean dynamic friction was 2.76N, ranging from 2.20 to 3.80N, with a standard deviation of 0.67N.\par
Gemini/Leone brackets displayed a mean static friction of 0.08N, ranging from 0.06 to 0.08N, and a standard deviation of 0.01N. Mean dynamic friction was 0.04N, ranging from 0.00 to 0.10N, with a standard deviation of 0.05N.\par
Gemini/Ligature brackets showed a mean static friction of 3.27N, ranging from 2.58 to 4.38N, with a standard deviation of 0.73N. Mean dynamic friction was 5.12N, ranging from 4.80 to 5.50N, with a standard deviation of 0.29N.\par
Vision LP brackets showed a mean static friction of 0.04N, ranging from 0.03 to 0.06N, and a standard deviation of 0.01N. All five specimens of this type of bracket showed no dynamic friction.\par
BioQuick brackets showed a mean static friction of 2.78N, ranging from 2.62 to 3.11N, with a standard deviation of 0.19N. Mean dynamic friction was 2.56N, ranging from 2.50 to 2.80N, with a standard deviation of 0.13N.\par
In-Ovation brackets exhibited a mean static friction of 1.83N, ranging from 1.61 to 2.06N, and a standard deviation of 0.16N. Mean dynamic friction was 5.12N, ranging from 4.80 to 5.50N, with a standard deviation of 0.29N.\par
SmartClip brackets displayed a mean static friction of 0.08N, ranging from 0.07 to 0.08N, with a standard deviation of 0.01N. All five specimens of this type of bracket showed no dynamic friction.\par
Inferential results showed that the static (p<0.001) and dynamic (p<0.001) friction levels are not statistically identical across the different types of brackets (Graphs 1 and 2).\par
? Gemini/EMs brackets have higher static friction than Gemini/Ligature (p<0.001), Gemini Leone (p<0.001), Empower (p<0.001), Vision LP (p<0.001), BioQuick (p<0.001), In-Ovation (p<0.001) and SmartClip (p<0.001) brackets.\par
? Gemini/EMs brackets have higher dynamic friction than Gemini/Ligature (p<0.001), Gemini Leone (p<0.001), Empower (p<0.001), Vision LP (p<0.001), BioQuick (p<0.001), In-Ovation (p<0.001) and SmartClip (p<0.001) brackets. 
\section[{IV. Discussion}]{IV. Discussion}\par
In preparing the patient for sliding mechanics, one should insert rectangular steel archwires as of one to two months prior to applying the mechanics itself. This preparation allows all brackets to express their torques and angulations more efficiently. The goal is to make the archwire as passive as possible to avoid interfering with the archwire as it slides along the bracket slot. Thus, the brackets were placed passively, applying sliding mechanics as much as possible in its clinical form as well.\par
During in vivo sliding mechanics, the steel wire slides along the molar and premolar brackets performing incisor and canine retraction while simultaneously closing spaces. This study used incisor, canine and premolar brackets to minimize bonding errors since it would be quite a challenge to place the appliance passively with tubes bonded to the molars. This may have slightly altered the absolute results, but given that the intention was to compare ligation systems, any changes would apply to all systems.\par
In a critical review of the literature in 2009 Burrow \hyperref[b3]{3} defined friction as a minor component in the set of forces that cause resistance to tooth movement. Possibly, sliding mechanics is an exception to this rule, given (a) the way in which the wire slides along the premolar and molar brackets with no forces being applied directly to the tooth, but rather to a hook welded to the wire, and (b) preparation involves the use of rectangular steel wires. These factors help to reduce the binding effect, which occurs when force is applied directly to the tooth being moved, rendering this type of mechanics highly dependent on the friction between wire and bracket. Some forms of sliding mechanics described in the literature 1 apply force to the tooth being moved, such as canines. This would completely change the force components of the system, making binding its major component.\par
Other limitations stem from not considering the moment caused by the elastomeric modules during movement. As described in the study by Budd et al \hyperref[b19]{17} in 2008, a typodont with brackets bonded to it, and dipped in a fluid would undergo variations in the movements that occur during the mechanics. Pliska et al \hyperref[b22]{18} in 2014 concluded that friction induced by ligation has little influence on the overall resistance to slinding when moment forces are combined. It should be underscored that the main objective of this investigation was to compare ligation systems. If rotation were to be incorporated during movement the variables would be far too numerous making it impossible to compare the systems themselves. Thus, not all clinical conditions were simulated in their entirety, and the number of variables was deliberately reduced to facilitate the study. For example, Leal et al \hyperref[b23]{19} in 2014, clarified the significant role of lubricant, like artificial saliva in friction forces between self-ligating brackets and wires. Nevertheless, the main results agreed with those reported by Budd et al \hyperref[b19]{17} in 2008, which included momentum in their laboratory model.\par
Furthermore, there is no denying that there are limitations in this study given that the laboratory environment does not provide clinical factors such as: The action of saliva, possible occlusal forces, muscle interference, interferences with oral functions such as mastication and swallowing, different degrees of malocclusion, thickness and compressibility of the periodontal ligament, rotated teeth, torque at the wire/ bracket interface, angulations and temperature.\par
Many studies \hyperref[b2]{2,}\hyperref[b19]{17,}\hyperref[b24]{20} used various wire sizes for comparison. The goal here was to simulate sliding mechanics, which is always performed with 0.019"x0.025" steel wire. Most studies also test the friction in a single bonded bracket and not in a set of brackets, as was done here. Future research should consider other rectangular steel wire sizes, such as 0.018"x0.025".\par
The static friction is the force that opposes the beginning of movement at the moment when activation is performed. The results showed a statistically significant difference (p<0.001) between the Gemini/ EMs group and the other groups. These results demonstrate that during sliding mechanics other ligation systems are better suited than elastomeric modules given the substantial difference in the friction force generated. It should be remembered that the lower the friction force, the less force is required to initiate movement, and the more optimized and physiological this movement will be.\par
Ehsani et al 2 in a review of the literature written in 2009 report that five studies were conducted and found no significant differences between self-ligating and conventional brackets in terms of friction force when ( D D D D ) J rectangular wires of greater caliber are utilized. Moreover, in seven other studies, self-ligating brackets produced lower friction than conventional brackets. All seven agree with the results of this study, if one considers conventional brackets tied with elastomeric modules. Regarding metal ligatures and Slide ligatures, the results agree with the first group. A 2007 study \hyperref[b24]{20} compared the use of metal ligatures with SmartClip selfligating brackets and found no statistically significant difference during en masse sliding mechanics. The literature review's conclusions disagree with the results of this investigation by admitting that there was not enough evidence to prove that self-ligating brackets produce lower friction forces than conventional brackets with rectangular archwires. This divergence may have occurred due to the fact that the authors could not specify comparisons amidst such an overwhelming number of articles. In this study, for example, if one were to compare the ligature system with the Empower or BioQuick brackets, no differences would be found. Holtmann et al \hyperref[b14]{12} in 2014 demonstrated that self-ligating and steel-ligated brackets are more effective to correct misalignment and exertion of lower forces at the same time, than brackets with elastic ligatures. However, since the comparison was made with elastomeric modules the difference was statistically significant. Perhaps because literature reviews are so comprehensive one may miss some important details that might clarify certain issues.\par
As shown in Table  {\ref 3}, the mean static friction forces of the Gemini Leone (0.08N), Vision LP (0.04N) and SmartClip (0.08N) ligation systems are clearly lower than the forces found in the other groups. This may be related to the fact that in these three ligation systems the wire is tied to the brackets passively. Slide ligatures (Leone ® ,Italy) cover the open part of the slot leaving the wire completely passive within it. Vision LP brackets feature an opening with the same passive cover design to keep the wire into the slot. Moreover, SmartClip brackets also have clips that appear not to compress the archwire inside the slot. Studies comparing active and passive self-ligating brackets concluded that passive brackets produce statistically lower friction forces \hyperref[b25]{21} .\par
With the Gemini/Ligature ligation system (3.27N), Empower brackets (3.24N) and BioQuick brackets (2.78N) have also been shown to generate similar mean values of static friction forces during sliding mechanics. These forces are obviously higher than in the groups discussed above, but still lower than in the Gemini/EMs group.\par
Metal ligatures push the wire against the base of the slot but because they are made from stainless steel they produce less friction. The Empower bracket is equipped with a chromium cobalt clip which with thicker wires acts by pressing the wire against the bracket base. The BioQuick bracket, in turn, has a steel clip that also exerts a continuous force on the wire.\par
The In-Ovation bracket has a mean static friction of 1.83N. This bracket also features a chromium cobalt spring that compresses the wire inside the bracket slot when thicker wires are inserted. This spring, however, can exert forces that are lighter than the springs. These data agree with Budd et al \hyperref[b19]{17} , who in 2008, after analyzing several variables, concluded that the binding mechanism is the main variable affecting frictional forces in the different ligation systems.\par
Dynamic friction is here defined as the force that opposes the force that allows the movement to continue. It is known that in sliding mechanics the force intensity applied in the initial activation usually weakens with each passing hour, and will probably be extinguished before the next activation. Thus, the lower the dynamic friction, the longer it takes this force to subside completely. Additionally, it is more effective, which optimizes the mechanics.\par
The results found in this study were very similar to the results found for static friction. Elastomeric ligatures (5.12N) showed a dynamic friction force statistically higher (p=0.001) than all other ligation systems. These other systems are therefore not indicated for use with sliding mechanics.\par
With the Gemini/Leone group (0.04N), SmartClip (0.00N) and Vision LP (0.00N) brackets exhibited the lowest mean dynamic friction. This is probably since these are passive systems.\par
On the other hand, the Gemini/Ligature group (2.76N), as well as the Empower (2.66N) and BioQuick (2,56N) brackets also showed values that are similar to dynamic frictional forces.\par
The In-Ovation bracket group showed a mean friction force of 1.44N, which was remained unchanged between the lowest and the intermediate values.\par
It is the authors' view that due to similarities between the results for static vs. dynamic friction, the arguments expressed in the literature probably apply to both types of friction. A study conducted in 2010 by Stefanos et al 21 also found significant similarities between the results of both types of friction. A 2009 literature review by  {\ref Burrow 3} argued that for practical purposes dynamic friction is irrelevant in orthodontic tooth movement. The author goes on to explain that the continuous movement of a tooth along an archwire is a rare phenomenon and that in sliding mechanics one is dealing with a quasi-static thermodynamic process. This means that the process occurs slowly and leads to a sequence of quasi-equilibrium states. Force and resistance to sliding change as the tooth moves along the archwire. It then inclines and responds by producing a biological response, i.e., bone remodeling, then inclines once again \hyperref[b3]{3} . This process is seen by  {\ref Burrow 3} as quasi-static, although for many other researchers it could be considered as an ongoing process. The results showed a striking similarity between the two types of friction, which led the authors to believe that regardless of its relevance or irrelevance dynamic friction can be considered as complementary to static friction. It can be present on rare occasions during orthodontic movement but should never be ignored, irrespective of relevance. Certain types of materials used in this study could influence friction. The first such material is steel, sliding underneath elastomeric modules present in the Gemini/EMs and Gemini Leone groups, since the steel archwire slides along a metal slot covered with an elastomeric module. The second type is steel with steel, as in the Gemini/Ligature and Quick groups. The third type is steel and chromium cobalt alloy in the Empower and In-Ovation groups, since the covers are made of cobalt chromium. The fourth and last type is steel with nickel-titanium, as in the Vision and SmartClip groups.\par
It became unequivocally clear that in types 1 and 3 substantial differences were found in the results, which rules out the possibility that the materials affected the tests in any way. These findings contrast with some studies \hyperref[b19]{17} that consider the material from which the cover was made as a factor capable of influencing the amount of friction that occurs in each bracket type. This may have occurred since this study involved at least two different ligation systems for each type of material, which was not the case in the study by Budd in 2008, which examined a more limited range of brackets \hyperref[b19]{17} . 
\section[{V. Conclusions}]{V. Conclusions}\par
Friction was influenced by the type of bracket and by the ligating systems. During sliding mechanics, frictional forces generated by the conventional ligation system (Gemini brackets + elastomeric modules) were statistically higher than the forces generated by selfligating brackets and other ligating systems. Specifically, SmartClip and Vision LP brackets as well as Leone's Slide ligating system generated the lowest frictional forces during sliding mechanics.    \begin{figure}[htbp]
\noindent\textbf{}\includegraphics[]{image-2.png}
\caption{\label{fig_0}}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{}\includegraphics[]{image-3.png}
\caption{\label{fig_1}Figure1:}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{21}\includegraphics[]{image-4.png}
\caption{\label{fig_2}Figure 2 : 1 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{1} \par 
\begin{longtable}{P{0.3486842105263158\textwidth}P{0.3526315789473684\textwidth}P{0.017105263157894738\textwidth}P{0.007894736842105263\textwidth}P{0.07894736842105263\textwidth}P{0.013157894736842105\textwidth}P{0.031578947368421054\textwidth}}
Group\tabcellsep \tabcellsep Brackets\tabcellsep \tabcellsep \multicolumn{3}{l}{Torque Angulation (tip) In/out}\\
Gemini/EMs, Gemini/Ligatures and Gemini Leone\tabcellsep \multicolumn{3}{l}{Maxillary right central incisor}\tabcellsep 17\tabcellsep 4\tabcellsep 0.82\\
Gemini/EMs, Gemini/Gemini Leone and Ligatures\tabcellsep \multicolumn{3}{l}{Maxillary right lateral incisor}\tabcellsep 10\tabcellsep 8\tabcellsep 1.06\\
Gemini/EMs, Gemini/Gemini Leone and Ligatures\tabcellsep \multicolumn{2}{l}{Maxillary right canine}\tabcellsep \tabcellsep -7 or 0\tabcellsep 8\tabcellsep 0.8\\
Gemini/EMs, Gemini/Gemini Leone and Ligatures\tabcellsep \multicolumn{3}{l}{Maxillary first right premolar}\tabcellsep -7\tabcellsep 0\tabcellsep 0.83\\
Gemini/EMs, Gemini/Gemini Leone and Ligatures\tabcellsep \multicolumn{3}{l}{Second right pre-molar}\tabcellsep -7\tabcellsep 0\tabcellsep 1.06\\
Empower\tabcellsep \multicolumn{3}{l}{Maxillary right central incisor}\tabcellsep 17\tabcellsep 4\tabcellsep -\\
Empower\tabcellsep \multicolumn{3}{l}{Maxillary right lateral incisor}\tabcellsep 10\tabcellsep 8\tabcellsep -\\
Empower\tabcellsep \multicolumn{2}{l}{Maxillary right canine}\tabcellsep \tabcellsep 0 or -7\tabcellsep 8\tabcellsep -\\
Empower\tabcellsep \multicolumn{3}{l}{Maxillary right first premolar}\tabcellsep -7\tabcellsep 0\tabcellsep -\\
Empower\tabcellsep Maxillary premolar\tabcellsep right\tabcellsep second\tabcellsep -7\tabcellsep 0\tabcellsep -\end{longtable} \par
 
\caption{\label{tab_0}Table 1 :}\end{figure}
 \begin{figure}[htbp]
\noindent\textbf{2} \par 
\begin{longtable}{P{0.85\textwidth}}
Volume XVII Issue II Version I\\
D D D D ) J\\
(\\
Medical Research\\
Global Journal of\end{longtable} \par
 
\caption{\label{tab_1}Table 2 :}\end{figure}
 			\footnote{© 2017 Global Journals Inc. (US)} 			\footnote{© 2017 Global Journals Inc. (US)Year 2017} 		 		\backmatter  			 \par
Graph 2: One-dimensional scatter diagram of dynamic friction of specimens according bracket type.			 			  				\begin{bibitemlist}{1}
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\end{document}
