Author’s Accepted Manuscript Influence of Bio-lubricants on the orthodontic friction A. Dridi, W. Bensalah, S. Mezlini, S. Tobji, M. Zidi www.elsevier.com/locate/jmbbm
PII: DOI: Reference:
S1751-6161(15)00500-7 http://dx.doi.org/10.1016/j.jmbbm.2015.12.026 JMBBM1741
To appear in: Journal of the Mechanical Behavior of Biomedical Materials Received date: 10 November 2015 Revised date: 16 December 2015 Accepted date: 18 December 2015 Cite this article as: A. Dridi, W. Bensalah, S. Mezlini, S. Tobji and M. Zidi, Influence of Bio-lubricants on the orthodontic friction, Journal of the Mechanical Behavior of Biomedical Materials, http://dx.doi.org/10.1016/j.jmbbm.2015.12.026 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of Bio-lubricants on the orthodontic friction A. Dridia, W. Bensalahb*, S. Mezlinia, S. Tobjic, M. Zidia a
Laboratoire de Génie Mécanique (LGM). ENIM, Rue Ibn Eljazzar 5019, Monastir
University, Tunisia. b
Laboratoire de Génie des Matériaux et Environnement (LGME). ENIS. B.P.W. 1173-3038,
Sfax University, Tunisia. c
Département d’orthodontie, Faculté de Médecine Dentaire de Monastir, Avenue
Avicenne 5019 Monastir University, Tunisia. *Corresponding author email: [email protected]
Abstract The Friction force of Stainless Steel (SS) and Nickel-Titanium (Ni-Ti) rectangular archwires against stainless steel brackets was investigated. Two types of brackets were used namely: Self-ligating brackets (SLB) and conventional brackets (CB). The friction tests were conducted on an adequate developed device under dry and lubricated conditions. Human saliva, olive oil, Aloe Vera oil, sesame oil and sunflower oil were used as bio-lubricants. The friction force was examined as a function of the ligation method and oil temperature. It is found that under oil lubrication, the friction behavior in the archwire/bracket assembly were the best. The SLB ligation was better than the conventional ligation system. The enhancement of the frictional behavior with natural oils was linked to their main components: fatty acids.
Keywords: Natural oils; Friction Force; Archwire/bracket; Orthodontic.
1. Introduction Since the discovery of positioning devices teeth, frictional force between archwire and bracket has held great importance in the field of orthodontic research. In fact, teeth movement can happen when orthodontic force overcomes the frictional force. Indeed a large part of this force is lost in friction. This loss varies from 12 to 60% (Kusy and Witley, 1997). Frictional force has been ascribed to many factors namely: the bracket type (Heano and Kusy, 2005; Ehsani et Al., 2009), the alloy, shape, dimension of the wire, bracket shape and the angulation between them (Readlich et al., 2003). Recently, the use of the self-ligating bracket (SLB) instead of the conventional bracket has been increased (Monteiro et al., 2014). The main benefit of SLB is 1
the decrease of the frictional force by avoiding the use of ligature (Monteiro et al., 2014; Hain et al., 2006). Regarding the archwire alloy, it was found that the friction coefficient of NiTi was higher than that of stainless steel (Kusy and Witley, 1997). Many authors have developed some kinds of solutions to reduce the frictional force such as coated wires using carbon nitride film or the Diamond-Like Carbon (DLC) (Wei et al., 2011, Muguruma et al., 2011), ion impregnation (Vaughan et al.,1995), polydimethylsiloxane (Lung et al., 2015) and epoxy resin treatment (Mendes and Rossouw, 2003). Others have proposed esthetic polymeric archwire with low friction (Hiroce et al., 2012). Actually, the use bio-lubricants is a very promising and attractive alternative. This solution has been widely used in the hip joint systems (Myant et al., 2012; Kobayashi et al. 2014; Meng et al. 2015; Guezmil et al. 2016) but still not well tried in orthodontic. Kusy and Witley (Kusy and Witley, 2003) have studied the effect of the artificial saliva on friction. They showed that artificial saliva cannot improve frictional behavior comparing to human saliva. Although human saliva lubricates the oral cavity and facilitates sliding between brackets and archwires, a very important part of orthodontic force is still lost in friction. Throughout attempts intended to reduce the friction force in the archwire-bracket assembly, we have retained to try adequate natural oils, as lubricants. These oils will ensure, in the same time, low frictional forces and therapeutic effects. The use of natural oils has shown good tribological performances when used as lubricant in the hip joint biomaterials contact (Guezmil et al. 2016). Natural oils are biodegradable, non-toxic and can be used as ingredient in medicinal and cosmetic products. Among natural oils, we have retained four oils: Olive, Sunflower, Sesame, and Aloe Vera. These oils are known to have good tribological performance (Anilakumar et al. 2010; Salih et al. 2011; Balestra et al. 2011; Mannekote and Kailas, 2012; Al Mahmud et al. 2013; Reeves et al. 2015) and widely used in pharmaceutical and medical applications. The mentioned oils are already used for therapeutic treatments such as skin and gum. In the orthodontic field, they are often used as a mouthwash. Their use does not cause skin irritation and have an anti-inflammatory character. In fact, Olive oil removes stains and bacteria that remain after brushing and strengthen the teeth (Ghobashy and El-Tokhey, 2012). Both, Sesame and Aloe Vera oils are used in oral hygiene and for caries prevention (Asokan et al., 2008; Sundarkar et al., 2011; Periasamy et al. 2014). However, Sunflower oil absorbs germs residing at gums and leads to the disappearance of inflammatory zones (Reynolds and Dweck, 1999).
The aim of the present study is to investigate the frictional force between archwire and bracket under dry and lubricated conditions. Five lubricants were used: human saliva, Olive, Sunflower, Sesame, and Aloe Vera oils. The Self ligating assembly was compared to the conventional ligation system using different ligation methods in the presence of lubricants. Stainless steel and NiTi archwire alloys were used. Special attention was given to the relation between the friction force and the chemical composition of the used natural oils.
2. Experimental details Two types of stainless steel (SS) brackets were used namely: “Mini Sprint” conventional bracket (CB) and “Quick” self ligating bracket (SLB) from FORESTADENT (Germany). Both brackets are incorporating -7° of torque. In traditional or conventional brackets, an elastomeric tie is used to hold the archwire inside the slot (Figure 1). The Self-ligating means that the archwires get held into the brackets without using any elastic ligatures. The bracket has a little slot that is closed after the archwire is positioned inside. Both SS brackets have a slot of 0.022’’×0.028’’ (0.59mm×0.71mm). NiTi and SS archwires were used with transverse section of 0.018’’×0.025’’ (0.46mm×0.64mm). Each archwire has a total length of 52 mm and a curvature radius of 130mm. In each test, 4 brackets (one for the lateral incisor, one for the canine and two for bicuspids) were aligned on the sample holder of the device (Figure 1a). The distance between brackets was set to 7mm. Brackets were glued with Cyano-C adhesive to the sample holder. Then, the archwire was placed into the slots of brackets (Figure 1a). Elastomeric tie modules were used to ligate CB (Figure 1b). For the conventional brackets, two ligation methods were investigated: the O-ligation (LO) and the 8-ligation (L8) (Figure 1b). The experimental device (Mzali et al., 2013) (Figure 2) is composed of a mono-dimensional displacement table driven by a step controlled motor. The archwire was, equally, tied at one of its end on a fixed arm (Figure 2). The displacement velocity was set to 10 mm min-1. The frictional force between the archwire and the brackets was digitally recorded using a load transducer attached to the table via an acquisition card. The load resolution is equal to 1mN. Each result is the average of five tests. Five lubricants were used: human saliva obtained from a healthy adult female, olive, sunflower, sesame and aloe Vera oils. The retained natural oils were pure and used in cosmetic, pharmaceutical and nutritious applications. 3
Six configurations were then investigated under dry and lubricated conditions at 25 and 35°C (Table 1).
3. Results and discussion Before conducting frictional tests, the influence of the elastomeric ligatures relaxation on the friction force was investigated. For these tests, frictional tests were carried out after 0, 1, 24, 48 and 72 hours of the mounting under lubricated conditions. For illustration, conventional brackets (CB) with LO elastomeric ligatures and NiTi archwires were used in the presence of olive oil lubrication at 25°C (Figure 3). The examination of figure 3 shows that, just after ligation, the frictional force was the highest. After 1 hour, this force decreased and the loss of friction was about 20 %. Beyond 24 hours, the decrease of the frictional force is not significant. Accordingly, all the frictional force measurements were taken after at least 1 hour after ligature mounting.
During the friction tests, curves of the frictional force against displacement were recorded. Figure 4 shows a typical response of the variation of the frictional force against displacement of a NiTi/SLB assembly under dry and lubricated conditions. It can be seen that, in all responses there is an increase of the frictional force up to a value called “Peak Static Friction”, and then the response evolves almost constantly. In the first part, static friction happens until the force is great enough to overcome the initial resistance between the archwire and the bracket. Then the movement of the archwire starts: the kinetic part. It is well known that, sliding inside the archwire/bracket assembly induces different friction interactions. Kusy and Whitley (Kusy and Witley, 1999) sorted these friction forces into three types namely: (i) classical friction: caused by the action of the contact between the archwire and the bracket surfaces, (ii) binding: created when the archwire flexes against the corners of the bracket walls and (iii) notching: caused when permanent deformation of the archwire happens at the bracket corners. It is obvious that, the use of natural oils reduces the friction force and it seems that these oils act in the interaction regions by forming a lubrication layer (Figure 4). It is important to note that natural oils are more efficient than the human saliva which has a tribological performance close to the dry condition and the lowest value was observed with olive oil.
All the obtained results from frictional tests under dry and lubricated conditions using SS archwire are gathered in Figure 5. Prior examination of this figure shows that human saliva does not improve significantly the frictional behavior compared to dry conditions. This result is in accordance with that of Kusy and witley (Kusy and Witley, 1997). Figure 5 shows that the use of CB with elastomeric ligatures generates more high frictional forces compared to those generated by SLB. In fact, SLB bracket generates frictional force 30% less than the CB with LO-ligation and 55% less than the CB with L8-ligation. The CB with L8-ligation produces the highest frictional force compared to the other configurations. These results are, also in agreement with those of Voudouris et al. (Voudouris et al., 2010) and Shivapuja and Berger (Shivapuja and Berger, 1994). In fact, with L8-ligation the magnitude of the normal force pushing the archwire is superior to that of the other methods of ligation. This is due to the great number of contact points between the pressing elastomeric ligature and the archwire. In the other side, Monteiro et al. (Monteiro et al., 2014) linked the lower friction forces in the Self-ligating brackets compared to those of conventional brackets to the difference in the angulation of the bracket. On the other side, all natural oils have ameliorated the frictional response regardless of all the ligation type. The remarkable decrease of the friction force is mainly ascribed to their chemical composition, more particularly their fatty acids content (Kusy and Witley, 2003; Anilakumar et al. 2010; Salih et al. 2011; Balestra et al. 2011; Mannekote and Kailas, 2012; Al Mahmud et al. 2013; Reeves et al. 2015; Guezmil et al. 2016). The difference between human saliva and natural oils is equally related to the difference in their chemical composition. In fact, the human saliva contains approximately 98% of water and a variety of proteins and electrolytes (Zhou and Jin, 2015). Salivary proteins can adsorb between teeth and mucosal tissues and tongue to form a boundary lubrication system in the oral cavity. It was widely established that the salivary proteins adsorb layer by layer and form a heterogeneous structure with thick, viscoelastic and highly hydrated outer layer and dense and thin inner layer (Zhou and Jin, 2015). The inner layer decreases effectively the friction of teeth due to its high adhesion on the tooth surface (Zhou and Jin, 2015). It is to mention, also, that the saliva play an important role to protect teeth surfaces from acid attack due to its buffer character (Zhou and Jin, 2015). The use of natural oils was to imitate the action of human saliva while ensuring more tribological performances.
The obtained results suggest that the chemical adsorption of the different natural oils on the archwire and the bracket surfaces depends on the oil itself and in a difference in adsorption ability. The decrease of the frictional force with oils might be ascribed to their main components: fatty acids. It is established that the fatty acids have very attractive tribological performances (Salih et al. 2011; Mannekote and Kailas, 2012; Reeves et al. 2015; Guezmil et al. 2016). In fact, they give to natural oils good lubricity, high shear stability and high viscosity index with a very low volatility (Reeves et al. 2015). Natural oils are based on triglyceride structural molecules having long chains of 8 to 22 carbon atoms of fatty acids (Fox and Stachowiak, 2007; Sahoo and Biswas, 2009; Loehlé et al., 2015). They are beneficial in boundary lubrication as they can attach to metallic surfaces and form a multi molecular layer leading to less friction and wear (Sahoo and Biswas, 2009; Loehlé et al., 2015).
Natural oils are distinguished by the amount of unsaturated and saturated fatty acids. Table 2 presents the percentages of the main fatty acids of different oils retained in this study. All oils are entirely composed of triacylglycerol with 95% of fatty acids content. These acids include saturated (stearic and palmitic acids) and unsaturated (oleic and linoleic acids). Table 2 shows that the high amount of fatty acids contained in the studied oils are mainly unsaturated namely the oleic and the linoleic acids. The amount of the oleic acid in the different oils is, in the ascending order: olive oil>Aloevera>sesame>sunflower. With regards to the composition analysis of these oils, it seems that there is strong correlation between the friction force in the contact archwire/bracket and the amount of unsaturated fatty acids. Similar results were found by Reeves et al. (Reeves et al. 2015). They established that natural oils with high oleic acid content have good tribological behavior. As can be seen from Table 2, olive oil has the higher oleic acid content and the lower friction force. Reeves et al. (Reeves et al. 2015), related the observed tribological performance to the structure of the fatty acids. The linoleic acid contains two double bonds but oleic acid has only one double bond (Reeves et al. 2015, Guezmil et al. 2016). Oleic acid will forms between the archwire and bracket surfaces a lubrication film. The inner layer is formed by an adsorbed dense multilayer. The outer consists of an entanglement of molecule chain (figure 6). The formed lubrication film will prevent friction inside the assembly. The unsaturation number (UN) of the retained oils gives the average number of double bonds in the triacylglycerol molecule. This number can be strongly correlated to the tribological
behavior of a given oil lubricant. The UN is calculated according to the following equation (Reeves et al. 2015, Guezmil et al. 2016): [(
Table 3 gives the calculated UN of the different oils and the frictional force in the different assemblies. Table 3 shows that there is a correlation between the frictional force and the UN number. In fact, the frictional force increases with the increase of the UN. These results suggest that a high number of double bonds hinders the fatty acid chains to create an adsorbed protective layer on the metallic surface of the archwire and the bracket. These results are in agreement with those of Reeves et al. (Reeves et al. 2015). In order to investigate the effect of the archwire alloy on the friction response inside the archwire/bracket assembly, SS and NiTi archwires were used. Different tests were conducted using a CB with an elastomeric ligature (L-O), in the presence of different natural oils (Figure 7). Figure 7 shows that, the highest friction force was observed when working with the NiTi archwire. This result is expected. In fact, Tidy (Tidy, 1990) have demonstrated that the friction coefficient between NiTi and SS alloys (f = 0.39) was higher than that between SS and SS alloys (f= 0.25). It was established that, the adsorption ability of natural oils depends on the substrate nature (Sahoo and Biswas, 2009). Fundamentally, the different observed behavior can be related to the difference in the adsorption capacity of fatty acids on these tow materials. According to Sahoo and Biswas (Sahoo and Biswas, 2009), the strong coupling between the high electronic charge density with metallic substrate is very good. This phenomenon is linked to the double bonds in the fatty acid molecule. Simic and Kalin (Simič and Kalin, 2013) have founded that the adsorption of fatty acid on steel is much superior to that on diamond-like-carbon (DLC). In fact, Kalin et al. (kalin et al., 2006) have found a better wear behavior in the contact steel/steel than that of DLC/DLC. From this we infer that, the retained bio-lubricants will interact differently with stainless steel and NiTi archwire alloy. It remains that, more elaboration on this point is needed to give the best advantageous solution in terms of the choice of the archwire/bracket alloy in presence of these natural oils.
On the other side, the thermal-oxidative property of the retained oils is an important data to be considered. In this part the effect of temperature on friction is investigated. We used for this an adiabatic chamber to set the temperature at 35°C which is similar to the oral cavity temperature. Figure 8 shows that the friction force increases with the temperature. The same tendency is conserved with regards to the oil tribological performances. It was established that the physical and chemical properties of natural oils can be affected by thermal oxidation (Fox and Stachowiak, 2007). The occurred changes have to induce a potential impact on lubrication (Fox and Stachowiak, 2007). During thermal oxidation of natural oils some compounds are produced. Triacylglyceride hydroproxides are the first oxidation compounds; all the secondary products are derived from hydroproxides decomposition (Fox and Stachowiak, 2007). Fox and Stachowiak have demonstrated that the wear increase during boundary lubrication using oxidized natural oil was linked to the increase of hydroproxides levels and to the degradation of the triglyceride fatty acid structures (Fox and Stachowiak, 2007). The thermal oxidation of oil decreases its viscosity. According to Mannekote and Kailas (Mannekote and Kailas, 2012), the decrease of the oil viscosities is basically ascribed to the oxidation and the formation of free fatty acid. They, equally, observed that the formed amount of free fatty acid is high when the percentage of the unsaturated fatty acids is high in the oil composition. The formation of free fatty acids reduces the lubricating performance of the oil by reducing the adsorption ability of the oil to the metallic surface. Finally, it is to mention that, in the case of oral use, the application time is too short so that to induce oil oxidation.
Natural oils seem to be interesting bio-lubricant base stock applicants in orthodontic. In fact, these oils can be developed in a more elaborated aerosol solution for optimal use. In order to be well utilized, many other properties are to be studied such as oxidative, corrosive action, hydrolytic stabilities and the interaction with the dental and the oral cavity.
4. Conclusion In the light of this work, the following findings can be retained: •
all natural oils have improved the friction behavior in orthodontic sliding. Olive oil is the best lubricant compared to others,
in the presence of natural oils: (i) the Self ligating systems produced less friction compared to the conventional ligation systems (LO and L8), (ii) the conventional
bracket (CB) with LO-ligation generated lower friction than CB with L8-ligation and (iii) the SS archwires ensured lower friction compared to the NiTi ones, •
the relative difference in the friction behavior between the used natural oils is linked to the unsaturated fatty acids content. In fact, good correlation was found between the unsaturation number (UN) and the friction force in different archwire/bracket assemblies,
the increase of oil temperature increases the friction force. This phenomenon was related to the decrease of the oil adsorption ability due to the formation of oxidative products.
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Vaughan, J.L., Manville, G., Duncanson, Jr., Ram, S., Nanda, Currier, G.F., 1995. Relative kinetic frictional forces between sintered stainless steel brackets and orthodontic wires. Am. J. Orthod. Dentofacial Orthop. 107, 20-27. Voudouris, J.C., Schismenos, C., Lackovic, K., Kuftinec, M.M., 2010. Self-ligation Esthetic Brackets with low Frictional Resistance. Angle Ortho. 80,188-194. Zhou, Z.R., Jin, Z.M., 2015. Biotribology: Recent progresses and future perspectives. Biosurface and Biotribology 1, 3-24. Figures List Figure 1. (a) The sample holder with different dimensions and (b) the ligation types. Figure 2. The used experimental device Figure 3. Effect of elastomeric ligature’s relaxation on the frictional force in the NiTi archwire/CB assembly at T=25°C in the presence of olive oil. Figure 4. Friction force against displacement in the contact NiTi/SLB in the presence of different oils at 25°C. Figure 5. Effect of the ligation method and bio-lubricant on the frictional force response of SS archwire against SLB and CB at T=25°C. Figure 6. Schematic presentation of fatty acid multilayer adsorption in the archwire/bracket assembly. Figure 7. Frictional force in the SS/SLB and NiTi/SLB assemblies in the presence of different natural oils at T=25°C. Figure 8. Effect of temperature on frictional force of the SS/SLB assembly in the presence of the natural oils.
Frictional force (N)
12 10 8 6 4 2 0 t = 0s
18 16 14
SLB-SS CB-LO-SS CB-L8-SS
8 6 4 2 0 Olive oil
Aloe Vera Sesame oil Sunflower oil oil
Frictional force (N)
12 10 8 6 4 2 0 Olive oil
Aloe Vera Sesame oil Sunflower oil oil
SS/SLB-35 °C SS/SLB-25°C
Frictional force (N)
10 8 6 4 2 0 Olive oil
Aloe Vera oil
Figure 8. Tables list Table 1. Definition of the retained configurations Table 2. Fatty acids composition of the used natural oils Table 3. Unsaturation number against frictional force obtained with different natural oils
Table 1. NiTi Archwire
Configuration 1 (C1)
Configuration 2 (C2)
Configuration 3 (C3)
Configuration 4 (C4)
Configuration 5 (C5)
Configuration 6 (C6)
Table 2. Fatty acid name Palmitic (C16:0) Palmitoleic (C16:1) Stearic (C18:0) Oleic (C18:1) Linoleic (C18:2) Linolenic (C18:3) Arachidic (C20:0) Behenic (C22:0) Others
Olive oil 8.5 0.5 1.8 72.50 15.50 0.50 0.30 0.15 Rest
Aloe Vera oil 3.50 3.0 62 24 Rest
Sesame oil 10.70 0.20 5.20 41.50 40.30 0.50 0.60 0.40 Rest
Sunflower oil 5.8 0.1 3.9 16 71.7 0.7 0.3 0.7 Rest
Table 3. Frictional force (N) Natural oil
We study the friction force behavior in the archwire/bracket under bio-lubricated conditions.
Natural oils reduced remarkably the frictional force compared to dry and human saliva.
Natural oils decrease the friction force due to their fatty acids content.
Fatty acids adsorb between the archwire/bracket surfaces and form a lubrication layer.