ROLE OF FRICTION IN SLIDING MECHANICS

JERUN JOSE 3RD MDSDEPT. OF ORTHODONTICSROLE OF FRICTION IN SLIDING MECHANICS

CONTENTSINTRODUCTIONPHYSICSVARIABLES AFFECTING FRICTIONRECENTLY INTRODUCED MATERIALS TO REDUCE FRICTIONCONCLUSIONREFERENCE

INTRODUCTIONFriction is the force that resists against the movement of one surface in relation to another and that acts on the opposite direction of the desired movementThe friction present during orthodontic sliding mechanics represents a clinical challenge to the orthodontists because high levels of friction may reduce the effectiveness of the mechanics, decrease tooth movement efficiency and further complicate anchorage control.

Friction and sliding mechanicsOrthodontic tooth movement during space closure may be performed with two different types of mechanics. The first is the Segmented Arch Mechanics (SAM), which consists in bending loops on stainless steel (SS) or titanium molybdenum (TMA) wires.SAM is also called frictionless mechanics because the brackets and tubes do not slide along the archwire.

The other space closure mechanics used in Orthodontics is the Sliding Mechanics (SM), which involves the actual sliding of brackets and tubes along the wire.

When two surfaces slide one over the other, two force components are created: Frictional Force (FF), tangent to the Contacting Surface (CS) and Normal Force (N), perpendicular to the FF and to the CS

Mariana Ribeiro Pacheco, Dental Press J Orthod. 2012

Kusy and Whitley divided friction into three components: (1) classic friction, static and kinetic, due to contact of the wire with the bracket surface; (2) binding, created when the tooth tips or the wire flexes so that there is contact between the wire and the corners of the bracket; and (3) notching, when permanent deformation of the wire occurs at the wire-bracket corner interfaceKusy RP, Whitley JQ. Influence of archwire and bracket dimensions on sliding mechanics: derivations and determinations of the critical contact angles for binding. Eur J Orthod. 1999

Only classical friction is imporatnt because binding and notching are non existent.There are two types of FF: Static Friction (SF) and Kinetic Friction (KF)

SF is the smallest force needed to initiate a movement between two solid bodies that were static in relation to each other.Kinetic friction is the force that resists against the sliding movement of a solid object against another at a constant speed.

SF is always greater than KF since it is harder to change a body from its inertial situation than to maintain it moving.When an orthodontist tries to slide a tooth along the archwire, the tooth is subjected to an alternate movement of tipping and uprighting, thus moving in a sequential number of small and consecutive movements.Therefore, SF is more important than KF during space closure.

The application of the retraction force during space closure with SM generates a moment on the tooths crown that causes an initial crown tipping and later root uprighting. This moment is determined by the location of the point of force application in relation to the center of resistance of the tooth or group of teeth.

A number of successive crown tippings and root uprightings take place in the same plane of space towards the direction of the applied force. When the tooth inclines, the orthodontic wire binds against the edge of the bracket slot (binding effect), increasing friction and further restricting OTM.

Greater frictional forces mean that an increased number of tipping and uprighting must take place. Thus, friction should be minimized to achieve a more efficient sliding movement of the tooth along the arch wire

When the orthodontic wire slides through the bracket slot and the tubes, some resistance to sliding always takes place at the bracket/wire interface. This phenomenon is observed during leveling and alignment, space closure and even during torque expression at the end of treatment

A percentage of the orthodontic force applied to the teeth is lost as static friction and the rest is transferred to the tooth and its periodontium, generating the actual OTM. Kojima e Fukui evaluated the influence of friction on OTM using the finite element method and reported that approximately 60% of the orthodontic force applied to a tooth is lost as SF.

Thus, the biological tissue response to the mechanical stimulus takes place only if the force is strong enough to overcome SFTherefore, higher levels of friction during sliding mechanics require the application of higher orthodontic forces and may compromise the amount of OTM obtained as well as complicate anchorage control.

Physics -frictionIn the late 15th century, Leonardo Da Vinci proposed the two basic laws of sliding friction. Frictional resistance to sliding is, Proportional to the loadIndependent of the area of the sliding interfaces

Amontons-Coulomb LawsExperimental verification by Amontons and Coulomb, in 1699 and 1781, respectivelyAmontons' First Law: The force of friction is directly proportional to the applied load.Amontons' Second Law: The force of friction is independent of the apparent area of contact.Coulomb's Law of Friction: Kinetic friction is independent of the sliding velocity

Coulombs friction law was not applicable at an extremely low sliding velocity.the frictional forces between SS brackets and wires increased with the decrease of sliding velocity

Angle Orthod. 2014

Static friction (occurring instantaneously up to the onset of sliding) and kinetic friction (occurring continuously after the onset of sliding)

static frictional resistance (fs) is equal to the normal force or load (F) multiplied by a coefficient of static friction fs=s FThe kinetic frictional resistance is equal to the normal force or load (F) multiplied by a coefficient of kinetic friction fk = k F

The classic Amontons-Coulomb laws relate static and kinetic friction as follows:s and k are independent of F and 0s and k are materials dependent; andusually k< s

Bowden-Tabor Microscopic Interpretation

Schematic depiction of a variety of asperity configurations

Contributions of Bowden and Tabor1. The effective area of contact eff between solids is independent of the nominal surface area 0, and is determined only by normal force F (load)2. A microscopic consideration of dry friction accounts for shearing of interlocking and/or adhered asperities

3. The elastic-plastic properties of localized point contacts between microscopically irregular surfaces are a key feature of dry friction

Static Coefficient as a Function of Time the static coefficient of friction varies as a function of increasing time t before the onset of sliding.The observation is that the longer a mass M is at rest on a flat surface S (or an archwire at rest on a bracket) the greater the resistance to pulling force f parallel to the contact surface of nominal area 0.

When plastic deformation occurs at the level of the softer asperities, the frictional force f becomes a function of shear stress localized to point contacts among surface asperities of mass M and surface S.

So frictional force

Kinetic Coefficient as a Function of VelocityConstancy of the kinetic frictional coefficient k is dependent on maintenance of a steady sliding velocity VCDifferent materials exhibit unique kinetic frictional characteristics as a function of velocity {ie, k (V)}.

The Stick-Slip PhenomenonA single stick-slip cycle involves a stick state associated with elastic loading of the system, followed by a sudden slip corresponding to stress relaxation

Stick-slip motion, as observed over a broad velocity range in frictional sliding, can potentiate consequences resulting in noise (chatter), energy loss (friction), surface damage (wear), and component failure (breakage).

Formula for frictional forceThe coefficient of friction at the archwire-bracket interface was calculated using the appropriate formula,

where P is frictional resistance, F = equivalent force acting at a distance,W= bracket slot width, h = 12 mm, and = coefficient of friction.

Tidy DC. Frictional forces in fixed appliances. Am J Orthod Dentofacial Ortho 1989;54:249254

Variables influencing Friction during OTM

Mechanical/PhysicalBiological Friction: An OverviewP. Emile Rossouw, SEMINARS IN ORTHODONTICS-2003

1.Physical/mechanical factorsArchwire properties: a) materialb) cross sectional shape/sizec) surface textured) stiffness

ii) Bracket to archwire ligationligature wires elastomerics method of ligation.

iii) Bracket properties: Material surface treatment manufacturing processslot width and depth bracket designbracket prescription (first-order/in-out;second-order/toe-in; third-order/torque).

iv) orthodontic appliances: a) interbracket distance b) level of bracket slots between teethc) forces applied for retraction

2. Biological factors such as:SalivaPlaqueacquired pellicleCorrosionfood particles

Physical /mechanical factors

Bracket propertiesMaterial Metal brackets present lower friction coefficients than ceramic and plastic brackets and they are considered the golden standard to perform sliding mechanicsPlastic brackets showed lower values of friction than polycrystalline ceramic brackets

At high ligation forces (500 g), the monocrystalline brackets had the highest level of friction whereas at lower levels there was no difference in the friction levels when compared with the polycrystalline brackets

Omana and coworkers found difference in crystalline structure did not produce any significant reduction in friction, moreover, they also found injection molded ceramic brackets had less friction than other ceramic brackets.Omana HM, Moore RN, Bagby MD. Frictional properties of metal and ceramic brackets. J Clin Orthod. 1992

The insertion of a metal slot in ceramic brackets has showed relatively good success to reduce the levels of SF on this type of esthetic orthodontic brackets.Ceramic brackets with metal slots did show lower levels of SF than pure ceramic brackets

Their levels of SF remained higher than those registered with metal bracketsThis difference may be due to the different ceramic and metal expansion coefficients, or to the presence of a gap between the ceramic bracket body and the metal slot

Bracket Width and Interbracket WidthWide brackets demonstrate increased friction at small second order tip, whereas narrow brackets do not bind until twenty degrees of tip.Greater interbracket width allows the longer lengths of wire between brackets larger amounts of deflection, thus greater flexibility

Frank and Nikolai found interbracket distance to have little effect on frictional resistance.

Arch wire propertiesIn general, increasing wire size or cross-sectional shape (round or rectangular) for a constant bracket size increased the frictional resistance at binding and nonbinding angulations.

Cacciafesta et al evaluated the amount of friction related to the most commonly alloys used in orthodontic archwire manufacture: Stainless steel (SS), nickel-titanium (NiTi) and TMA. 0.016-in, 0.017 x 0.025-in and 0.019 x 0.025-in wires were tested The results showed that SF increases when thicker wires of the same alloy were testedTMA wires presented higher frictional resistance than SS and NiTi wires of same diameterEvaluation of friction of stainless steel and esthetic self-ligating brackets in various bracket archwire combinations. Am J Orthod Dentofacial Orthop. 2003;

In sliding mechanics, stainless steel wires generally showed the least friction followed by cobalt- chromium (Elgiloy), nickel-titanium, and beta-titanium (TMA).Friction appears to depend primarily on the vertical dimension of the wire, thus the frictional resistance of a 0.016 inch wire is not much different than a 0.016 * 0.022 inch wireFriction: Validation of Manufacturers ClaimKevin Mendes and P. Emile Rossouw, SEMINARS IN ORTHODONTICS-2003

Bracket to archwire ligationSteel or elastic ligatures may contribute differently to friction increase depending on how they are used.Steel ligatures influence resistance to sliding according to the intensity of the ligation. They can generate higher amounts of SF than elastic ligatures if they are tightly used.

if they are loosely inserted, small gaps between the wire and the bracket slot remain present and smaller SF values may be registered.Loosely tied stainless steel ligatures generally produced negligible friction in both dry and wet situations

The amount of SF may also vary depending on the type of elastic ligature used to ligate the archwire to the brackets.Hain et al reported that elastic ligatures with decreased surface roughness generated lower amounts of frictionHain M, Dhopatkar A, Rock P. The effect of ligation method on friction in sliding mechanics. AJODO 2003

Elastic ligatures tied as the number 8 generated increased levels of SF with all ligatures tested.

The use of small and medium elastomeric ligatures determines a 1317% decrease in static friction compared with large ligatures. Silicone- lubricated modules can reduce static friction by 2334% with respect to the small and medium nonlubricated elastomeric ligatures and by 3643% compared with nonlubricated large ligatures

Angle Orthod -2005

Biological factorsWhen human saliva is present, frictional forces and coefficients may increase, decrease, or not change depending on the arch wire alloy tested.The greatest differences between dry and wet states occurred with TMA archwire, in which the kinetic coefficients of friction in the wet state were reduced to 50% of the values in the dry state. At this point they were comparable to nickel-titanium but still higher than stainless steelAsian Journal of Oral Health & Allied Sciences - Volume 1, Issue 1, Jan-Mar 2011

Saliva can cause increase in friction in elastomeric modules, but does not cause any significant difference in friction when stainless steel or Teflon coated stainless steel ligatures are used.Baty et al., has noted that when extended and exposed to an oral environment, elastomeric materials absorb water and saliva, permanently stain, and suffer a breakdown of internal bonds that leads to permanent deformation. They also experience a rapid loss of force due to stress relaxation, resulting in a gradual loss of effectiveness.Aparna et al,Effects of Different Ligature Materials on Friction in Sliding Mechanics, Journal of International Oral Health 2015

Studies have shown that debris accumulation on the wire surface increases roughness and generates higher levels of friction.Recent studies have demonstrated that self-ligating brackets favor a higher colonization of Streptococcus mutans and accumulate more biofilm compared with conventional brackets with steel wire ligation

Self-ligating and conventional brackets showed a significant increase in debris and friction after intraoral exposure for 8 weeks. Accumulation of debris was higher for self-ligating brackets.Debris and friction of self-ligating and conventional orthodontic brackets after clinical useAngle orthodontist-2015

Orthodontic appliance

Bracket prescriptionIncreasing torque when using SLBs causes an increase in friction, since contact between the bracket slot wall and the wire edge becomes greater; the design of brackets influences static friction

Angle Orthod. 2014;

With increase in tip friction also increase

Ajodo -2004

Manufacturing process

Evaluated surface rougness

The correspondence between all three methods is in the range of 20 per cent, except for the wires SSA and NSA(GAC sentalloy,neosentalloy )The roughness of these two products is affected by the application temperature, which alters their microstructure and, thus, their external topographyChristoph et al, Europian journal of orthodontics-1998

Sliding VelocityThe friction of cobalt chromium wire decreased with increasing sliding velocityThe coefficient of friction increased for Beta-titanium (TMA) with increasing sliding velocity possibly because of cold welding.Seminars in orthod-2003

Recently introduced orthodonticmaterials to reduce frictionThe main technological innovations that have been tried to create low-friction orthodontic materials could be divided in design innovations and surface treatmentsAmong the various attempts to change the bracket design to reduce friction, the use of self-ligating brackets (SLB) has been the most tested.

SLB present a clip incorporated to its buccal surface that locks the wire within the slot and transforms the bracket in a tube-like device, thus eliminating the need for elastic or steel ligatures.

There were no significant differences between the amounts of friction registered when passive and active SLB where tested with round wiresHowever, when heavier rectangular wires were implemented active SLB showed more resistance to sliding than passive SLBPacheco MR, Oliveira DD, Smith-Neto P, Jansen WC. Evaluation of friction in selfligating brackets subjected to sliding mechanics: an in vitro study. Dental Press J Orthod. -2011

There were no evidence to support that SLB brackets generate significantly less friction than conventional brackets in the following clinical situations: (1) when rectangular stainless steel wires were used; (2) with marked dental tipping or torquing and (3) when treating complex malocclusions.Ehsani et al .Frictional resistance in selfligating orthodontic brackets and conventionally ligated brackets. A systematic review. Angle Orthod. 2009

Ligature material and designA polyurethane elastic ligature-SLIDEThis ligature combined to a conventional bracket forms a tube-like structure.significant lower resistance to sliding with the Slide ligature than with conventional elastic ligatures

Baccetti T, Franchi L. Friction produced by types of elastomeric ligatures in treatment mechanics with the preadjusted appliance. Angle Orthod. 2006

Metafasix (Super Slick Elastic Modules)Consisting of a water resistant polymeric coating, thus making the elastic ligature extremely slippery in the presence of saliva. Hain et al reported approximately 60% of friction reduction when these elastic ligatures were usedHain M, Dhopatkar A, Rock P. The effect of ligation method on friction in sliding mechanics. Am J Orthod Dentofacial Orthop. 2003

Surafce coated wires/ion implantation Burstone and Farzin-Nia showed that ion implantation increases archwire hardness, reduces flexibility, and improves surface finish Nitrogen Ion implanted -titanium orthodontic archwires (honeydew and purple).The ion implanted TMA has also been known as low-friction and is offered in different colors

Youssef et al .Titanium Nitride and Nitrogen Ion Implanted Coated Dental Materials .Coatings -2012

Ion implantation produces no sharp interface.Does not alter wire dimension, mechanical propertiesDepth, distribution of implantaion can be controlled by varying the ion dosage and energy.

Burstone CJ, Farzin-Nia F. Production of low friction and coloured TMA by Ion implantation. JCO. 1995

Bioforce iongurad NiTi wire on which 3-micron nitrogen coating that is produced by ion bombardment of the wire surface.Reduced friction

Viazis A D; Atlas of advanced orthodontics

Before slidingAfterUncoated TMAPurpleHoney dew

Comparative Evaluation of Frictional Properties, Load Deflection Rate and Surface Characteristics of Different Coloured TMA Archwires - An Invitro Study, JCDR-2015

Diamond-like carbon (DLC) surface coating of SS and NiTi orthodontic wires have been suggested to decrease static frictional forceThese ions were incorporated to the surface of wire during the manufacturing process, increasing the wire hardness and significantly reducing SF when compared to the conventional orthodontic wiresMuguruma T, Iijima M, Brantley WA, Mizoguchi I. Effects of a diamond-like carbon coating on the frictional properties of orthodontic wires. Angle Orthod. 2011

WC/C (tungsten carbide)coated archwires with their thin nature and smooth surface showed low frictional properties when compared with uncoated and TiAlN coated archwires making it ideal for space closure stage of orthodontic mechanics, when sliding mechanics is used.

Angle orthodontist-2012

CONCLUSIONFriction cannot be eliminated from materials. Therefore, the clinician should be aware of the characteristics of the orthodontic appliance that contribute to friction during sliding mechanics and the extent of the amount of force expected to be lost to friction. This will help allow efficient reproducible results to be achieved.

REFERENCESMariana Ribeiro Pacheco, Dental Press J Orthod. 2012Kusy RP, Whitley JQ. Influence of archwire and bracket dimensions on sliding mechanics: derivations and determinations of the critical contact angles for binding. Eur J Orthod. 1999Friction: An Overview P. Emile Rossouw, SEMINARS IN ORTHODONTICS-2003

Omana HM, Moore RN, Bagby MD. Frictional properties of metal and ceramic brackets. J Clin Orthod. 1992Evaluation of friction of stainless steel and esthetic self-ligating brackets in various bracket archwire combinations. Am J Orthod Dentofacial Orthop. 2003;Hain M, Dhopatkar A, Rock P. The effect of ligation method on friction in sliding mechanics. AJODO 2003

Asian Journal of Oral Health & Allied Sciences - Volume 1, Issue 1, Jan-Mar 2011Effects of Different Ligature Materials on Friction in Sliding Mechanics, Journal of International Oral Health 2015Debris and friction of self-ligating and conventional orthodontic brackets after clinical use .Angle orthodontist-2015

Christoph et al, Europian journal of orthodontics-1998Pacheco MR, Oliveira DD, Smith-Neto P, Jansen WC. Evaluation of friction in selfligating brackets subjected to sliding mechanics: an in vitro study. Dental Press J Orthod. -2011Ehsani et al .Frictional resistance in selfligating orthodontic brackets and conventionally ligated brackets. A systematic review. Angle Orthod. 2009

Baccetti T, Franchi L. Friction produced by types of elastomeric ligatures in treatment mechanics with the preadjusted appliance. Angle Orthod. 2006Hain M, Dhopatkar A, Rock P. The effect of ligation method on friction in sliding mechanics. Am J Orthod Dentofacial Orthop. 2003Youssef et al .Titanium Nitride and Nitrogen Ion Implanted Coated Dental Materials .Coatings -2012

Burstone CJ, Farzin-Nia F. Production of low friction and coloured TMA by Ion implantation. JCO. 1995Comparative Evaluation of Frictional Properties, Load Deflection Rate and Surface Characteristics of Different Coloured TMA Archwires - An Invitro Study, JCDR-2015Muguruma T, Iijima M, Brantley WA, Mizoguchi I. Effects of a diamond-like carbon coating on the frictional properties of orthodontic wires. Angle Orthod. 2011

Viazis A D; Atlas of advanced orthodonticsKrishnan V, Ravikumar KK, Sukumaran K, Kumar KJ. Invitro evaluation of physical vapor deposition coated beta titanium orthodontic archwires. Angle Orthod. 2012Yumi Yanase; Hideki Ioi; Masato Nishioka; Ichiro Takahashi Effects of sliding velocity on friction An in vitro study at extremely low sliding velocity approximating orthodontic tooth movement.AO-2014

Baker KL, Nieberg LG, Weimer AD, Hanna M. Frictional changes in force values caused by saliva substitution. Am J Orthod Dentofacial Orthop 1987;91(4):316-20Tidy DC. Frictional forces in fixed appliances. Am J Orthod Dentofacial Orthop. 1989;54:249254