Friction

FRICTIONFRICTION
ININ
ORTHODONTICSORTHODONTICS
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WHAT IS FRICTIONWHAT IS FRICTION

Defn:Defn: “ Friction is defined as a force tangential“ Friction is defined as a force tangential
to the common boundary of two bodies into the common boundary of two bodies in
contact that resists motion or tendency tocontact that resists motion or tendency to
motion of one relative to another”motion of one relative to another”
 Friction is the resistance to motionFriction is the resistance to motion
encountered when one body slides or tends toencountered when one body slides or tends to
slide over another.slide over another.
 It’s a feature of surface characteristic .It’s a feature of surface characteristic .

 Why is frictionWhy is friction “BIG-DEAL“BIG-DEAL”?”?

When we apply inWhen we apply in
ORTHODONTICSORTHODONTICS
 Orthodontic sliding mechanics using edgewiseOrthodontic sliding mechanics using edgewise
brackets is a common approach to translation ofbrackets is a common approach to translation of
tooth or a block of teeth .tooth or a block of teeth .
 During overjet reduction , a distally directed forceDuring overjet reduction , a distally directed force
retracts anterior teeth by sliding an archwireretracts anterior teeth by sliding an archwire
through brackets and tubes in the buccalthrough brackets and tubes in the buccal
segments. Significant resistance to movementsegments. Significant resistance to movement
may arise due to frictional force generated.may arise due to frictional force generated.
 During individual tooth movement esp. when doingDuring individual tooth movement esp. when doing
canine retraction the bracket of the tooth slides oncanine retraction the bracket of the tooth slides on
the wire,there by causing frictional resistance .the wire,there by causing frictional resistance .

 This frictional force slows or resists the movement ofThis frictional force slows or resists the movement of
the archwire through the bracket slots .the archwire through the bracket slots .
 It consists of complex interactions between bracketIt consists of complex interactions between bracket
slot, archwire and methods of ligation.slot, archwire and methods of ligation.
 To start the motion the magnitude of force is muchTo start the motion the magnitude of force is much
higher than to keep it into motion .higher than to keep it into motion .
 Sliding friction is generated between archwire and theSliding friction is generated between archwire and the
bracket when the archwire is slipped through posteriorbracket when the archwire is slipped through posterior
brackets and molar tubes. e.g. retraction of anteriorbrackets and molar tubes. e.g. retraction of anterior
segment .segment .
 Decreasing friction allows the clinician to lowerDecreasing friction allows the clinician to lower
treatment forcestreatment forces

COEFFICIENT OF FRICTIONCOEFFICIENT OF FRICTION
 Depends upon type of surfaces movingDepends upon type of surfaces moving ..
 FF==µµNN
FF== Frictional force {Newton}.Frictional force {Newton}.
µµ == Coefficient of friction.Coefficient of friction.
NN== normal force acting on thenormal force acting on the
contactcontact ..

CLASSICAL MODEL OF FRICTIONCLASSICAL MODEL OF FRICTION
BLOCK AND PLANE DIAGRAM

Orthodontic criterionOrthodontic criterion
 The block and plane are analogous to the wire andThe block and plane are analogous to the wire and
bracket and gives a good fundamental basis forbracket and gives a good fundamental basis for
examining the problem of friction between archwire andexamining the problem of friction between archwire and
bracket slot .bracket slot .
 The orthodontic problem is three dimensional in that theThe orthodontic problem is three dimensional in that the
wire may contact the bracket slot and the accompanyingwire may contact the bracket slot and the accompanying
ligation at as many as four locations simultaneously.ligation at as many as four locations simultaneously.

SPACE CLOSURE ANDSPACE CLOSURE AND
FRICTIONFRICTION
 On the basis of friction the mechanics of space closure isOn the basis of friction the mechanics of space closure is
divided into two :divided into two :
A-A- Friction or Sliding mechanicsFriction or Sliding mechanics : space closure by means: space closure by means
of e-chain , NiTi coil spring, elastics etc.of e-chain , NiTi coil spring, elastics etc.
B-B- Frictionless or Segmented mechanicsFrictionless or Segmented mechanics: space closure: space closure
by loops incorporated into the wire . e.g.- T-loop.by loops incorporated into the wire . e.g.- T-loop.

Role of friction in sliding mechanicsRole of friction in sliding mechanics
 Storey and SmithStorey and Smith developed the concept of optimaldeveloped the concept of optimal
forces as the minimum value of force that results in theforces as the minimum value of force that results in the
maximum rate of tooth movement within the limits ofmaximum rate of tooth movement within the limits of
biologic response. However realization of the optimalbiologic response. However realization of the optimal
force value for the tooth movement has been elusiveforce value for the tooth movement has been elusive..
 When slidingWhen sliding mechanics is used friction occurs at themechanics is used friction occurs at the
bracket wire interface. Some of the applied force isbracket wire interface. Some of the applied force is
therefore dissipated as friction and the remainder istherefore dissipated as friction and the remainder is
transferred to supporting structures of the tooth totransferred to supporting structures of the tooth to
mediate tooth movement .mediate tooth movement .

 Therefore maximum biological tissue response occursTherefore maximum biological tissue response occurs
only when the applied force is of sufficient magnitude toonly when the applied force is of sufficient magnitude to
adequately overcome the friction and lie within theadequately overcome the friction and lie within the
optimum range of forces necessary for the movement ofoptimum range of forces necessary for the movement of
the tooth.the tooth.

FACTORS AFFECTING FRICTIONALFACTORS AFFECTING FRICTIONAL
RESISTANCE DURING TOOTHRESISTANCE DURING TOOTH
MOVEMENTMOVEMENT
A : PHYSICALA : PHYSICAL
1. Archwire1. Archwire
A. Material{ surface qualities} .A. Material{ surface qualities} .
B. Cross-section { shape and size} .B. Cross-section { shape and size} .
C. Stiffness .C. Stiffness .
2. Brackets2. Brackets
A. Material .A. Material .
B. Manufacturing process .B. Manufacturing process .
C. Slot width .C. Slot width .
D. Design of bracket .D. Design of bracket .

3. Ligation3. Ligation
A. Stainless steel ligature wires .A. Stainless steel ligature wires .
B. Elastomeric module .B. Elastomeric module .
C .Teflon coated ligaturesC .Teflon coated ligatures
D. Self ligating brackets .D. Self ligating brackets .
E .Methods of tying .E .Methods of tying .
4. Orthodontic appliance4. Orthodontic appliance
A Mechanics AppliedA Mechanics Applied
B Self added errors at Point of forceB Self added errors at Point of force
application .application .
BB.. BiologicalBiological
A. SalivaA. Saliva
B. DepositsB. Deposits
C HabitsC Habitswww.indiandentalacademy.comwww.indiandentalacademy.com

BRACKET ARCHWIRE INTERPLAYBRACKET ARCHWIRE INTERPLAY
 When sliding mechanics are employed threeWhen sliding mechanics are employed three
mechanical phenomenon exist to cause themechanical phenomenon exist to cause the
resistance to slidingresistance to sliding { RS}{ RS} . {Kusy and Whitley 1997}.. {Kusy and Whitley 1997}.
RS =Fr + Bi + NoRS =Fr + Bi + No
FrFr = classical friction= classical friction
BiBi = binding= binding
NoNo = notching= notching

 THE ARCHWIRE AND THE BRACKET HASTHE ARCHWIRE AND THE BRACKET HAS
TWO CONFIGURATIONSTWO CONFIGURATIONS
A-A- PassivePassive-- there is clearance between archwire andthere is clearance between archwire and
the bracketthe bracket..
B-B- Active configuration-Active configuration- clearance disappears andclearance disappears and
interference fit occurs.interference fit occurs.

θθ== Contact angleContact angle {Proffit 1993}{Proffit 1993} ..
angulation between the archwire and theangulation between the archwire and the
bracket.bracket.
Influence by bracket width, archwireInfluence by bracket width, archwire
dimensiondimension
θθcc == Critical contact angle,Critical contact angle,
 Angle of interference.Angle of interference.
The angle where the wire contacts bothThe angle where the wire contacts both
ends ofends of the bracket slot.the bracket slot.


Passive configurationPassive configuration
 Its the force that resists the movement between the twoIts the force that resists the movement between the two
objects .Orthodontic literature has documented classicalobjects .Orthodontic literature has documented classical
friction as a function of arch wire bracket materialsfriction as a function of arch wire bracket materials
 It also occurs because of ligation .It also occurs because of ligation .
 Its of small magnitude but controls sliding because bindingIts of small magnitude but controls sliding because binding
or notching do not exist at this juncture.or notching do not exist at this juncture.
 brackets have clearance and are inbrackets have clearance and are in passive configurationpassive configuration . in this. in this
condition:condition:
θθ<< θθ cc

 Its exists as only component of sliding resistanceIts exists as only component of sliding resistance
when the archwire and the brackets have clearancewhen the archwire and the brackets have clearance
and are inand are in passive configurationpassive configuration . in this condition:. in this condition:
θθ<< θθ cc
θθ== Contact angleContact angle {Proffit 1993}{Proffit 1993} ..
angulation between the archwire and the bracket.angulation between the archwire and the bracket.
Influence by bracket width, archwire dimensionInfluence by bracket width, archwire dimension
θθcc == Critical contact angle,Critical contact angle,
 Angle of interference.Angle of interference.
The angle where the wire contacts both ends ofThe angle where the wire contacts both ends of
the bracket slot.the bracket slot.

F FR
N
θ < θc
PASSIVE CONFIGURATION
F=APPLIED FORCE
FR= CLASSICAL FRICTION
N = NORMAL FORCE

ACTIVE CONFIGURATIONACTIVE CONFIGURATION
Under these conditions three distinct normal forcesUnder these conditions three distinct normal forces
exist.exist.
– FrFr= classical friction= classical friction
– BiBi= binding force which increases with further= binding force which increases with further
increase inincrease in θθ ..
– NoNo=notching: Defined as the observed mechanical=notching: Defined as the observed mechanical
damage to an archwire that occurs during the laterdamage to an archwire that occurs during the later
stages of binding which manifests itself asstages of binding which manifests itself as
recognizable defects of varying no, pattern, andrecognizable defects of varying no, pattern, and
severity.severity.
– If notching occurs its suspected to cease sliding allIf notching occurs its suspected to cease sliding all
together {Kusy 1998}together {Kusy 1998}

Br
N
FR
N
N
F
ACTIVE CONFIGURATION
θ = θc
BI
BI
NBI = INTERFERENCE FIT WHERE BINDING
TAKES PLACE

 In later stages whenIn later stages when θθ is much greater thanis much greater than θθcc ,both,both
classical friction and binding becomes negligible relativeclassical friction and binding becomes negligible relative
to notching.to notching.
 NO>>Bi>>FRNO>>Bi>>FR. As a consequence sliding is all but. As a consequence sliding is all but
impossible thereafter .RS approaches to infinityimpossible thereafter .RS approaches to infinity
 RS=NO=INFINITYRS=NO=INFINITY.{ Kusy ; Angle:2000}.{ Kusy ; Angle:2000}

Orthodontic bracketsOrthodontic brackets
 Orthodontists today have multitude of optionOrthodontists today have multitude of option
when it comes to selecting a bracket. There arewhen it comes to selecting a bracket. There are
ample no. of choices in bracket material, width,ample no. of choices in bracket material, width,
design, ligation capabilities and in prescription.design, ligation capabilities and in prescription.
Effect of each feature should be well understoodEffect of each feature should be well understood
to minimize friction .to minimize friction .

EFFECT OF BRACKETEFFECT OF BRACKET
MATERIAL ON FRICTIONMATERIAL ON FRICTION
 Stainless steelStainless steel::
 Most popular brackets till today.Most popular brackets till today.
 The friction of stainless steel bracket is minimum inThe friction of stainless steel bracket is minimum in
comparison to other brackets of different materials.comparison to other brackets of different materials.
 This is attributed to the fact that stainless steelThis is attributed to the fact that stainless steel
bracket has the “bracket has the “smoothest surfacesmoothest surface” as seen in” as seen in
scanning electron microscopescanning electron microscope..

EFFECT OF MANUFACTURINGEFFECT OF MANUFACTURING
PROCESS IN S.S BRACKETSPROCESS IN S.S BRACKETS
 Till some years back stainless steel bracketsTill some years back stainless steel brackets
were made by the process of casting orwere made by the process of casting or
milling.They were crude methods.milling.They were crude methods.
 However conventional cast stainless steelHowever conventional cast stainless steel
brackets has met its competitor in the sinteredbrackets has met its competitor in the sintered
variety.variety.

 For the most wire size the sintered stainless steel bracketsFor the most wire size the sintered stainless steel brackets
produced significantly lower friction than cast stainless steelproduced significantly lower friction than cast stainless steel
brackets.brackets.
 The friction of the sintered stainless steel variety was 38%The friction of the sintered stainless steel variety was 38%
less than the cast stainless steel bracket in .018 slot, whereless than the cast stainless steel bracket in .018 slot, where
as it was 44% less than the friction of cast stain less bracketsas it was 44% less than the friction of cast stain less brackets
in .022 slot.in .022 slot.
 The difference in frictional forces between the two bracketThe difference in frictional forces between the two bracket
may be attributed to the smoother surface texture of themay be attributed to the smoother surface texture of the
sintered stainless steel material.sintered stainless steel material.

CERAMIC BRACKETSCERAMIC BRACKETS
 The recent introduction ofThe recent introduction of
ceramic brackets toceramic brackets to
orthodontics is only part oforthodontics is only part of
the rapidly expandingthe rapidly expanding
ceramic technology inceramic technology in
many industries.many industries.

Ceramic bracketsCeramic brackets

 There are two types of ceramic brackets :There are two types of ceramic brackets :
1. Polycrystalline1. Polycrystalline
2. Single crystal alumina{99% aluminum oxide}2. Single crystal alumina{99% aluminum oxide}
 The mono crystalline alumina brackets were observed toThe mono crystalline alumina brackets were observed to
be smoother than polycrystalline ones.be smoother than polycrystalline ones.
 Laser specular reflectance and scanning electronLaser specular reflectance and scanning electron
microscopemicroscope studies have shown polycrystalline ceramicstudies have shown polycrystalline ceramic
brackets to posses very rough surface which actuallybrackets to posses very rough surface which actually
scribed grooves into the archwire.{scribed grooves into the archwire.{Ajo 1997}Ajo 1997} ..

 Ceramic brackets became quite popular in 1980sCeramic brackets became quite popular in 1980s
because of their improved esthetics, but problem ofbecause of their improved esthetics, but problem of
frictional resistance limited there use.frictional resistance limited there use.
 The rough but hard ceramic material is likely toThe rough but hard ceramic material is likely to
penetrate the surface of even a steel wire during slidingpenetrate the surface of even a steel wire during sliding
creating considerable resistance to sliding.creating considerable resistance to sliding.
 With ceramic brackets most of the wire size and alloyWith ceramic brackets most of the wire size and alloy
combinations with both .018 slot and .022 inch slot sizescombinations with both .018 slot and .022 inch slot sizes
demonstrated significantly higher frictional forces thandemonstrated significantly higher frictional forces than
with stainless steel brackets. {Angolkar et al, Ajo 1990} .with stainless steel brackets. {Angolkar et al, Ajo 1990} .

S.E.M OF SLOT OF S,S VSS.E.M OF SLOT OF S,S VS
CERAMIC BRACKETCERAMIC BRACKET

NOTCHING OF S.S WIRE INNOTCHING OF S.S WIRE IN
CERAMIC BRACKET SLOTCERAMIC BRACKET SLOT

PLASTIC BRACKETSPLASTIC BRACKETS
 In an attempt to create an esthetic bracket with lowerIn an attempt to create an esthetic bracket with lower
frictional resistance and easier depending features thanfrictional resistance and easier depending features than
the ceramic , a variety of new ceramic - reinforced plasticthe ceramic , a variety of new ceramic - reinforced plastic
brackets with or without metal slots have beenbrackets with or without metal slots have been
introduced.introduced.
 There was no distinct difference in frictional resistanceThere was no distinct difference in frictional resistance
between the composite brackets with or without the metalbetween the composite brackets with or without the metal
slot insert.{Ajo 1997:112:143:}slot insert.{Ajo 1997:112:143:}

TITANIUM BRACKETSTITANIUM BRACKETS
 Introduced in response to reports about the corrosion ofIntroduced in response to reports about the corrosion of
stainless steel brackets and sensitivity to nickel that isstainless steel brackets and sensitivity to nickel that is
present in the alloy.present in the alloy.
 Its proven to be biocompatible.Its proven to be biocompatible.
 But its very rough . as the titanium content of the alloyBut its very rough . as the titanium content of the alloy
increases , the surface reactivity increases , and theincreases , the surface reactivity increases , and the
surface chemistry is the major influence on frictionalsurface chemistry is the major influence on frictional
behaviour .behaviour .
 More chances of “cold weld” formation with the titaniumMore chances of “cold weld” formation with the titanium
brackets at the bracket and archwire interface thusbrackets at the bracket and archwire interface thus
increased frictional resistance. {increased frictional resistance. {Kusy and Whitley: 1997}Kusy and Whitley: 1997}
TITANIUM BRACKETSTITANIUM BRACKETS
 Introduced in response to reports about the corrosion ofIntroduced in response to reports about the corrosion of
stainless steel brackets and sensitivity to nickel that isstainless steel brackets and sensitivity to nickel that is
present in the alloy.present in the alloy.
 Its proven to be biocompatible.Its proven to be biocompatible.
 But its very rough . as the titanium content of the alloyBut its very rough . as the titanium content of the alloy
increases , the surface reactivity increases , and theincreases , the surface reactivity increases , and the
surface chemistry is the major influence on frictionalsurface chemistry is the major influence on frictional
behaviour .behaviour .
 More chances of “cold weld” formation with the titaniumMore chances of “cold weld” formation with the titanium
brackets at the bracket and archwire interface thusbrackets at the bracket and archwire interface thus
increased frictional resistance. {increased frictional resistance. {Kusy and Whitley: 1997}Kusy and Whitley: 1997}

EFFECT OF BRACKET WIDTH ONEFFECT OF BRACKET WIDTH ON
FRICTIONFRICTION
 The effect of bracket width on friction has beenThe effect of bracket width on friction has been
controversial.controversial.
 Some studies have found out that altering bracket widthSome studies have found out that altering bracket width
made no difference . {Andresen et.al J.Biomech. I970}.made no difference . {Andresen et.al J.Biomech. I970}.
 Others have found that frictional resistance increased withOthers have found that frictional resistance increased with
increase in bracket width. {Frank ,Nikolai,Ajo 1980, Ogataincrease in bracket width. {Frank ,Nikolai,Ajo 1980, Ogata
et.al Ajo 1995}.et.al Ajo 1995}.
 Frictional resistance also have been reported to decreaseFrictional resistance also have been reported to decrease
with increase in bracket width.with increase in bracket width.

 Frank and NikolaiFrank and Nikolai { Ajo 1980} related the greater friction{ Ajo 1980} related the greater friction
with wider brackets to the fact that binding occurswith wider brackets to the fact that binding occurs
frequently with wider brackets at smaller degrees offrequently with wider brackets at smaller degrees of
second order angulation than narrow bracketssecond order angulation than narrow brackets ..
 Drescher et al.{Ajo 1989}, bednar et al.{Ajo 1991} andDrescher et al.{Ajo 1989}, bednar et al.{Ajo 1991} and
Omana et al.{ Jco 1992}Omana et al.{ Jco 1992} suggested that with a narrowsuggested that with a narrow
bracket , the tooth could tip considerably before bindingbracket , the tooth could tip considerably before binding
occurred and that once binding occurred , it was ofoccurred and that once binding occurred , it was of
severe nature and resisted further sliding of thesevere nature and resisted further sliding of the
archwire.archwire.

BRACKET WIDTH AND ITSBRACKET WIDTH AND ITS
INFLUENCEINFLUENCE
θ=10°
θ=6°
Single bracket Twin bracket

Other bracket designOther bracket design
 Begg bracketBegg bracket
 Tip – edge bracketTip – edge bracket
 Self- ligating bracketSelf- ligating bracket
 Friction free bracketFriction free bracket

BEGG BRACKET :Vertical slot pointingBEGG BRACKET :Vertical slot pointing
gingivally, allows free tipping and sliding up to 60gingivally, allows free tipping and sliding up to 60
degrees because of single point contact.degrees because of single point contact.

TIP- EDGE BRACKETTIP- EDGE BRACKET
 By removing predetermined diagonally opposite cornersBy removing predetermined diagonally opposite corners
from the conventional edgewise archwire slot, the tip-edgefrom the conventional edgewise archwire slot, the tip-edge
bracket is created. This permits the desired distal crownbracket is created. This permits the desired distal crown
tipping required for the differential tooth movement.tipping required for the differential tooth movement.
 The slot is designed so that initial second order changes.The slot is designed so that initial second order changes.
Mesial -distal crown tipping can be accomplished in theMesial -distal crown tipping can be accomplished in the
presence of a straight orthodontic round archwire poweredpresence of a straight orthodontic round archwire powered
by light intra - oral forces , elastics or coil springs.by light intra - oral forces , elastics or coil springs.

TIP EDGE BRACKETTIP EDGE BRACKET

 Binding which normally occurs betweenBinding which normally occurs between
conventional edgewise slot and archwire duringconventional edgewise slot and archwire during
space closure is non existent.space closure is non existent.
 Its because the corners of the slot that usuallyIts because the corners of the slot that usually
press against the upper and lower surfaces of thepress against the upper and lower surfaces of the
archwire as the teeth tip and which causes 90% ofarchwire as the teeth tip and which causes 90% of
the friction in the appliance are eliminated. Thusthe friction in the appliance are eliminated. Thus
friction is greatly reduced.friction is greatly reduced.

FRICTION FREE BRACKETSFRICTION FREE BRACKETS
 Three brackets American friction free brackets {A.OThree brackets American friction free brackets {A.O
Sheboygan}, GAC{ shoulder bracket} and RMOSheboygan}, GAC{ shoulder bracket} and RMO
{Synergy bracket } were introduced to restrict the{Synergy bracket } were introduced to restrict the
amount of force exerted on the wire by ligature.amount of force exerted on the wire by ligature.
 These brackets generated lower mean frictionalThese brackets generated lower mean frictional
forces at second order deflection of .00 and .25 mmforces at second order deflection of .00 and .25 mm
than conventionally ligated brackets .{Ogata et.althan conventionally ligated brackets .{Ogata et.al
1995 } .1995 } .

SYNERGY BRACKETS

SELF LIGATING BRACKETSSELF LIGATING BRACKETS
 Orthodontic brackets are now available that possess theOrthodontic brackets are now available that possess the
feature of self ligation {feature of self ligation {SSpeed bracketpeed bracket}}.It has a potential of.It has a potential of
reduced friction as well as chair side time.reduced friction as well as chair side time.
 Studies have consistently shown reduced frictionalStudies have consistently shown reduced frictional
resistance up to 23% that of stainless steel bracketresistance up to 23% that of stainless steel bracket
irrespective of wire shape and ligation.irrespective of wire shape and ligation.
 The feature of self ligation especially with theThe feature of self ligation especially with the “S“Speedpeed
bracket”bracket” is associated with its unique anatomicis associated with its unique anatomic
characteristic that is its highly resilient spring clip.characteristic that is its highly resilient spring clip.

 In absence of ligature the spring clip appears to trapIn absence of ligature the spring clip appears to trap
the archwire loosely in the slot .the archwire loosely in the slot .
 In contrast the stainless steel ligature or elastomericIn contrast the stainless steel ligature or elastomeric
module :module :
 Binds both the mesial and the distal aspect .Binds both the mesial and the distal aspect .
 The cut ends of pig tail of ligature may contactThe cut ends of pig tail of ligature may contact
with archwire thus increasing friction .with archwire thus increasing friction .

SPEED Bracket

Damon S.L. bracket

ACTIVA Bracket

EDGELOK
Bracket
MOBIL-Lock
Bracket

ARCHWIREARCHWIRE
Wire alloyWire alloy
 The role of wire alloy in the frictional characteristics 0fThe role of wire alloy in the frictional characteristics 0f
sliding mechanics has been exclusively studied .sliding mechanics has been exclusively studied .
 Specular reflectance studies have shown that S.S wiresSpecular reflectance studies have shown that S.S wires
have the smoothest surface followed by co-cr,have the smoothest surface followed by co-cr, ββ--titaniumtitanium
and NiTi wires in order of increasing surface roughness.and NiTi wires in order of increasing surface roughness.
{ Kusy and Whitley, Angle Ortho 1988 } .{ Kusy and Whitley, Angle Ortho 1988 } .

S.E.M VIEW OF SURFACES OFS.E.M VIEW OF SURFACES OF
S.S V/S NiTi WIRES.S V/S NiTi WIRE

 ββ - Titanium wires may form micro weld with stainless- Titanium wires may form micro weld with stainless
steel brackets in dry conditions further increasing thesteel brackets in dry conditions further increasing the
frictional forces.frictional forces.
 Frank and Nikolai {Ajo 1980Frank and Nikolai {Ajo 1980} found that stainless steel} found that stainless steel
wires had less friction than nickel titanium at non bindingwires had less friction than nickel titanium at non binding
angulations. But as angulations increased and bindingangulations. But as angulations increased and binding
was present ,the reverse was true.was present ,the reverse was true.

 One possible solution to the high coefficient of frictionOne possible solution to the high coefficient of friction
ofof ββ--titanium and nickel titanium wires is ion-titanium and nickel titanium wires is ion-
implantation, an area that is currently beingimplantation, an area that is currently being
investigated.investigated.
 Burstone and Farzin NiaBurstone and Farzin Nia {Jco 1995} demonstrated{Jco 1995} demonstrated
that ion implantedthat ion implanted ββ - titanium wires produced about- titanium wires produced about
the same level of friction as stainless steel wires.the same level of friction as stainless steel wires.

ION IMPLANTATIONION IMPLANTATION
 Ion implantation is a process in which any element canIon implantation is a process in which any element can
be embedded into the surface of a substrate.be embedded into the surface of a substrate.
 In this process various elements or compounds areIn this process various elements or compounds are
ionized and then accelerated toward the target . in ourionized and then accelerated toward the target . in our
case orthodontic wire .case orthodontic wire .
 It takes place into a vacuum chamber where a flux of ionIt takes place into a vacuum chamber where a flux of ion
is generated with an electron beam evaporator andis generated with an electron beam evaporator and
deposited on the substrate.deposited on the substrate.

 The ions penetrate the surface of the wire onThe ions penetrate the surface of the wire on
impact, building up a extremely hard layer andimpact, building up a extremely hard layer and
creates a considerable amount of compressivecreates a considerable amount of compressive
force in material at atomic level .force in material at atomic level .
 The increase compressive force and increaseThe increase compressive force and increase
surface qualities improve the fatigue resistancesurface qualities improve the fatigue resistance
and reduce the coefficient of friction of the wire.and reduce the coefficient of friction of the wire.

Teflon coated archwiresTeflon coated archwires
 Found to reduce the kinetic friction.Found to reduce the kinetic friction.
 It has a costing ofIt has a costing of PTFE {polytetrafluoroethylene }PTFE {polytetrafluoroethylene }
coating is sprayed on stainless steel or Niti.coating is sprayed on stainless steel or Niti.
 Results in esthetics and less resistance to sliding.Results in esthetics and less resistance to sliding.
 With time,coating disappears and notching occursWith time,coating disappears and notching occurs

 More recentlyMore recently diamond-like carbon coatings {DLC} anddiamond-like carbon coatings {DLC} and
plasma depositionplasma deposition have been applied to arch wires withhave been applied to arch wires with
various degrees of success all in efforts to get estheticvarious degrees of success all in efforts to get esthetic
wires with reduced friction.wires with reduced friction.

WIRE SIZEWIRE SIZE
 Several studies have found an increase in wire size toSeveral studies have found an increase in wire size to
be associated with increased bracket- wire friction. Inbe associated with increased bracket- wire friction. In
general rectangular wires produce more friction thangeneral rectangular wires produce more friction than
round wires.round wires.

 At greater angulations of bracket the determiningAt greater angulations of bracket the determining
factor is the point at which the wire contacts the edgefactor is the point at which the wire contacts the edge
of the bracketof the bracket..
 With round wires the bracket slot can “bite” into theWith round wires the bracket slot can “bite” into the
wire at one point ,causing an indentation in the wire .wire at one point ,causing an indentation in the wire .
 However with a rectangular wire the force is distributedHowever with a rectangular wire the force is distributed
over a larger area that is the entire facio lingualover a larger area that is the entire facio lingual
dimension of the wire resulting in less pressure anddimension of the wire resulting in less pressure and
less resistance to movement.less resistance to movement.
 This can account for the finding byThis can account for the finding by Frank andFrank and NikolaiNikolai
that .020 inch wire is associated with more friction thanthat .020 inch wire is associated with more friction than
the .O17 x.025 inch wire .the .O17 x.025 inch wire .

EFFECT OF LIGATIONEFFECT OF LIGATION
 The normal force exerted by the ligature has beenThe normal force exerted by the ligature has been
estimated to be between 50 to 300 mg.estimated to be between 50 to 300 mg.
 BazakidouBazakidou showed that there was no significant differenceshowed that there was no significant difference
between frictional resistance offered by the conventionalbetween frictional resistance offered by the conventional
tied stainless steel ligature and elastomeric module.tied stainless steel ligature and elastomeric module.
 Elastomeric moduleElastomeric module are adversely affected by oralare adversely affected by oral
environment demonstrates stress relaxation with time andenvironment demonstrates stress relaxation with time and
exhibit great individual variation in properties.exhibit great individual variation in properties.

 On the other hand stainless steel ligatures can be tooOn the other hand stainless steel ligatures can be too
tight or too loose , depending upon the technique andtight or too loose , depending upon the technique and
needs of the clinician.needs of the clinician.
 Frictional forces can be reduced by stretching theFrictional forces can be reduced by stretching the
elastomeric modules prior to placement on theelastomeric modules prior to placement on the
brackets or by tying stainless steel ligatures looselybrackets or by tying stainless steel ligatures loosely
around the brackets .around the brackets .
 One of the method of reducing the frictional forceOne of the method of reducing the frictional force
generated by ligatures and nodules is to use selfgenerated by ligatures and nodules is to use self
ligating brackets .ligating brackets .

Teflon coated ligaturesTeflon coated ligatures
 The teflon coated stainless steel ligature wires wereThe teflon coated stainless steel ligature wires were
shown to be having very less friction compared toshown to be having very less friction compared to
stainless steel ligature wires.stainless steel ligature wires.
 The teflon coated ligatures are useful in minimizingThe teflon coated ligatures are useful in minimizing
the friction of ceramic brackets when an estheticthe friction of ceramic brackets when an esthetic
appliance is imperative. { Franco et al 1995 Angle}appliance is imperative. { Franco et al 1995 Angle}

Method of ligationMethod of ligation
Twist ligationTwist ligation. –. – { Prollit, Jco 1995}.{ Prollit, Jco 1995}.
 When the ligatures were twisted in gingival toWhen the ligatures were twisted in gingival to
occlusal direction, the ligature wire did not appear toocclusal direction, the ligature wire did not appear to
enter the bracket and the binding of the pigtail withenter the bracket and the binding of the pigtail with
the archwire does not take place.the archwire does not take place.
 When the archwire was twisted in another directionWhen the archwire was twisted in another direction
the section of ligature oriented almost parallel to thethe section of ligature oriented almost parallel to the
slot and in contact with the arch wire resulting inslot and in contact with the arch wire resulting in
binding, which is incorrect.binding, which is incorrect.

A Bwww.indiandentalacademy.comwww.indiandentalacademy.com

 Number One enemy of conventionalNumber One enemy of conventional
Orthodontics is the way arch wires are tiedOrthodontics is the way arch wires are tied
DamonDamon

BIOMECHANICAL CONSIDERATIONBIOMECHANICAL CONSIDERATION
Effect of point of application of forceEffect of point of application of force
 Mesial or distal tipping of the bracket is dependent uponMesial or distal tipping of the bracket is dependent upon
the location of force application relative to the centre ofthe location of force application relative to the centre of
resistance and biological resistance of the tooth.resistance and biological resistance of the tooth.
 Tipping of the bracket produces pressure at the contactTipping of the bracket produces pressure at the contact
area between the bracket and the archwire with aarea between the bracket and the archwire with a
resultant increase in frictional resistance .resultant increase in frictional resistance .
 Farther away the retraction force is applied from theFarther away the retraction force is applied from the
centre of resistance, greater is the moment arm andcentre of resistance, greater is the moment arm and
greater the tipping and binding.greater the tipping and binding.

EFFECT OF SECOND ORDEREFFECT OF SECOND ORDER
ANGULATIONSANGULATIONS
 Second order deflections of the wire between brackets held inSecond order deflections of the wire between brackets held in
series can have significant effects on the bracket - wire friction.series can have significant effects on the bracket - wire friction.
 Several studies have found that increasing the angulationSeveral studies have found that increasing the angulation
between bracket and wire produced greater friction.between bracket and wire produced greater friction. Frank andFrank and
NikolaiNikolai found that frictional resistance increased in a non-linearfound that frictional resistance increased in a non-linear
manner with increased bracket angulation.manner with increased bracket angulation.
 Therefore in the management of maximum anchorage case ,theTherefore in the management of maximum anchorage case ,the
clinical situation requires complete leveling of deflections in aclinical situation requires complete leveling of deflections in a
series of bracketed teeth , not just a single bracket before theseries of bracketed teeth , not just a single bracket before the
retraction is started .retraction is started .

FRICTIONAL EFFECTS ONFRICTIONAL EFFECTS ON
ANCHORAGEANCHORAGE
 We know that one moving object contacts anotherWe know that one moving object contacts another
friction at their interface produces resistance to thefriction at their interface produces resistance to the
direction of movement .direction of movement .
 The frictional force is proportional to the force withThe frictional force is proportional to the force with
which the contacting surfaces are pressed togetherwhich the contacting surfaces are pressed together
and is affected by the nature of the surfaces at theand is affected by the nature of the surfaces at the
interface.interface.

EFFECT OF SALIVAEFFECT OF SALIVA
 It has been suggested that saliva serves as anIt has been suggested that saliva serves as an
excellent lubricant in the sliding of the bracket alongexcellent lubricant in the sliding of the bracket along
the wire .the wire .
 BakerBaker{{Ajo 1987} using artificial saliva found a 15% toAjo 1987} using artificial saliva found a 15% to
19% reduction in friction.19% reduction in friction. KusyKusy{ Angle orth.1991}{ Angle orth.1991}
found lubricous as well as adhesive behaviourfound lubricous as well as adhesive behaviour
depending upon the archwire bracket combinations.depending upon the archwire bracket combinations.

 The function of saliva as a lubricant is to reduce theThe function of saliva as a lubricant is to reduce the
strength and number of the bridges formed between thestrength and number of the bridges formed between the
asperities of sliding surface ;that means it preventsasperities of sliding surface ;that means it prevents
solid to solid contact thus reducing friction .solid to solid contact thus reducing friction .
 When dealing with a adult patient ,giving a history ofWhen dealing with a adult patient ,giving a history of
xerostomia, oral radiation therapy or ant cholinergicxerostomia, oral radiation therapy or ant cholinergic
medication , varying the force level to move the teethmedication , varying the force level to move the teeth
should be noted.should be noted.

MAGNITUDE OF FRICTIONMAGNITUDE OF FRICTION
 The most important information to be gained from aThe most important information to be gained from a
consideration of friction is an appreciation of itsconsideration of friction is an appreciation of its
magnitude .magnitude .
 A .019 x.025 steel wire in a .022 inch slot bracket tiedA .019 x.025 steel wire in a .022 inch slot bracket tied
with a ligature , the minimum frictional resistance towith a ligature , the minimum frictional resistance to
sliding a single bracket is aboutsliding a single bracket is about 100gm.100gm.

 If a canine tooth is to slide along an archwire aIf a canine tooth is to slide along an archwire a
100gm net force is needed for the tooth movement ,100gm net force is needed for the tooth movement ,
approximately another 100 gm is needed toapproximately another 100 gm is needed to
overcome friction.overcome friction.

 In terms of orthodontic anchorage ,the problem createdIn terms of orthodontic anchorage ,the problem created
by friction is not so much its presence as the difficultyby friction is not so much its presence as the difficulty
of knowing itof knowing it ..
 Its difficult toIts difficult to avoid the temptation to estimate frictionavoid the temptation to estimate friction
generously and add enough force to be certain thatgenerously and add enough force to be certain that
tooth movement will occur.tooth movement will occur.
 The effect of any force beyond what was really neededThe effect of any force beyond what was really needed
to overcome friction is to bring the anchor teeth up ontoto overcome friction is to bring the anchor teeth up onto
the plateau of the tooth movement curve and thenthe plateau of the tooth movement curve and then
unnecessary movement of anchor teeth occurs.unnecessary movement of anchor teeth occurs.

CLINICAL SIGNIFICANCECLINICAL SIGNIFICANCE
 An appreciation of the magnitude of the friction isAn appreciation of the magnitude of the friction is
crucial for the orthodontist who employs slidingcrucial for the orthodontist who employs sliding
mechanics during the treatment.mechanics during the treatment.
 With best of the wire -bracket combination atWith best of the wire -bracket combination at
least 40gm of friction must be included in theleast 40gm of friction must be included in the
force applied to initiate movement.force applied to initiate movement.
 High levels of bracket -wire friction my result inHigh levels of bracket -wire friction my result in
binding of the bracket with little or no toothbinding of the bracket with little or no tooth
movement.movement.

 Further binding of anterior tooth under retraction mayFurther binding of anterior tooth under retraction may
lead to “lead to “tent-pegging “tent-pegging “ effect , with the resultant forceeffect , with the resultant force
being optimal for for the movement for thebeing optimal for for the movement for the
movement of the posterior teeth resulting in themovement of the posterior teeth resulting in the
anchor loss.anchor loss.
 The ideal situation is one in which no friction exists.The ideal situation is one in which no friction exists.
 Since this does not occur in sliding mechanics theSince this does not occur in sliding mechanics the
orthodontist must be aware of the magnitude oforthodontist must be aware of the magnitude of
friction in the appliance.friction in the appliance.
 All available measures should be taken care of toAll available measures should be taken care of to
minimize friction in the applianceminimize friction in the appliance

METHODS TO MINIMIZEMETHODS TO MINIMIZE
FRICTIONFRICTION
 Case requiring critical posterior anchorage such asCase requiring critical posterior anchorage such as
those requiring large overjet reduction , a bestthose requiring large overjet reduction , a best
possible combination of bracket and arch wire mustpossible combination of bracket and arch wire must
be selected so as to minimize friction and thusbe selected so as to minimize friction and thus
conserve anchorage.conserve anchorage.
 Stainless steel arch wires in stainless steel bracketsStainless steel arch wires in stainless steel brackets
are considered to beare considered to be goldgold standard for the purposestandard for the purpose
of retractionof retraction
 Stain less steel ligatures should not be tied too tightlyStain less steel ligatures should not be tied too tightly
on premolar bracket during retraction . elastomericon premolar bracket during retraction . elastomeric
ligature must be pre-stretched before ligating. Itsligature must be pre-stretched before ligating. Its
advisable to use self ligating brackets to reduceadvisable to use self ligating brackets to reduce
friction.friction.

Slide and StepSlide and Step
Anushi1.tif

 Retraction should be done with hooks on the anteriorRetraction should be done with hooks on the anterior
segment closer to the centre of resistance so the thesegment closer to the centre of resistance so the the
tipping and binding is minimum therefore decreasedtipping and binding is minimum therefore decreased
frictional resistance.frictional resistance.
 Bumps on the bracket slot walls and floor whichBumps on the bracket slot walls and floor which
reduce the surface contact with the wire , help reducereduce the surface contact with the wire , help reduce
the friction at the bracket wire interface.the friction at the bracket wire interface.

 Since sintered brackets reduce friction by 40% suchSince sintered brackets reduce friction by 40% such
a bracket manufacturing process is advantageous.a bracket manufacturing process is advantageous.
 Ceramic brackets with metal slot should be usedCeramic brackets with metal slot should be used
when going for it. Polycrystalline ceramic bracketswhen going for it. Polycrystalline ceramic brackets
with smoother surfaces should be used . itswith smoother surfaces should be used . its
preferred not to go for ceramic brackets in highpreferred not to go for ceramic brackets in high
anchorage cases.anchorage cases.

 Deflection of second order discrepancies of as littleDeflection of second order discrepancies of as little
as 0.5mm between brackets can significantlyas 0.5mm between brackets can significantly
increase friction. Complete leveling of the arch is anincrease friction. Complete leveling of the arch is an
important factor in reducing friction during sliding.important factor in reducing friction during sliding.
 It should be noted that so much advertisedIt should be noted that so much advertised ββ--
titanium wires produce more friction than stainlesstitanium wires produce more friction than stainless
steel. Thesteel. The ββ - titanium wires form micro welds with- titanium wires form micro welds with
brackets.brackets.
 Accurate bracket positioning right at time of bondingAccurate bracket positioning right at time of bonding
should be done to keep the second ordershould be done to keep the second order
discrepancy at the minimum.discrepancy at the minimum.

CONCLUSIONCONCLUSION
 Sliding tooth movement is a necessary accompanimentSliding tooth movement is a necessary accompaniment
in orthodontic practice. The clinical discipline requiresin orthodontic practice. The clinical discipline requires
complete understanding if its complexities.complete understanding if its complexities.
 Just as the orthodontist today has been provided withJust as the orthodontist today has been provided with
several options of appliance systems from which toseveral options of appliance systems from which to
choose , he must be cognizant of their limitations.choose , he must be cognizant of their limitations.

 The multitude of possible appliance combinations inThe multitude of possible appliance combinations in
sliding systems posses serious challenge in producingsliding systems posses serious challenge in producing
a force system considered in producing the appliancea force system considered in producing the appliance
best suited for the patient.best suited for the patient.
 The level of frictional force must be taken into accountThe level of frictional force must be taken into account
in order to successfully achieve the treatmentin order to successfully achieve the treatment
objectives.objectives.

Friction

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Friction