FRICTION.doc

7/27/2019 FRICTION.doc

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FRICTION

A force resisting the relative displacement of two contacting bodies in a

direction tangent to the plane of contact is called friction.

It is the force that resists the movement of one surface past another and acts

in a direction opposite the direction of movement. Because of friction, part

of the mechanical energy intended for movement of the two bodies relative

to each other is dissipated as thermal energy

Friction may exist between:

Two solid surfaces

Solid fluid interface

Liquid/fluid layers

Types of friction

Rolling or sliding

Static or dynamic

When two surfaces in contact slide or tend to slide against each other,two components of total force arise:-

Frictional component

Normal force

Normal force:-

Perpendicular to one or both contacting surfaces and also to the

frictional component.

Fixed surface on which the block rests responds only to the weight of

the block with an upward force, perpendicular to the plane contact area.

This force is symbolized by N.

Also signifies the force pushing two surfaces together


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Frictional force :-

Parallel in the direction to the intended or actual sliding motion and

opposes the motion.

2. Static and Dynamic Friction :-

Resistance that precludes actual motion is termed static friction.

That which exists during motion is called Dynamic friction.

Both static and dynamic forms of sliding friction are of orthodontic interest.

Static friction is the component of frictional force that has to be overcome

to initiate motion

Dynamic (kinetic) friction is the component of frictional force that has to

be overcome to maintain motion .

Static frictional force is usually higher than dynamic frictional force

Frictional coefficient( )the law of friction theorized by coulomb statesthat the magnitude of the frictional force F is equal to the product of normal

force N acting perpendicular to the contact surface multiplied by frictional

coefficient

The frictional coefficient depends on the surface roughness of the

combination of the materials involved .it does not depend on the area of

contacting surfaces and varies only slightly with velocity of movement

Coefficient of static friction:

It reflects the force necessary to initiate movement.

Coefficient of Kinetic friction:-

Reflects the force necessary to perpetuate the motion.

It takes more force to initiate motion than to perpetuate it.


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LAWS OF FRICTION :

As early 17th and 18th centuries, Amontons and coulomb were formally

investigating frictional forces. From their efforts, fundamental laws of

friction evolved :-

a) Frictional force (f) is proportional to the applied normal force (N)

multiplied by the coefficient of friction () i.e f = N.

b) Frictional force (f) is independent of the apparent area of contact b/n two

sliding surfaces.

This is because all surfaces, no matter how smooth, have irregularities

that are large on a molecular scale and real contact occurs only at a limited

number of small spots at the peaks of surface irregularities

ASPERITIES :

Spots called Asperities, carry all the load b/n two surfaces. Even

under light loads,, local pressure at the asperities may cause appreciable

plastic deformation of small areas beacuse of this the true contact area is to a

considerable extent determined by the applied load and is directly

proportional to it

C) Frictional force is independent of the sliding velocity (v) i.e the so called

coulombs 3rd law.

Orthodontic tooth movement during space closure is achieved through

two types of mechanics:-

1. Friction

2. Friction free or frictionless


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Friction Mechanics (Sliding Mechanics) - It involves either :-

Moving the brackets along an archwire

Sliding the archwire through brackets and tubes.

ROLE OF FRICTION IN SLIDING MECHANICS :

Most fixed appliance techniques involve some degree of sliding

between bracket and archwire.

When sliding mechanics are used, friction occurs at the wire bracket

interface. Some of the applied force is therefore dissipated as friction and the

remainder is transferred to supporting structures of the tooth to mediate tooth

movement.

Maximum biological tissue response occurs only when the applied

force is of sufficient magnitude to adequately overcome friction and lie

within the optimum range of forces necessary for movement of the tooth

in vitro resolution of static and kinetic frictional resistance into

separate and distinct phases is arbitrary and potentially misleading because

at low velocity, such as exists in orthodontics, static and kinetic frictional

resistances are dynamically related.

The first two laws are usually obeyed in orthodontics whereas the

third usually is not.

In wire/bracket couples of stainless steel/stainless steel and NiTi/ stainless

steel, the third law is obeyed; however for Co-Cr/SS and B-Ti/SS couples,

the values of u slightly increase and markedly decrease respectively with

velocity.


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If a stationary mass (M) is at equilibrium (at rest with zero velocity

V0) on a solid flat surface (S), the contact between the mass and the surface

is the result of a normal force (F) acting on the mass

The normal force F may be the net system force or the weight of

mass M. The interfacing area between mass M and surface S may be

approximated as the nominal area by the macroscopic surface area of M in

contact with S To overcome the static frictional resistance from the rest position, a

minimum pulling or shear force (f), which is parallel to the contact surface

of the nominal area, is required to move mass M at velocity V. This static

frictional resistance (fs) is equal to the normal force or load (F) multiplied by

a coefficient of static friction .Once a steady sliding motion of a constant

velocity (Vc) is achieved, then a minimum force to overcome the kinetic

frictional resistance (fk) is required to maintain velocity Vc of mass M. The

kinetic frictional resistance is equal to the normal force or load (F)

multiplied by a coefficient of kinetic friction (fk).

Static friction (occurring instantaneously up to the onset of sliding)

and kinetic friction (occurring continuously after the onset of sliding) are

two distinct phases that by definition cannot coexist. The classic Amontons-

Coulomb laws relate static and kinetic friction as follows:

1. s and k are independent of F and area;

2. s and k are materials dependent; and

3. usually k < s.


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The two distinct frictional phases (ie, fs and fk are defined by zero

velocity Vo and constant sliding velocity Vc, respectively.

The transition from zero velocity Vo to constant sliding velocity Vc

must involve an acceleration of mass M. The acceleration phase is of

particular interest when Vc approximates Vo (ie, low velocity friction).

Refinement of the Amontons-Coulomb principles is required

especially when it becomes evident that Vc approximates Vo. Apparent

deviation from the Amontons-Coulomb principles includes

Time-dependency of the static coefficient S(t) (ie, static coefficient as

a function of time) and Velocity-dependency of the kinetic coefficient K(V) (ie, kinetic

coefficient as a function of velocity).

The Stick-Slip Phenomenon

At low speeds a Stick - slip phenomenon may occur as enough

force builds up to shear the junctions and a jump occurs, then the surfaces

stick again until enough force again builds to break them.

A single stick-slip cycle involves a stick state associated with elastic

loading of the system, followed by a sudden slip corresponding to stress

relaxation.

Static Coefficient as a Function of Time {s (t)}

The static coefficient of friction varies as a function of increasing

time t before the onset of sliding.

Increases in the coefficient of static friction, as a function of stick

time vary over a wide time interval range

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 fparallel to the

contact surface of nominal area.


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Age strengthening of localized point contacts among the asperities of

mass M and surface S is correlated with experimental observation of slow

plastic deformation occurring at the stressed asperities, leading to an

increasing effective interface area as a function of time

When plastic deformation occurs at the level of the softer asperities,

the frictional force becomes a function of shear stress localized to point

contacts among surface asperities of mass M and surface S As a result,

increases in both area and shear produce a proportionate increase in f.

Assuming that the normal force F (ie, load) remains constant, the

coefficient of static friction increases as a function of timeKinetic Coefficient as a Function of Velocity

The second refinement of the Amontons-Coulomb principles is that

constancy of the kinetic frictional coefficient is dependent on maintenance

of a steady sliding velocity Vc. Different materials exhibit unique kinetic

frictional characteristics as a function of velocity

Within very low velocity ranges, most materials exhibit decreasing

coefficients of kinetic friction as the low-velocity range increases (ie,

velocity weakening).

Subsequent velocity strengthening as velocity progressively increases

beyond the low-velocity range.

At low velocity, such as occurs with in vivo tooth movement, steady

sliding instability may lead to oscillations of motion characterized by cycles

of sticking and slipping.

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).


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Stick-slip processes are caused when the frictional force does not

remain constant as a function of some other variable such as distance, time,

or velocity

Orthodontic evidence of repetitive stick-slip oscillations at the

archwire-bracket interface may be inferred from scanning electron

micrographs that reveal permanent deformation of archwires subjected to

intermittent binding and sliding at bracket surfaces.

Loss of Applied Force

Orthodontic tooth movement is dependent on the ability of the clinician to

use controlled mechanical forces to stimulate biologic responses within theperiodontium. It has been concluded that the rate of tooth movement

increases proportionally with increases in applied force up to a point, after

which additional force produces no appreciable increase in tooth movement.

With orthodontic mechanotherapy, a biologic tissue response with

resultant tooth movement will occur only when the applied forces

adequately overcome the friction at the bracket wire interface.

Mechano therapy to move a tooth via a bracket relative to a wire

results in friction localized at the bracket wire interface that may prevent the

attainment of an optimal force in the supporting tissues.

The portion of the applied force lost because of the resistance to

sliding can range from 12% to 60%.

If frictional forces are high, the efficiency of the system is affected,

and the treatment time may be extended or the outcome compromised

because of little or no tooth movement and/or loss of anchorage.

The amount of frictional resistance will impact on the moment to-

force ratios of the teeth and, consequently, their centers of rotation.


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When the archwire and the bracket have clearance, classical friction

exists as the only component to the resistance to sliding.

When clearance disappears and an interference fit occurs between the

bracket and the arch wires, binding arises as a second component to the

resistance to sliding superimposed on the classical friction.

Movement of the crown mostly precedes displacement of the root

because a tipping moment is placed on the crown of the tooth

This tipping leads to increased friction from binding between the

archwire and bracket restricting movement of the entire tooth.

Friction is influenced by :-

- the nature of contacting surface ,but is independent of area of contact, this

is due to the interlocking of surface irregularities

-the extent to which asperites on the harder material plough into the surface

of the softer material

Total frictional resistance is the sum of

-Force necessary to shear all junctions

-Resistance caused by interlocking roughness

-Ploughing component of total frictional force

Orthodontic Model of friction

BETWEEN ARCHWIRE AND BRACKET

Sliding friction is generated between arch wire and bracket when

The wire guides the bracket during M-D movement of an

individual tooth.

The wire is slipped through posterior crown attachment in, e.g:-

Retraction of anterior dental segment.

Possible Components of this force are :-


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Engagement of arch wire in brackets that are out alignment.

Ligatures pressing the wire against base of slot

Active torque in rectangular wire

Bodily tooth movement in which tipping tendency is resisted by two

point contact between the bracket and archwire.

The relative magnitudes of these components of frictional force vary

according to the clinical situation

VARIABLES AFFECTING FRICTIONAL RESISTANCE DURING

TOOTH MOVEMENTA) PHYSICAL :-

1) Archwire

a. Material

b. Cross sectional shape/size.

c. Surface texture.

d. Stiffness.

2) Ligation of archwire to bracket

Ligature wires.

Elastomerics

Method of ligation : Method of tying, bracket designs to limit force of

ligation, self ligating brackets

2) Bracket

a. Material

b. Manufacturing process : Cast or sintered stainless steel.

c. Slot width and depth

d. Design of bracket : Single or twin

e. First order bend ( in - out)


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f. Second order bend (angulation)

g. Third order bend (torque

Orthodontic appliance

Interbracket distance.

Level of bracket slots between adjacent teeth.

Forces applied for retraction.

B. BIOLOGICAL

Saliva

Plaque.

Acquired pellicle.

Corrosion

With so many variables affecting frictional force, it is difficult to accurately

determine them in a clinical situation. the problem is further complicated by

wide array of brackets, wires and ligatures available that provide a multitude

of combinations for use during various stages of orthodontic treatment.

EXPERIMENTAL METHOD USED TO STUDY FRICTION :

1. SIMULATED TOOTH MOVEMENT:

Most of studies within orthodontic literature have carefully simulated

different clinical conditions b/n bracket and archwire to measure sliding

frictional resistance.2. SURFACE ROGHNESS :

Some studies have quantified surface roughness of various bracket

and archwire materials.


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Most common method of estimating surface roughness

SPECULAR REFLECTANCE involves determination of amount of light

that is reflected back from a surface.

Smooth surface :- Reflects much of light shone on it in a narrow

pattern.

Rough surface :- Scatters light and reflects it back in amore dispersed

pattern.

3. CONTACT FLATS :-

Coefficients of friction have also been evaluated using orthodonticwire held between two parallel plates (Contact flats) made of material

similar to that used in orthodontic brackets such as SS, polycrystalline

alumina or Teflon various levels of normal force were applied to plates and

wire is pulled through them to measure friction generated.

4. DESCRIPTIVE STUDIES :-

These have involved discussion of frictional resistance of brackets

and wires based on clinical experience and anecdotal information.

EFFECT OF BRACKET MATERIAL, DESIGN, MANUFACTURING

PROCESS ON FRICTION:-

Various Bracket materials today available are :-

1. Stainless steel - Cast Sintered

2. Ceramic brackets

Polycrystalline alumina

Single Crystalalumina (SCA) (i.e Sapphirc)

3. Zircoma brackets

4. Plastic brackets


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1. Stainless Steel Brackets :-

Most popular bracket material Stainless steel brackets are associated

with lowest frictional force values amongst the available bracket materials.

Kapila et al (1990)

Evaluated friction b/n Edgewise SS brackets and orthodontic wires of

4 alloys (SS, Co- Cr, NiTi and B-Ti)

Mean frictional forces with conventional cast stainless steel brackets

ranges between 40-336 g.

Level of frictional forces observed in :-

0.018inch SS brackets Ranged from 49g with 0.016 inch SS wires

in narrow single brackets to 336g with 0.017 x 0.025 inch B-Ti wires wide

twin brackets.

0.22 inch SS brackets Friction ranged from 40g 0.018 inch SS

wires in narrow brackets to 222g with 0.019 x 0.025 inch NiTi wires in wide

brackets.Several SS bracket wire combinations generated low levels of

frictional forces less than 100g.

SINTERED STAINLESS STEEL BRACKETS

Sintering : Process of fusing individual particles together after compacting

them under heat and pressure.

Sintering allows individual bracket to be premolded in a smooth

streamlined manner. The SS particles are compressed in a contoured smooth

rounded shape as apposed to older casting procedure in which milling or

cutting process left sharp angular brackets that were bulky and rough.

Sintered edgewise brackets


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RMO Mini Taurus. (RMO Mini Taurus Synergy

Unitek Mini Twin.)

Sintered SS brackets produce significantly lower friction than cast

stainless steel brackets overall friction of sintered SS brackets is approx 40%

- 45% less than friction of conventional cast stainless steel brackets.

2) CERAMIC BRACKETS :

With ceramic brackets, most of wire size and alloy combinations with

both 0.018 and 0.022 inch slot sizes demonstrate significantly higher fric-

tional forces than with SS brackets.

Characteristics of Ceramic Bracket Material or Slot Surface Texture:

Highly magnified views have revealed numerous generalized small

indentations in the ceramic bracket slot while the SS bracket appeared

relatively smooth.

Hardness of the material

All currently available ceramic brackets are composed of Aluminium oxide.

Aluminium oxide is extremely hard.

The rough but hard ceramic material is likely to penetrate the surface

of even a steel wire during sliding, creating a considerable resistance and

this is worse with titanium wires

The interaction of metal wire - ceramic slot interface leads to leveling

of ceramic slot. This results in drop in friction as ceramic peaks are removed

and valleys become clogged with metal

Types of Ceramic brackets :

Single Crystal alumina (SCA)

Polycrystalline alumina (PCA)


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Monocrystalline alumina :- Single crystal ceramic brackets are

derived from large single crystals of Alumina which are milled into desired

shape and dimensions by ultrasonic cutting, diamond cutting or combination

of two techniques. Because Alumina is third hardest known material, this

procedure is difficult and may explain granular and putted surface of

ceramic brackets seen in SEM.

Polycrystalline brackets - have also been observed under SEM to

possess very rough surfaces which actually scribed grooves into the archwire

Monocrystalline brackets were observed to be smoother than PCA

brackets but their frictional properties were comparable.

The most apparent difference b/n polycrystalline and single. crystal

brackets is their optical clarity. Single crystal brackets are noticeably clearer

than PCA brackets which tend to be translucent

Clinical significance :-

Combination of metal archwires and Ceramic brackets produce high

magnitudes of frictional force; therefore greater force is needed to move

teeth with ceramic brackets compared to SS brackets in sliding mechanics.

Since ceramic brackets on anterior teeth are often used in

combination with SS brackets and tubes on premolar and molar teeth,

retracting canines along archwire may result in greater loss of anchoragebecause of higher frictional force associated with Ceramic than SS

brackets.To reduce frictional resistance Ceramic brackets with smoother slot

surfaces and consisting of metallic slot surface are available.


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ZIRCONIA BRACKETS :

Besides high friction, Ceramic brackets have very low fracture

resistance Due to their brittle nature even smallest crack or flaw can

propagate rapidly through the material.

Zirconia brackets have been offered as an alternative to ceramic

brackets since surface hardening treatments to increase fracture toughness

are available for Zirconium oxide.

Frictional coefficients of Ziconia brackets were found to be greater

than or equal to those of poly crystalline alumina brackets in both dry and

wet states (Keith et al , 1994). Surface changes consisting of wire debris and

surface damage in Zirconia brackets after sliding of archwires were also

observed.

PLASTIC BRACKETS :

In an attempt to create an esthetic bracket with lower frictional

resistance and easier debonding features than ceramics a varity of new,

ceramic reinforced plastic brackets with or without metal slot inserts have

been introduced.

Plastic brackets can deform because of compression from ligation and

thus binding of the wire, and higher frictional resistances were recorded than

stainless steel brackets. Recently introduced composite brackets with andwithout metal slots faired better in friction studies showing lower frictional

resistance than both ceramic and stainless steel brackets in one of the

studies.

EFFECT OF BRACKET WIDTH ON FRICTION


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Effect of bracket width on friction has been controversial

Some studies have found that altering bracket width made no

difference in friction (Peterson et al 1982, Andereasen et al, 1970).

Frictional resistance has been reported to increase with increase in

bracket width (Tidy 1989, Drescher et al, 1989).Whereas others found that

frictional resistance decrease as bracket width increased.

Franks and Nikolai (1980):- Related greater friction with wider

brackets to the fact that binding occurs frequently with wider brackets.

Omana et al suggested that with a narrow bracket the tooth could tip

considerably before binding could occour,and once binding occurs it was of

severe nature

Kapila et al (1990 and Ogata et al (1996) :

Suggested that with a wider bracket the elastomeric ligature used was

stretched more than with a narrower bracket which exerted a greater normal

force on the wire

Bracket slot size may not influence the frictional resistance,

Some studies suggested that frictional resistance decreased as slot

size increased from 0.018 inch to 0.022 inch because of reduced binding

probably from increased wire stiffness. And also because of the increased

play in the slot with final archwire.

ADDITIONAL DESIGN FEATURES IN BRACKETS TO REDUCE

FRICTION

Bumps on the bracket slot walls and floor which decreased surface

contact with the wires, help decreased friction in bracket wire interface.

(Ogata etal)


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Begg bracketshaveachieved low friction by virtue of an extremely

loose fit betweena round archwire and a very narrow bracket, but this is at

the cost of making full control of tooth position correspondingly more

difficult. Some brackets with an edgewise slot have incorporated shoulders

to distance the elastomeric from the archwire and,thus, reduce friction, but

this type of design also produces reduced friction at the expense of reduced

control.

EFFECT OF SECOND ORDER DEFLECTION OF FRICTION

Second order defection of wire b/n brackets held in series can have

significant effects on brackets wire friction.

Several studies have found that increasing the angulation between

bracket and wire produced greater friction.

Frank and Nikolai (1980) :- Found that frictional resistance increased in a

nonlinear manner with bracket angulation.

With brackets out of alignment archwire stiffness, strongly influences

forces normal to the points of contact and hence friction.

In a well aligned arch forces that result from archwire deflection are

not important and friction is largely independent of archwire stiffness.

Ogata et al (1986)Evaluated the effects of different bracket wire

combinations and 2nd order deflections on kinetic friction. The brackets

were offset deflecting wire in increments of 0.25 mm.

As 2nd order deflection increased frictional resistance increased for

every bracket wire combination - With lower deflections a smooth sliding

phase appeared in which friction increased in approximate a linear manner.

As deflection increased further a binding phase occured in which

friction increased at a much greater rate and was not necessarily linear.


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Binding generally occured between 0.75 and 1.00 mm of 2nd order

deflection.

The relationship between frictional resistance and second order

angulation may not be linear and may become more important as the

angulation increases. The active configuration for binding occurred between

3 to 7.

When tipping occurs the frictional resistance of nickel-titanium has

been reported to be less than stainless steel,

Because of the lower modulus of elasticity of nickel-titanium

compared with stainless steel, lower normal force that was induced by

binding occurred resulting in less resistance to sliding.

Active third order torque with rectangular wires would increase the

friction even more.

Similarly, greater friction with larger rectangular wires results from

the possible introduction of torque because an 0.021x 0.025 wire has 3.9

of play compared with an 0.018 x0.025 wire that has 14.8 of play when

engaged into an 0.022 bracket slot

EFFECT OF ARCH WIRE

WIRE ALLOY:-

The role of wire alloy in frictional characteristics of sliding

mechanics has been extensively studied.

Most studies have found SS wires to be associated with the least

amount of friction and Beta titanium with the most. from lowest to highest

friction SS, Co - Cr, NiTi and B-Ti.


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Frank and Nikolai (1980) :- Found that SS wires had less friction

than nickel titanium at non binding angulations, but as the angulation

increased and binding was present reverse was true.

SURFACE TEXTURE :-

Specular reflectance studies have shown that SS wires have the

smoothest surface followed by Co-Cr, B-Ti and NiTi wires in order of

increasing surface roughness.

Since B-Titanium had the most friction but was not the roughest Kusy

and Whitly concluded that one cannot use surface roughness as an indicator

of frictional characteristics in sliding mechanics.

NiTi has greater surface roughness Beta Ti has greater frictional

resistance.

As the titanium content of an alloy increased its surface reactivity increases

and surface chemistry is a major influence on frictional behaviour.

-Ti at 80% Titanium has higher coefficient of friction than NiTi at

50% titanium. There is enough titanium reactivity for wire to COLD

WELD itself to steel bracket and therefore -Ti wire exhibits more of stick

slip phenomenon.

ION IMPLANTATION :-

Alteration of the surface of titanium wires by implantation of ions

into the surface.

Gas ions (Nitrogen and Oxygen) are implanted in to the wire surfaceresulting in a surface that is extremely hard.

Ion implantation produces no interface b/n the coating and the wire

neither does it alter the dimensions of the wire.


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Burstone and Farzin Demonstrated that ion implanted -Ti wires

produced about the same level friction as SS wires.

Braided arch wires

Berger (1990) Studied friction produced by 0.0175 inch braided

archwire in a 0.022 slot and found very high friction levels.

1.5 times compared to 0.018 inch round SS wire with elastomeric

ligation. 5 times with stainless steel ligation.

This can be attributed to interwoven pattern and irregular surface of

Braided arch wire.

Mechanical interlocking of the archwires with the edges of the

bracket slot increase friction as the wire moves relative to the bracket.

Efforts to reduce friction with teflon coating are being made

WIRE SIZE

Several studies have found that an increase in wire size is to be

associated with increased bracket wire friction .

The main reason for the increase in friction as the wire size increased

can be attributed to an increase in the stiffness of the wire.Wires of greater

stiffness will create a greater normal force with binding of the archwire with

the edges of the bracket.

Rectangular wires produce more friction than round wires. At

nonbinding angulations the contact area between bracket and archwire is

important factor in friction and would therefore expect more friction with

rectangular wire. (Nanda)

Placement of a rectangular wire can dramatically increase the friction

because of the concomitant increase in wire stiffness.(tidy)


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At greater angulation of the bracket, the determining factor is the

point at which the wire contacts the edge of the bracket

With round wires bracket slot can bite into the wire at one point

,causing an indentation in the wire

With rectangular wire the force is distributed over a large area i.e the

entire faciolingual dimension of the wire, resulting in less pressure and

therefore less resistance to movement

Frank and Nikolai (1980)Found that an 0.020 inch wire was

associated with more friction than 0.017 x 0.025 inch wire.

ROLE OF WIRE STIFFNESS AND CLEARENCE

Mechanically speaking orthodontic archwires are elastic beams

supported at either one or both ends.

Wire stiffness depends upon :-

Diameter or cross section of the wire

Length of beam.

e.g. Doubling length of cantilever beam decreases stiffness by 8 times.

By altering inter-bracket distance stiffness of wire can be altered.

During canine retraction in a premolar extraction case the increased inter-

bracket span of the unsupported wire over the extraction site decreases the

stiffness of wire.

Retraction force therefore has a greater chance of deflecting the wire

resulting in buckling. To prevent such deflections of the wire that may

increases friction and chances of bracket binding, diameter of wire should be


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increased to compensate for decrease in stiffness when inter bracket span is

greater than normal.

Another reason for not using flexible small size archwires during

sliding canine retraction is that flexible small size archwires can deflect as

canine crown tips distally which can lead to incisor extrusion.

CROSS SECTIONAL DIMENSION IN DIRECTION OF BENDING

0.017 x 0.022 inch wire placed edgewise is more springy in vertical

dimension than when placed in ribbon mode

Drescher et al (1989) Stated that vertical dimension of the wire was an

important factor in frictional resistance

NATURE OF END SUPPORTS OF A BEAM

Rigidly supported beam at both ends has stiffness 4 times as

compared to cantilever beam.Therefore During sliding space closure the

wire therefore should be tied into the supporting brackets tightly to increase

stiffness.

e.g. During canine retraction premolar and lateral incisor brackets should be

tied tightly to the archwire

CLEARANCE OF ARCH WIRE :-

Adequate clearence should be provided between bracket and wire to

prevent binding. Clearance or play in 2nd order i.e tipping depends upon slot

size, Bracket width, Archwire size

3RD ORDER PLAY IN RECTANGULAR ARCH WIRES.

In 0.018 slot 16.7 for 0.016 x 0.016 wire 4.5 for 0.017 x 0.025

wire.


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0.022 27.4 for 0.016 x 0.022 inchwire.

2 for 0.0215 x 0.028 wire.

EFFECT OF LIGATION TECHNIQUE ON FRICTION:-

The normal force exerted by ligature has a significant influence in

determining the frictional resistance developed within an orthodontic

appliance.

Ligation technique signifies the force that pulls the wire into the

bracket.

Elastomeric modules:

Affected by the oral environment.

Demonstrate stress relaxation with time

Stainless steel ligatures:

Can be tied either too tight or too lose.

Properties of an ideal ligation system

Be secure and robust;

Ensure fullbracket engagement of the archwire;

Exhibit low friction betweenbracket and archwire;

Be quick and easy to use;

Permit highfriction when desired;

Permit easy attachment of elastic chain;

Assist good oral hygiene;

Be comfortable for the patient

Edwards et al (1995) : Compared the effect of 4 ligation techniques.

E modules tied conventionally and in figure 8 pattern.

Stainless steel ligatures.

Teflon coated ligatures.


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Static frictional resistance greatest in figure of 8 emodules ; -

No significant differences between frictional resistance offered by

conventionally tied E-modules and steel ligature.teflon coated ligatures

produce lowest friction.

Shivapuja et al (1994) : E-modules produced greater frictional resistance as

compared to steel ligature ties. This combined with rapid rate of decay for

these E-modules and their predliction for harboring large quality of plaque

suggests little merit in their use especially in sliding mechanics

NEW SLICK ELASTOMERIC MODULE SYSTEM :

A new slick elastomeric module system incorporating metafasixtechnology (TP orthodontics) has recently been introduced claims to

combine ease of use with low friction.

The new slick E - modules reduced friction by upto 60% compared

with their regular counterparts when tied normally.

BRACKET DESIGNS LIMITING FORCE OF LIGATION:

Three brackets were introduced to restrict the amount of force placed

on wire by the ligature.

American friction free bracket (Am. Orthod)

GAC shoulder bracket (GAC central I slip NI)

RMO synergy bracket (RMO)

These brackets generated lower mean frictional forces at 2nd order

deflections of 0.00 & 0.25 mm than conventionally ligated brackets.

E.g. Synergy bracket:


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Includes 6 wings 3 on each side of bracket slot. The lateral wings

may be included in ligation for correction of rotation of teeth but only center

wings may be ligated during sliding mechanics to reduce force of ligation.

SELF LIGATING BRACKETS:

Orthodontic brackets are now available that possess the feature of self

ligation. First Edgewise self ligating bracket Russelhock (1946).

SELF LIGATING BRACKET SYSTEMS : LIGATIONS SYSTEM

* Edgelok bracket (Ormco) 1972 Sliding cap

* Speed bracket (Strite industries) - 1980 Spring clip

* Activa bracket (A company) - 1986 Lever arm.

Damon 1994 vertical slide

Berger (1990) Compared between speed bracket and stainless steel bracket

revealed that friction with self ligating bracket was between 12% and 23%

that of stainless steel bracket irrespective of wire shape and ligation

technique.

The unique anatomic characteristic associated with speed bracket -

highly resilient and flexible spring clip was determined to be causative factor

in critically lowering level of applied force.

Shivapuja et al (1994)

Self ligating brackets displayed significantly lower level of friction

both static and dynamic as compared to conventional ligating system.

Significantly less chair side time was required for archwire removal

and insertion with self ligating system as compared to conventional ligating

systems.


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Kapur found dramatically lower frictionwith both stainless steel and

nickel-titanium wires for Damonbrackets compared to conventional brackets

Advantages of self ligating system

More certain fullarchwire engagement;

Low friction between bracket and archwire;

Less chairside assistance;

Faster archwire removal and ligation

Gac in ovation bracket similar in design to speed bracket with sliding

spring clip.

The clip places a diagonally

directed lingual force on the wire, which

does not contribute to any third order interaction between the wire corners

and the walls of the bracket slot, which is the origin of torquing force

This increasesthe slop between the rectangular wire and theslot, and also

reduces the moment arm of the torquing mechanism.

Activa bracket with clip Clip retaining groove is visible on the

gingival surface.

With low friction, the net tooth-moving forces are more predictably

low and the reciprocal forces correspondingly smaller.

Lower net forces deflect archwires less and, therefore, facilitate

release of binding forces between wire and bracket, enhancingsliding of

brackets along a wire.

BIOLOGICAL FACTORS:

I. Effect of saliva on kinetic friction:

It has been suggested that saliva substitute serves as an excellent

lubricant in sliding of the bracket along the wire.


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BAKER ET AL (1987):

Using an artificial saliva substitute found a 15% - 19% reduction in friction.

KUSY ET AL (1991):

Found that saliva could have lubricious as well as Adhesive

behaviour depending on which archwire bracket combination was under

consideration

SS WIRES: Showed an adhesive behaviour with saliva and a

resultant increased in coefficient of friction in wet state.

-TI wires: In wet state kinetic coefficients of friction were 50% ofthe values in dry state.when sliding through SS brackets, the titanium rich

oxide layer in -Ti archwire breaks down, reacts, adheres and breaks away

,resulting in a stick-slip phenomenon.

Hypothesis: Saliva probably acts by preventing solid to solid contact.

CLINICAL SIGNIFICANCE:

In an adult patient: H/O of Xerostomia or decreased salivary, Oral

radiation therapy, Anticholinergic medication.

- Should be noted as possible factors in varying force levels necessary to

move teeth.

Surface Characteristics Affecting Friction

Most metals are subject to oxidation and an associated oxide layer

growth. Friction between specific sliding metallic surfaces significantly

decreases with proportionate increases in oxide layer thickness, although

sliding characteristics in the presence of an oxide layer vary from material to

material.


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Bioflims may reduce the coefficient of friction by producing a

boundary lubrication effect through salivary protein adsorption and plaque

accumulation.

CLINICAL SIGNIFICANCE OF FRICTION

Based on information gathered from studies of friction several points of

clinical significance can be identified.

An appreciation of magnitude of friction is crucial for the

orthodontist who employs sliding mechanics during treatment - With best of

wire bracket combinations atleast 40g of friction must be included in force

applied to initiate tooth movement. New bracket designs and manufacturing techniques have been

introduced to decreased the amount of friction generated between wire and

bracket slot.

- Sintered SS brackets.

- Bracket designs limiting force of ligation

Self ligating brackets

Clinicians using esthetic tooth colored brackets - important to know

the level of friction generated by these brackets before initiating tooth

movement

Selection of wire shapes and sizes:

e.g.: 0.018 inch SS steel best choice for canine retraction in 0.022 slot.

- If overall torque control is required:

0.016x 0.022 inch - 0.018 slot.

0.019x0.025 inch - 0.022 slot.

Archwire can also be thinned down in region distal to canine so as to further

facilitate movement. Care must be taken not to over reduce the wire

dimensions which could decreased strength of wire


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Complete leveling of arch - important factor in reducing friction

during tooth movement. e.g.: Space closure using 0.019 x 0.025 wire in

0.022 slot.

Before starting space closure rectangular wires need to be place for at least 1

month.

- To ensure proper leveling and freedom from posterior torque pressure.

Sliding mechanics can proceed smoothly

- To optimize use of sliding mechanic sufficient time must be allowed for

distal root movement to occur.

- A common mistake is to change the E-chain too often thus maintaininghigh force levels and a M/F ratio that produces distal tipping only.

Documents

FRICTION.doc