Role of friction in orthodontics /certified fixed orthodontic courses by Indian dental academy

Role of Friction in
Orthodontics
www.indiandentalacademy.com
INDIAN DENTAL ACADEMY
Leader in continuing dental education

Contents
Introduction
Static & dynamic friction
Laws of friction
Sliding mechanics
effect of bracket
archwire
ligation
biological factors

What is FRICTION ?
A force resisting the relative displacement
of two contacting bodies in a direction
tangent to the plane of contact .
Because of friction ,part of the mechanical
energy intended for movement of the two
bodies relative to each other is dissipated
as thermal energy

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

It is the force that resists the movement
of one surface past another and acts in a
direction opposite the direction of
movement.
Friction may exist between :
- Two solid surfaces
- Solid fluid interface
-Liquid/fluid layers

Types of friction
1. Rolling or sliding
2. 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

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.

Frictional coefficient(µ ) –the law of friction
theorized by coulomb states that 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 :
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.

. 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 (µ) i.e f = µ 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 bcoz of this the true contact
area is to a considerable extent deter
mined by the applied load and is directly
proportional to it

Surfaces with relatively large asperity wavelengths may
be considered smooth but not flat, whereas, surfaces
with relatively short asperity wavelengths may be
considered flat but not smooth. With preparation
methods such as machining, milling, grinding, or
lapping, peak height and wavelength may vary

Frictional force is independent of the
sliding velocity (v) i.e the so called
coulomb’s 3rd law.

Orthodontic tooth movement
during space closure is achieved
through two types of mechanics :-

Friction
FrictionFree

Friction Mechanics (Sliding Mechanic)
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.

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 www.indiandentalacademy.com

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.

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 f parallel to the contact surface of
nominal area.

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.www.indiandentalacademy.com

. Assuming that the normal force F (ie,
load) remains constant, the coefficient
of static friction increases as a function
of time

Kinetic 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).

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 the periodontium.
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.
mechanotherapy to move a tooth via a
bracket relative to a wire results in friction
localized at the bracketwire 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.

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 :-
-       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 MOVEMENT
A) 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

) 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)
f.     Second order bend (angulation)
gThird order bend (torque

4. Orthodontic appliance
a. Interbracket distance.
b.Level of bracket slots between adjacent
teeth.
c. Forces applied for retraction.
B. BIOLOGICAL
1. Saliva
2.Plaque.
3.Acquired pellicle.
4. 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.
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 orthodontic wire 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
AND 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

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
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)

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 anchorage
because 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
avaliable

) 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.

Trade Name
Company Type
E.g: Silkon American Orthodontics Plastic reinforced with ceramic
Spirit Ormco Plastic with metal insert
Image GAC Plastic reinforced with glass
Clarity Unitek Ceramic with metal insert
Cermaflex TP Metal with plastic base

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 and without 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
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 Width and Interbracket Distance (IBD)
Bracket width is closely related to IBD
Narrower the bracket
↓
Greater the length of interbracket wire
↓
Greater the flexibility of wire
↓
Decrease in stiffness of wire
↓
Greater chance of binding with more flexible 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 archwirewww.indiandentalacademy.com

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)

Begg brackets—haveachieved low friction by
virtue of an extremely loose fit betweena round
archwire and a very narrow bracket, but this is at
thecost of making full control of tooth position
correspondinglymore difficult. Some brackets
with an edgewise slot have incorporatedshoulders
to distance the elastomeric from the archwire
and,thus, reduce friction, but this type of design
also producesreduced 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.
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 ARCHWIRE
) 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.

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.
β-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 β-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 surface resulting 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.
Burstone and Farzin Demonstrated that ion
implanted β-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)
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 interbracket 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 increased to
compensate for decrease in stiffness when
interbracket 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.
eg 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.
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.
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 metafasix technology (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

eg.Synergy bracket:
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.
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,
Damon sl 2 with vertical
slide

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 increases the ‘slop’ between the rectangular wire and the slot, 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 morepredictablylow and the
reciprocal forces correspondingly smaller
Lower net forces deflect archwires less
and, therefore, facilitaterelease 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.
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

SSWIRES: Showed an adhesive behaviour with
saliva and a resultant increased in coefficient of
friction in wet state.
β-TI wires: In wet state kinetic coefficients of
friction were 50% of the values in dry state.when
sliding through SS brackets ,the titanium rich
oxide layer in β-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.
.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

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 atleast 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 maintaining high force levels and a
M/F ratio that produces distal tipping only.

Thank you
For more details please visit

Role of friction in orthodontics /certified fixed orthodontic courses by Indian dental academy

Recommandé

Recommandé

Contenu connexe

Tendances

Tendances (20)

En vedette

En vedette (11)

Similaire à Role of friction in orthodontics /certified fixed orthodontic courses by Indian dental academy

Similaire à Role of friction in orthodontics /certified fixed orthodontic courses by Indian dental academy (20)

Plus de Indian dental academy

Plus de Indian dental academy (20)

Dernier

Dernier (20)

Role of friction in orthodontics /certified fixed orthodontic courses by Indian dental academy