Friction in orthodontics /certified fixed orthodontic courses by Indian dental academy

Friction in orthodontics

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Contents

Introduction
Components of friction
Active and passive configuration
Critical contact angle
Effect of bracket material on friction
Effect of wire material on friction
Effect of bracket design on friction
Effect of bracket ligation on friction
Effect of arch wire size on friction
Resistance to sliding in leveling and aligning
Resistance to sliding in canine retaction
Effect of saliva on friction
Conclusion

Introduction
The straight wire appliance was the first
orthodontic mechanism that was based on
sliding mechanics .
When one moving object contacts another
tangentially friction at the interface resists the
movement .
Consequently , orthodontists have to apply more
force to overcome the frictional force to achieve
the desirable result, due to which there is more
patient discomfort and pain and also increases
anchorage demands.

•
A.

Factors effecting frictional resistance during
tooth movement.

PHYSICAL

1.
Archwire
. Material
cross section size and shape
surface texture
stiffness
2. ligation of arch wire and bracket
ligture wire
elastomerics
method of ligation
3. Bracket
material
Manufacturing process
Slot width and depth
Design of bracket
Second order angulation
Third order bend

4.Orthodontic appliance
interbracket span
level of bracket slots b/w adjacent teeth
forces applied for retraction
B BIOLOGICAL
1 saliva
2. Plaque
3. corrosion


Two main phenomena that resists sliding
movement are fricton and binding
• FRICTION – is classically described as a force
acting tangentially at the surface of two moving
bodies in contact.
• Friction acts parallel to and opposes the tooth
movement.
• Friction can be static or kinetic.
• Static friction is force required to start the
movement .
• Kinetic friction is the force required to maintain
movement oncewww.indiandentalacademy.com
started.

Components of resistance to sliding (RS)
1 st component – classical friction (FR) –is a product of
co –efficient of friction (µ) and normal force.
Co –eff of friction- it is the objects frictonal proportionality
constant. i.e surface roughness of the material.
N- is the amount of force acting perpendicular to the surface
of the object such as ligation force.on the bracket.


• 2 nd component – Binding
• SECOND ORDER ANGULATON – it is the angle
b/w base Of The wire (vertical dimension of wire)
and the bracket.{θ}.
• Critical contact angle – the level where the wire
contacts both the ends of the bracket slot. .{θc}.
• When the second order angulation increases to
critical angle binding occurs .


• Passive configuration- FR
exsists has an only
component when the arch
wire and bracket have
clearance, in this
angulation b/w arch wire
and bracket is less than
critical angulation.
• Active configurationwhen clearance is
absent and interferance
occurs (θ = θ c) binding
occurs. Under these
conditions two forces
exsists i.e. N and binding

• Factors determining critical contact angle are-

• Slot size
• Bracket width
• Arch wire size

• Engagement index – size/ slot
• Bracket index – WIDTH / SLOT


Bracket
width

Wire
size

Slot size


3rd component - notching


Influence of arch wire and bracket dimensions on sliding mechanics :
derivations and determinations of the critical contact angles for binding
( robert .p. kusy and whitley -1999)

Derived an equation

size/ slot = width/ slot(sin θc)+cos θc.
In this equation size/slot defines the engagement
index .This index defines the fraction of the bracket
slot filled by the arch wire .
width/ slot defines the bracket index – the number of
times the bracket width is more than slot dimension.
Toghether , these two dimensionless indices define all
that is necessary to determine θc as the point at
which the binding starts.
This study derived the maximum and minimum
engagement and bracket indices possible,

• .
For maximum bracket index - large bkt width/ small bkt slot
= size= 250 mil/18 = 13.9
For minimum bracket index - small bkt width/ large bkt slot
size=125mil/22=5.7
The range bracket index is 5.7 to 13.9
For maxi engagement index – large wire size/ small bkt
slot= 16/18=.86
For mini engagement index – small wire size/ large bkt slot=
14x22=.64
The range for engagement index is .5 to1
When the nominal parameters of arch wire and bracket used
in sliding mechanics were estimated for critical contact
angle .three important conclusions were drawn

1) Narrow bracket showed θc double the value
when compared to the wider brackets.
2) Smaller bracket slot showed decreased θc
value .Hence, more precise aligning and
leveling is required before retraction.
3)Smallest wires used for retraction i.e. 16 size
wire in 22 slot, 125mil width . θc =2.8 degrees.
Same wire in 18 slot showed θc =0.9 degrees.
Even in the best scenario the practitionar must
align and level so that the angulation b/w wire
and Bracket is within the range of 1-4 degrees
or else binding increases and sliding ceases.
To accomplish the best scenario most easily
within the strength and stiffness requirements.
The bracket width and wire size should be small
and bracket slot should be larger.

INFUENCE OF BINDING OF THIRD ORDER TORQUE TO
SECOND ORDER ANGULATION (ROBERT P. KUSY
2004)

As base dimensions of arch wire increases, bracket
width decreases, bracket slot size decreases the
critical contact angle for binding decreases.
But when torque is incorporated into the wire ,the
height of the wire is also considered. ( depth of the
wire).

As the torque angle is increased, clearance b/w arch
wire and bracket decreases reducing the critical
contact angle.
Thus, increasingwww.indiandentalacademy.com
the chances of binding.

By incorporating the torque in the
posterior region would help to preserve
the anchorage by increasing the
resistance to sliding in posterior
segment.


stainless steel brackets
• These are the most popular brackets till today.
• Friction is minimum in these brackets due to
smoother surface.
• Sintered stainless steel brackets had low friction
then cast stainless steel brackets due to more
smoother surface texture.


CERAMIC BRACKETS

demonstrated significantly higher
frictional forces than ss
brackets.highly magnified views
revealed numerous small
indentations in ceramic brackets.
monocrystalline alumina brackets
had smoother surface than
polycrytalline bracket,but their
frictional characteristics are
comparable.
since ,greater forces are required
to slide the teeth. Caution in
preserving anchorage must be
exerted in such situations.
ceramic brackets with metal slots
showed decreased friction as wire
is contacting the smoother metal
slot.

Zincornia brackets
• Zincornia brackets has been offered as an
alternative to the ceramic brackets since surface
hardening treatments to increase fracture
toughness are available for zincornium oxide .
However, the frictional co-efficients for these
brackets were found to be greater than or equal
to polycrystalline brackets in both wet and dry
states. Surface changes consisting of wire
debris and surface damage in zincornia brackets
after sliding of wires were observed.

Titanium brackets
Introduced in response to reports of the
corrosion of stainless steel brackets and
increased sensitivity to nickel content of the
alloy.
It is proven to be biocompatible.
It is very rough as the titanium content of the
alloy is increased.
More chance of cold weld formation with the
titanium brackets at bracket wire interface which
increases resistance to sliding.

Plastic brackets
In an attempt to make a esthetic bracket with low
frictional resistance and easier debonding features than
ceramic ,a wide variety of new ceramic reinforced plastic
brackets with or without metal slots were introduced.
several studies showed that when these brackets were
tightly ligated with steel ligatures deformed slightly to
squeeze the bracket slot thereby increasing friction.


Tip edge brackets
Tip edge bracket design- since,
diagonally wedges of bracket is
removed from each side which
allows the slot size change from
0.022 slot to 0.028 slot as the tooth
tips. This bracket design
decreases the resistance to sliding
during tooth movement.


WIRES
Specular reflectance studies have shown that stainless
steel wire have smoothest surface followed by co –cr,
beta–ti, niti in the order of inceasing surface roughness.
Beta titanium wires may form micro-welds in dry states
and further increase the frictional forces.
frank and nicoli found that stainless steel wires had least
friction at non binding sites ,but as angulation increased
and binding was present , the reverse was true.


MECHANICAL AND SURFACE CHARACTERISTICS OF
3 ARCH WIRE ALLOYS. ( VINOD KRISHNAN AND
JYOTHINDRA KUMAR 2002 ).

SEM of STAINLESS STEEL
SS IS STRONG , HAS
SMOOTH SURFACE
SEM of BETA TITANIUM
BETTER LOAD DEFLECTON,
LESS STIFFNESS THAN
SS,ROUGH SURFACE
SEM of TIMOLIUM
IT IS A α AND BETA
TITANIUM, HAS
INTERMEDIATE
PROPERTIES TO SS AND
BETA TITANIUM.

Co-efficient of friction and surface roughness
Kusy et al used laser spectroscopy to study
surface roughness of orthodontic wires . Among
the four wire alloy types that are commonly
used in orthodontics , stainless steel appeared
to be lowest followed by cobalt chromium, beta
titanium and nickel titanium.
kusy and whiteley were the first to look at
the effect surface roughness on friction coefficient . The results showed that low surface
roughness was not sufficient condition for low
friction co efficient.
For ex: beta titanium has decreased roughness
than niti wires but frictional force resistance is
more for beta titanium.

Ion implantation
Gas ions like nitrogen and oxygen are implanted
into the wire surface, resulting in a surface that
is hard and creates a considerable compressive
force at atomic level.
This improves the surface characteristics and
reduce co=-efficient of friction.
Burstone and farzin –nia demonstrated that ion
implanted beta titanium wires produced the
same level friction as stainless steel,

•

BioForce®, Nickel Titanium (NiTi) wires have
transformed the orthodontic profession. The current
trends in orthodontic treatment are low force, reduced
friction, and shorter treatment time. BioForce
uniquely addresses all of these issues, all within one
wire.
• BioForce is the only single strand, superelastic
orthodontic arch wire that provides forces that range
gradually from 80 grams in the central to 320 grams
at the molars: specific biologically determined forces
to move specific teeth.
• All in One Arch Wire. At any point on the wire the
force is near constant.

•

The Quest for a Low Friction System!!

.

The truth is that Friction is not simply a component of the
bracket alone but is instead tied to the interaction of the
bracket slot and the wire within that slot. The wire is as
responsible for Friction as the bracket.
BioForce With Ion Guard will reduce friction in your
current system!!
Cu NiTi Has 78% more friction Than IonGuard.
IonGuard = Greater BioCompatability
Quicker retraction
Similar to Steel smoothness
No Breakage over Wide Spans.
Bio Compatibility / No measurable nickel release

Round vs rectangular wires.
Several studies have found that an increase in
wire size increase bracket wire friction.
Rectangular wires produce more friction than
round wires.
At non binding sites contact area between arch
wire and bracket is an important factor in friction
hence more friction with rectangular wires.
At binding sites with rectangular wire the force is
distributed over a large surface area resulting in
less pressure and less resistance to movement.
At binding sites with round wires the bracket slot
can bite the wire causing indentations resulting
in more friction.

Ligation techniques.
Normal ligation force ranges from 50 to 300 grams.
Edward et al in 1995 compared the frictional forces
produced elastomeric modules applied conventionally or
figure of eight , stainless steel ligatures and teflon coated
stainless steel when used for arch ligation.
Figure of 8 configuration appeared to create highest
friction.
No significant difference between normal conventional
module and stainless steel ligature.
Teflon coated stainsteel had lowest frictional force.
Even the composition of the ligature is another variable
in determining the co_efficient of friction.

As the elastomeric ligatures are polyurethane based
polymers , studies have shown that when exposed to
oral environment they undergo stress relaxation and
hydrolytic decompensation over time which will effect
the properties of the module.
Frictional forces by ligation can be reduced by pre
streching the module , or by using stainless steel
ligatures or using self ligating brackets.
Backing of one quarter turn after tying steel ligatures.


Conventional ties such as O-rings and stainless steel ligatures make
using optimal forces impossible due to friction and binding.
Elastomeric O-rings will lose half their elasticity within days of initial tie
in, thus compromising tooth control.
O-rings are extremely plaque retentive and greatly increase the number
of microorganisms attached to appliances during treatment, increasing
the incidence of decalcification during treatment.

Self ligating brackets

• The first self ligating bracket was the Russel
lock.
• Self ligating brackets are ligatureless bracket
system that have mechanical device built into
the bracket to close off the edgewise slot .
• These brackets show low frictional resistance.
• They are 2 types –
1) Active-spring clip which presses against
archwire.
2) passive- slides which does not press against
wire and produces less friction.
• { This difference in friction is seen only when the
bigger sized arch wire is used.}

Smart clip


•

It Begins with Low Force

•

.* Damon System brackets are
designed to allow the clinician to use
these optimum low forces throughout all
phases of treatment. This is only
possible with a completely passive
system.

•

Damon brackets produce significantly
less friction than conventional or active
self-ligating appliances. By greatly
reducing the amount of friction in the
Damon bracket system, low-force
archwires can work to peak expression,
thereby stimulating a more biologically
compatible tooth movement. Low forces
throughout treatment also mean greater
patient comfort.

Effect of arch wire and material on the resistance to
sliding of self ligating brackets with second order
angulations in the dry state.
( robert p kusy 2002)

The results of this study showed that when the
wire is passive ,the ligation force applied by
slides of bracket was absent.
But when the clips were used the amount of
ligation appeared to be proportional to
dimensions of the wire relative to lumen of the
slot.
When clearance no longer existed , i.e. when the
second order angulations increased more than
critical contact angle . The rate of binding was
independent of www.indiandentalacademy.com
these bracket designs.

A new low force ligation system (jco 2005)
•

•

•

This article describes an
alternative to self ligating
systems a ligature that
markedly reduces the friction
b/w the arch wire and
bracket.
The slide ligature is made of
special polyurethane ,is
applied in the sameway as a
conventional elastomeric
ligature
.Like a passive self ligating it
forms a fourth wall and
allows the archwires to slide
freely in the slot while
transmitting its full force to
the teeth.

•This ligature also forms the buffer b/w the bracket and
soft tissues considerably improving patient comfort.


Sliding mechanics often occurs in one of the 2ways ;
1) During the space closure at the extraction site
( mesio distally).
Dr . kusy

2) Aligning and leveling of the misaligned teeth.
( labio lingually, occlusogingivally, mesio distally)


Importance of wire stiffness and clearance during canine retraction.

Stiffer wires are less springy and deflect less for a given
force .stiffness is proportional to modulus of elasticity
and wire dimensions
The interbracket span is increased due to extractions
which further reduces the stiffness
The retraction force therefore has a greater chance of
deflection and result in buckling and increases friction.
To compensate , the wire size should be increased.
Stiffness of the wire is dependent on vertical dimensions
of the wire ex. 17x25 ( 17 is the vertical dimension.

• Another reason for not using flexible wires during
retraction is that flexible wires can deflect as the canine
crown tips distally which would result in extrusion of the
centrals.
During canine retraction it ia advisable to use 19x25 wire
which provides adequate stiffness and clearance with
acceptable tip control during space closure..
Movement of the brackets along the 0.021x0.025inch
wires ss produced three times more friction than
0.019x0.025 ss wire due to decreased clearance. The
overall means being 3.0N and 1.2N respectively.
An increase in bracket tip from 1 to 3 degrees almost
quadrupled the friction. With 0.021x0.025 wire at 3
degrees tip friction lock was seen and ceased sliding
completely. (Moore and Rock 2004,EJO)

Biomechanical consideration
Effect of point of application of force

Mesial and distal tipping of the teeth is dependent
upon the the location of the force application relative
to the centre of resistance and biological
consideration of the teeth.
Tipping of the bracket produces pressure at the
contact areas b/w arch wires and bracket with a
resultant increase in the frictional resistance.
Farther away the retraction force is applied from the
centre of resistance greater is the moment arm and
greater the tipping and binding.

Aligning and leveling
During the initial stages of treatment when the teeth
might be severly misaligned relative to each other .
Small flexible wires provide low resistance to sliding ,by
increasing the intra bracket space and critical contact
angle, which allows the greater portion of the applied
force available for unraveling the teeth.


Intrabracket space and inter bracket span: critical factors in clinical
orthodontics .(SChudy and SChudy 1989)

•

Intrabracket space- ( slop, play) is the space around the
wire.

•

Inter bracket span – is the amount of wire between two
adjacent brackets.

•

The results also demonstrate several general facts that
the inherent behavior of the wire and the effect of the
appliance modification on that behavior as follows.

•
•
•
•

less force and greater range result from use of
1) smaller wires.
2) narrow brackets.
3) increased intra www.indiandentalacademy.com
bracket space.

They are many positive manifestations of more intrabracket space.
1) less patient discomfort 2) less complex wires 3) fewer arch
removals 4) faster and more efficient leveling 5) reduction of
permanent sets.
The only disadvantage of increased intrabracket span is lose
of torque control.
Round wires used over a longer period of time allow buccal and
lingual rolling of posterior teeth and labial dumping of
anterior teeth.
Advancing to rectangular wire early in treatment makes it
possible to control axial inclinations.
In addition to increased flexibility and range , the intra bracket
span reduces the amount of friction b/w archwire and bracket

Resistance to sliding of stainless steel multistrand
archwires and comparision with single stranded
leveling wires.(rucker and kusy 2002).
Niti wires have low stiffness because the modulus of elasticity is
20% of stainless steel.
Alternatively for the same wire dimensions and material, a
multistrand wire has low stiffness to single stranded wire of ss.
This study compared the frictional behavior of the single stranded niti
and stainless steel and multistranded wires of ss in wet and dry
conditions.
Multistranded wires used were 1.3 stranded round wire
2. co axial wire.
3. 3 stranded rectangular wire.

4. 8 stranded rectangular wire.

Results of this study showed thatIn passive configuration co- eff of friction in presence of saliva was
same as dry state in stainless steel wires, more for multistrand wires
( adhesive effect), and was less with niti wires ( lubricating effect).
In passive configuration the co eff of friction was more for niti wires
than single and multistanded stainless steel due to rougher surface
of niti.
In active configuration has the binding component of the friction
dominates the co eff of friction , Which is dependent on the stiffness
of the wire. Niti single stranded wires show low resistance to sliding
than single stranded wires of ss for the same dimension of wire.
Multistrand wires have low stiffness than niti wires and hence low
resistance to binding .Co axial wtre produced low RS.


INFLUENCE OF CERAMIC BRACKETS AND STAINLESS STEEL
BRACKETS ON NOTCHING OF ARCHWIRES DURING CLINICAL
TREATMENT.(KUSY 2000)

•

Notching – is defined as observed mechanical damage to an archwire
that occurs during later stages of binding which manifests itself as
recognizable defects of varying number, pattern and severity.

•

the direction and movement determines the type of damage and
subsequent appearance of NO p, NO s,No n.

•

Fourteen different defect patterns are only required to describe
notching.

•

Specific wire aspects ( lingual aspect) and anatomical regions ( canines
and incisors ) are more prone to notching.

• Two mechanisms of notch formation are suggested
• 1) fretting- in which the tooth movement is in vertical
plane often causes pairs of shallow parabolic effects .
• 2) sliding- in which the translational movement in the
horizontal plane often leaves a elliptical defect.
•
• The notch activity and severity were nearly three times
more greater in ceramic brackets than in stainless steel
brackets.
• Over 1/3 rd of the notches in ceramic brackets had
severity numbers greater than 3 and penetrated at least ¼
of the wire.

Three main other reasons that interferes
with sliding mechanics
• 1) involuntary irregularities introduced into the
arch wires while construction.

• 2) corrosion of the arch wires and brackets
which effect the polished surface.
• 3) appearance of adherent concretions due to
hard tartar and calculus.

Frictional resistance in orthodontic brackets
with repeated use.( ram.s. nanda 1999)

• This study measured and compared the level
of frictional resistance generated by repeated
and non- repeated brackets to evaluate the
bracket slot will influence the frictional
resistance.
• The results of the study show that there was
a distinct trend for the mean frictional force to
be higher with repeated use of the brackets .
• The grooving and wearing of the brackets
and mechanical interlocking at the bracket
wire interface www.indiandentalacademy.com frictional
resulted in higher
resistance.

vibration

Vibration is a motion that creates forces.
chewing food bolus requires forces and creates
vibrations which are of low frequency and high
magnitude.
Clenching of teeth produces vibrations of high
frequency and low magnitude.
These vibrations that stimulate teeth can impede
motion or facilitate it.
Eg. These vibrations can help an arch wire to jump
out of notched wire region occurred due to
excessive angulations.
These are undeterminable.

Effect of saliva on kinetic friction

• Saliva serves as an excellent lubricant in the sliding of
the bracket along the wire.
• Kusy et al 1991 found that saliva could be lubricous as
well as adhesive behaviour depending upon which
archwire bracket combination is used .
• The kinectic co eff of friction of the beta titanium wires in
wet state were 50% of the vaues in the dry state. When
sliding through stainless steel wire , titanium rich oxide
layer in beta titanium wires break down , adheres and
reacts breaks away , resulting in stick-slip phenomenon.


Conclusion
Sliding mechanics is a necessary accompaniment in
orthodontic practice .The clinician requires complete
understanding of its complexities. The clinician must
consider bracket material, design and wire alloy size
,material ,as well as resistance factors .The multitude of
possible appliance combinations in sliding mechanics
posses a serious challenge in producing a force system
that is optimal for tooth movement. Mechanical and
biological factors must be considered in producing the
appliance best suited for the patient. The level of force
systems must be take into account the frictional force levels
in order to successfully achieve the treatment objectives.

Thank you


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