محمد الخولاني-اليمن

Egypt, Mansoura University
Faculty of Dentistry
Orthodontics Department

Friction in clinical orthodontics now is receiving much attention
because orthodontic companies have decided that low friction is
good and are using that concept to market their self-ligating
brackets.
Sometimes low friction can be important, as in retracting
a tooth along a continuous archwire or in consolidating
space; sometimes high friction is needed, as in closing loop
mechanics, anchorage, and 2-couple systems (torquing arch).
Often friction is not an issue, as in a 1-couple
system (intrusion or extrusion arch) or for repositioning
an impacted tooth with a cantilever.
This presentation evaluates friction in the context of resistance to
sliding of brackets along an archwire or an archwire through
brackets, when friction is just 1 component of the total resistance.

WHAT IS THE FRICTION?
Friction is the resistive force between surfaces
that opposes motion. It is not a fundamental
force, because it is derived from electromagnetic
forces between atoms.

There are 2 types of friction: static and kinetic. Static friction
opposes any applied force. Its magnitude is exactly what it
must be to prevent motion between 2 surfaces, up to the
point at which it is overcome and movement starts.
Kinetic friction, which usually is less than static friction,
then opposes the direction of motion of the object.

• For all practical purposes, kinetic friction is irrelevant in
orthodontic tooth movement because continuous motion
along an archwire rarely if ever occurs. In sliding
mechanics, we are dealing with a quasi-static
thermodynamic process, which means that the process
happens slowly and goes through a sequence of states
that are close to equilibrium.
• Forces and resistance to sliding change as the tooth
moves down the wire, tips, has a biologic response,
uprights as bone remodels around the root, and then tips
again.
• In orthodontic tooth movement, friction (static or kinetic)
results from the interaction of an archwire with the sides
of an orthodontic bracket or a ligature. Friction is only a
part, and usually a small part, of the resistance to
movement as a bracket slides along an archwire.

Kusy and Whitley divided resistance to sliding (RS) into 3
components: (1) friction, static or kinetic (FR), due to contact
of the wire with bracket surfaces; (2) binding (BI), created
when the tooth tips or the wire flexes so that there is contact
between the wire and the corners of the bracket (when a force
is applied to a bracket to move a tooth, the tooth tips in the
direction of the force until the wire contacts the corners of the
bracket, and binding occurs); and (3) notching (NO), when
permanent deformation of the wire occurs at the wire-bracket
corner interface. This often occurs under clinical conditions.
FRICTION (FR) BINDING (BI) NOTCHING (NO)

When teeth slide along an arch wire, force is needed for two purposes:
to overcome frictional resistance, and to create the bone remodeling
needed for tooth movement. Controlling the position of anchor teeth is
accomplished best by minimizing the reaction force that reaches them.
Use of unnecessarily heavy force to move teeth creates problems in
controlling anchorage. Unfortunately, anchor teeth usually feel the
reaction to both frictional resistance and tooth movement forces, so
controlling and minimizing friction is an important aspect of anchorage
control.

To retract a canine by sliding it along an archwire (in this case
with 100-g distal force), conventional wisdom dictates that
additional force, beyond what is required to move the tooth, is
necessary to overcome friction (in this case also assumed to be
100 g). Some authors suggest that additional frictional force
increases loading on anchor molar to value equal to canine
retraction force plus frictional force (in this case to 200 g) and,
consequently, increases molar anchorage loss.

When one moving object contacts another, friction at
their interface produces resistance to the direction of
movement.
The frictional force is proportional to the force with
which the contacting surfaces are pressed together and is
affected by the nature of the surface at the interface
(rough or smooth, chemically reactive or passive,
modified by lubricants, etc.).
Interestingly, friction is independent of the apparent
area of contact. 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 the surface
irregularities (Figure).

These spots, called asperities, carry all the load between the two
surfaces. Even under light loads, local pressure at the asperities
may cause appreciable plastic deformation of those small areas.

• If a force is applied to a canine from a chain elastic or a coil
spring, the tooth will not feel the full force if there is friction
in the appliance. What the tooth feels is the effective force
(FE), not the applied force (FA):
FE= FA – Frictional force (FF)
• When the frictional force is the same as the applied force,
the tooth will feel no force from the spring. As long as there
is frictional force, effective force is always less than the
applied force. Of course, it is the effective force that is
relevant for the clinician.

The
nature of friction is still being debated between
adhesion and interlocking theory, even among
modern physicists; however, classic friction
theory tells us that forces perpendicular to the
archwire are responsible for friction. Figure-a
shows a canine sliding along an archwire. For
simplicity, all moments are ignored.

The coefficient of friction is not an inherent property of a material, such
as modulus of elasticity. It is a dimensionless property that represents the
amount of friction between two materials and is determined by
experiment only and not by theory. If the material used at the interface of
two materials reduces the coefficient of friction, it is called a lubricant. If
it increases the coefficient of friction, it is called an adhesive. For a
stainless steel wire and stainless steel bracket in the mouth, an average
value for the coefficient of friction (µ) is 0.16. The magnitude of normal
force can be unpredictable because of the many variables, including three
material interfaces that can be present: wire, bracket, and polymeric O-
ring. Suppose a 50-g normal force is applied to a bracket. The frictional
force can be calculated, and the effective distal force is 92 g.

The coefficient of friction is the lowest with stainless steel wires
and the highest with beta-titanium wires.
. It is
often assumed that the smoother the material, the lower is the
coefficient of friction; however, the relationship is not so
simple. If the forces are high, destructive changes can occur in
either the bracket or the wire, changing the subsequent
behavior. Examples include wire notching, as depicted in Fig . A
tipped tooth can notch a wire, producing effects not easily
predicted.

Some surface treatments, such as ion impregnation
by nitrogen bombarding, increase the hardness and
reduce the coefficient of friction of a wire. Figure
below shows a group of beta-titanium archwires; the
various colors are produced after titanium nitride
particles are distributed in the wire’s surface by ion
impregnation.
Colored -TMA
wires

Doshi et al (AJO-DO 2011) investigated the static frictional
resistance between 3 modern orthodontic brackets—ceramic
with gold-palladium slot, ceramic, and stainless steel—and 4
archwires (0.019×0.025-in)—stainless steel, nickel-titanium,
titanium-molybdenum alloy (TMA), and low-friction colored
TMA. They reported that frictional values for colored TMA were
comparable with SS wires and thus seem a good alternative to SS
wires during space closure in sliding mechanics.
SEM microphotographs showing A. colored TMA , B. TMA
Vs
BA

Frictional forces are evident at all stages of orthodontic
treatment. They involve any mesiodistal sliding between
wire and bracket. This occurs not only with purposeful
sliding mechanics such as canine retraction but also in
alignment arches where, if the wire cannot slide, buccal or
lingual forces can be attenuated.

Forces perpendicular to the wire can come from a
number of sources and in any direction: buccal,
lingual, occlusal, or apical (Fig a). In the passive wire,
the O-ring produces a lingual force in (Fig b)that can
lead to a frictional force. Thus, the ligation method is
only one source of friction.
Any other forces required for tooth movement, if
perpendicular to the archwire, can also lead to
friction and in many situations can produce much
more friction than the ligature tie.
a b

Of particular importance are forces originating from pure
moments or couples. By definition, couples are equal and
opposite forces not in the same line of action. Normal forces
exist on the wire, although the sum of the forces is zero (Fig
below). Moments are used in a first-order direction to rotate
teeth, in a second-order direction to change axial mesiodistal
inclinations, and in a third-order direction to change
buccolingual axial inclinations. A moment (couple) at the
bracket is required to give an equivalent force system for full
control of a tooth. This moment is one major source of friction
with the edgewise appliance.

Some brackets are designed to allow a tooth to tip
or rotate. With this type of bracket, this source of
friction can be eliminated, but control of tooth
movement is lost as a result.
(a) Even in a low-friction self-
ligating bracket, frictional force
operates at the distal of the
bracket in a mesial direction. (b)
The frictional force produced a
side effect that opened up space,
and the crown moved mesially.
In clinical situations, forces on
the wire are a major source of
friction, not just the ligature tie.

During canine retraction, the canine rotates
distal in, and the crown tips distally. The
archwire elastically deforms and, during
recovery, prevents or minimizes the rotation
(a) and tipping (b) by exerting couples on the
teeth. (c and d) The same diagram with the
couples (curved arrows in a and b) replaced
by two normal forces (arrows) to further show
the origin of the frictional force.

To figure out how much frictional force occurs during canine
retraction, we must consider the phase of canine retraction as
evaluated from both the facial and occlusal views.
Four phases can be recognized. After a distal force is placed,
the canine may have play between the wire and the bracket,
and initially the tooth will display uncontrolled tipping.
This is phase I. No moments or normal forces operate in this
plane. For now, ligation forces are ignored. The tooth
continues to tip more, and the play is eliminated.

Increasing moments are created by the elastically deformed
wire, and a controlled tipping phase is produced (phase II).
Perhaps we have a tipping center of rotation at the apex.
Note that normal forces are produced in phase II as the
tipping is being minimized, but only low levels of friction are
produced. When the tooth tips some more and a sufficiently
high moment is delivered by the wire, translation occurs
(phase III).

The greatest frictional forces are produced during
translation. During phase IV, as the force is reduced, no
more distal sliding occurs, and the axial inclination is
corrected. Here, of course, there is a high frictional force
that is acceptable because sliding is not desired at this stage
(see also Fig 14-9).

In short, frictional force varies depending on the
stage of canine retraction: none initially with play
and the highest levels during translation. Even with
rigid edgewise arches, a retracted tooth will go
through these four phases; however, the angle of tip
will be smaller.
The angle of tip during translation is mainly a
function of wire stiffness and the applied distal
force. Clinically, it may appear that the tooth has
translated in one phase. In reality, however, it has
first tipped, then translated, and then finally
uprighted. Ligation forces and forces in other planes
are considered separately in this .
As the bracket width decreases, the friction will
increase because the normal force must increase to
provide the same amount of moment.

However, the mechanism of narrow brackets (eg,
Begg bracket) is different. They produce only a single
force and negligible frictional forces because they do
not prevent tooth tipping (no control moments) and
do not demonstrate phases II, III, and IV of space
closure.
In Begg treatment, a separate individual root spring
is used for tooth uprighting during phase IV.

From the facial view, frictional forces are developed because the CR is
apical to the bracket. In a similar evaluation from the occlusal view, the
bracket is labial to the CR and, hence, a distal force will rotate the
canine distal in. The archwire prevents or minimizes canine rotation in
four phases (Fig 19-16).
During phase I, if play exists between the wire and the bracket, the
canine is free to rotate. No wire restraining of the rotation occurs;
therefore, there is no friction in this phase in the occlusal view. During
phase II, the tooth continues to rotate; however, the archwire is
minimizing the rotation by elastic deformation. Because of the
restraining archwire moments, friction increases and finally reaches its
maximum during phase III translation. No sliding occurs in phase IV
when the rotation is being corrected.

The amount of frictional force from the occlusal view
depends on the perpendicular distance of the bracket to the
CR. The greater this distance, the larger is the moment
rotating the canine and the greater is the moment needed
from the archwire to prevent this rotation.

It has been seen that moments associated with the
prevention of tipping and rotation of a canine can lead to
high frictional forces.
In addition, third-order moments (ie, torque) can lead to
particularly high frictional forces. Figure(a and b)compares
two activations on a canine; both have the same moment
magnitude of 1,000 g mm, but one is in the bending mode
(Fig- a), and the other is in the torsion mode (Fig-b).

The torque produces the largest vertical force of 2,000 g because
the distance is small across the wire cross section. Because the
normal forces from torque are greater than those from the
second-order couple, the friction will be eight times higher in
torque than tipping for the same moment. (In this example, the
ratio of the moment arms is 4 mm/0.5 mm = 8; hence, the
normal force is eight times greater.) For this reason, it is not
recommended to use edgewise wires that fully engage the
brackets (with possible unwanted torque) for canine retraction.
The high friction can potentially make for inefficient or
unpredictable retraction. Round or undersized wires are
preferable to eliminate possible unwanted torque problems.

Let us consider two bracket design parameters: (1) method
of ligation and (2) bracket width. A wire can be placed
passively into a bracket, and a ligature or locking
mechanism holds it in place. No force is exerted on the
tooth, and the tie function is purely restraint (Fig - a). In (Fig
–b), the tie mechanism activates the wire, producing an
active force for desired tooth movement. Displacing the
ligature tie with more force will cause the wire to more fully
seat in the bracket. After the wire is fully seated, a greater
ligature tie force does not increase the force to move the
tooth (Fig -c).

The added perpendicular force will only produce a frictional
force that most likely is not required or wanted. This friction
from tight ties is sometimes used to keep teeth from sliding.
Normal force from metal ligature ties are difficult to control
if predictable ligating forces are to be achieved. Elastomeric
O-rings can deliver initially higher forces than a lightly tied
metal ligature wire.
However, elastomers will undergo degradation (or
relaxation) over time, making the ligation force
unpredictable; after degradation, their normal forces may be
as low as some self-ligating brackets. If one only considers
friction from ligation, so-called self ligating brackets do have
the advantage of more predictably delivering lighter
restraining forces (forces at 90 degrees to the archwire) and,
hence, lower friction.

Both active and passive self-ligating systems can produce lower
normal forces by ligation alone than elastomeric rings or metal
ties. On the other hand, after degradation, elastomers can
deliver low tie forces; also, some clinicians are very adept at
forming light metal ties. If the frictional forces are known, they
can be overridden.
It should be remembered that, during treatment, the
orthodontist applies forces perpendicular to the arch during
wire placement and that it is these forces that can produce the
most friction during sliding mechanics; self-ligating brackets
are not an exception. The same forces are required for
delivering the correct force system with self-ligating brackets as
with more traditional brackets; hence, friction is similar.

Narrow brackets may show faster tooth movement initially; therefore, it
may be assumed to have less friction, but this concept is wrong. The tooth
movement in this case is not directly related to the friction. The reason
narrow brackets seem to show initial faster tooth movement during
sliding mechanics is due to the play between the bracket slot and the wire
in phase I of sliding mechanics (Fig a & b). With the same amount of play
(clearance) between the bracket and the wire, the narrow bracket can tip
(rotate) more during phase I of space closure. In this phase, the friction
comes only from the normal force ligature mechanism. To find the
frictional force, we must use a moment (couple) thatproduces vertical
forces.

where FF is frictional force, N is normal force, M is moment at the
bracket, and W is bracket width.
Figure 19-23 compares two brackets: a narrow 2-mm bracket and a wide
4-mm bracket. Let us suppose both teeth need a counterclockwise
moment of 1,000 gmm for translation. The narrow bracket requires
equal and opposite 500-g forces (500 g × 2 mm = 1,000 gmm), and the
wide bracket needs 250-g forces (250 g × 4 mm = 1,000 gmm). The
narrow bracket has twice the frictional force because the normal force is
two times that of the wide bracket. Therefore, the wide bracket has less
friction during phases II and III of space closure.

Smaller cross-section wires may have more clearance between the wire
and the bracket and therefore may have an extended phase I (no
friction). Also, these wires have lower wire stiffness and associated
lower normal forces during other phases of canine retraction. But
remember that the lowerfriction found in small round wires is not
caused by the smaller contact area.

It could be theorized that vibration in the mouth could
relieve some frictional forces. This certainly is a commonly
observed phenomenon in laboratory friction.
Liew et al has shown a 60% to 85% reduction of frictional
force using O-rings and round wire. O’Reilly et al also
demonstrated a 19% to 85% friction reduction in both
rectangular and round wires.

Different phenomena may operate to reduce the magnitude of
friction. The horizontal component of occlusal forces can
produce lateral tooth displacement that can loosen the ligature
tie or O-ring. Thus, vibration or tooth displacement could be an
important factor in eliminating the frictional force from the
ligation mechanism.
The frictional forces produced in response to tipping during
sliding of a tooth along an archwire are an entirely different
phenomenon, because it is the elastically bent wire that
produces the normal forces, not the force from ligation.
Occlusal forces may not relieve the friction unless the chewing
force is placed in a direction to temporarily reduce the normal
force between the wire and the bracket.

This suggests once again that friction from the ligation
mechanism may not be as important as friction from tooth-
moving forces—the forces from the elastically bent wires.
One of the main advantages of a self-ligating bracket is that
the ligation mechanism produces less normal force in the
passive state of the wire. This advantage may be minimized
because vibratory forces seem to be successful in reducing
friction from conventional ligature ties or O-rings.

Patients could have identical brackets, malocclusions, and wires and
still not have the same frictional forces based on anatomical
variation in root length and alveolar and periodontal support.
only consider the translation phase during canine retraction for the
four teeth. To translate the teeth, a force must be placed through the
CR (yellow arrows). That force is usually replaced at the bracket
level with a force and a couple (red arrows). The magnitude of this
couple is the force times the distance from the bracket to the CR.
Thus, the greater the M/F ratio, the higher are the vertical normal
forces that create the frictional force.

The tooth in Fig- a is a typical tooth with average periodontal support as a
reference. The CR is away from the bracket; therefore, a high M/F ratio at
the bracket is required. This moment produces much friction, as discussed
in this chapter. The teeth in Figs b and c have shorter roots, with their CRs
closer to the bracket. Here, the M/F ratios are low with subsequent low
frictional force. Root resorption (see Fig c) is certainly unwanted, but it does
have the advantage of minimizing the friction produced at the level of the
bracket. The tooth from an adult showing alveolar bone loss (Fig d) has the
largest distance to the CR and would have the greatest friction during
translation. Clinically, the tooth might not move so rapidly by translation,
and we would be disappointed in the response. We might blame the poor
response on the age of the patient and biologic factors, but perhaps the
greater frictional force is the real culprit.

The applied force can be placed more apically by an
extension arm or by an equivalent force system at the
bracket from an additional wire or spring. Apical levers and
lingual placement of the force can readily be utilized. The
spring to store and release energy can be part of the canine
retraction spring and its apical extension. To eliminate or
minimize the friction from canine retraction, rotational
forces from a chain elastic or a coil spring can be attached
on the lingual surface of the canine (Fig 19-33).

If an auxiliary retraction spring
or loop is used, activations can be
placed to minimize tipping and
rotation during canine retraction
three-dimensionally so that the
sliding archwire can deliver a
smaller frictional force. An
archwire is still present to give
positive control with minimal
friction (Fig 19-34).
En masse space closure requires
sliding of the archwire at the
posterior brackets. Because the
mesial force is buccal to the CR of
the posterior teeth, molars tend
to rotate mesial in (Fig 19-35).

The use of a buccal archwire can barely
prevent this side effect, and friction will
be produced. Lingual or transpalatal
arches can preserve arch form without
producing friction from a wire observed
in the occlusal view.
Finally, space closure can be
accomplished without sliding or friction
mechanics by a so-called frictionless
spring. In Fig 19-37, canine retraction
springs were used. All needed anti-tip
and antirotation moments are bent and
twisted into the springs.
No sliding on an archwire is required.
With sliding mechanics, the required
moments are obtained by perpendicular
normal forces from the archwire
inevitably producing friction. With
frictionless springs, the same forces and
moments may be required and are
present, but because no sliding occurs,
there is no friction.

Frictional forces can be present and
influence results at all stages of
treatment from leveling to finishing.
Two effects that occur with lighter
alignment arches merit mention.
Frictional forces produce a
component of force that is parallel to
the archwire.
Sometimes this is good and other
times bad. The positive effect of
mesiodistal forces due to friction
is the opening of space for tooth
alignment.

Many patients have moderate crowding, and an increase of
arch length is desirable. If the wire is not free to slide, the
wire will open space by pushing teeth laterally, causing an
increase in arch length.
It is a well-known principle that teeth cannot be aligned or
rotated unless there is enough space for them. Because there
are limitations in the ability of a main archwire to
sufficiently increase arch length, auxiliary or secondary
wires such as coil springs, intrusion arches, and bypass
arches can be used to increase arch length. If there is
adequate space, low friction in an archwire is desirable.

The negative effect of friction during leveling
is that the wire may not be free to slide
mesially or distally through the brackets;
therefore, the desired buccal forces are not
free to express themselves.
Longitudinal frictional forces prevent
deactivation of the wire (Fig-a). Large lateral
deflections of the wire that cannot be
recovered to the original shape because of
friction necessitate removal and reinsertion
of the archwire. Friction at the canine and
the first molar prevents the wire from fully
deactivating (Fig-b). If full deactivation does
not occur and the wire does not slide
spontaneously, it can be removed and retied.
Leaving a wire in place to deactivate it can
open space and relieve the offending
friction; however, these mesiodistal forces
may not be efficient or wanted.

A reverse articulation of the maxillary lateral incisor is treated by a
nickel-titanium (Ni-Ti) overlay wire (Fig). If the ligature is too tight, the
Ni-Ti wire cannot fully deactivate. It is important to allow sliding at the
tie (green arrows in Figs 19-41a to 19-41c). Note that the overlaid Ni-Ti
wire has a hook on each side (blue arrows in Figs 19-41a to 19-41c) and
that the elastics are activated with light force in the direction of the axis
of the wire.
The mesiodistal forces to the wire will thereby unlock the friction and
will allow full labial force expression to the lateral incisor (Figs 19-41c
and 19-41d). Another approach is to remove the Ni-Ti overlay and retie
the ligature to eliminate the unwanted longitudinal forces.

Tying of archwires into irregular teeth can either increase the arch
length or reduce the arch length, even when identical forces are
applied, because of friction.
To simplify this explanation, let us consider a cantilever force system
with a single force delivered at the free end. Figure 19-42 shows an
intrusion arch with a V-bend placed anterior to the molar tube.
Its configuration after initial intrusion will also produce flaring of the
incisors; however, let us not consider this effect. We could assume
that the intrusion force is acting at the CR of the anterior teeth.
Because it is a cantilever, the location of the V-bend is not very
important. It can be placed at many locations further anteriorly
along the intrusion arch to produce identical intrusive forces; yet
different configurations would produce varying amounts of
horizontal force from the described friction effect during
deactivation.

The nature of friction in orthodontics is multi-
factorial, derived from both a multitude of
mechanical or biological factors. Numerous
variables have been assessed using a variety of
model systems with nearly equally varying
results

Variables affecting frictional resistance in orthodontic
sliding mechanics include the following:
1. Physical/mechanical factors such as:
i) Archwire properties:
a) material, b) cross sectional shape/size, c) surface texture, d) stiffness.
ii) Bracket to archwire ligation:
a) ligature wires, b) elastomerics, c) method of ligation.
iii) Bracket properties:
a) material, b) surface treatment, c) manufacturing process, d) slot
width and depth, e) bracket design, f) bracket prescription (first-
order/in-out; second-order/toe-in; third order/torque).
iv) orthodontic appliances:
a) interbracket distance, b) level of bracket slots between teeth, c)
forces applied for retraction.

2. Biological factors such as:
a) saliva,
b) plaque,
c) acquired pellicle,
d) corrosion,
e) food particles.

Wire material: Most studies have found stainless steel
wires to be associated with the least amount of friction. This
is further backed up by specular reflectance studies which
show that stainless steel wires have the smoothest surface,
followed by Co-Cr, ß-Ti, and NiTi in order of increasing
surface roughness.
SEM microphotographs of archwires (1000 times
magnification): A, SS; B, NiTi; D,TMA.

Kusy & Whitney (1990) investigated the
correlation between surface roughness &
frictional characteristics. They found Stainless
steel to have least coefficient of friction & the
smoothest surface. However ß- titanium showed
greater friction compared to Ni Ti , though the
latter was rougher. Hence they concluded that
surface roughness cannot be used as an indicator
of frictional characteristics in sliding mechanics.

The reason why ß-titanium has a higher
coefficient of friction than Ni-Ti is because of its
higher titanium content (79W/W%), which results
in increased adherence or cold welding of wire to
bracket slot.
found that Stainless Steel
had less friction than NiTi at nonbinding
angulations but as angulation increased & binding
was present, SS showed more friction.

Nishio et al (AJO-DO Jan 2004) performed an in
vitro study to evaluate frictional forces between
various archwires & ceramic brackets. They found
that ß-titanium showed the highest frictional force,
followed by NiTi & SS wires. They suggest that elastic
properties of the wire are secondary & surface texture has more
influence on frictional force.
Zufall & Kusy (Angle Orthod. 2000) Studied the
sliding mechanics of composite orthodontic
archwires with a coating of polychloro-p-xylene.
The coating eliminated the risk of glass release from
the wire. Also frictional & binding coefficients were
within the limits outlined by the conventional
orthodontic wire-bracket couples.

Wire Size: Several studies have found an increase in wire
size to be associated with increased bracket-wire friction. In
general, at non-binding angulations, rectangular wires produce
more friction than round wires. However, at binding
angulations, the bracket slot can bite into the wire at one point,
causing an indentation in the wire.
However, with a rectangular wire, the force is distributed over
a larger area ie. the facio-lingual dimension, resulting in less
pressure & less resistance to movement. This may account for
the finding of Frank & Nikolai that an 0.020” wire was
associated with more friction than the .017 x .025” wire.

Wire stiffness & clearance: Mechanically
speaking, orthodontic wires are elastic beams,
supported at one or both ends. A force applied on
such an elastic beam causes a deflection, which is
reversible within elastic limit of the material.
Stiffer wires are less springy & deflect less for a
given force.
Changing the diameter or the cross-sectional
greatly changes the stiffness. Doubling the
diameter of a wire increases the stiffness by a
factor of 16, when supported at one end, & by a
factor of 4, when supported between two
brackets.
Doubling the length of a cantilever beam
decreases stiffness by a factor of 8.

During canine retraction in a premolar extraction
case the increased inter-bracket span of 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, which may increase friction &
chances of bracket binding, the diameter of wire
should be increased to compensate for decrease in
stiffness when interbracket span is greater than
normal.

Yet another reason for not using flexible, small-size
archwires during sliding canine retraction is that flexible
archwires can deflect as the canine crown tips distally, which
could result in incisor extusion. This situation can be
axacerbated with the use of preadjusted canine brackets with
a built-in distal root angulation.
For rectangular wires, stiffness is also dependent on cross
sectional dimension in the direction of bending. In other
words an 017 x 022” wire is more springy in the vertical
direction when it is placed edgewise rather than flatwise.

Drescher et al (AJO-DO 1989) stated that friction depends
primarily on the vertical dimension of the wire. An 016”
stainless steel round wire and an 016 x 022” stainless steel
rectangular wire showed virtually the same amount of
friction. This was however, lower than that for 018X025”
wires.
The authors state that for mesiodistal tooth movement,
rectangular wire is preferred because of its additional
feature of buccolingual root control.

As the stiffness of a beam is dependent on the
support at both ends of the beam, during canine
retraction, the premolar and lateral incisor brackets
should be tied tightly to archwire. This will increase
the stiffness of the wire as well as increase friction in
the premolar bracket, thus minimizing anchorage
loss.

An adequate clearance should be provided between the
bracket and the wire to prevent binding. The clearance or
play in the second order, i.e., tipping, depends on a
combination of slot size, bracket width, and archwire size.

Third order play for rectangular wires in an 0.018 inch slot
range between 16.7 for 0.016× 0.016 inch wire, to 4.5º for
0.017x0.025 inch wires. For the 0.022 inch slot, third order
play ranges between 27.4º for the .016 × 0.022 inch wire to 2º
for 0.0125×0.028 wire.
Since rectangular wires produce significantly higher friction than round
wires, the authors recommend the use of 0.018 inch wires in the 0.022
inch slot during space closure and canine retraction. The round wire
results in less friction, and the 0.018 inch diameter provides adequate
stiffness, reducing the buckling tendency of the wire.

Marques et al (Angle Orthod. 2010), investigated the debris,
roughness and friction of stainless steel archwires following
clinical study. S.S rectangular wires exposed to the intraoral
environment for 8 weeks. They showed a significant
increase in the degree of debris and surface roughness,
causing an increase in friction between the wire and bracket
during the mechanics of sliding.
SEM images (200×) showing debris on the wires. (A)
Score 0. (B) Score 1. (C) Score 2. (D) Score 3.

Orthodontists today have a multitude of
options when it comes to selecting a
bracket. In the edgewise design itself, there
are choices in slot size, bracket width,
number of wings, presciption in
preadjusted designs, ligation capabilities,
and bracket material. The most popular
bracket material remains stainless steel;
however, conventional cast S.S has met its
competitor in the sintered variety.

Angle used a gold prototype of edgewise brackets
over 75 years ago. In 1933, Dr. Archie Brusse
presented a table clinic on the first stainless steel
appliance system. Since then SS brackets have
displaced gold. Because they were stiffer & stronger,
SS brackets could be made smaller, in effect
increasing their esthetics via their reduced
dimension. Their frictional characteristics were so
satisfactory that they are today’s standard of the
profession. However, conventional cast stainless
steel has met its competitor in the sintered variety.

Stainless steel, cobalt-chromium, nickel-titanium, and B-
titanium archwires were ligated with elastomeric ligatures
into sintered stainless steel brackets (Mini-Taurus, Rocky
Mountain Orthodontics, Denver, CO); Miniature Twin
(Unitek Corp, Monrovia, CA) and conventional stainless
steel brackets. The friction of sintered stainless steel
brackets was approximately 40% to 45% less than the
friction of the conventional stainless steel brackets. The
authors attribute this difference to differences in surface
texture of the brackets, but they do not provide any
evidence of this.

The technology of sintering, the process of fusing
individual particles together after compacting them
under heat & pressure allows each bracket to be
premolded in a smooth streamlined manner. The SS
particles are compressed in a contoured, smooth,
rounded shape as opposed to the older casting
procedure in which the milling or cutting process
left sharp, angular brackets, which were bulky and
rough.

Investigations comparing these two
varieties with various archwire sizes at the
Univ. of Oklahoma revealed that for most
wire sizes, sintered stainless steel brackets
produced significantly lower friction than
cast SS brackets. (up to 38.44% less
friction.) This difference in frictional
forces could be attributed to smoother
surface texture of sintered S.S material.

With ceramic brackets, most of the wire size and alloy
combinations with both 0.018 and 0.022 inch slot sizes
demonstrated significantly higher frictional forces than with S.S
brackets.
This difference in friction between St.St and ceramic brackets
may be attributed to characteristics of the ceramic bracket
material or slot surface texture.
Highly magnified views have revealed numerous generalized
small indentations in the ceramic bracket slot, while the S.S
bracket appeared relatively smooth.

Monocrystalline ceramic brackets are derived from
large single crystals of alumina, which are milled
into the desired shape and dimensions by
ultrasonic cutting, diamond cutting, or a
combination of both techniques.
Polycrystalline ceramic brackets have also been
observed under SEM to possess very rough
surfaces, which actually scribed grooves into the
archwires.

The monocrystalline allumina brackets were
observed to be smoother than polycrystalline ones,
but their frictional characteristics were comparable.
The combination of metal archwires and ceramic
brackets produce high magnitude of frictional forces;
therefore, greater force is needed to move teeth with
ceramic brackets compared with St.St brackets in
sliding mechanics.
Since ceramic brackets on anterior teeth are often
used in combination with St.St brackets and tubes on
the premolar and molar teeth, retracting canines
along an archwire may result in greater loss of
anchorage because of the higher frictional force
associated with ceramic than steel brackets. Greater
caution in preserving anchorage must be exerted in
such situations.

Clinical significance:
Since ceramic brackets on anterior teeth are often
used in combination with stainless steel brackets
and tubes on premolar and molar teeth, retracting
canines along an archwire may result in greater loss
of anchorage because of higher frictional force
associated with ceramic than steel brackets. Greater
caution in preserving anchorage must be exerted in
such situation.

Plastic brackets first appeared in around 1970, and
these were injection molded from an aromatic
polymer called polycarbonate. These were meant to
be esthetic but were subject to stains & odors.
Moreover these plastic brackets deformed plastically
under load & showed creep with time.

About 10 years passed before the first ceramic
brackets were developed. In spite of their superior
esthetics, their frictional properties are far inferior
to stainless steel. Highly magnified views have
revealed numerous generalized small indentations
in the ceramic bracket slot, while S.S brackets
appear relatively smooth. Single crystal ceramic
brackets are derived by milling large single alumina
crystals into the derived shape & size via ultrasonic
or diamond cutting or a combination of these two
processes. Polycrystalline ceramic brackets are
sintered together using special binders to fuse the
particles together. Laser speculance & SEM have
shown monocrystalline brackets to be smoother
than polycrystalline ones, but their frictional
characteristics were comparable.

• In order to overcome the problem of brittleness &
low fracture resistance associated with ceramic
brackets, Zirconia brackets were offered as an
alternative. But these were found to have friction
coefficients equal to or greater than ceramic
brackets. They also showed surface changes
consisting of wire debris and surface damage to
brackets after sliding of arch wires.

Friction from the slot is especially a problem in case
of ceramic and plastic brackets. To reduce friction
some manufacturers have replaced the slot with
metal usually stainless steel, titanium, gold and
niobium (Figure 4.14)

• In the last few years, it has been recognized that
ceramics have desirable esthetics but other materials
have superior frictional characteristics.
Consequently, as stainless steel and a gold liner have
now been placed in a polycrystalline Alumina bracket.
• These metal inserted products capitalize on the best
of both worlds, namely, pleasing esthetics and
competitive frictional characteristics, both in the
presence & absence of saliva.

Frank & Nikolai (1980) found that frictional resistance
increased in a NON LINEAR manner with increased bracket
angulation. • Ogata et al (AJO DO 1994) also noted that as
second order deflection increased, frictional resistance was
found to increase for every bracket-wire combination
evaluated by them. The friction increased appeared in 2
phases: • With lower deflections: - A smooth sliding phase
appeared in which friction increased in an approximately
linear manner. • As deflection increased further: A binding
phase occurred in which friction increased at a higher, non-
linear rate.

Clinical Significance:
• For patients requiring maximum anchorage protection,
complete leveling of the arch prior to using sliding mechanics
is imperative. This will reduce the force required for
retraction of the teeth because the frictional resistance will be
decreased.

Articolo & Kusy (AJO-DO 1999) studied the resistance to
sliding as a function of five angulations (0 , 3 , 7 , 11 , 13 )
using a different combinations, of SS, monocrystalline, or
polycrystalline ceramic brackets against SS, NiTi or -Ti
archwires. When the couples were in the passive
configuration at low angulation, all stainless steel wire
bracket couples had the least resistance to sliding. When
angulation was >3 , active configuration emerged and
binding quickly dominated, with RS increasing over 100 fold.
• Under these conditions, couples of SS had the highest RS.
While couples of the more compliant alloys such as NiTi had
the least.

3 d) The role of third order torque: •
When torque is applied to the wire, its projected size is larger than
the actual size of the wire. This further decreases the clearance
between the archwire & the bracket and contributes to frictional
resistance to sliding.
Kusy (AJO-DO 2004) evaluated the onset of binding for 3
scenarios • Second order angulation alone. • Third order torque
only. • Combination of second order angulation & third order
torque. • He found that each wire-slot combination has a common
maximum torque angle, independent of bracket width. He suggests
that the use of a metric 0.5mm slot might have some advantages
with regard to torquing. Wires can be used that apply lighter forces
while maintaining angulation and torque capabilities, which were
once possible only with larger wires.

• Greater interbracket width allows the longer
lengths of wire between brackets larger amounts
of deflection, thus greater flexibility and more
initial arch leveling.
• The effect of bracket width and interbracket
width on friction appears unclear as indicated
by Frank and Nikolai who found interbracket
distance to have little effect on frictional
resistance.

The fourth wall of Bracket slot
Wires once inserted into the slot should remain within the slot
till next appointment. As the edgewise bracket slot has three
fixed walls, so there is a fair chances that the wire will come
out of the slot opening until or unless a mechanism is present
that make up the fourth wall of the slot and prevent the wire
from coming out.
This fourth wall is traditionally been provided by ligatures.

Traditional wire ligatures were used to keep the
wire within the slot. For many decades thin
stainless steel wires were used as ligatures which
provide durable, cheap and effective ligation.
Though stainless steel ligature are still
used but due to increased chair side time which is
on the average 11 minutes to tie these ligature,
steel ligatures are taken over by elastic ligatures.

Elastic ligatures are mostly used in contemporary orthodontics
for ligation of wire within the slot.
Elastic ligatures though provide very good ligation at the time of
insertion have a rapid force decay rate and almost half of the
force is lost in the first 24 hours.
They also get discolor with time so increases esthetic concern of
the patients.
To overcome these problems associated with steel and elastic
ligatures self-ligating brackets were introduced. Though the
history of self-ligating bracket is very old starting back to 1935
but they have only gained much popularity in the last decade .

Self-ligating brackets are available in all type of materials in
which conventional brackets are available. Self-ligating
brackets are of two types depending upon the type of ligation
they provide.
1. Active self-ligating brackets (Figure A).
2. Passive self-ligating brackets (Figure B).
Active self-ligating brackets are one in which ligating clip is
occupying some of the slot space.
This clip is flexible and caries some energy. While the passive
self-ligating clip doesn't cover the slot space and is usually
hard. So an active clip will push a rectangular wire into the slot
and in some grossly displaced teeth round wire is also pushed
in, while a passive clip will simply prevent the wire whether
round or rectangular from coming out of the slot.

Too much have been written on self-ligating brackets and
its proposed benefits in different orthodontic books. In
following text only evidence based findings would be
given.
Oral hygiene
A systematic review by Nascimento found no evidence of
self-ligating brackets related to less formation of
streptococcus mutans colonies as compared to
conventional brackets. So claims by manufacturers that
these brackets are more hygienic are not evidence based.
Treatment time and initial pain
A systematic review by Celar found no evidence that self-
ligating brackets are related with less initial pain, less
number of visits and less treatment time than
conventional brackets.

Friction resistance
Ehsani in a systematic review concluded that self-
ligating brackets show less friction resistance on
round wires if used on well aligned arches but
there is no evidence of decrease friction resistance
on rectangular wires. A low level of evidence
suggested that there is no clinically significant
difference in terms of friction resistance between
active and passive self-ligating brackets on SS
wires.

Torque Expression
Archambault found that active stainless steel self-
ligating brackets show less wire play than passive
self-ligating brackets. So there would be more torque
expression from active self ligating brackets than
passive self-ligating brackets.

Steel ligatures though largely have been taken
over by elastic ligatures are still used in cases
where there is a need to express more torque i-e
lower arch in growth modification cases,
impacted canines and cases in which teeth are
palatally or buccally displaced. Steel ligature are
also a reliable mechanism of ligation in rotated
teeth, piggy back mechanics and surgical cases.
Steel ligatures are also used on teeth undergoing
translation because if wisely ligated they offer
less friction as compared to elastic ligatures.

Edwards et al (BJO 1995) compared the frictional
forces produced when elastomeric modules were
applied conventionally or in a “figure of –8”
configuration, stainless steel ties or Teflon coated
ligatures were used for archwire ligation. The “figure of
8” modules appeared to create the highest friction.
There was no significant difference in mean frictional
force between the conventional module and the St.St
ligature, but the Teflon coated ligature had the lowest
mean frictional force.
Dowling et al (BJO 1998) investigated the
frictional forces of differently colored modules & found
the clear modules to exhibit significantly lower friction
than other modules. This study however was carried
out in absence of saliva.

Khambay et al (EJO 2004) compared the effect of
elastomeric type and stainless steel ligation on frictional
resistance and these were further compared with self
ligating Damon II brackets. There was no consistent
pattern in the mean frictional forces across the various
combinations of wire size, type, and ligation method. The
polymeric coated module did not produce the lowest mean
frictional force. The introduction of a 45 bend into the
module (Alastik Easy-to-use) reduced mean frictional
force to that of a St.St ligature when using 19 x 25” SS wire.
The use of metal ligatures with 7 turns produced the lowest
friction confirming the findings of Bazakidon et al (AJO-
DO 97). They concluded that the use of passive self ligating
brackets is the only may of almost eliminating friction.

Thorstenson & Kusy (AJO 2002): investigated
the RS for 3 self ligating brackets with passive slides
(Activa, Damon & Twinlock) and 3 self ligating
brackets with active clips (In-ovation, SPEED, Time),
with second order angulation, in dry and saliva states.
They reported that for second order angulations c, the
RS of self ligating brackets is small to non-existent
regardless of saliva state, thus facilitating siding
mechanics, but compromising root position. The RS of
brackets with active clips was higher being in range of
1247CN (dry state) and (22-54CN) wet State,
respectively. • They reported that in the active state (
> c), the rate of binding is similar, regardless of
presence of passive slide or active clip. • According to
them “The desire to minimize the RS should be
moderated by the necessity to control tooth

Henao & Kusy (Angle orthod. 2004) compared the
frictional resistance of conventional & self ligating brackets
using various archwire sized. They reported that self
ligating brackets exhibited superior performance when
coupled with smaller wires used in early stages of
orthodontic treatment. However when larger 016 x 022”
and 019 x 025” AW were tested, the differences between
self ligating & conventional brackets were not so evident.
This shows that self ligating brackets have the ability to
maintain low frictional resistance only up to a certain size
of archwire. It also emphasizes the importance of leveling
and alignment before using larger wires & sliding
mechanics.

Ion Implantation
• Greenberg and Kusy coated orthodontic arch wires with a
polymer composite and a polytetrafluorethylene-based coating
(Teflon, Dupont Co.) and preliminary results showed a
reduction in the coefficients of friction. Unfortunately, the
surface coatings tended to stain, peel off or crack on bending.
• As the titanium content of an alloy increases, its surface
reactivity increases and the surface chemistry is a major
influence on frictional behavior.12 Thus, -titanium, at 80%
titanium, has a higher coefficient of friction than nickel-titanium
at 50% titanium, and there is greater frictional resistance to
sliding (“stick-slip” phenomena) with either than with steel.
• A solution to this is to alter the surface zone of the titanium
wires by implantation of ions into the surface, thereby altering
the surface chemistry.

• Implantation of boron or phosphorus into steel produce an
amorphous, “glassy” structure on the surface of steel which is free
from the grain boundaries of a steel surface and is impervious to
pitting corrosion.
• Kusy and Andrews tested stainless steel, cobalt-chromium, nickel-
titanium, and ß-titanium archwires against simulated brackets
(stainless steel cylinders). In addition to control samples, the
polished flats of these cylinders were implanted with N+, N+/Cr+,
N+/C+, C+, Ti+/ C+, Ti+/N+, and Ti+ ions. Each arch wire was
drawn in an Instron Universal Testing Machine, at 1 cm/min
between flats of two cylinders at 34°C in saliva. The stainless steel
control cylinders/brackets yielded lower μk values than ion
implanted cylinders. This unexpected result may be because the
optimal ion distributions for wear resistance were too penetrating
for frictional reduction and in addition subjected to low stress, no
wear regimes.

wet and dry environment
one would think that saliva acts as a lubricant, but
unfortunately, the literature is divided with regards to
saliva’s role in reducing friction between orthodontic wires
and brackets. No differences were measured in friction
levels between trials with saliva and those without saliva.
When human saliva is present, frictional forces and
coefficients may increase, decrease, or not change
depending on the arch wire alloy tested. The greatest
differences between dry and wet states occurred with ß-
titanium (TMA) archwires, in which the kinetic coefficients
of friction in the wet state were reduced to 50% of the
values in the dry state. At this point they were comparable
to nickel-titanium but still higher than stainless steel.

BIOLOGIC VARIABLES : Saliva
It is suggested that saliva or a saliva substitute serves as an excellent
lubricant in the sliding of the bracket along the wire.
Baker et al (AJO-DO 1987) using an artificial saliva substitute found a
15% to 19% reduction in friction. • Kusy et al (Angle Orthod 1991):
found that saliva could have lubricous as well as adhesive behavior
depending on which archwire-bracket combination was under
consideration. Stainless steel wires showed an adhesive behavior with
saliva & a resultant increase in the coefficient of friction in the wet
state. The kinetic coefficients of friction of the -Ti archwire in the wet
state were 50% of the values of the dry state. This probably occurred
because saliva prevented the solid to solid contact between the -Ti
archwires and SS brackets, & thus prevented the slipstick
phenomenon from occurring. (The slip-stick phenomenon occurs
when -Ti wire slides through SS brackets & the TiO2 layer breaks
down, adheres & breaks away)

• Therefore, especially in adult patients, a history
of xerostomia or reduced salivary flow, oral
radiation therapy, or anticholinergic medication
should be noted as possible factors in varying
the force levels necessary to more teeth.

محمد الخولاني-اليمن

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