Orthodontic tooth movement during space closure is
achieved through 2 types of mechanics
The first type, segmental or sectional mechanics,
involving closing loops fabricated either in a full or
sectional arch wire.
The second type, sliding mechanics involves either
moving the bracket along an arch wire or sliding the
archwire though brackets and tubes.
The main differentiating factor between 2 types of
mechanics pertains to FRICTION. Since sectional
mechanics do not involve friction it is called friction
free or friction less technique.
On the other hand, friction plays a significant role in
sliding space closure; therefore it is called friction
Friction is the resistance to motion that exists when a solid
moves tangentially with the respect to the surface of another
FRICTIONAL FORCE FFR : Is parallel but opposite to the
direction of force (F) causing motion.
The frictional force produced 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.
Friction is independent of the apparent area of contact.
There are two components of friction – static and
STATIC FRICTIONAL FORCE:
Tangential force that does not cause any motion of the
contacting parts. It reflects the force necessary to
KINETIC FRICITIONAL FORCE: Is the
tangential force which acts between the sliding
surface moving at a constant speed and it reflects the
force necessary to perpetuate this motion.
Kinetic co-efficient of friction is smaller than static
co-efficient of friction.
When a tangential force is
applied to cause one
material to slide past the
other, the junctions begin
At low sliding 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
A single stick-slip
cycle involves a stick
state associated with
elastic loading of the
system, followed by a
corresponding to stress
To move a tooth bodily, the force applied has to
pass through the center of resistance of the tooth.
However as the force is applied at the bracket level
of the crown, the concerned tooth experiences both
force and moment.
Moment of force is created in 2 planes of space. One
moment tends to rotate the canine mesial out as the force
application is buccal to the center of resistance and the
other tends to cause distal tipping of the tooth as the point
of force application is occlusal to the center of resistance.
The wire bracket interaction tends to counteract this
moment by applying an opposite moment. When
distal tipping of the crown takes place the M/F ratio
will be around 7:1, the tooth slides along the
archwire till binding occurs between the archwire and
the bracket. This produces a COUPLE at the bracket.
Now the M/F ratio increases to 12:1, which results
in distal root movement and uprighting of the
tooth.As the tooth uprights, the moment decreases
until the wire no longer bends. The process is
repeated until the tooth is retracted and force is
When sliding mechanics are used, friction occurs
at the bracket wire interface. Some of the applied
force is dissipated as friction and the remainder is
transferred to the supporting structures of tooth to
mediate tooth movement.
Upon activation, only when the delivered force is
sufficient to overcome the static frictional force,
teeth are displaced. Movement continues until the
resistance of deformed pdl builds to a value, which
when added to kinetic frictional force offsets the
delivered force and tooth movement temporarily
As time proceeds, periodontal remodeling affects the
resistance potential and other factors like wire
resiliency and masticatory action alter the mean
resultant normal force between the bracket and the
wire, so that friction lock is broken & tooth
movement resets again.
1. Elastic module with ligature
2. E - chains
3. closed coil springs
4. J - hook head gear
5. Mulligan’s V - bend sliding mechanics (JCO 1980
July 1994 Sep)
6. Employing Tip-edge Bracket on canines
There are two ways in which anterior teeth are
1. By retracting the canine first followed by retraction of
other four anteriors enmasse.
2. Enmasse retraction of six anterior teeth.
Among the above mentioned force delivery system,
commonly used are the elastomeric materials and the
closed coil spring.
They are either attached directly to attachments on the teeth
(canine hooks) or more usually to hooks on the archwire
Methods of applying traction to the archwire include
Fabricated tie back loops (in shape of boot or inverted
Soldered brass hooks (0.7mm)
Stainless steel hooks (0.6mm)
Preposted archwires are also available
Tie back loops are difficult to bend in preformed
archwires and negate many of their advantages
Soldering requires chair side or laboratory
equipment and is time consuming. It may lead to
annealing of the archwire.
Preposted archwires with soldered brass or SS hooks
allow large inventory of stock and has cost
They are available in 3 sizes
The average distance between the
hooks are 38mm in the upper arch
and 26 mm in lower arch (this
suits more than half of the
Additional sizes 41 mm (U) &
35 mm (L)
(U) & 24mm (L)
Crimpable arch wire hooks allow quick and simple
placement of hooks in the desired position on the arch
wire in (or) out of the mouth.
Cost saving both in time and materials.
It is associated with minimum discomfort.
Excessive force during crimping caused GABLING of
the wire and introduction of unwanted force into the
GRIFFIN & FERRACANE (1998) examined the effect of
addition of sandblasting and dental adhesive on the stability
of the crimpable hooks. The combination increased the force
required to dislodge the hook by a factor of 10. The typical
crimping force was found to be 18 kg.
Employed during canine retraction and in the
settling phase. Prefabricated ones are also available.
It can be custom made from 0.09” or 0.010” ligature
Composition and structure
Elastomeric modules and E-chains are polyurethanes,
which are thermosetting polymers. The polymers posses
rubber like elasticity and have long chain which are
lightly cross-linked. The cross-links between chains
must be relatively few to facilitate large extension with
no rupture of primary bonds.
1. ELASTIC MODULES WITH LIGATURE : ( Active
tiebacks)- This method was popularized by BENETT &
MCLAUGHLIN (controlled space closure with
preadjusted appliance system) JCO 1990 April. They
recommend placing 0.019” x 0.025” SS in a 0.022 slot for
atleast one month with passive tie backs before attempting
Hooks of 0.024” SS or .028” brass are soldered to upper
and lower archwires.
An elastic module is stretched by 2-3mm(ie) twice its normal
length and it usually delivers 0.5 - 1.5mm of space closure per
month. Tie backs are replaced every 4-6 weeks. Modules
generate 50-100 gm of force if module was pre stretched before
use. If used directly from manufacture without pre-stretching
force delivered is greater. There are 3 methods of placing active
tie backs with elastic modules .
Type 1: active tiebacks. This is the most frequently used method.
The .019x .025 rectangular steel archwire is placed, with modules
or wire ligatures on all brackets. The elastomeric module is
attached to the first or second molar hook. A .010 ligature is used,
with one arm beneath the archwire. This makes the active tieback
more stable, and helps to keep the ligature wire away from the
gingival tissues. An elastomeric module is stretched to twice its
This follows the same principle as type 1, but the
elastomeric module is attached to the soldered brass hook on
the archwire. The.019x.25 rectangular steel archwire is
placed with elastomeric modules or wire ligatures on all
brackets, except the premolar brackets. A.010 wire ligature
is attached to the first or second molar hook, with several
twists in the wire, and then attached to an elastomeric
module on the archwire hook. Finally, a normal module is
placed on the premolar brackets to cover the tieback wire
and the archwire.
This is a recent innovation introduced by Dr. Trevisi, which he
is currently evaluating. It is modification of the type 2 active
tieback. It may have the advantage of being cleaner, with less
friction. The 0.19x.025 rectangular steel archwire is placed with
elastomeric modules on all brackets, except the premolar
brackets. A. 010 wire ligature is attached to the first or second
molar hook, with several twists in the wire. The 0.10 wire
ligature is also used to loosely ligate the premolar bracket, and is
then attached to an elastomeric module on the archwire hook.
It has been shown that space closure can continue for
several months in a patient who has failed to present
for normal adjustments even when the elastomeric
module is in poor condition and delivering very little
force. This might be due to a trampoline effect which
occurs during mastication and which can result in an
intermittent pumping activation.
This jiggling motion is greater in the lower arch and in
low – angle cases with heavier masticatory forces. These
presumed differences would conveniently meet clinical
requirements as normally more forces are required for
lower space closure and in low angle cases.
It was introduced in 1960 and used in orthodontics
for canine retraction, diastema closure, rotation
correction and arch constriction.
Most of the elastomeric chains generally lose 50% -
70% of their initial force during the 1st day of load
application and at 3 weeks retain only 30 – 40% of
their original force.
In view of the wide variation of initial force levels of
different types of power chains, a force gauge should
be used to determine the desired initial force.
To overcome the problem of rapid force decay
rate and provide for a more constant and consistent
force delivery, prestreching of elastomeric chains has
Elastomeric chains are available in 3
Closed loop chain
Short filament chain
Long filament chain :
- generally deliver a lower initial force
and exhibit a greater rate of force
decay at the same extension.
A common mistake is to change the elastic chain or
module too often, thus maintaining high force levels and
a moment to force ratio that produces distal crown tipping
only. This also causes excessive mesial out rotation of the
canines. Constantly high force levels can cause excessive
hyalinization of the periodontal ligaments and inhibit
direct resorption around the canine.
If the posterior segment undergoes direct bone
resorption at the same time, loss of anchorage may
result. It is therefore recommended that elastic
modules or chain should be changed at an intervals
of 4-6 weeks to optimize sliding retraction of the
absorb water and saliva and
permanent staining takes place.
Stretching causes permanent
The loss of force (FORCE
DECAY) with time leads to
variable forces levels during the
treatment, this results in
Can be easily applied
without arch wire
No patient co-
Coil springs were introduced to the orthodontic world as early
as 1931. During the manufacturing process, the material is
subjected to winding that includes tensional and torsional
components and hence spring properties may be slightly
different from the wires made from the same material. The
various materials that have been used for making springs are
Co-Cr Ni alloy
Stainless steel coil springs are efficient methods
of retraction. They apply more predictable
levels of forces compared to elastic based
systems described before. They are easy to apply
However, stainless steel springs have a relatively
higher load deflection rate compared to some
other material springs like NiTi springs. So, as
the space starts to close, there is some force
degradation due to lessening activation.
Moreover, the force applied depends to some
extent on the level of activation. Hence there
may be a tendency to apply excessive force
initially by over activation. Thus, the amount of
activation has to be monitored to maintain ideal
Nickel titanium alloys were introduced to the
dental profession by William. P. Bleur in the
1960's. He demonstrated the unique
combination of properties of shape memory
and super elasticity in addition to low modulus
of elasticity, moderately high strength, high
resilience and less corrosion.
The force degradation is very less due to the low load
deflection rate. They deliver constant amount of force
till they reach the terminal end of deactivation stage
and generally close space with single activation. They
are available in lengths of 9 mm and 11 mm.
Can be easily placed and removed without archwire
Do not need to be reactivated at each appointment
Patient co-operation not required.
Relatively unhygienic compared to elastic force
In 1992 Angolkar et al examined the force
degradation of SS, COCR and NiTi coil springs and
found major force loss to occur after 24 hours. 17%
for SS, 10% for cobalt chromium, 3% NiTi spring.
Thus the clinician should consider a force decay of
8% to 20% when retracting with coil springs against
the force decay of 70% with modules and 42% with
A standardised NiTi closed coil springs can produce
a relatively constant force during deactivation.
Super elastic (SE) NiTi coil spring are extremely
temperature sensitive and thus produce a large force
variation at different mouth temperatures. However
in a narrow temp range, this variation is small.
NiTi springs with clinically targeted values of
100gm, 150gm and 200gm ever found to have little
The force magnitude varied depending on mouth
The use of SE Niti coil springs resulted in significant
greater and more consistent rate of space closure
than elastic module.
Optimal force for space closure is 150gm when
using NiTi coil spring and found to be more effective
than 100gm springs.
When examined clinically there was no difference in
tooth position produced by 2 system after space
No evidence of greater patient discomfort with the
EFFECT OF ENVIRONMENTAL FACTORS ON E-
CHAIN & NITI-COIL SPRINGS. Claire Nattrass (EJO-
Temp variations affected both E-chain and NITI coil springs
E-chains – Effect of temp was more profound. Force loss
greater at higher temp.
NITI springs- overall effect of temp was smaller. Force loss
greater at lower temp.
This is due to modifications in the crystal structure of the
E-Chains are also affected by other environmental factors
such as food. The gross colour change is a common clinical
finding in patients who consume spicy foods.
Force decay of E-chain was more in carbonated drinks than in
water which may be due to low PH.
But Ferriter in 1990 found that acidic fluoride environment
reduced the level of force decay exhibited by elastomers.
EFFECT OF ENVIRONMENTAL FACTORS ON E-
CHAIN & NITI-CORT SPRINGS. Claire Nattrass
J hook headgear, either of the straight pull or high pull
type is clipped on the archwire mesial to the canines to slide
Straight pull headgear allows swifter canine retraction
than the high pull type. However, this may cause anterior
extrusion (Perej et al 1980; Hickham 1974) and
unfavourable occlusal plane rotations (Bowden 1978). This
might specially be a problem in high maxillomandibular
High pull headgear may cause more bodily
retraction. However, it is not as efficient for distal
movement, needing prolonged periods or wear for
During the retraction, direction of force may be
varied between straight pull and high according to
the individual requirements of the case.
Anchorage conservation is
good. Additional molar
support by head gear may
Overjet reduction might
occur during canine
retraction due to the distal
force and binding of the
Can be applied to both
upper and lower arches
As force application is
intermittent this is slower
than other methods of canine
Highly dependent on patient
The molar and buccal
segment correction is
usually a later event in
treatment compared to other
Canine tipping and anterior
extrusion can occur with the
straight pull headgear.
introduced by Mulligan in the 1970's.
The basic principle was to apply different moments to the
teeth via bends in the continuous archwire while force for
retraction was applied by auxiliaties like elastic chain, coil
In the 0.018 slot, 0.016 SS wire is used for retraction
while in the 0.022 slot, 0.016, 0.018 or 0.020 wires may be
The archwire is not tied
into incisor brackets
during cuspid retraction.
The wire is tied in for 4
to 6 weeks for alignment.
The 45o V bends are
added to the wire and
200 gms of force are
applied between the
canine and the molar.
If the bend is placed offcenter it creates a short and a long
segment. The shorter segment is more rigid and hence applies
greater moments. So, if maximal canine retraction is required
the bend is placed very close to molar . The longer span of
wire towards the canine allows some tipping to occur as the
moment is less. Thus the canine gets retraced by tipping and
uprighting. As the canine retracts, the bend goes on becoming
less offcentered and mesial crown uprighting moments on the
Space closure typically occurs more easily in
high angle patterns with weak musculature. The
rate of closure can be increased either by
increasing the force or using thinner arch wire.
However more rapid space closure leads to
loss of control of torque,
results in upper incisors being too upright at the end of
space closure with the spaces distal to the canines and a
consequent unesthetic appearance. The lost torque is
difficult to regain. Also, rapid mesial movement of the
upper molars can allow the palatal cusps to hang down,
resulting in functional interferences, and rapid movement
of the lower molars causes “rolling in “.
can be seen mainly in
the teeth adjacent to
extraction sites, which
also tend to roll in if
spaces are closed too
produces unwanted movement of canines, premolars, and
molars, along with a tendency for lateral open bite. In high-
angle cases, where lower molars tip most freely, the elevated
distal cusps create the possibility of a molar fulcrum effect. In
some instances, excessive soft-tissue hyperplasia occurs at the
extraction sites. This can prevent full space closure or allow
spaces to reopen after treatment. Local gingival surgery may
be necessary in such cases.
1. Occlusal interference - To prevent this proper aligning
and leveling of the arches is required.
2. Friction and binding between bracket and archwire
may place heavy demand on anchorage.
3. Poor canine control can be a problem : Doing canine
retraction on heavier arch wire reduces the problem.
4. Cortical plate resistance (Narrowing of alveolar bone in extraction
5. Excessive forces causes lower molar tipping and extrusion of
6. Soft tissue build up in the extraction site can prevent space closure
(or) reopen spaces after treatment.
7. Rotation of canines mesio-bucally and molar mesiopalatally.This
occurs due to the use buccal traction. It can be prevented
simultaneous palatal traction using lingual cleats or buttons.
Cross sectional shape/ size.
2. Ligation of archwire to bracket:
Method of ligation / Self ligating brackets
3. Bracket properties:
Slot width and depth.
Design of bracket: single/twin.
First order bend (in-out)
Second order bend (angulation).
Third order bend (torque).
4. Orthodontic appliance:
Inter bracket distance.
Level of bracket slots between adjacent
Forces applied for retraction.
Various Bracket materials today available are :-
1. Stainless steel Cast
2. Ceramic brackets Polycrystalline
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
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
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
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
frictional 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
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
) 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.
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 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,
- 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
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
some studies suggested that frictional
resistance decreased as slot size increased
from 0.018 inch to 0.022 inch because of
reduced binding probably from increased
wire stiffness. And also because of the
increased play in the slot with final archwire
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—have achieved low friction by virtue of an
extremely loose fit between a round archwire and a very
narrow bracket, but this is at the cost of making full
control of tooth position correspondingly more difficult.
Some brackets with an edgewise slot have incorporated
shoulders to distance the elastomeric from the archwire
and, thus, reduce friction, but this type of design also
produces reduced friction at the expense of reduced
Second order defection of wire b/n brackets held in
series can have significant effects on brackets wire
- Several studies have found that increasing the
angulation between bracket and wire produced
Frank and Nikolai (1980) :- Found that frictional
resistance increased in a nonlinear manner with
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
Binding generally occured between 0.75 and 1.00 mm of 2nd
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.
The potential risks of friction and binding are
now reduced but not totally eliminated from
sliding mechanics. However it remains the most
widely used method of canine retraction in
clinical practice. So the orthodontist must be
aware that even with best archwire / bracket
combination (SS / SS), atleast 40 gms of friction
must be included in the force applied to the tooth
to initiate movement during sliding mechanics.