Download A guide to Orthodontic Archwires

ARCH WIRES
DR AYAZ AHMAD KHAN
WHAT IS AN ARCH WIRE
• An arch wire is the wire that attaches to your
braces
• It is called an "arch wire" because it engages in
the brackets in the upper and lower arches
• It releases energy as mechanical work that
correctly aligns your teeth
• Arch wires can be considered as beams supported on
both ends spanning between the attachments
• An arch wire is just like an engine that guides and moves
your teeth. Without an arch wire to connect, you would be
wearing braces just for fun and your teeth would never
move!
• When you first start treatment with braces, your
teeth are crooked.
• The arch wire attaches to your braces must be
able to return to its original shape when it is
deformed or bent.
• The force that returns the wire to its original shape
is what moves your teeth.
• The wire should be springy and exert a gentle
force so that your brackets do not pop off when
the orthodontist ties in your arch wire.
• For orthodontic purposes, three major properties of
beam materials are critical in defining their clinical
usefulness:
• Strength
• Stiffness
• Range
STRENGTH
• It is a measurement of force applied and
deflection produced as a result.
• A graph drawn between stress and strain shows
the maximum load that a material can resist. It is
measured as the elastic limit which is the point at
which a deflection of 0.1% is produced.
• The new titanium alloys have much more ultimate
tensile strength than steel wires. Strength is
measured in strength units gm/cm2
STIFFNESS
• Stiffness is a measure of wire resistance to bending.
• Stiffness can indicate at the rate at which it can exert force.
• There must be a presence of lower stiffness for the arch wires
which helps for the lower and constant force for the
movement of the teeth as needed.
RANGE
• It is the measure how far the wire can be deflected within its
elastic limit before permanent deformation.
• It is measured in millimetres.
• If the wire is deflected beyond its yield strength, it will not
return to its original shape.
Thus a wire should possess
High strength
High range
Low stiffness
High formability: it helps to make the arch
bend into desired
configurations like loops, coils etc.
• Resilience:
the measure of the maximum
force an arch wire can withstand.
• Joinability:
The arch wire must have the ability
to get soldered or welded so that
hooks and stops can be
incorporated in it
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•
•
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• Friction:
There must be least friction at the
arch wire-bracket interface. Least
friction shows greater efficiency
• Biocompatibility and Environment Stability:
This is one of the important feature of the
orthodontic arch wire since the orthodontic
treatment normally takes a year or two so it
should be resistant to tarnish and corrosion and
should also be non-toxicity.
• It should be reasonable in cost
COMPOSITION OF ARCH WIRES
• Three main types of compositions for arch wires :
• Stainless Steel
• Nickel-Titanium (Ni-Ti)
• Beta-Titanium.
• Precious Metal alloys of Platinum and palladium
with gold and copper compositions were initially
used in the 20th century. The Crozat appliance is
still made from Gold exclusively.
NICKEL-TITANIUM (NI-TI)
• Nitinol wire is made from an alloy of Nickel, Titanium
and Cobalt.
• The Austenite form exists at higher temperatures and
called A-Niti which is a preferred material for
orthodontic wires where a long range of activation
with relatively constant force is needed e.g coil
springs.
• The Martensite form exists at lower temperatures and
called M-Niti used in the later stages of treatment
when flexible but stiffer wires are used.
• It is available in preformed arch wire shape and in
straight lengths.
• They are used in the beginning stages of orthodontic
treatment to put gentle forces.
• It is more expensive, is brittle and has low formability
so available in commercially preformed shape where
a certain shape is set while the alloy is maintained at
a higher temperature.
• The Niti offers the greatest working range and is more
resistant to distortion in the mouth
• Friction forces are lowest but it has a greater
tendency to fracture.
• Orthonol has similar strength and springiness but
better formability.
They have two
important properties
Shape Memory
Super Elasticity
Shape Memory is the
property to “remember”
and return to their
original shape when
deformed.
HEAT ACTIVATED NITI
• Heat Activated Nickel Titanium (NiTi) wires comfortably level
and align tough cases with easy engagement and low forces.
• They exert lowest forces for gentle movement.
• They become dead soft when chilled below 20°C and
achieve optimum activation at 37°C.
• Their centerline is etched.
• They are ideal for easier bracket engagement.
STAINLESS STEEL WIRES
• Stainless steel wires have been used for decades due
to their high strength
• It is resistant to rust because of its high chromium
content
• A typical formulation has 18% chromium and 8%
nickel, thus referred as 18-8 stainless steel.
• Steel may be softened by annealing and hardened
by cold working. Fully annealed steel wires are soft
and are highly formable. This dead soft wire is also
used to make steel ligatures.
• They deliver a perfect finish
• High force values ideal for finishing cases.
• Made of medical grade stainless steel
• It is also made in braided form, with up to 6 or
more separate wires making a multi strand arch
wire. They have reduced stiffness and find their
application early in treatment when there are
marked tooth displacements within the arch.
BETA- TITANIUM
• Beta-Titanium, Beta-Ti/TMA arch wires are composed of
alloy of Titanium, Molybdenum, Zirconium, and Tin.
• These produces forces which are 40% stronger than that
of NiTi, but have only 42% of the stiffness of SS.
• They offer an intermediate range of elasticity and
strength and serve as a good intermediary wire
between Ni-Ti and stainless steel.
• They have a good formability.
• They may be used at the start of the treatment to
produce initial alignment.
• In larger sizes, it can be used as a finishing arch wire
towards the end of the treatment and is useful for final
detailing of tooth positioning.
ELGILOY
• Made up of Cobalt, Chromium, Nickel alloy.
• It can be used in the soft state to apply light forces
and can be hardened by heating to increase its
stiffness.
• After heat treatment, it becomes equal to stainless
steel in properties.
• The inconvenience of heat treatment is a
disadvantage keeping in view of the availability of
a variety of wire types available.
COMPOSITE PLASTICS
• There has been a corresponding increase in demand
for more esthetic orthodontic appliances
• By adjusting the ceramic/polymer proportions, these
wires can be manufactured in a wide range of
clinically relevant levels of elastic stiffness.
• Allergic reactions to nickel are also averted with
composite materials.
• With further developments, in the near future, fiber
reinforced composite materials are expected to
replace metals as the material of choice for
orthodontic arch wires.
SIZE OF ARCH WIRES
• When orthodontists talk about the “size” of an
arch wire, they are referring to the cross-section or
thickness of the wire.
• Considering arch wires made from identical
materials, the smaller the cross-section, the more
elastic and less stiff the wire will be.
• Wires come in two types of cross-sections:
1) Round 2) Rectangular.
• Round wires are obviously round in cross-section.
• Rectangular wires can be square or rectangular in
cross section.
• In the beginning stages of treatment, round
wires are typically used to level and align the
teeth because round wires are more elastic and
so the orthodontist will be able to engage all
your teeth into the wire without popping off
brackets.
• If he uses a wire that is too stiff and tries to tie the
arch wire to a really crooked tooth, the wire will
put too much pressure on the bracket, and the
bracket may break off from the tooth.
• After the teeth get straighter, orthodontists usually
advance to rectangular wires.
• Because the wire slot of the bracket is rectangular,
a rectangular wire fits into the bracket like a hand
fits into a glove.
• By fitting snugly into the bracket, the rectangular
wire controls tooth movement better than a round
wire.
ARCH WIRES FOR INITIAL
ALIGNMENT:
• At the beginning of the treatment, the aim is to
reduce individual discrepancies of teeth by
achieving initial resolution of labio lingual
displacement, rotation and apical displacement
to begin the process of “levelling”.
• Nickel titanium and multi stranded stainless steel
wires are used
• Achieve full engagement of arch wire in all the
brackets before moving to next size
• After preformed Niti 0.012, next wire can be 0.014
or 0.016 in Niti, or straight length 0.0175 in coaxial
multistranded stainless steel.
• All of them allow too much tipping of anchorage
units because of the torque.
• Initial alignment is usually completed within 3
months of the commencement of the treatment.
MID TREATMENT ARCHWIRES
• The flexible arch wires are replaced by “working”
arch wires of greater stiffness offering more control
over tooth movements.
• Round single stranded arch wires are used such as
0.014 progressing to 0.016 and 0.018. Elastics can
be used safely with SS 0.016 wires and above.
• In the early stages, Overbite and over jet are
corrected and spaces are closed.
• When the principal tooth movements have been
achieved, final detailing of tooth position and
then retention are done.
Step of
Choice’s rules
First
Second
Third
treatment
of wire
choice
choice
choice
Initial step
Low force
•leveling
Wide activation range
Nitinol HA
•tipping
Low elastic modulus
ThermoActiv
•rotation
Low friction
Intermediate step
•spaces closure
•correction of arch
•leveling occlusal plane
Nitinol SE
Nitinol
Classic
Intermediate force
Intermediate activation range
Intermediate elastic modulus
Intermediate malleability
Nitinol
Beta III
Perma
Classic
Titanium
chrome
Low friction
Finishing step
•vertical details
Intermediate force
•individual rotations
Short activation range
Beta III
Perma
•finishing
High elastic modulus
Titanium
chrome
•interdigitation
High malleability
•contention
Flexiloy