ORTHODONTICS wires 1

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Orthodontic wires-I
Dr.Meenakshi Vishwanath

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Contents
 Introduction
 Evolution of materials
 Basic properties of materials
 Mechanical & Elastic properties
 Physical properties
 Requirements of an ideal arch wire
 Properties of wires
 Orthodontic arch wire materials

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Introduction
“All you can do is push, pull or turn a tooth. I have given
you an appliance and now for God’s sake use it”
Edward.H.Angle
 The main components of an orthodontic appliance
-brackets and wires.
 Active and reactive elements (Burstone)
 Wires Brackets Bonding

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Introduction
 Orthodontics involves correction of the position of teeth
–requiring moving teeth.
 Forces and Moments
 Optimum orthodontic tooth movement- light continuous
force.

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Introduction
 The challenge –
Appliance which produces forces that are neither too
great nor variable.
 Different materials and type of wires introduced to
provide forces.

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Evolution of Materials
1. Material Scarcity, Abundance of Ideas (1750-1930)
 Before Angle’s search;
 Noble metals and their alloys.
- Gold (at least 75%), platinum, iridium and silver
alloys
 Good corrosion resistance
 Acceptable esthetics
 Lacked flexibility and tensile strength
 Inappropriate for complex machining and joining.

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 Angle listed few materials appropriate for work:
 Strips of wire of precious metals.
 Wood
 Rubber
 Vulcanite
 Piano wire
 Silk thread

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 Angle (1887)  German silver (a type of brass)
 “according to the use for which it was intended”-varying
the proportion of Cu, Ni & Zn and various degrees of cold
work.
 Neusilber brass (Cu 65%, Ni 14%, Zn 21%)
 jack screws (rigid)
 expansion arches (elastic)
 Bands (malleable)
 Opposition by Farrar – discolored

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 Stainless steel (entered dentistry -1919).
 Dumas ,Guillet and Portevin-(France), qualities
reported in Germany –Monnartz (1900-1910).
 Discovered by chance before W W I.
 1919 – Dr. F Hauptmeyer –Wipla (wie platin).
 Simon, Schwarz, Korkhous, De Coster- orthodontic
material.
 Angle used steel as ligature wire (1930).

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 Opposition
 Emil Herbst
-Gold wire was stronger than stainless steel (1934).
 “The Edgewater" tradition-
-1950-2 papers presented back to back-competition
between SS & gold.
- B/w Dr.Brusse (The management of stainless steel)
and Drs.Crozat & Gore (Precious metal removable
appliances).
 Begg (1940s) with Wilcock-ultimately resilient arch
wires-Australian SS.

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2. Abundance of materials, Refinement of
Procedures (1930 – 1975).
 Kusy-after 1960s-proliferation abounds.
 Improvement in metallurgy and organic chemistry –
mass production(1960).
 Farrar’s dream(1878)-mass production of orthodontic
devices.

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 Cobalt chrome (1950s)-Elgin watch company
developed a complex alloy-
Cobalt(40%),Chromium(20%),iron(16%)&nickel(15%).
 Rocky Mountain Orthodontics- ElgiloyTM
 1958-1961
various tempers
Red – hard & resilient
green – semi-resilient
Yellow – slightly less formable but ductile
Blue – soft & formable

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Variable cross-section orthodontics-
Burstone
 To produce changes in load-deflection rate- wires of
various cross sections were used.
 Load deflection rate varies with 4th
power of the wire
diameter.

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 1962 - Buehler discovers nickel-titanium dubbed
NITINOL (Nickel Titanium Naval Ordnance Laboratory)
 1970-Dr.George Andreason (Unitek) introduced NiTi to
orthodontics.
 50:50 composition –excellent springback, no
superelasticity or shape memory (M-NiTi).
 Late 1980s –NiTi with active austenitic grain structure.

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 Exhibited Superelasticity (pseudoelasticity in
engineering).
 New NiTi by Dr.Tien Hua Cheng and associates at the
General Research Institute for non Ferrous Metals, in
Beijing, China.
 Burstone et al–Chinese NiTi (1985).
 In 1978 Furukawa electric co.ltd of Japan produced a
new type of alloy
1. High spring back.
2. Shape memory.
3. Super elasticity.
Miura et al – Japanese NiTi (1986)

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Variable – modulus orthodontics-Burstone
(1981)
 Wire size was kept constant and material of the wire is
selected on the basis of clinical requirements.
 Fewer wire changes.
 Different materials-maintaining same cross-section.

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 Cu NiTi – (thermoelasticity) - Rohit Sachdeva.
•Quaternary metal – Nickel, Titanium, Copper,
Chromium.
•Copper enhances thermal reactive properties and creates a
consistent unloading force.
Variable transformation temperature
orthodontics

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3. The beginning of Selectivity (1975 to the present)
 Orthodontic manufacturers
 CAD/CAM – larger production runs
 Composites and Ceramics
 Iatrogenic damage

Nickel and en-masse detachments
New products-
control of government agencies, private organizations

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β titanium –Burstone and Goldberg-1980
 β phase –stabilized at room temperature.
 Early 1980s
 Composition
 Ti – 80%
 Molybdenum – 11.5%
 Zirconium – 6%
 Tin – 4.5%
 Burstone’s objective  deactivation characteristics
1/3rd
of SS or twice of conventional NiTi
 TMA – Titanium Molybdenum alloy - ORMCO

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 Titanium-Niobium- M. Dalstra et al.
 Nickel free Titanium alloy.
 Finishing wire.
 Ti-74%,Nb-13%,Zr-13%.
 TiMolium wires (TP Lab)-Deva Devanathan (late 90s)

Ti - 82% ,Mo - 15% , Nb-3%

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 β III- Ravindra Nanda (2000-2001)
• Bendable,inc. force-low deflection
• Ni free
• Versatility of steel with memory of NiTi.

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Fiber reinforced polymeric composites:
 Next generation of esthetic archwires
 Many orthodontic materials adapted-Aerospace industry
 Pultrusion – round + rectangular
 ADV – tooth colored  enhanced esthetics
- reduced friction
 DISADV – difficult to change its shape once manufactured

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Basic Properties of Materials
To gain understanding of orthodontic wires – basic
knowledge of their atomic or molecular structure
and their behavior during handling and use in
the oral environment .

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 Atom - smallest piece of an element that keeps
its chemical properties.
 Element - substance that cannot be broken
down by chemical reactions.

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Electrons – orbit
around nucleus.
Floating in shells of diff
energy levels
Electrons form the
basis of bonds

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 Pure substances are rare-eg. Iron always contains carbon,
gold though occurs as a pure metal can be used only as an
alloy.
 An ore contains the compound of the metal and an
unwanted earthly material.
 Compound - substance that can be broken into elements
by chemical reactions.
 Molecule - smallest piece of a compound that keeps its
chemical properties (made of two or more atoms).

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 Cohesive forces-atoms held together.
Interatomic bonds
Primary Secondary
Ionic Hydrogen
Covalent Van der Waals
Metallic forces

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 Ionic-mutual attraction between positive and negative
ions-gypsum, phosphate based cements.
 Covalent-2 valence electrons are shared by adjacent
atoms-dental resins.

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 Metallic –increased spatial extension of valence-electron
wave functions.
 The energy levels are very closely spaced and the
electrons tend to belong to the entire assembly rather
than a single atom.
 Array of positive ions in a “sea of electrons”

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 Electrons free to move
 Electrical and thermal conductivity
 Ductility and malleability
-electrons adjust to deformation

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IONIC BOND METALLIC BOND
Ionic bond Metallic bond

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 Materials broadly subdivided into 2 categories -
Atomic arrangement
Crystalline structure Non-crystalline structure
Regularly spaced Possess short range
config-space lattice. atomic order.
Anisotropic –diff in Isotropic-prop of material
mechanical prop due remains same in all
directional arrangement directions.
of atoms. Amorphous

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Characteristic properties of metals
 An opaque lustrous chemical substance that is a good
conductor of heat and electricity & when polished is a
good reflector of light – Handbook of metals.
 Metals are-
• Hard
• Lustrous
• Dense (lattice structure)
• Good conductors of heat & electricity
• Opaque (free e- absorb electromagnetic energy of
light)
• Ductile & Malleable

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Crystals and Lattices
1665-Robert Hooke simulated crystal shapes –musket ball.
250 years later-exact model of a crystal with each
ball=atom.
Atoms combine-minimal internal energy.
Space lattice- Any arrangement of atoms in space in which
every atom is situated similarly to every other atom. May
be the result of primary or secondary bonds.

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 Crystal  combination of unit cells, in which each shell
shares faces, edges or corners with the neighboring cells
There are 8 crystal systems:
Cubic system –Important as many metals belong to it.

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There are 14 possible lattice forms.( Bravais lattices)
The unit cells of 3 kinds of space lattices of practical
importance –
1.Face-centered cubic:
Fe above 910°C & Ni.

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2.Body centered cubic:
Fe-below 910°C &above 1400°C.
Cr &Ti above 880°C.

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3.Hexagonal close packed:
Co & Ti below 880°C

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 Perfect crystals - rare - atoms occupy well-defined
positions.
 Cation-anion-cation-anion-
 Distortion strongly opposed -similarly charged atoms
come together.
 Single crystals- strong
 Used as reinforcements –whiskers (single crystals- 10
times longer, than wide)

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 Crystal growth-atoms attach themselves in certain
directions.
 Perfect crystals-atoms-correct direction.
 In common metals the crystals penetrate each other such
that the crystal shapes get deformed.
 Microscopic analysis of alloys-grains (microns to
centimeters).

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 Grain boundaries-area-crystals meet.
 Atoms-irregular
 Decrease mechanical strength
 Increase corrosion
 imperfections beneficial-interfere with movement along
slip planes
 Dislocations cannot cross boundary- deformation
requires greater stress.

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 Usually crystals have imperfections- Lattice defects.
1.Point defects:
a. Impurities
•Interstitials – Smaller atoms that penetrate the lattice Eg –
Carbon, Hydrogen, Oxygen, Boron.
•Substitutial Element – Another metal atom of approx same
size can substitute . E.g. - Nickel or Chromium substituting iron in
stainless steel.

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b.Vacancies:
2.Line defects: Dislocations along a line. Plastic
deformations of metals occurs –motion of dislocations.
These are empty atom sites.

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 Edge dislocation
 Sufficiently large force-
bonds broken and new bonds
formed.
 Slip plane
+
 Slip direction
=
 Slip system

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 Significance of slip planes-
Shear stress  atoms of the crystal can glide.
More the slip planes easier is it to deform.
Slip planes intercepted at grain boundaries.

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Elastic deformation
Plastic deformation
Greater stress - fracture

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 Twinning – alt. mode of permanent deformation.
 Seen in metals-few slip planes (NiTi & α-titanium)
 Small atomic movements on either side of a twinning
plane results in atoms with mirror relationship

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 Also the mechanism for reversible transformation-
austenite to martensite.
• A movement that
divides the lattice into 2
planes at a certain
angle.
•NiTi – multiple
twinning
•Subjected to a higher
temperature, stress
de - twinning
occurs (shape memory)

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 Cold working ( strain hardening or work
hardening)
• Dislocations pile up along the grain boundaries.
• Hardness & strength ductility
• Plastic deformation-difficult.
• During deformation - atomic bonds within the crystal
get stressed
 resistance to more deformation

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 An interesting effect of cold work-crystallographic
orientation in the distorted grain structure.
 Anisotropic (direction dependant) mechanical
properties.
 Slip planes align with shear planes.
 Wires – mechanical properties different when measured
parallel and perpendicular to wire axis.

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 Implications:
 Fine grained metals with large no. of grains
- stronger
•Enhancing crystal nucleation by adding fine particles with a
higher melting point, around which the atoms gather.
•Preventing enlargement of existing grains. Abrupt cooling
(quenching) of the metal.
•Dissolve specific elements at elevated temperatures. Metal is
cooled
Solute element precipitates barriers to the
slip planes.

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 The effects of cold working can be reversed-heating the
metal to appropriate temperature- Annealing
• Relative process-heat below the melting temperature
•More the cold work, more rapid the annealing
•Higher melting point – higher annealing temp
•Rule of thumb-½ the melting temperature (°K)

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 Recovery-cold work disappears.
• Ortho appliances heat treated (recovery
temperature)-
• stabilizes the configuration of the appliance and
• reduces-fracture.
 Recrystallization –severely cold worked-after recovery-
radical change in microstructure.
• New stress free grains
• Consume original cold worked structure.
• Inc. ductility ,dec. resiliency

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 Grain growth - minimizes the grain boundary area.
•Coarse grains

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 Before Annealing
 Recovery – Relief of stresses
 Recrystallization – New grains from severely cold
worked areas
 Grain Growth – large crystal “eat up” small ones

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Polymorphism
 Metals and alloys exist as more than one type of
structure
 Transition from one to the other-reversible- Allotropy
Steel and NiTi

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 Steel -alloy of iron and carbon
 Iron – 2 forms-
• FCC-above 910°c
• BCC-below-Carbon practically insoluble.(0.02%)
•Iron  FCC form
(austenite)
•Lattice spaces greater
•Carbon atom can easily
be incorporated into the
unit cell

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 On Cooling
 FCC  BCC
 Carbon diffuses out
as Fe3C
 Cementite adds
strength to ferrite
and austenite
Rapidly cooled (quenched)
Carbon cannot escape
Distorted body centered
tetragonal lattice called
martensite
Too brittle-tempered-heat b/w
200-450°C –held at a given temp
for known length of time-cooled
rapidly.

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Austenite (FCC)
slow cooling rapid cooling
Mixture of: Tempering Martensite (BCT)
Ferrite(BCC) distorted lattice-
& Pearlite hard & brittle
Cementite(Fe3C)

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 NiTi-
• Transformations –temperature & stress.
• Austenite (BCC)
• Martensitic (Distorted monoclinic, triclinic,
hexagonal structure.
Austenite- high temperature & low stress.
Martensite –low temperature & high stress.
Twinning-Reversible below elastic limit
Transformations and reverse-not same temperature-
hysteresis

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 Bain distortion
• Transformations occur without chemical change or
diffusion
• Result-crystallographic reln b/w parent and new
phase
• Rearrangement of atoms-minor movements

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 Gold
 1887-Neusilber brass (Cu,Ni,Zn)
 1919-Stainless steel
 1950s-Cobalt chromium
 1962-NiTiNOL-1970-Orthodontia
 Early 1980s-β-titanium
 1985,86-superelastic NiTi
 1989-α-Titanium
 1990s- Cu NiTi, Ti Nb and Timolium
 2000-β-III

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 Metallic bond-properties
 Crystals & lattices
 Imperfections
 Edge dislocations, Twinning
 Cold working
 Annealing
 Polymorphism
 Bain distortion

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Making an orthodontic wire
 Sources
 Stainless steel- based on standard formulas of AISI.
 After manufacture –further selection to surpass the
basic commercial standard
 Orthodontists –small yet demanding customers
 Chrome – cobalt and titanium alloys- fixed formulas
 Gold –supplier’s own specification.

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 4 steps in wire production
1. Melting
2.The Ingot
3.Rolling
4.Drawimg

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 Melting
-Selection and melting of alloy materials-important
-Physical properties influenced
-Fixes the general properties of the metal
 The Ingot
-Critical step- pouring the molten alloy into mold
- Non –uniform chunk of metal
- Varying degrees of porosities and inclusions of slag.

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-Microscopy –grains –influence mechanical properties.
-Size and distribution of grains –rate of cooling and the
size of ingot.
-Porosity -2 sources
o Gases dissolved or produced
o Cooling and shrinking –interior cools late
-Ingot – trimmed
Important to control microstructure at this stage –Important to control microstructure at this stage –
basis of its physical properties and mechanicalbasis of its physical properties and mechanical
performanceperformance

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 Rolling
- 1st
mechanical step-rolling ingot –long bars
-Series of rollers – reduced to small diameter
-Different parts of ingot never completely lose identity
-Metal on outside of ingot-outside the finest wire,
likewise ends
- Different pieces of wire same ingot differ depending on
the part they came from
-Individual grains also retain identity

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-Each grain elongated in the same proportion as the ingot
-Mechanical rolling-forces crystals into long finger-like
shapes –meshed into one another
-Work hardening-increases the hardness and brittleness
-if excess rolling-small cracks
-Annealing –atoms become mobile-internal stresses
relieved
-More uniform than original casting
-Grain size controlled

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 Drawing
-Further reduced to final size
-Precise process –wire pulled
through a small hole in a die
- Hole slightly smaller than
the starting diameter of the
wire – uniformly squeezed
-Wire reduced to the size of
die

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- Many series of dies
- Annealed several times at regular intervals
- Exact number of drafts and annealing cycles depends
on the alloy (gold <carbon steel<stainless steel)

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 Rectangular wires
-Draw through rectangular die or roll round wires to
rectangular shape
-Little difference in the wires formed by the 2 processes
-Drawing –produces sharper corners –advantageous in
application of torque

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 Hardness and spring properties depend–entirely on the
effects of work hardening during manufacture
 Drawing –Annealing schedule –planned carefully with
final properties & size in mind
 Metal almost in need of annealing at final size-maximum
spring prop.
 Drawing carried too far-brittle, not enough-residual
softness.

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Mechanical properties
 Strength-ability to resist stress without fracture or
strain (permanent deformation).
 Stress & strain-internal state of the material.
 Stress-internal distribution of load – force/ unit area
(Internal force intensity resisting the applied load)
 Strain- internal distortion produced by the load-
deflection/unit length
(change in length/original length)

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 Material can be stressed in 4 ways-
• Compression
• Tensile
• Shear
• Complex force systems

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 Evaluation of mechanical properties –
• Bending tests
• Tension tests
• Torsional tests
 Bending tests : 3 types
• A cantilever bending test-Oslen stiffness tester (ADA-
32)
• 3 point
• 4 point

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Universal testing machine

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 The modulus of elasticity calculated
from the force-deflection plot, using
equations from solid mechanics.
 Cantilever bending test-incompatible
with flexible wires-(NiTi and
multistranded).
 Disadvantage of 3 point-bending
moment-maximum at loading point
to zero at the 2 supports.
 4 point –uniform bending moment-
specimen fails at the weakest point.

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 Nikolai et al proposed a 5 point bending test:
-2 loading points at each end-simulate a couple.
-simulates engagement of arch wire in bracket.
 Tensile testing-strain - rate mechanical testing machine
is used.

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Elastic properties
 Stress-Strain relationship (ductile material)

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Elastic properties
STRAIN
STRESS
Elastic portion
Wire returns back to original
dimension when stress is
removed
(Hooke’s law)(Hooke’s law)

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Elastic properties
0.1%
stress
strain
Elastic limit
Proportional limit
Yield point

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Elastic properties
 Elastic /Proportional limit-used interchangeably
 Proportional limit –determined by placing a straight
edge on the stress-strain plot.
 Elastic limit -determined with aid of precise strain
measurement apparatus in the lab.
 Yield strength (Proof stress) -PL-subjective ,YS used to
for designating onset of permanent deformation.0.1% is
reported.
 Determined by intersection of curved portion with 0.1%
strain on horizontal axis.

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Elastic properties
Ultimate tensile
strength Fracture point
stress
strain
Plastic deformation

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Elastic properties
 Ultimate tensile strength -the maximum load the wire
can sustain (or)
maximum force that the wire can deliver.
 Permanent (plastic) deformation -before fracture-
removal of load-stress-zero, strain = zero.
 Fracture -Ultimate tensile strength higher than the
stress at the point of fracture
 reduction in the diameter of the wire (necking)

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Elastic properties
Slope α Stiffness
Stiffness α 1
Springiness
stress
strain

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Elastic properties
 Slope of initial linear region- modulus of elasticity (E).
(Young’s modulus)
• Corresponds to the elastic stiffness or rigidity of
the material
• Amount of stress required for unit strain
• E = σ/ε where σ does not exceed PL (Hookean
elasticity)
• The more horizontal the slope-springier the wire;
vertical-stiffer

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Elastic properties
Springback deflection
force
Range
YP
Point of arbitrary clinical loading

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Elastic properties of metals
 Range-
• Proffit-Distance that the wire bends elastically, before
permanent deformation occurs
• Kusy – Distance to which an archwire can be activated-
• Thurow – A linear measure of how far a wire or material
can be deformed without exceeding the limits of the
material.

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 Springback-
• Proffit- Portion of the loading curve b/w elastic limit and
ultimate tensile strength.
•Kusy -- The extent to which the range recovers upon
deactivation
•Ingram et al – a measure of how far a wire can be
deflected without causing permanent deformation.
•Kapila & Sachdeva- YS/E

96
Elastic properties
resiliency
formability
YP
PL
stress
strain

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Elastic properties
 Resiliency-Area under stress-strain curve till
proportional limit.
-Maximum amount of energy a material can absorb
without undergoing permanent deformation.
When a wire is stretched, the space between the
atoms increases. Within the elastic limit, there is
an attractive force between the atoms.
Energy stored within the wire.
Strength + springiness

98
Elastic properties
 Work = f x d
• When work is done on a body-energy imparted to
it.
• If the stress not greater than the PL elastic energy
is stored in the structure.
• Unloading occurs-energy stored is given out

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Elastic properties
 It depends on –
Stiffness and Working Range
 Independent of –
Nature of the material
Size (or)
Form

100
Elastic properties
 Formability –
• Amount of permanent deformation that the wire can
withstand before failing.
• Indication of the ability of the wire to take the shape
• Also an indication of the amount of cold work that it can
withstand

101
Elastic properties
 Flexibility –
• Amount a wire can be strained without undergoing
plastic deformation.
• Large deformation (or large strain) with minimal force,
within its elastic limit.
• Maximal flexibility is the strain that occurs when a wire
is stressed to its elastic limit.
Max. flexibility = Proportional limit
Modulus of elasticity.

102
Elastic properties
stress
strain
Toughness

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Elastic properties
 Toughness –Amount of elastic & plastic deformation
required to fracture a material. Total area under the
stress – strain graph.
 Brittleness –Inability to sustain plastic deformation
before fracture occurs.
 Fatigue – Repeated cyclic stress of a magnitude below
the fracture point of a wire can result in fracture. Fatigue
behavior determined by the number of cycles required to
produce fracture.

104
Elastic properties
 Poisson’s ratio (ν)
ν = - εx/εy=-εy/εz
Axial tensile stress (z axis) produces elastic tensile strain
and accompanying elastic contractions in x in y axis.
The ratio of x,y or x,z gives the Poissons ratio of the material
It is the ratio of the strain along the length and along the
diameter of the wire.

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Elastic properties
 Ductility –ability to sustain large permanent
deformation under tensile load before fracturing.
Wires can be drawn
 Malleability –sustain deformation under compression-
hammered into sheets.

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Requirements of an ideal arch wire
 Robert P.Kusy- 1997 (AO)
1. Esthetics
2. Stiffness
3. Strength
4. Range
5. Springback
6. Formability
7.Resiliency
8.Coefficient of friction
9.Biohostability
10.Biocompatibility
11.Weldability

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 Esthetic
•Desirable
•Manufacturers tried-coating -White coloured wires
• Deformed by masticatory loads
•Destroyed by oral enzymes
•Uncoated-transparent wires-poor mechanical properties
•Function>Esthetics
•Except the composite wires

108
 Stiffness / Load –Deflection Rate
•Proffit: - Slope of stress-strain curve
•Thurow - Force:Distance ratio, measure of resistance to
deformation.
•Burstone – Stiffness is related to – wire property &
appliance design
Wire property is related to – Material & cross section.
•Wilcock – Stiffness α Load
Deflection

109
 Magnitude of the force delivered by the appliance for a
particular amount of deflection.
Low stiffness or Low LDR implies that:-
1) Low forces will be applied
2) The force will be more constant as the appliance
deactivates
3) Greater ease and accuracy in applying a given force.

110
 Strength
• Yield strength, proportional limit and ultimate tensile &
compressive strength
• Kusy - Force required to activate an archwire to a
specific distance.
• Proffit - Strength = stiffness x range.
• Range limits the amount the wire can be bent, stiffness is
the indication of the force required to reach that limit.

111
 Range
•Distance to which an archwire can be activated
• Distance wire bends elastically before permanent
deformation.
•Measured in millimeters.

112
 Springback
• The extent to which the range recovers upon deactivation
•Clinically useful-many wires deformed
-wire performance-EL & Ultimate strength

113
 Formability
• Kusy – The ease in which a material may be permanently
deformed.
• Clinically- Ease of forming a spring or archwire

114
 Resiliency
• Store/absorb more strain energy /unit volume before
they get permanently deformed
• Greater resistance to permanent deformation
• Release of greater amount of energy on deactivation
High work availability to move the teeth

115
 Coefficient of Friction
• Brackets (and teeth) must be able to slide along the wire
• Independent of saliva-hydrodynamic boundary layer
• High amounts of friction  anchor loss.
• Titanium wires inferior to SS

116
 Biohostability-
•Site for accumulation of bacteria, spores or viruses.
• An ideal archwire must have poor biohostability.
•Should not-actively nurture nor passively act as a substrate
for micro-organisms/spores/viruses
•Foul smell, discolouration, build up of material-compromise
mechanical properties.

117
 Biocompatability
• Ability of a material to elicit an appropriate biological
response in a given application in the body
• Wires-resist corrosion –products – harmful
• Allergies
• Tissue tolerance

118
 Weldability –
• Process of fusing 2 or more metal parts though
application of heat, pressure or both with/out a filler
metal to produce a localized union across an interface.
• Wires –should be easily weldable with other metals

119
Elastic properties
 Thurow
- 3 characteristics of utmost importance
- Important for the orthodontist –selection of the
material and design-any change in 1 will require
compensatory change in others.
Strength = Stiffness x Range

120
Elastic properties
 Clinical implications:
• The properties can be expressed in absolute terms -in
orthodontics-simple comparison.
• Main concern-change in response – if there is change
in material, wire size or bracket arrangement.
• Knowledge- force and movement can be increased or
decreased in certain circumstances
Comparing the 3 properties

121
Elastic properties
 Stiffness indicates-
 rate of force delivery
how much force
how much distance can be covered
 Strength –measures the load or force that carried at its
maximum capacity
 Range-amount of displacement under maximum load

122
Elastic properties
 Factors effecting the 3 components
- Mechanical arrangement-includes bracket width,
length of arch wire.
-Form of wire-size, shape & cross-section
- Alloy formula, hardness, state of heat treatment

123
Optimal Forces & Wire Stiffness
Varying force levels produced during deactivation of a
wire: excessive, optimal, suboptimal, & subthreshold.
During treatment by a wire with high load deflection
rate the optimal zone is present only over a small
range

124
Optimal Forces & Wire Stiffness
Overbent wire with low load-deflection rate (Burstone)
Tooth will reach desired position before subthreshold force
zone is reached.
Replacement of wires is not required

125
Effects of wire cross-section
 Variable-cross section orthodontics
How does change in size and shape of wire effect
stiffness, strength & springiness?
Considering a cantilever beam;

126
 Doubling diameter makes beam 8 times stronger
 But only 1/16 times springy
 ½ the range.
 Strength changes as a cubic fn of the ratio of the 2 cross
sections.
 Springiness-4th
power
 Range-direct proportion

127
 Rectangular wire
The principle is same
 In torsion more shear stress rather than bending stress
in encountered
However the principle is same
 Increase in diameter – increase in stiffness & strength
rapidly– too stiff for orthodontic use & vice-versa
Ideally wire should be in b/w these two extremes

128
 Wire selection-based on
 load -deflection rate requirement
-magnitude of forces and moments required
Is play a factor?
 Wire ligature minimizes the play in I order direction as
wires can seat fully.
 Narrow edgewise brackets-ligature tie tends to minimize
 No point-0.018” over 0.016-diffrence in play.

129
Should a smaller wire be chosen to obtain greater elastic
deflection?
 Elastic deflection varies inversely with diameter of
wire but differences are negligible-
 0.016 has 1.15 times maximum elastic deflection as 0.018
wire.
Major reason- load deflection rate
 Small changes in the wire produce large changes in L-D
rate
 Determined by moment of inertia.

130
Shape Moment of
Inertia
Ratio to stiffness of round
wire
Пd4
64
1
s4
12
1.7
b3
h
12
1.7 b3
hd4

131
 The clinician needs a simplified system to determine the
stiffness of the wire he uses.
 Cross-sectional stiffness number (CS)-relative stiffness
 0.1mm(0.004in) round wire-base of 1.

132

133
0
500
1000
1500
2000
2500
3000
3500
Stiffnessnumber
(Burstone)
14 16 18 20 22 16x16 18x18 21x21 16x22 22x16 18x25 25x18 21x25 25x21 215x28 28x215
Wire dimension
Relative stiffness

134
 Rectangular wires
• Bending perpendicular to the larger dimension (ribbon
mode)
• Easier than bending perpendicular to the smaller
dimension (edgewise).
•The larger dimension  correction is needed.
•The smaller dimension  the plane in which more stiffness
is needed.

135
 > first order, < second order – RIBBON
 > Second order, < first order - EDGEWISE
•> 1st order correction in anterior segment
•> 2nd order in the posterior segment, wire can be
twisted 90°
•Ribbon mode in anterior region and edgewise in posterior
region.

136
 Both, 1st
& 2nd
order corrections are required to the same
extent, then square or round wires.
 The square wires - advantage - simultaneously control
torque
better orientation into a rectangular slot.
(do not turn and no unwanted forces are created).

137
 Mechanical & Elastic properties
 Ideal requirements of an arch wires
 Strength, stiffness & range
 Optimal forces and wire stiffness
 Effects of cross-section
 Strength changes as a cubic fn of the ratio of the 2 cross
sections.
 Springiness-4th
power
 Range-direct proportion
Orthodontic wires

138
Effects of length
 Changing the length-dramatically affects properties
 Considering a cantilever ;

139
Effects of length
If length is doubled-
• Strength – cut by half-(decreases proportionately)
• Springiness – inc. 8 times ( as a cubic function)
• Range – inc 4 times (increases as a square.)
In the case of torsion, the picture is slightly different.
Increase in length –
•Stiffness decreases proportionately
•Range increases proportionately
•Strength remains unchanged.

140
Effects of length
 Way the beam is attached also affects the values
 Cantilever, the stiffness of a wire is obviously less
 Wire is supported from both sides (as an archwire in
brackets), again, the stiffness is affected
• Method of ligation of the wire into the brackets.
•Loosely ligated, so that it can slide through the brackets, it
has ¼th the stiffness of a wire that is tightly ligated.

141
Effects of material
 Modulus of elasticity varied by changing the material
 Material stiffness number-relative stiffness of the
material
 Steel -1.0(Ms)

143
Nomograms
 Developed by Kusy
 Graphic representation-comparing wire materials and
sizes
 Fixed charts that display mathematical relationships-
scales
 Nomograms of each set drawn to same base, any wire on
1 of 3 can be compared to any other.

144
A reference wire is
chosen (0.012”SS) and
given a value of 1 . The
strength , stiffness and
range of other wires are
calculated to this
reference
Nomograms

147
Clinical implications
 Balance between stiffness, strength & range
 Vary - material ,cross-section or length as the
situation demands.

148
 Variation in Cross-Section
Wires with less cross-section-low stiffness (changes by
4th
power)
 Used initial part of treatment
 Thicker-stiffer wires used later

149
 Multi-stranded wires
 2 or more wires of smaller diameter are twisted
together/coiled around a core wire
 Twisting of the two wires causes the strength to increase,
so that the wire can withstand masticatory forces.
 The properties of multistranded wires depend on the
individual wires that are coiled, and on how tightly they
are coiled together.

150
 Variation in length
•Removable appliance -cantilever spring
•The material of choice is usually steel. (Stiff material)
•Good strength to resist masticatory and other oral
forces.

151
 Increase the length of the wire-
 Proportionate decrease in strength, but the stiffness
will decrease as a cubic function
 Length is increase by either bending the wire over
itself, or by winding helices or loops into the spring

152
 Fixed appliance
 The length of wire between brackets can be increased
 Loops, or Smaller brackets,
or Special bracket designs –Mini-unitwin bracket,Delta

153
 Variation in the material
 Relatively constant dimension important for the third
order control
 Titanium wires-low stiffness-used initial part of
treatment
 Steel-when rigidity-control and torque expression
required

155
Stage Wires Reason
Aligning Multistranded SS,
NiTi
Great range and light
forces are reqd
Space closure Β-Ti (frictionless),
SS – if sliding
mechanics is
needed
Increased formability,
springback , range and
modest forces per unit
activation are needed
Finishing SS , preferably
rectangular
More stability & less
tooth movement reqd

156
Stage Wires Reason
Aligning Multistranded SS,
Low LDR-SS
Great range and light
forces are reqd
Space closure SS(high resilience
aust.wire) –
sliding mechanics
Increased formability,
springback , range and
modest forces per unit
activation are needed
Finishing SS , α-titanium More stability & less
tooth movement reqd

157
A rough idea can be obtained clinically
 Forming an arch wire with the thumb gives an indication
of the stiffness of the wire.
 Flexing the wires between the fingers, without deforming
it, is a measure of flexibility
 Deflecting the ends of an archwire between the thumb
and finger gives a measure of resiliency.

158
Physical properties
 Corrosion
Chemical or electrochemical process in
which a solid, usually a metal, is attacked by an
environmental agent, resulting in partial or complete
dissolution.
 Not merely a surface deposit –deterioration of metal
 Localized corrosion-mechanical failure
 Biological effects-corrosion products

159
Physical properties
Nickel -
1. Carcinogenic,
2. Mutagenic,
3. Cytotoxic and
4. Allergenic.
 Stainless steels, Co-Cr-Ni alloys and NiTi are all rich in
Ni
 Co & Cr can also cause allergies.

160
Physical properties
 Studies-Ni alloy implanted in the tissue
 Although-more invasive –reactivity of the implanted
material is decreased –connective tissue capsule
 Intraoral placement-continuous reaction with
environment
Corrosion resistance of steel-
 SS- passivating layer-Cr-also contains Fe, Ni, Mo

161
Physical properties
 Passivating film-inner oxide layer-mainly-Cr oxide
outer- hydroxide layer
 Elgiloy-similar mechanism of corrosion resistance
 Titanium oxides-more stable
 Corrosion resistance of SS inferior to Ti alloys

162
Physical properties
-Forms of corrosion
1. Uniform attack –
 Commonest type
 The entire wire reacts with the environment
 Hydroxides or organometallic compounds
 Detectable after a large amount of metal is dissolved.
2. Pitting Corrosion –
 Manufacturing defects
 Sites of easy attack

163
Physical properties
 Excessive porous surface-as received wires
Steel NiTi

164
Physical properties
3. Crevice corrosion or gasket corrosion -
 Parts of the wire exposed to corrosive environment
 Non-metallic parts to metal (sites of tying)
 Difference in metal ion or oxygen concentration
 Plaque build up  disturbs the regeneration of the
passivating layer
 Depth of crevice-reach upto 2-5 mm
 High amount of metals can be dissolved in the mouth.

166
Physical properties
4.Galvanic /Electrochemical Corrosion
 Two metals are joined
 Or even the same metal after different type of treatment
are joined
 Difference in the reactivity 
Galvanic cell.
 
Less Reactive More Reactive
(Cathodic) (Anodic) less noble metal

167
Physical properties
 Less noble metal-oxidizes-anodic-soluble
 Nobler metal-cathodic-corrosion resistant
 “Galvanic series”
 SS-can be passive or active depending on the nobility of
the brazing material

168
Physical properties
5.Intergranular corrosion
 Sensitization - Precipitation of CrC-grain boundaries
-Solubility of chromium carbide
6.Fretting corrosion6.Fretting corrosion
 Material under load
 Wire and brackets contact –slot – archwire interface
Friction  surface destruction
 Cold welding -pressure  rupture at contact points-
wear oxidation pattern

169
Physical properties
7.Microbiologically influenced corrosion (MIC)
 Sulfate reducing-Bacteroides corrodens
 Matasa – Ist to show attack on adhesives in
orthodontics
 Craters in the bracket
 Certain bacteria dissolve metals directly form the wires.
 Or by products alter the microenvironment-accelerating
corrosion

171
Physical properties
8.Stress corrosion
 Similar to galvanic corrosion-electrochemical potential
difference-specific sites
 Bending of wires - different degrees of tension and
compression develops locally
 Sites-act as anodes and cathodes.

172
Physical properties
 9.Corrosion9.Corrosion Fatigue:Fatigue:
 Cyclic stressing of a wire-aging
 Resistance to fracture decreases
 Accelerated in a corrosive medium such as saliva
 Wires left intraorally-extended periods of time under
load

173
Physical properties
 Corrosion – Studies
 In vitro Vs In vivo
 Never simulate the oral environment
 Retrieval studies
 Biofilm-masks alloy topography
 Organic and inorganic components
 Mineralized –protective esp. low pH

174
Physical properties
 Ni hypersensitivity-case reports-very scarce
 Insertion of NiTi wires –
 rashes
 swelling
 Erythymatous lesions
 Ni and Cr
 impair phagocytosis of neutrophils and
 impair chemotaxis of WBCs.

175
Physical properties
 Ni at conc. released from dental alloys
 Activating monocytes and endothelial cells,
 Promote intercellular adhesion(molecule 1)
 Promotes inflammatory response in soft tissues.
 Arsenides and sulfides of Ni - carcinogens and mutagens.
 Ni at non toxic levels - DNA damage.

177
Stainless steel
 Gold
 1960s-Abandoned in favour of stainless steel
 Crozat appliance –original design
 1919 – Dr. F Hauptmeyer –Wipla (wie platin).
•Extremely chemically stable
•Better strength and springiness
• High resistance to corrosion-Chromium
content.

178
Stainless steel
 Properties of SS controlled-varying the degree of cold
work and annealing during manufacture
 Steel wires-offered in a range of partially annealed states
–yield strength progressively enhanced at the cost of
formability compromised
 Fully annealed stainless steel  extremely soft, and highly
formable
 Ligature wire-“Dead soft”

179
Stainless steel
 Steel wires with high yield strength- “Super” grade wires-
brittle-used when sharp bends are not needed
 High formability- “regular” wires-bent into desired
shapes

180
Stainless steel
 Structure and composition
 Iron –always contains carbon-(2.1%)
 When aprrox 12%-30% Cr added- stainless
 Cr2O3-thin transparent, adherent layer when exposed to
oxidizing atm.
 Passivating layer-ruptured by chemical/mechanical
means-protective layer reforms
 Favours the stability of ferrite (BCC)

181
Stainless steel
 Nickel(0-22%) – Austenitic stabilizer (FCC)
 Loosly bound
 Copper, manganese and nitrogen – similar function
 Mn-dec corrosion resistance
 Carbon (0.08-1.2%)– provides strength
 Reduces the corrosion resistance

182
Stainless steel
 Sensitization.
 400-900o
C-looses corrosion resistance
 During soldering or welding
 Chromium diffuses towards the carbon rich areas
(usually the grain boundaries)-chromium carbide-most
rapid 650°C
 Chromium carbide is soluble- intergranular corrosion.

183
Stainless steel
 3 methods to prevent sensitization-
1. Reduce carbon content-precipitation cannot occur-not
economically feasible
2. Severely cold work the alloy-Cr carbide ppts at
dislocations-more uniform
 Stabilization
 Addition of an element which precipitates carbide more
easily than Chromium.
 Niobium, tantalum & titanium

184
Stainless steel
 Usually- Titanium.
 Ti 6x> Carbon
 No sensitization during soldering.
 Most steels used in orthodontics are not stabilized-
additional cost

185
Stainless steel
 Other additions and impurities-
 Silicon – (low concentrations) improves the resistance
to oxidation and carburization at high temperatures and
corrosion resistance
 Sulfur (0.015%) increases ease of machining
 Phosphorous – allows sintering at lower temperatures.
 But both sulfur and phosphorous reduce the corrosion
resistance.

186
Stainless steel
 Classification
 American Iron and Steel Institute (AISI)
 Unified Number System (UNS)
 German Standards (DIN).

187
Stainless steel
 The AISI numbers used for stainless steel range from
300 to 502
 Numbers beginning with 3 are all austenitic
 Higher the number 
 Less the non-ferrous content
 More expensive the alloy
 Numbers having a letter L signify a low carbon
content

188
Austenite (FCC)
slow cooling rapid cooling
Mixture of: Tempering Martensite (BCT)
Ferrite(BCC) distorted lattice-
& Pearlite hard & brittle
Cementite(Fe3C)

190
Stainless steel
Austenitic steels (the 300 series)
 Most corrosion resistance
 FCC structure,  non ferromagnetic
 Not stable at room temperature,
 Austenite stabilizers Ni, Mn and N

191
Stainless steel
 Type 302-basic alloy -17-19%
Cr,8-10% Ni,0.15%-C
 304- 18-20%-Cr, 8-12%-
Ni,0.08%-C
 Known as the 18-8 stainless
steels- most common in
orthodontics
 316L-10-14%-Ni,2-3%-
Mo,16-18%-Cr,O.03%-C-
implants

192
Stainless steel
 The following properties-
 Greater ductility and malleability
 More cold work-strengthened
 Ease –welding
 Dec. sensitization
 Less critical grain growth
 Ease in forming
 X-ray diffraction-not always single phase-Bcc
martensitic phase present

193
Stainless steel
Khier,Brantly,Fournelle(AJO-1998)
Austenitic structure-
metastable
Decomposes to martensite-
cold work & heat treatment
Manufacturing process

194
Stainless steel
Martensitic steel (400)
 FCC  BCC
 BCC structure is highly stressed. (BCT)
 More grain boundaries,
 Stronger
 Dec. ductulity-2%
 Less corrosion resistant
 Making instrument edges which need to be sharp and
wear resistant.

196
Stainless steel
Ferritic steels – (the 400 series)
 Name derived from the fact-microstr (BCC) same as iron
 Difference-Cr
 “super ferritics”-19-30% Cr-used Ni free brackets
 Good corrosion resistance, low strength.
 Not hardenable by heat treatment-no phase change
 Not readily cold worked.

197
Stainless steel
Duplex steels
 Both austenite and ferrite grains
 Fe,Mo,Cr, lower nickel content
 Increased toughness and ductility than ferritic steels
 Twice the yield strength of austenitic steels
 High corrosion resistant-heat treated –sigma-dec
corrosion resistance
 Manufacturing low nickel attachments-one piece
brackets

198
Stainless steel
Precipitation hardened steels
 Certain elements added to them  precipitate and
increase the hardness on heat treatment.
 The strength is very high
 Resistance to corrosion is low.
 Used to make mini-brackets.

199
Stainless steel
-General properties
1. Relatively stiff material
 Yield strength and stiffness can be varied
 Altering diameter/cross section
 Altering the carbon content and
 Cold working and
 Annealing
 High forces - dissipate over a very short amount of
deactivation (high load deflection rate).

200
Stainless steel
 In clinical terms-
•Loop - activated to a very small extent so as to
achieve optimal force but
•Deactivated by only a small amount (0.1 mm)
force level will drop tremendously
•Type of force-Not physiologic
•More activations

201
Stainless steel
 Force required to engage a steel wire into a severely mal-
aligned tooth.
 Either cause the bracket to pop out,
 Or the patient to experience pain.
 Overcome by using thinner wires, which have a lower
stiffness.
 Not much control.

202
Stainless steel
High stiffness can be advantageous
 Maintain the positions of teeth & hold the corrections
achieved
 Begg treatment, stiff archwire, to dissipate the adverse
effects of third stage auxiliaries

203
Stainless steel
2. Lowest frictional resistance
 Ideal choice of wire during space closure with sliding
mechanics
 Teeth will be held in their corrected relation
 Minimum resistance to sliding

204
Stainless steel
3.High corrosion resistance
Ni is used as an austenite stabilizer.
 Not strongly bonded to produce a chemical compound.
 Likelihood of slow release of Ni
 Symptoms in sensitized patients

205
Stainless steel
 Passivating layer dissolved in areas of plaque
accumulation – Crevice corrosion.
 Different degrees of cold work – Galvanic corrosion
 Different stages of regeneration of passivating layer –
Galvanic corrosion
 Sensitization – Inter-granular corrosion

206
Stainless steel
 1919-SS introduced
 Structure and composition-stainless
 Classifications
 FCC-BCC
 General properties

208
High Tensile Australian Wires
 Claude Arthur J. Wilcock started association with dental
profession-1936-37
 Around 1946-asssociation with Dr.Begg
 Flux, silver solder, lock pins, brackets, bands, ligature
wires, pliers & high tensile wire
 Needed-wires that were active for long
 Dr Begg-progressively harder wires

209
 Beginners found it difficult to use the highest tensile
wires
 Grading system
 Late 1950s, the grades available were –
 Regular
 Regular plus
 Special
 Special plus

210
 Demand-very high-1970s
 Raw materials overseas
 Higher grades-Premium
 Preformed appliances, torquing auxiliaries, springs
 Problems-impossibility in straightening for appliances
-work softening-straightening
-breaking

211
•Higher working range- E
(same) But inc. YS
Range=YS/E
•Higher resiliency
ResilαYS2/
E
•Zero stress relaxation
•Reduced formability

212
Zero Stress Relaxation
 If a wire is deformed and held in a fixed position, the
stress in the wire may diminish with time, but the strain
remains constant.
 Property of a wire to deliver a constant light elastic
force, when subjected to external forces (like occlusal
forces).
 Only wires with high yield strength-possess this desirable
property

213
 Relaxation in material- Slip dislocation
 Materials with high YS-resist such dislocations-internal
frictional force.
 New wires-maintain their configuration-forces generated
are unaffected

214
 Zero stress relaxation in springs.
 To avoid relaxation in the wire’s working stress
 Diameter of coil : Diameter of wire = 4 (spring index)
 smaller diameter of wires  smaller diameter springs (like
the mini springs)
 Higher grade wires (high YS), ratio can be =2, much
lighter force
 Bite opening anchor bends-
zero stress relaxation –infrequent reactivation

215
 Spinner straightening
 It is mechanical process of straightening resistant
materials in the cold-hard drawn condition
 The wire is pulled through rotating bronze rollers that
torsionally twist it into straight condition
 Wire subjected to tension-reverse straining.
 Disadv:
 Decreases yield strength (strain softened)
 Creates rougher surface

216
 Straightening a wire - pulling through a series of rollers
 Prestrain in a particular direction.
 Yield strength for bending in the opposite direction will
decrease.

217
 Bauschinger effect
 Described by Dr. Bauschinger in 1886.
 Material strained beyond its yield point in one direction,
then strained in the reverse direction,
its yield strength in the reverse direction is reduced.

218
roundning

219
 Plastic prestrain increases the elastic limit of
deformation in the same direction as the prestrain.
 Plastic prestrain decreases the elastic limit of
deformation in the direction opposite to the prestrain.
 If the magnitude of the prestrain is increased, the elastic
limit in the reverse direction can reduce to zero.

220
 JCO,1991 Jun(364 - 369): Clinical Considerations in the
Use of Retraction Mechanics - Julie Ann Staggers,
Nicholas Germane
 The range of action will be greatest in the direction of the
last bend
 With open loop, activation unbends loop; but with closed
loop, activation is in the direction of the last bend
-increases range of activation.
 Premium wire  special plus or special wire

222
 Pulse straightening
Placed in special machines that permits high tensile
wires to be straightened.
This method :
Permits the straightening of high tensile wires
1. Does not reduce the yield strength of the wire
2. Results in a smoother wire, hence less wire – bracket
friction.

223
 Dr.Mollenhauer requested –ultra high tensile SS
round wire.
 Supreme grade wire –lingual orthodontics-initial faster
and gentler alignment of teeth-brackets close
 Labial Begg brackets-reduces tenderness
 Intrusion  simultaneously with the base wires
 Gingival health seemed better

224
 Higher yield strength 
more flexible
 Supreme grade flexibility
= β-titanium.
 Higher resiliency  nearly
three times.
 NiTi  higher flexibility
but it lacks formability

225
Methods of increasing yield strength of Australian
wires.
1. Work hardening
2. Dislocation locking
3. Solid solution strengthening
4. Grain refinement and orientation

226
Twelftree, Cocks and Sims (AJO 1977)
 Wires-0.016-7 wires
 Premium plus, Premium and Special plus wires showed
minimal stress relaxation-no relaxation -3 days
 Special,
 Remanit,
 Yellow Elgiloy,
 Unisil.
 Special plus maintained original coil size, Unisil-inc.
curvature

227
 Hazel, Rohan & West (1984)
 Stress relaxation of Special plus wires after 28 days
was less than Dentaurum SS and Elgiloy wires.
 Barrowes (82)
 Sp.plus greater working range than stnd. SS but
NiTi,TMA & multistranded-greater
 Jyothindra Kumar (89) -evluated working range
 Australian wires-better recovery than Remanuim

228
 Pulse straightened wires – Spinner
straightened
(Skaria 1991)
 Strength, stiffness and Range higher than spinner
staightened wires
 Coeff. of friction higher-almost double
 Similar- surface topography, stress relaxation and
Elemental makeup.

229
 Anuradha Acharya (2000)
 Super Plus (Ortho Organizers) – between Special plus
and Premium
 Premier (TP) – Comparable to Special
 Premier Plus (TP)– Special Plus
 Bowflex (TP) – Premium

230
 Highest yield strength and ultimate tensile strength as
compared to the corresponding wires.
 Higher range
 Lesser coefficient of friction
 Surface area seems to be rougher than that of the
other manufacturers’ wires.
 Lowest stress relaxation.

231
 High and sharp yield points-freeing of dislocations and
effective shear stress to move these dislocations.
 Flow stress dependent on-
 Temperature
 Density of dislocations in the material
 Resulting structure-hard-high flow stress
 Plastic deformation absence of dislocation locking-low
YS
 Internal stress=applied stress x density of dislocations

232
Fracture of wires and crack propagation
Dislocation locking

High tensile wires have high density of dislocations and
crystal defects

Pile up, and form a minute crack

Stress concentration

233
Small stress applied with the plier beaks

Crack propagation

Elastic energy is released

Propagation accelerates to the nearest grain boundary

234
Ways of preventing fracture
1.Bending the wire around the flat beak of the pliers.
-Introduces a moment about the thumb and wire gripping
point, which reduces the applied stress on the wire.

235

236
2. The wire should not be held tightly in the beaks of the
pliers.
Area of permanent deformation to be slightly enlarged,
Nicking and scarring avoided
3.Wilcock-Begg light wire pliers, preferably not tungsten
carbide tipped

237

238
4. The edges rounded  reduce the stress concentration in
the wire. –sandpaper & polish if sharp.
5.Ductile – brittle transition temperature slightly above
room temperature.
Wire should be warmed – pull though fingers
Spools kept in oven at about 40o
, so that the wire
remains slightly warm.

239
Multistranded wires
 They are composed of specified numbers of thin wire
sections coiled around each other to provide round or
rectangular cross section
 The wires-twisted or braided
 When twisted around a core wire-coaxial wire

240
Multistranded wires
Co-axial
Twisted wire

241
Multistranded wires
 Individual diameter - 0.0165 or 0.0178
final diameter – 0.016" – 0.025“
 On bending - individual strands slip over each other ,
making bending easy.
 Strands of .007 inch twisted into .017 inch-(3 wires)
stiffness comparable to a solid wire of .010 inch

242
Multistranded wires
 Stiffness – decreases as a function of the 4th
power
 Range – increases proportionately
 Strength – decreases as a function of the 3rd
power
Result - high elastic modulus wire behaving like a low
stiffness wire

243
Multistranded wires
Elastic properties of multistranded archwires depend on –
1.Material parameters – Modulus of elasticity
2.Geometric factors – moment of inertia & wire dimension
3.Twisting or braiding or coaxial
4.Dimensionless constants
 Number of strands coiled
 Helical spring shape factor
 Bending plane shape factor

244
Multistranded wires
Helical spring shape factor
 Coils resemble the shape of a helical spring.
 The helical spring shape factor is given as –
2sin α
2+ v cos α
α - helix angle and
v - Poisson’s ratio (lateral strain/axial strain)
Angle α can be seen in the following diagram :-

246
Multistranded wires
Schematic definition of the helix angle (a). If one revolution of a wire
strand is unfurled and its base length [p(D-d)] and corresponding
distance traversed along the original wire axis (S*) are determined,
then a ratio of these two distances equals tan a. Everything else
being equal, the greater p(D-d) or the less S* is, the more compliant
a wire will be.

247
Multistranded wires
 Bending shape factor
 Complex property
 number of strands
 orientation of the strands
 diameter of the strands and the entire wire
 helix angle etc
.
 Different for different types of multistranded wires

248
Multistranded wires
 Deflection of multi stranded wire
= KPL3
knEI
K – load/support constant
P – applied force
L – length of the beam
K – helical spring shape factor
n- no of strands
E – modulus of elasticity
I – moment of inertia

249
Multistranded wires
Kusy (AJO 1984)
 Triple stranded 0.0175” (3x0.008”) SS
 GAC’s Wildcat
 Compared the results to other wires commonly used by
orthodontists- SS,NiTi & β-Ti

250
Multistranded wires
 The multistranded wire did not resemble the 0.018 wire
in any way except for the size and & slot engagement
 Stiffness was comparable to 0.010 SS wire but strength
was 20% higher
0.016 NiTi-equal in stiffness, considerably stronger and
50% more activation
0.016 β-Ti –twice as stiff, comparable to 0.012 SS

253
Multistranded wires
Ingram, Gipe and Smith (AJO
86)
 Range independent of wire
size
 Range seems to increase
with increase in diameter
 It varies only from 11.2-10.0-
largest size having slightly
greater range than smallest
wire.

254
Multistranded wires
 Oltjen,Duncanson,Nanda,Currier (AO-1997)
 Wire stiffness can be altered by not only changing the
size or alloy composition but by varying the number of
strands.
 Increase in No. of strands  stiffness
 Unlike single stranded wires
 stiffness varied as deflection varied.
Increase in No. of strands
 stiffness
Unlike single stranded
wires
stiffness varied as
deflection varied.

255
Multistranded wires
Rucker & Kusy (AO 2002)
 Interaction between individual strands was negligible.
 Range and strength Triple stranded = Co-axial (six
stranded)
 Stiffness  Coaxial < Triple stranded
 Range of small dimension single stranded SS wire was
similar.

257
Cobalt chromium
 1950s the Elgin Watch
“The heart that never breaks”
 Rocky Mountain Orthodontics - Elgiloy
 CoCr alloys –belong to stellite alloys
 superior resistance to corrosion (Cr oxide),
comparable to that of gold alloys exceeding SS.

258
Cobalt chromium
Composition
 Co-40%
 Cr-20%
 Ni-15% - strength & ductility
 Fe-16%,traces of Molybdenum, Tungsten, Titanium-
stable carbides –enhance hardenability and set resistance.

259
Cobalt chromium
Advantages over SS
1. Delivered in different degrees of hardening or tempers
2. High formability
3.Further hardened by heat treatment
4.Greater resistance to fatigue and distortion
5.Longer function as a resilient spring

260
Cobalt chromium
 The alloy as received
is highly formable,
and can be easily
shaped.
 Heat treated-
Considerable strength
and resiliency
 Strength 
 Formability 

261
Cobalt chromium
 Ideal temperature- 482o
C for 7 to 12 mins
 Precipitation hardening
  ultimate tensile strength of the alloy, without
hampering the resilience.
 After heat treatment, Elgiloy had elastic properties
similar to steel
 . Heating above 650o
C
 partial annealing, and softening of the wire
 Optimum heat treatment  dark straw color of the wire or
temperature indicating paste

262
Cobalt chromium
1958-1961-4 tempers
Red – hard & resilient
Green – semi-resilient
Yellow – slightly less formable
but ductile
Blue – soft & formable

263
Cobalt chromium
 Blue-bent easily -fingers or pliers
 Recommended –considerable bending, soldering or
welding required
 Yellow -bent with ease-more resilient
-inc. in resiliency and spring performance-heat
 Green –more resilient than yellow,can be shaped to
some extent-pliers
 Red- most resilient –high spring qualities,minimal
working
Heat treatment-inc. resilient but fractures easily.

264
Cobalt chromium
After heat treatment
 Blue and yellow =normal steel wire
 Green and red tempers =higher grade steel
 E very similar –SS & blue elgiloy (10% inc in E)
 Similar force delivery and joining characters

266
Cobalt chromium
 Comparable amount of Ni
 Coefficient of friction higher than steel -recent study-
comparable to steel-zero torque brackets are used.
 The high modulus of elasticity of Co-Cr and SS-
Deliver twice the force of β-Ti and 4times NiTi for equal
amounts of activation.

267
Cobalt chromium
 Stannard et al (AJO 1986)
 Co-Cr highest frictional resistance in wet and dry
conditions.
Ingram Gipe and Smith
(AJO 86)
•Non heat treated
•Range < stainless steel
of comparable sizes
•But after heat treatment,
the range was
considerably increased.

268
Cobalt chromium
 Kusy et al (AJO 2001)
 16 mil (0.4mm or .016 inch) evaluated
 E values –identical
-red –highest- YS & UTS
-blue-most ductile

269
Cobalt chromium
 The elastic modulus did not vary appreciably  edgewise
or ribbon-wise configurations.
 Round wires -
higher ductility than square or rectangular wires

270
Cobalt chromium
 The averages of E,YS,UTS and ductility plotted against
specific cross-sec area.
 Elastic properties (yield strength and ultimate tensile
strength and ductility) were quite similar for different
cross sectional areas and tempers.
 This does not seem to agree with what is expected of the
wires.

272
Cobalt chromium
 Conclusion- based on force-deactivation
characteristics- interchangeably – SS
 Can choose different tempers and amounts of formability
 Inc the YS by heat treating
 Fine in principle-but-lack of control of the processing
variables in the as received state.

273
To strive, to seek to find ,and not to yield
- Lord Tennyson ( Ulyssess)

274
References
 Proffit – Contemporary orthodontics-3rd
ed
 Graber vanarsdall – orthodontics – current principles
and techniques-3rd
ed
 Phillips’ science of dental materials-Anusavice -11th
ed
 Orthodontic materials-scientific and clinical aspects-
Brantly and Eliades
 Edgewise orthodontics-R.C. Thurow-4th
ed
 Notes on dental materials-E.C.Combe-6th
ed

275
References
 Frank and Nikolai. A comparative study of frictional
resistance between orthodontic brackets and archwires.
AJO 80;78:593-609
 Burstone. Variable modulus orthodontics. AJO 81; 80:1-
16
 Kusy and Dilley. Elastic property ratios of a triple
stranded stainless steel archwire. AJO 84;86:177-188
 Stannard, Gau, Hanna. Comparative friction of
orthodontic wires under dry and wet conditions. AJO
86;89:485-491Ingram, Gipe, Smith. Comparative range
of orthodontic wires AJO 1986;90:296-307

276
References
 Ingram, Gipe, Smith. Comparative range of orthodontic
wires AJO 1986;90:296-30
 Arthur J Wilcock. JCO interviews. JCO 1988;22:484-489
 Khier, Brantley, Fournelle,Structure and mechanical
properties of as received and heat treated stainless steel
orthodontic wires. AJO March 1988, 93, 3, 206-212
 Twelftree, Cocks, Sims. Tensile properties of Orthodontic
wires. AJO 89;72:682-687
 Kapila & Sachdeva. Mechanical properties and clinical
applications of orthodontic wires. AJO 89;96:100-109.

277
References
 Arthur Wilcock. Applied materials engineering for
orthodontic wires. Aust. Orthod J. 1989;11:22-29.
 Julie Ann Staggers, Nicolas ,Clinical considerations in the
use of retraction mechanics.. JCO June 1991
 Klump, Duncanson, Nanda, Currier ,Elastic energy/
Stiffness ratios for selected orthodontic wires.. AJO 1994,
106, 6, 588-596
 A study of the metallurgical properties of newly introduced
high tensile wires in comparison to the high tensile
Australian wires for various applications in orthodontic
treatment. – Anuradha Acharya, MDS Dissertation
September 2000.

278
References
 Kusy, Mims, whitley ,Mechanical characteristics of
various tempers of as received Co-Cr archwires.. AJO
March 2001, 119, 3, 274-289
 Eliades, Athanasios- In vivo aging of orthodontic alloys:
implications for corrosion potential, nickel release, &
biocompatibility –AO, 72,3,2002
 Kusy.Orthodontic biomaterials: From the past to the
present-AJO May 2002

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