Properties of orthodontic wires

1
Properties of Orthodontic
Wires
Part I
Dr. Punit Thawani

2
Introduction
Moving teeth and craniofacial harmony
Forces and moments
Wires
Light continuous forces

3
History
1. Material Scarcity, Abundance of Ideas
(1750-1930)
noble metals
Gold, platinum, iridium and silver alloys
good corrosion resistance
acceptable esthetics
lacked the flexibility and tensile strength

4
Angle (1887)  German silver (a type of
brass)
Opposition Farrar
Neusilber brass (Cu 65%, Ni 14%, Zn 21%)
various degrees of cold work
– jack screws,
– expansion arches,
– Bands
History

5
History
Wood, rubber, vulcanite, piano wire and silk
thread
Steel - as ligature wire.

6
History
Stainless steel (entered dentistry -1919)
Opposition  Emil Herbst

gold wire was stronger than stainless steel.
(1934)

7
History
2. Abundance of materials, Refinement of
Procedures (1930 – 1975)
 Improvement in metallurgy and organic
chemistry
Cobalt chrome (1950s)
Nitinol (1970s)

8
History
3. The beginning of Selectivity (1975 to the
present)
Beta titanium (1980)
CAD/CAM
Composites and Ceramics
Iatrogenic damage

Nickel and bis-GMA

9
Basic Properties of Materials
Elements – no simpler form
 Atoms
Electrons – orbits around nucleus
Electron shells
Electrons form the basis of bonds
In metals, the energy levels are very closely
spaced and the electrons tend to belong to
the entire assembly rather than a single
atom.

10
Molecules – 2 or more atoms
Amorphous – similar properties in all
directions – isotropy  Glass
atoms organize themselves into specific
lattices  geometry
CRYSTAL

anisotropy

11
anion – cation –anion – cation
extremely strong
Wiskers reinforce
If like ions are forced together, breakage
results. Unlike metals, crystals cannot
deform.

12
alloy crystals grow
anion – cation –anion – cation
Order is the dream of man,
and chaos the rule of nature

13
Crystals penetrate each other such that the
crystal shapes get deformed
Grains  microns to centimeters
Grain boundaries
Atoms are irregularly arranged, and this
leads to a weaker amorphous type structure.
Alloy  combination of crystalline (grains)
and amorphous (grain boundaries)

14

15
Vacancies – These are empty atom sites

16
Interstitials – Smaller atoms that penetrate the
lattice Eg – Carbon, Hydrogen, Oxygen, Boron.

17
Substitutial Element – another metal atom can
substitute one of the same or similar size. E.g. -
Nickel or Chromium substituting iron in stainless
steel.

18
The three dimensional arrangement of lines
that can be visualized as connecting the
atoms in undisrupted crystals, is called a
lattice.
Unit cell
Crystal  combination of unit cells, in which
each shell shares faces, edges or corners
with the neighboring cells
14 crystal lattices

19

20
The atoms, which are represented as
points, are not static. Instead, they
oscillate about that point and are in
dynamic equilibrium.

21
various defects  slip
planes

22
shear stress  atoms of the crystals
can glide
more the slip planes easier is it to
deform
Slip planes intercepted at grain boundaries

23
If the shearing force is:-
Small - atoms slip, and return back to their
original position (elastic deformation)
Beyond the elastic limit -
crystal suffers a slight deformation
permanent (plastic deformation)
Greater stress - fracture

24
During deformation - atomic bonds within the
crystal get stressed
 resistance to more deformation
Number of atoms that get stressed also
increases  resistance to more
deformation
Strain or work hardening or cold work

25
Strain hardening
 Hard and strong
Brittle.
Annealing – heat below melting point.
– More the cold work, more rapid the
annealing
– Higher melting point – higher annealing
temp.
– ½ the melting temperature (oK)

26
Recovery
Recrystallization
Grain Growth

27

28
Before Annealing
Recovery – Relief of stresses
Recrystallization – New grains
from severely cold worked areas
Grain Growth – large crystal “eat
up” small ones

29
Recovery Heat Treatment
– Relief of internal stresses
– Heating to low temperatures (370 – 480oC)
for 11 mins
– Increase in measured elastic properties
– Residual stresses + External stresses

30
Various methods of obtaining smaller grain size
1. Enhancing crystal nucleation by adding fine
particles with a higher melting point, around which
the atoms gather.
2. 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.

31
Twinning
Closed packed hexagonal type of crystals
Two symmetric halves
Fixed angle
NiTi
Subjected to a higher temperature,
de - twinning occurs

32

33
Polymorphism
Metals and alloys exist as more than one type
of structure
Transition from one to the other
Allotropy
At higher temperature, ironFCC structure
(austenite)
lower temperatures,  BCC structure
(ferrite)

34
Iron  FCC form
(austenite),
Lattice spaces greater,
Carbon atom can
easily be
incorporated into
the unit cell

35
On Cooling
FCC  BCC
Carbon diffuses out
as FeC
FeC adds strength
to ferrite and
austenite
TIME

36
Rapidly cooled
(quenched)
– Carbon cannot
escape
Distorted body
centered tetragonal
lattice called
martensite

37
Grain boundaries are more in number
Alloy is stronger and more brittle
Interstitials are intentionally incorporated
into the alloy to make it hard when it is
quenched.

38
Cooled slowly
Other crystal structures are formed at
intermediate temperatures Softer
Some are stable at room temperature
Ultimately, the final structure is softer and
more workable

39
Tempering –
Reheat the alloy to intermediate temperature
Partial formation of the softer alloys
More workable

40
Some alloys
FCC to BCC by rearrangement of atoms
Diagonal plane of the BCC unit becomes the
face of the FCC unit
Shape memory alloys – Easy switching from
one type of structure to another.
Bain distortion
Over a range of temperature {hysteresis}

41
Elastic Properties
Force applied to wire Deflection
External force Internal force
(equal and opposite)
Internal force = Stress
Area of action
Deflection change in length = Strain
Original length

42
Elastic Properties
Types of stress/strain
Tensile
Compressive
Shear
Complex force systems

44
Elastic Properties
Strain
Stress
Elastic Portion

45
Elastic Properties
When the stress is removed, the wire returns
back to its original dimension.
Hooke’s law

46
Elastic Properties
Strain
Stress
Elastic Limit
Proportional Limit
Yield strength
0.1%

47
Elastic Properties
Strain
Stress
Ultimate Tensile
Strength Fracture Point

48
Elastic Properties
ultimate tensile strength is higher than the
yield strength
important clinically  maximum force that
the wire can deliver
Ultimate tensile strength higher than the
stress at the point of fracture
–reduction in the diameter of the wire

49
Elastic Properties
Strain
Stress
Slope α Stiffness
Stiffness α 1 .
Springiness

50
Elastic Properties
Strain
Stress
Range Springback

51
Elastic Properties
Strength = Stiffness x Range

52
Elastic Properties
Resiliency -
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.

53
Elastic Properties
Formability - amount of permanent
deformation that the wire can withstand
without breaking
Indication of the ability of the wire to take the
shape
Also an indication of the amount of cold
work that they can withstand

54
Elastic Properties
Strain
Stress
Resilience Formability

55
Elastic Properties
Flexibility
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.

56
Elastic Properties
Toughness –force required to fracture a
material. Total area under the stress – strain
graph.
Brittleness –opposite of toughness. A brittle
material, is elastic, but cannot undergo
plastic deformation.
Fatigue – Repeated cyclic stress of a
magnitude below the fracture point of a wire
can result in fracture. This is called fatigue.

57
Requirements of an ideal
archwire (Kusy
1. Esthetics
2. Stiffness
3. Strength
4. Range
5. Springback
6. Formability
7. Resiliency
8. Coefficient of friction
9. Biohostability
10. Biocompatibility
11. Weldability

58
1. Esthetics
Desirable
No compromise on mechanical properties
White coloured wires discolour
Destroyed by oral enzymes
Deformed by masticatory loads
Except the composite wires

59
2. Stiffness / Load deflection
Rate
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

60
Stiffness / Load deflection Rate
Magnitude of the force delivered by the appliance for a
particular amount of deflection.
Low stiffness or load deflection rate 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.

61
Stiffness / Load deflection Rate
The shape and cross section of a wire have
an effect on the strength of the wire. The
effects of these will be considered
subsequently.

62
3. 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.

63
4. Range
Distance that the wire bends elastically,
before permanent deformation occurs
(Proffit).
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.

64
5. Springback
Large springback
Activated to a large extent.
Hence it will mean fewer archwire changes.
Ratio – yield strength
Modulus of elasticity

65
5. Springback
Kusy - The extent to which a wire recovers
its shape after deactivation
Ingram et al – a measure of how far a wire
can be deflected without causing permanent
deformation. (Contrast to Proffit).

66
6. Formability
Kusy – the ease in which a material may be
permanently deformed.

67
7. Resiliency
Amount of energy stored in a body.

68
8. Coefficient of friction
Brackets (and teeth) must be able to slide
along the wire
High amounts of friction  anchor loss.

69
9. Biohostability:- site for accumulation of
bacteria, spores or viruses. An ideal archwire
must have poor biohostability.
10. Biocompatibility:- Resistance of
corrosion, and tissue tolerance to the wire.
11. Weldability:- the ease by which the wire
can be joined to other metals, by actually
melting the 2 metals in the area of the bond.
(A filler metal may or may not be used.)

70
Properties of Wires
Before the titanium alloys were introduced
into orthodontics, the practitioners used
only steel wires. So the way to control the
stiffness of the wire was:-
1. Change the cross section of the wire
2. Increase the length of the wire ( inter
bracket distance, incorporate loops.)

71
Effects of Wire Cross Section
Wires of various dimensions and cross
sections.
Does the wire need to be move teeth over
large distances, or does it need to correct the
torque of the tooth?
Is it primarily going to be used to correct first
order irregularities, or second order?

72
primary factor 
load deflection rate or stiffness
play of the wire not very significant
ligature ties minimizes the play in the first
order

73
Play in the second order –
0.016” wire in 0.022” slot is only 1.15 times
the play of a 0.018” wire.
The play in the second order becomes
significant if the wire dimensions are
drastically different (0.010” and 0.020”)

74

75

76
Shape Moment
of Inertia
Ratio to stiffness of
round wire
Пd4
64
1
s4
12
1.7
b3h
12
1.7 b3h:d4

77
Stiffness of different dimensions of wires can
be related to each other.
0
500
1000
1500
2000
2500
3000
3500
Stiffness
number
(Burstone)
14 16 18 20 22 16x16 18x18 21x21 16x22 22x16 18x25 25x18 21x25 25x21 215x28 28x215
Wire dimension
Relative stiffness

78

79

80
A rough idea can be obtained clinically as well
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.

81
Rectangular wires  bending perpendicular
to the larger dimension (ribbon mode)
easier than bending perpendicular to the
smaller dimension (edgewise).
0
500
1000
1500
2000
2500
3000
3500
Stiffness
number
(Burstone)
14 16 18 20 22 16x16 18x18 21x21 16x22 22x16 18x25 25x18 21x25 25x21 215x28 28x215
Wire dimension
Relative stiffness

82
The larger dimension  correction is
needed.
The smaller dimension  the plane in which
more stiffness is needed.
> first order, < second order – RIBBON
> Second order, < first order - EDGEWISE

83
> 1st order correction in anterior segment
> 2nd order in the posterior segment,
wire can be twisted 90o
ribbon mode in anterior region and
edgewise in posterior region.

84
If 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).

85
In torsion - absolute values of strength,
stiffness and range are different,
but the overall effect of changing the
diameter of the wire is the same.
1. Strength – Increases with increase in
diameter
2. Stiffness – increases
3. Range – decreases.

86
Effects of Length
Loops,
 the inter-bracket distance
For bending
1. Strength – decreases proportionately
2. Stiffness – decreases as a cubic function
3. Range – increases as a square.

87
Effects of Length
In the case of torsion, the picture is slightly
different. Increase in length –
1. Stiffness decreases proportionately
2. Range increases proportionately
3. Strength remains unchanged.

88
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

90
Effects of Length
Stiffness is also affected by the 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.

91
Clinical Implications
LIGHT CONTINUOUS FORCES
Stiff wires should be taboo to the
orthodontist?
Springier wire, can be easily distorted in the
harsh oral environment.
Aim at balance.

92
Removable appliance cantilever spring
The material of choice is usually steel. (Stiff
material)
Sufficiently thick steel wire
Good strength to resist masticatory and other
oral forces.
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 helicals or loops into
the spring

93
In archwires of stiffer materials the same
principle can be used.
The length of wire between brackets can be
increased
– loops, or smaller brackets, or special
bracket designs.
Also, the use of flexible wires

94
Multi-stranded wires
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.

95
Variable cross-section orthodontics.
Variable modulus orthodontics.

97
variable modulus orthodontics – Advantage-
– relatively constant dimension
– important for the third order control
variable stiffness approach,
– compromise control for getting a wire with
adequate stiffness,
– had to spend clinical time bending loops
into stiffer archwires, which would offer less
play.

98
Take into account the amount of force that
wire can deliver.
For example, a NiTi wire  efficient in tipping
teeth to get them into alignment, but may not
be able to achieve third order corrections.
After using rectangular NiTi wires for
alignment, rectangular steel wire must
always be used to achieve the correct torque
of the tooth.

99
Corrosion
Nickel -
1. Carcinogenic,
2. mutagenic,
3. cytotoxic and
4. allergenic.
Stainless steels, Co-Cr-Ni alloys and NiTi
are all rich in Ni

100
Corrosion
Placement in the oral cavity

Greater peril than implanting

Implanted material gets surrounded by a
connective tissue capsule

In the oral cavity, the alloy is free to react
with the environment.

101
Corrosion
Passivating film  traces of Fe Ni and Mo.
Aqueous environment
– inner oxide layer
– outer hydroxide layer.
CrO is not as efficient as TiO in resisting
corrosion some Ni release
Improper handling  sensitization

102
Corrosion
1. Uniform attack –
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

104
Corrosion
3. Crevice corrosion or gasket corrosion -
Parts of the wire exposed to corrosive environment
Sites of tying to the brackets
Plaque build up  disturbs the regeneration of the
passivating layer
Reach upto 2-5 mm
High amount of metals can be dissolved in the
mouth.

105
Corrosion
4. Galvanic corrosion
two metals are joined
or even the same metal after different type of
treatment (soldering etc)
difference in the reactivity 
Galvanic cell.
 
Less Reactive More Reactive
(Cathodic) (Anodic)

106
Corrosion
Anodic

Looses Electrons

Soluble ions

Leach out
Cathodic

Accepts electrons

Even less reactive

107
Corrosion
5. Intergranular corrosion
Sensitization
6. Fretting corrosion
Wire and brackets contact
Friction  surface destruction
Pressure  rupture of the oxide layer
Debris get deposited at grain boundaries, grain
structure is disturbed.

108
Corrosion
7. Microbiologically influenced corrosion
Adhersive
Craters at the base of brackets
Or wires directly bonded on to teeth
shown by matasa.
Certain bacteria dissolve metals directly form
the wires.
Others affect surface structure.

110
Corrosion
8. Stress corrosion
Similar to galvanic corrosion
Bending of wires  different degress of
tension and compression.
Alter the electrochemical behavior
 
anode cathode

111
Corrosion
9. Fatigue corrosion –
Cyclic stressing of a wire
Accelerated in a corrosive medium such as
saliva

112
Corrosion
Analysis of used wires also indicated that a
biofilm was formed on the wire.
Eliades et al
Calcification
– Shielding the wire
– Protecting the patient form the alloy

113
Corrosion
Ni and Cr
– impair phagocytosis of neutrophils and
– impair chemotaxis of WBCs.
Ni at conc. Released from dental alloys
– activating monocytes and endothelial cells,
– promote intracellular adhesion.
– This promotes inflammatory response in soft
tissues.
Arsenides and sulfides of Ni - carcinogens and
mutagens.
Ni at non toxic levels - DNA damage.

114
Orthodontic Arch Wire Materials

115
Precious Metals
Upto about the 1950s
Gold alloys
Only wire which would tolerate the oral
environment
Crozat appliance – according to original
design

116
Stainless Steel
1919 – Germany  used to make prostheses.
Extremely chemically stable
High resistance to corrosion.
Chromium content.
The chromium gets oxidized,

Impermeable, corrosion resistant layer.

117
Stainless Steel
Variety of stainless steels
Varying the degree of cold
work and annealing during
manufacture
Fully annealed stainless
steel  extremely soft, and
highly formable
Ligature wire
“Dead soft”

118
Stainless Steel
stainless steel arch wires are cold worked to
varying extents,  yield strength increases,
at the cost of their formability
The steel with the highest yield strength, the
Supreme grade steels, are also very brittle,
and break easily when bent sharply.

119
Stainless Steel
Structure and composition
Chromium (11-26%)–improves the corrosion
resistance
Nickel(0-22%) – austenitic stabilizer
copper, manganese and nitrogen - similar
– amount of nickel added to the alloy
– adversely affect the corrosion resistance.

120
Stainless Steel
Carbon (0.08-1.2%)– provides strength
Reduces the corrosion resistance
Sensitization.
Improper cooling after heat treatment
– During soldering or welding
– 400-900oc
Chromium diffuses towards the carbon rich
areas (usually the grain boundaries)

121
Stainless Steel
Chromium carbides
Amount of chromium decreases
Chromium carbide is soluble, 
intergranular corrosion.
Stabilization

122
Stabilization –
– Element which precipitates carbide more
easily than Chromium.
– Usu. Titanium.
– Ti 6x> Carbon
– No sensitization during soldering.
– Most steels used in orthodontics are not
stabilized.
Stainless Steel

123
Stainless Steel
Silicon – (low concentrations) improves the
resistance to oxidation and carburization at high
temperatures.
Sulfur (0.015%) increases ease of machining
Phosphorous – allows sintering at lower
temperatures.
But both sulfur and phosphorous reduce the
corrosion resistance.

124
Manufacture
Various steps –
1. Melting
2. Ingot Formation
3. Rolling
4. Drawing
Stainless Steel

125
Melting
– Various metals of the alloy are melted
– Proportion influences the properties
Ingot formation
– Molten alloy into mold.
– Non uniform in compositon
– Porosities and slag.
– Grains seen in the ingot – control of grain
size.
Stainless Steel

126
– Porosities due to dissolved gasses
(produced / trapped)
– Vacuum voids due to shrinking of late
cooling interior.
– Important to control microstructure at this
stage.
Stainless Steel

127
Rolling –
– First mechanical process.
– Ingot reduced to thinner bars
– Finally form a wire
– Different wires, slightly different
composition.
Stainless Steel

128
– Altering of the shape of grains during rolling
– Grains get elongated, defects get rearranged
– Work hardening.
– Wires starts to crack if rolling continues
– Annealing is done
– Rolling continued
Stainless Steel

129
Drawing
– More precise
– Ingot  final size.
– Wire pulled through small hole in a die
– Progressively smaller diameter.
– Same pressure all around, instead of from 2
opposite sides.
Stainless Steel

130
– Annealing at regular intervals.
– Exact number of drafts and annealing cycles
depends on the alloy.
Stainless Steel

131
Stainless Steel
Classification
American Iron and Steel Institute (AISI)
Unified Number System (UNS)
German Standards (DIN).

132
Stainless Steel
The AISI numbers used for stainless steel range
from 300 to 502
Numbers beginning with 3 are all austenitic
Higher the number 
– More the iron content
– More expensive the alloy
– Numbers having a letter L signify a low carbon
content

134
Stainless Steel
Austenitic steels (the 300 series)
Corrosion resistance
FCC structure,  non ferromagnetic
Not stable at room temperature,
Austenite stabalizers Ni, Mn and N
Known as the 18-8 stainless steels.

135
Stainless Steel
Martensitic steel
FCC  BCC
BCC structure is highly stressed.
More grain boundaries,
– Stronger
– Less corrosion resistant
Making instrument edges which need to be
sharp and wear resistant.

136
Stainless Steel
Ferritic steels – (the 400 series)
Good corrosion resistance
Low strength.
Not hardenable by heat treatment.
Not readily cold worked.

137
Stainless Steel
Austenitic steels more preferable :-
Greater ductility and ability to undergo more cold
work without breaking.
Substantial strengthening during cold work.
(Cannot be strengthened by heat treatment).
Strengthening effect is due partial conversion to
martensite.
Easy to weld
Easily overcome sensitization
Ease in forming.

138
Stainless Steel
Duplex steels
Both austenite and ferrite grains
Increased toughness and ductility than
Ferritic steels
Twice the yield strength of austenitic steels
Lower nickel content
Manufacturing low nickel attachments

139
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.

140
Stainless Steel
General properties of steel
Relatively stiff material

142
Stainless Steel
Yield strength and stiffness can be varied
– Altering the carbon content and
– Cold working and
– Annealing
High forces - dissipate over a very short
amount of deactivation (high load deflection
rate).

143
Stainless Steel
Clinical terms:-
Loop - activated to a very small extent so as
to achieve optimal force
Deactivated by only a small amount (0.1 mm)
Force level will drop tremendously
Not physiologic
More activations

144
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.
Fit poorly loss of control on the teeth.

145
Stainless Steel
High stiffness 
Maintain the positions of teeth
Hold the corrections achieved
Begg treatment, stiff archwire, to dissipate
the adverse effects of third stage auxiliaries

146
Stainless Steel
Lowest frictional resistance
Ideal choice of wire during space closure
with sliding mechanics
Teeth be held in their corrected relation
Minimum resistance to sliding

147
Stainless Steel
Ni is used as an austenite stabilizer.
Not strongly bonded to produce a chemical
compound.
d likelihood of slow release of Ni

148
Stainless Steel
Nickel and chromium content in the steel
wires
Cause patients to be sensitized
Cause reactions in already sensitized
patients

149
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
Stainless Steel

150
High Tensile Australian Wires
History
Early part of Dr. Begg’s career
Arthur Wilcock Sr.
– Lock pins, brackets, bands, wires, etc
Wires which would remain active for long
No frequent visits
This lead wilcock to develop steel wires of high
tensile strength.

151
Beginners found it difficult to use the highest
tensile wires
Grading system
Late 1950s, the grades available were –
– Regular
– Regular plus
– Special
– Special plus

152
Newer grades were introduced after the 70s.
Raw materials directly from the suppliers
from out of Australia
More specific ordering and obtaining better
raw materials
Higher tensile strength wires – premium

153
Highly brittle, and broke
easily
Straightening the wire
 softening and loss of
high tensile properties.

154
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.

155

156
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.

157
Straightening a wire  pulling through a
series of rollers
Prestrain in a particular direction.
Yield strength for bending in the opposite
direction will decrease.
Premium wire  special plus or special wire

158
Special pulsing process.
This method :
1. Permits the straightening of high tensile
wires
2. Does not reduce the yield strength of the
wire
3. Results in a smoother wire, hence less wire
– bracket friction.

159
Higher yield strength 
more flexible.
Supreme grade
flexibility = β-titanium.
Higher resiliency 
nearly three times.
NiTi  higher flexibility
but it lacks formability

160
Mollenhauer 
Supreme grade wire  faster and gentler
alignment of teeth.
Intrusion  simultaneously with the base
wires
Gingival health seemed better
Originally in lingual orthodontics
Equally good for labial orthodontics as well.

161
Methods of increasing yield strength of
Australian wires.
1. Work hardening
2. Dislocation locking
3. Solid solution strengthening
4. Grain refinement and orientation

162
Clinical significance of high yield strength.
Flexibility – Yield strength
Elastic Modulus
Resilience - (Yield strength)2
Elastic Modulus
Plastic portion of the stress-strain graph is
smaller  wires are more brittle.

163
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 give constant light
force, when subjected to external forces (like
occlusal forces).

164
external forces

particles slip over each other

activation of the wire is lost
Overcome
 
Internal friction
Between particles
 yield strength

165
Zero stress relaxation in springs.
To avoid relaxation in the wire’s working
stress
Diameter of coil : Diameter of wire = 4
smaller diameter of wires  smaller diameter
springs (like the mini springs)
Higher grade wires, ratio can be 2

166
Twelftree, Cocks and Sims (AJO 1977)
Premium plus, Premium and Special plus
wires showed minimal stress relaxation.
Special,
Remanit,
Yellow Elgiloy,
Unisil.

167
Hazel, Rohan & West (1984)
– Stress relaxation of Special plus wires after
28 days was less than Dentaurum SS and
Elgiloy wires.
Barrowes (82) & Jyothindra Kumar (89)
– Higher working range among steel wires.

168
Pulse straightened wires – Spinner
straightened
(Skaria 1991)
– Strength, stiffness and Range higher
– Coeff. of friction higher
– Similar surface topography, stress
relaxation and Elemental makeup.

169
Anuradha Acharya (2000)
– Super Plus (Ortho Organizers) – between
Special plus and Premium
– Premier (TP) – Comparable to Special
– Premier Plus – Special Plus
– Bowflex – Premium

170
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.

171
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

172
Small stress applied with the plier beaks

Crack propagation

Elastic energy is released

Propagation accelerates to the nearest grain
boundary

173
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.

174

175
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.
The tips of the pliers should not be of
tungsten carbide.

176

177
3. The edges rounded  reduce the stress
concentration in the wire.
4. Ductile – brittle transition temperature
slightly above room temperature.
Wire should be warmed.
Spools kept in oven at about 40o, so that the
wire remains slightly warm.

178
Multistranded Wires
2 or more wires of smaller diameter are
twisted together/coiled around a core wire.
Diameter - 0.0165 or 0.0175, but the stiffness
is much less.
On bending  individual strands slip over
each other and the core wire, making
bending easy.

179
Multistranded Wires
As the diameter of a wire decreases –
Stiffness – decreases as a function of the 4th
power
Range – increases proportionately
Strength – decreases as a function of the 3rd
power
Multistranded wires  Small diameter wires,
High strength

180
Multistranded Wires
Elastic properties of multistranded archwires
depend on –
1. Material parameters – Modulus of elasticity
2. Geometric factors – moment of inertia &
wire dimension
3. Dimensionless constants
– Number of strands coiled
– Helical spring shape factor
– Bending plane shape factor

181
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 :-

182
Multistranded Wires
α = tan -1 l* .
π (D-d)

183
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

184
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.

187
Multistranded Wires
Did not resemble the 0.018” SS wire except -
Size
Wire-bracket relation.

188
Multistranded Wires
Ingram, Gipe and Smith
(AJO 86)
Range independent of
wire size
Consistent for different
configurations of wires.
For round as well as
rectangular
multistranded wires.

189
Multistranded Wires
Nanda et al (AO 97)
Increase in No. of
strands  stiffness
Unlike single stranded
wires
– stiffness varied as
deflection varied.

190
Multistranded Wires
Kusy (AO 2002)
Interaction between individual strands was
negligible.
Range and strength Triple stranded Ξ Co-
axial (six stranded)
Stiffness  Coaxial < Triple stranded
Range of small dimension single stranded
SS wire was similar.

192
Friction
Two bodies in contact,
Relative motion
2 components :-
Frictional force –contact force between the sliding
surfaces and is opposite to it.
Normal (perpendicular force) – forces that arise
which are normal (perpendicular) to the two surfaces
in contact

193
Friction
Frictional force = Coefficient of friction x
normal force.
Coefficient of friction
– static
– kinetic

195
Friction
Friction is determined by the relative
roughness of the two materials in contact.
This depends on :-
1. The material itself
2. manufacturing process (polishing, heat
treatment etc.)
3. Shelf and/or use time.(due to corrosion,
creep etc)

196
Friction
Normal force –
1. Stiffness of the wire in bending.

197
Friction
Friction is independent
of the area of contact
- Proffit
Rough at molecular
level – Asperities
Real contact only at
these spots.

198
Friction
Light loads can cause
plastic deformation.
Shearing of asperities.
“Stick-slip”
phenomenon.
Interlocking of
asperities.
Harder materials plow
into softer materials.

199
Friction
Frank and Nikoli (AJO-1980)
Friction depends on wire bracket angulation
– At lower angulations surface
– Higher angulations Normal force

200
Friction
Rectangular wires > Round wires
– Greater area of contact.
At higher angulations  0.020” higher
friction than some rectangular wires (17x25)

201
Friction
Influence of wire material
NiTi wires - rougher surface than steel or
Elgiloy – more friction at low angulations
at high angulations - normal force
– NiTi  lower stiffness than steel or Elgiloy
 lower frictional resistance.

202
Friction
Stannard et al (AJO 1986)
surface layer of oxides - harder than the
material on the interior
unpredictability of the exact area of contact
Friction cannot be determined accurately
every time.

203
Friction
Before and after sliding-
SS had a smooth and polished structure
before sliding & surface roughness and wear
tracks were seen after sliding
β titanium had a grainy surface before and a
smoother surface after sliding.
NiTi and cobalt chrome had a fibrous surface
before and no change after sliding.

204
Friction
Dry and wet conditions
Wet condition  the coefficient of friction
Except cobalt chromium
Liquid  the adhesion b/w molecules of the
wire and bracket
Saliva does not act as a lubricant

205
Friction
Stainless steel and β titanium - lowest
friction.
NiTi
Cobalt Chrome. - Highest

206
Friction
Tidy (AJO 1989)
stainless steel - lowest friction
NiTi second
TMA
TMA 5 times > stainless steel

207
Friction
At high wire bracket angulations – normal
force more important.
– Initial stages – High friction of steel wires
– NiTi and β titanium have lower friction.
(lower stiffness)
After alignment, surface characteristics more
important
– Steel has smoother surface.

208
Friction
Round or Rectangular for sliding?
– After alignment, difference is negligible.
– Rect. Wire Better torque control
Leave rect. Wire in place for final corrections
Start sliding after 1 visit.

209
Welding of Steel
3 useful properties –
1. Comparatively low melting point,
2. High electrical resistance and
3. Low conductivity of heat.

210
Welding of Steel
Important to
– minimize the time of passing the current
– minimize the area of heating
Sensitization - between 400 and 900oC
Chromium carbides need time for their
formation.

211
Welding of Steel
Join two thin sheets of metal
Same thickness
Joining tubes, wires and springs, soldering
is generally recommended.
Electrodes - small tips, not exceeding 1 mm
in diameter.

212
Cobalt Chromium
1950s the Elgin Watch
Rocky Mountain Orthodontics
Elgiloy
CoCr alloys - satellite alloys
– superior resistance to corrosion,
comparable to that of gold alloys.

213
Cobalt Chromium
Cobalt – 40-45%
Chromium – 15-22%
Nickel – for strength and ductility
Iron, molybdenum, tungsten and titanium to
form stable carbides and enhance
hardenability.

214
Cobalt Chromium
Strength and formability modified by heat
treatment.
The alloy is highly formable, and can be
easily shaped.
Heat treated.
– Strength 
– Formability 

216
Cobalt Chromium
Heat treated at 482oc 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.

218
Cobalt Chromium
Blue – soft
Yellow – ductile
Green – semiresilient
Red – resilient

219
Cobalt Chromium
Blue considerable bending, soldering or
welding
Red  most resilient and best used for
springs
– difficult to form, (brittle)
– After heat treatment , no adjustments can be
made to the wire, and it becomes extremely
resilient.

220
Cobalt Chromium
After heat treatment 
Blue and yellow ≡ normal steel wire
Green and red tempers ≡ higher grade steel

221
Cobalt Chromium
Heating above 650oC
– partial annealing, and softening of the wire
Optimum heat treatment  dark straw color
of the wire
advantage of Co-Cr over SS is –
– Greater resistance to fatigue and distortion
– longer function as a resilient spring

222
Cobalt Chromium
Properties of Co-Cr are very similar to that of
stainless steel.
Force
– 2x of β titanium and
– 1/4th of niti.

223
Cobalt Chromium
Frank and Nikoli (1980)
– Co-Cr alloys ≡ stainless steel.
Stannard et al (AJO 1986)
– Co-Cr highest frictional resistance in wet
and dry conditions.

224
Cobalt Chromium
Ingram Gipe and Smith
(AJO 86)
Non heat treated
– Range < stainless
steel of comparable
sizes
But after heat
treatment, the range
was considerably
increased.

225
Cobalt Chromium
Kusy et al (AJO
2001)
The elastic modulus
did not vary
appreciably 
edgewise or ribbon-
wise configurations.

226
Cobalt Chromium
Round wires 
higher ductility than
square or
rectangular wires.

227
Cobalt Chromium
As expected, the
modulus of elasticity
was independent of
temper of the wire.

228
Cobalt Chromium
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.

230
Cobalt Chromium
Different tempers with different physical
properties – attractive
More care taken during the manufacture of
the wires.

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