Orthodontic wires /certified fixed orthodontic courses by Indian dental academy

Orthodontic Wires

INDIAN DENTAL ACADEMY
Leader in Continuing Dental Education
www.indiandentalacademy.com
1

Introduction

Forces & moments
Various alloys
Newer techniques –
differential tooth movement
Light continuous forces

2

Contents
 Evolution of biomaterials
 Evolution of orthodontic wire materials

 Basic structure of metals

 Mechanical properties of wires

 Ideal criteria of archwire

 Effects of change in shape and size on
elastic properties of wires
 nomograms

3

Evolution of Biomaterials
1. Material Scarcity, Abundance of Ideas
(1750-1930)

2. Abundance of materials, Refinement of
Procedures (1930 – 1975)

3. The beginning of Selectivity (1975 -
present)

4

Evolution of Biomaterials
1. Material Scarcity, Abundance of Ideas (1750-1930)
 Quest for newer materials by Angle
 Wood, rubber, vulcanite, piano wire and silk thread
 No restrictions

2. Abundance of materials, Refinement of Procedures (1930
– 1975)
 Improvement in metallurgy and organic chemistry – mass
production (1960)
 Development of newer materials

3. The beginning of Selectivity (1975 to the present)
 CAD/CAM , CNC
 Composites and Ceramics
5

Evolution of Orthodontic wire materials

 Noble metals – Gold (75%), platinum, iridium & silver
alloys
c o rro s io n re s is ta nt - fle x ibility & te ns ile s tre ng th
 Angle (1887)  German silver (a type of brass)
O p p o s itio n Fa rra r – d is c o lo ra tio n
 Change in compostion -(Cu 65%, Ni 14%, Zn 21%)
various degrees of cold work (diff prop) Neusilber
brass
 Rigid -jack screws,
 Elastic -expansion arches, Easy solderability
 Malleable -Bands 6


Stainless steel (entered dentistry -1920;
world war I)
 1930’s – popular –refinement of drawing
process
 1934 - Opposition  Emil Herbst gold > SS
Angle – steel as ligature wire
 1950’s – type 300 series – most orthodontic
appliances
 17-25% Cr
 8-25% Ni
 Balance Fe
7

 Rocky Mountain – 2 tempers of cold worked
steel
 Standard
 Extra hard grade
 American orthodontics
 Standard
 Gold tone
 Super gold tone
 M/s A J Wilcock – Australian wires
 Regular ; regular plus
 Special ; special plus
 Premium P ; premium Plus P+
 Supreme S
www.indiandentalacademy.com 8

Cobalt chromium
 1950s-Elgin watch co.
 Co – 40%
 Cr – 20%
 Fe – 16%
 Ni – 15%

 Rocky Mountain Orthodontics- Elgiloy™
⊙ various tempers
Red – hard & resilient
green – semi-resilient
Yellow – slightly less formable but ductile
Blue – soft & formable 9


 Nickel Titanium alloys (late1960s)
 Buehler - Office of Navy – alloys – shape memory effect (SME)

Nitinol- Nickel Titanium Naval Ordnance laboratory
 deformed, clamped, heated & cooled - specified shape

 1970s George Andreasen. UNITEK - orthodontics
 50:50 Ni and Ti

 TYPES
1. Conventional NiTi
2. Pseudoelastic NiTi
3. Thermoelastic NiTi
Superelastic NiTi

10

 Pseudoelastic Niti
 Pseudoelasticity – stress induced austenitc
martensitc phase transformation
Copper NiTi™- Cu 5 – 6%
Cr 0.2 – 0.5%
Thermoelastic NiTi - Miura
 Thermoelasticity – thermally induced austenitic
martensitic
phase transformation
Sentalloy™-GAC
Chinese NiTi – General research institute for Non
Ferrous Metals
Japanese NiTi – FURUKAWA electric co Ltd
11

β titanium
 Early 1980s
 Composition
 Ti – 80%
 Molybdenum – 11.5%
 Zirconium – 6%
 Tin – 4.5%
 ORMCO – Burstone’s objective  deactivation
characteristics 1/3rd of SS or twice of conventional
NiTi
 TMA – Titanium Molybdenum alloy
12


Fiber reinforced polymeric composites
 Next generation of esthetic archwires

 Aerospace industry

 Pultrusion – round + rectangular

 ADV – tooth colored  enhanced esthetics

 DISADV – difficult to change its shape once
manufactured
13

Classification based on cross
section

14

Wire dimensions
 In US - thousandths of an inch
0.016 “ =16 mils
 In Europe and many other areas –
millimeters

 CONVERSION

Divide the dimensions in mils by 4 and place
a decimal point behind it.
eg – 16 mils = 0.4mm
15

Basic Properties of Metals

 Defn :- 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

 Alloy – A crystalline substance with metallic
properties that is composed of two or more
chemical elements, at least one of which is a
metal
16

 Lustre
 Malleability& ductility
 Thermal conductivity
 Electric conductivity
 Toughness

 Why metals behave the way they do?
Metallic bond
Crystalline stc

17

 atom - smallest piece of an element that keeps its chemical
properties

 element - substance that cannot be broken down by chemical
reactions

 ion - electrically charged atom (i.e., excess positive or
negative charge)

 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)
18

SHELL VALENCE ELECTRONS METALLIC BONDING

“SEA OF ELECTRONS”

ELECTRIC & THERMAL CONDUCTIVITY

19
DUCTILITY & MALLEABILITY

Ductility and malleability

IONIC BOND METALLIC BOND

20

Lattices and crystals
LATTICE - An infinite array of
points in space, in which each
point has identical
surroundings to all others.

CRYSTAL - any arrangement of
atoms in space in which every
atom is situated similarly to
every atom.

It can be described by
associating with each lattice
point a group of atoms called
the MOTIF (BASIS)

21

Solidification of metals into
crystals

Freezing point /
melting point

supercooling

22

Structure of metal crystals
 14
crystal lattices are present in materials of
dental use

 Metals arrange in any of the 3 foll stc
 Body centred cubic lattice
 Hexagonal close packed lattice
 Face centred cubic lattice

23

 Simple cubic packing

24

 Body centered cubic packing

25

 Hexagonal closest packing

26

 Face centered closest packing

27

Table 1: Crystal Structure for some Metals
(at room temperature)
 Nickel FCC
 Chromium BCC
 Platinum FCC
 Cobalt HCP
 Silver FCC
 Copper FCC
 Titanium HCP
 Gold FCC
 Iron BCC

28

Grains and grain boundaries
 Grains  microns
to centimeters
 Grain boundaries

 Irregular
arrangement of
atoms weaker,
non-crystlline stc.

 Decreased
mechanical strength Diagram of grains and grain boundaries.
and reduced
corrosion resistance 29

Grains and grain boundaries

Stages in the
formation of metallic
grains during the
solidification of a
molten metal

30

Crystal imperfections
 Vacancies -
Empty atom
sites

 Interstitial
s – smaller
atoms Carbon,
Hydrogen,
Oxygen, Boron

 Substitutia
Replacement atoms E.g. - Nickel or
ls - Chromium substituting iron in stainless 31
steel.

Lattice deformations:

 dislocations  linear deformations
 slip planes  along which dislocation moves

32

Lattice deformations:

 shear stress  dislocations move along slip planes
 more slip planes easier is it to deform

33

Elastic deformation

Plastic deformation

Greater stress - fracture

Work hardening
 During deformation - atomic bonds within the
crystal get stressed
 resistance to more deformation

Strain or work hardening or cold work

 Principle of Strain hardening hardness
  Hard and strong, tensile strength

Brittle.
35

Role of grains and grain
boundaries in work hardening
 Fine grained metals with large no. of grains
stronger

 Grain boundaries hinder movement of
dislocations which further increases
resistance to deformation

36

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.
 Dissolving specific elements at elevated
temperatures and cooling the metal
Solute element precipitates  barriers to the
slip planes.
37

Clinical implication of work
hardening

 When a wire is bent back and forth beyond
the proportional limit , eventually fracture
occurs after extensive permanent
deformation

38

Twinning

 Alternate method of
permanent deformation
to cold working
 Two symmetric halves -
Fixed angle
 NiTi – multiple twinning
 Subjected to a higher
temperature,
de - twinning occurs
(shape memory)
39

ANNEALING

 Process of softening
the metal to reverse the
effect of cold working

 heat below melting
point.
 More the cold work, more
rapid the annealing
 Higher melting point –
higher annealing temp.
 ½ the melting
temperature
40

ANNEALING:

STAGES

⊙ Recovery ⊙Recrystallization ⊙ Grain Growth

41

Before Annealing

Recovery – Relief of stresses

Recrystallization – New grains
from severely cold worked areas
-original soft and ductile condition

Grain Growth – large crystal “eat
up” small ones-ultimate coarse grain
structure is produced
42

Polymorphism
 Few Metals crystallize into more than one stc.
 Transition from one stc to the other with varying temp
 Reversible - Allotropy
 Eg – 1. Iron
 At higher temperature, FCC structure (austenite)
 lower temperatures,  BCC structure (ferrite)

 2. NiTi – transition from FCC to BCC takes place by
rearrangement of atoms in the lattice – BAIN DISTORTION

 This occurs over a range of temperature - HYSTERISIS
43

44

Mechanical properties
 Stress & strain
 Elastic properties
 Young’s modulus (modulus of elasticity)
 Range
 Springback
 Formability
 Resiliency
 Flexibility
 Strength properties
 Proportional limit (elastic limit)
 Yield strength
 Plastic deformation
 stiffness/load deflection rate
45

 Assessed by tensile, bending and torsional tests

Specimen

Universal testing machine 46

Stress and strain
The mechanical properties are measures of
resistance to deformation or fracture under an
applied force.

 Stress- internal distribution of load
F/A

 Strain- internal distortion produced by load
deflection/unit length
47

Types of stress/strain

 Tensile –stretch/pull

 Compressive – compress towards each

other

 Shear – 2 non linear forces in opp direction

which causes sliding of one part of a body

over another

 Complex force systems

48

Elastic Properties of orthodontic
wires

 Force applied to wire Deflection
 Internal force = Stress
Area of action
 change in length = Strain Elastic - reversible

Original length Plastic - permanent

49

Elastic properties
 Stress strain graph – 3 major properties of wires
strength , stiffness and Range

50
A B

Elastic Properties – stress
strain graph Wire returns back to original
dimension when stress is
removed
Stress

Elastic Portion

www.indiandentalacademy.com Strain 51

Elastic Properties – strength
analysis
3 points on the stress strain graph can be represented to explain

“STRENGTH”

1. Proportional limit

2. Yield strength

3. Ultimate tensile strength

52

analysis
 Proportional limit
point at which first deformation is seen
 proportional limit
elastic limit

 At this point if the stress is
removed the wire returns
back to its original form

53

analysis
 Yield strength
 Experimentally it is difficult
To measure the proportional
Limit

 0.1% of plastic deformation

54

analysis
 Ultimate tensile strength
 Max. load a wire can substain

 Is greater than the yield
Strength & occurs after
Some plastic deformation

 Clinically imp – determines
Max force a wire can deliver

55

Elastic Properties

Modulus of elasticity (Young’s modulus)

 Measures the relative stiffness

or rigidity of the wire

 Hooke’s law – stress and strain

(elastic or compressive) are proportional

to each other

 Represented by a st.line designated as ‘E’

 Spring stretch in proportion to applied force uptil the proportional limit

 Modulus of elasticity – constant for a given material
56

 Stiffness and springback
-are proportional to ‘E’
stiffness α E ie load / deflection
springiness α 1/ E

stiffness = 1/ springiness

The more horizontal the slope the
more springier the wire, the more
vertical the slope the more stiffer
the wire

57

 Range – distance the wire will bend elastically
before permanent deformation occurs
 measured upto the yield strength on X axis

58

Clinical implication
Relationship b/w strength, stiffness & range
Clinically optimal springback occurs when the
wire is bent b/w its elastic limit and ultimate
strength
The greater the springback, the more the wire
can be activated

Ultimate strength = stiffness x range

59

Resiliency & formability
 Are 2 other characteristics of some clinical
importance
 Resiliency – represents the energy storage
capacity of the wire
Strength + springiness
 wire is stretched- space between the atoms
increases.
 Within the elastic limit, there is an attractive force
between the atoms.
60

Resiliency
Itis represented by the area under the stress strain graph upto the
proportional limit.

Yield strength
Stress

Proportional limit

Resilience Formability

Formability -
 amount of permanent deformation that the wire
can withstand before breaking
 Indication of the permanent bending the wire
will tolerate while bent into springs , archforms
etc
 Also an indication of the amount of cold work
that they can withstand

62

Formability
It is represented by the area under the stress strain graph b/w the yield
strength and fracture point.

Fracture point
Yield strength
Stress

Proportional limit

Resilience Formability

Other mechanical properties
1. Flexibility
2. Toughness
3. Brittleness
4. Fatigue

Flexibility
 large deformation (or large strain) with minimal force, within its
elastic limit
FLEXIBLE

 Maximal flexibility is the strain that occurs when a wire is
stressed to its elastic limit.
Max. flexibility = Proportional limit
Modulus of elasticity. 64

Other mechanical 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 given
magnitude below the fracture point. This is called
fatigue.
65

Requirements of an ideal archwire
(Kusy )
1. Esthetics 7. Resiliency

2. Stiffness 8. Coefficient of friction

3. Strength 9. Biohostability

4. Range 10. Biocompatibility

5. Springback 11. Weldability

6. Formability

66

1. Esthetics
 Desirable compromise on mechanical
properties
 White coated wires
 Destroyed by oral enzymes
 Deformed by masticatory loads
 Exception  composite wires

67

2. Stiffness / Load deflection
Rate
 Proffit: - proportional to the modulus of elasticity &
represented by slope of stress-strain curve
 Wilcock – Stiffness α Load
Deflection Thurow and
Burstone have given definitions which imply the
same meaning

68

Stiffness / Load deflection Rate
 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) More constant force delivery as the appliance
deactivates

3) Greater ease and accuracy in applying a given force.

69

3. Strength
 proportional limit , Yield strength, and ultimate
strength (tensile/compressive)
 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,
while the Stiffness is the indication of the force
required to reach that limit.
70

Strength
 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.

71

4. Range
 Distance that the wire bends elastically,
before permanent deformation occurs
(Proffit).
 Kusy – Distance to which an archwire can be
activated- working range.
 Thurow – A linear measure of how far a wire
or material can be deformed without
exceeding the limits of the material. 72

5. Springback
 Proffit
– the ratio of yield strength and
modulus of elasticity YS/E

 Kusy -- The extent to which a wire recovers
its shape after deactivation

73

5. Springback

 Large springback - Activated to a large
extent.
 Hence it will mean fewer archwire changes.

74

6. Formability

 Kusy – the ease in which a material may be
permanently deformed.
 Ease of forming a spring or archwire
 Proffit: amount of permanent deformation a
wire can withstand without breaking

75

7. 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

76

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

77

 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.)
78

Effects of size and shape on
elastic properties
 Each of the major elastic properties
strength , stiffness and range are affected by
the geometry of the beam

Two such variables
1. Change in cross section

2. Change in length

79

Effects of Wire Cross Section
 Cantilever spring – round wire – double the diameter

80

 Rectangular wire
The principle is same

 Intorsion more shear stress rather than
bending stress in encountered
However the principle is same

81

 Increase in diameter – increase in stiffness
threshold point – too stiff for orthodontic use

 Decrease in diameter – decrease in stiffness
threshold point – too soft for orthodontic use

Ideally wire should be in b/w these two extremes

82

 The phenomenon is same for different materials but
the useful sizes vary from material to material

83

Stiffness – deflection behaviour
 Stiffnessis directly related to the
cross – sectional size and shape

 Orthodonticforce & deflection within elastic
range depend on stiffness

correct dimension of wire depending upon
purpose of use

84

 Main criteria for selection of wire is stiffness -
Burstone

Varying force levels produced during deactivation of a wire:
excessive, optimal, suboptimal, and subthreshold. During
treatment by a wire with high load deflection rate the optimal zone
is present only over a small range 85


Overbent wire with low load-deflection rate. Tooth will
reach desired position before subthreshold force zone is
reached. Replacement of wires is not required

86


CROSS – SECTIONAL STIFFNESS NO. (CS)
 simple numbering system has been
developed using engineering formulas to
denote the stiffness of wires of various cross
section of the same material.
 0.1 mm (0.004”) round wire is considered as
the base wire with Cs no. 1
Eg - 0.006 wire has Cs no. 5 indicating that it
produces 5 times much force for the same
amount of activation
87

 Stiffness of different dimensions of wires can
be related to each other.
Relative stiffness

3500
3000
Stiffness number

2500
(Burstone)

2000
1500
1000
500
0
14 16 18 20 22 16x16 18x18 21x21 16x22 22x16 18x25 25x18 21x25 25x21 215x28 28x215

Wire dimension
88

 Round wires
 Deflection rate varies as the fourth power of the diameter.
 Small change in diameter - considerable change in load –
deflection rate

 Rectangular wires 
 Ribbon mode – less stiffness
 Edgewise mode – more stiffness

 Clincal implication
This property can be utilized to orient the wire in the plane
towards which more correction is needed

89


1. > first order, < second order – RIBBON
2. > Second order, < first order – EDGEWISE
3. > 1st order correction in anterior segment
> 2nd order in the posterior segment,
wire can be twisted 90o
4. If both, 1st & 2nd order corrections are required to the
same extent, then square or round wires.

90

Effects of length and attachment
Cantilever beam – double the length

91

 Generallywhen the length of a cantilever
beam is increased
 The strength decreases proportionally
 The springiness increases as the cubic function of
the ratio of the length
 Range increases as the square of the ratio of the
length

92

 Supported beam

As length increases there is proportionally decrease in
strength and exponential increases in springiness and
range

 In torsion
Springiness and range increase proportionally with length
while torsional strength is not affected

93

In ligation – if the wire is rigidly attached at the ends
it’s strength is doubled for the same length but is 1/4 th
spring and range decreases by half compared to loosely
ligated wires which allow sliding over attachments

94

Nomograms
 Developed by Kusy

 Provides
comparison of stiffness , strength
and range of wires of diff materials and
dimensions

A reference wire is choosen (0.012”SS) and
given a value of 1 . The strength , stiffness
and range of other wires are calculated to this
reference
95

Nomograms

96

Nomograms

97

clinically
1. Forming the archwire with the thumb gives a
rough idea about the stiffness
2. Flexing the wires b/w the fingers gives an
idea about the flexibility
3. Deflecting the ends of an archwire b/w the
thumb and forefinger - resiliency

98

99

Carbon steel
 Steel = iron + carbon >2.1%
 Cast irons = >4% carbon

Transition of iron
Carbon steels - 3 major crystal stc’s
 < 9120 - iron - BCC - with Carbon as interstitial stc
-FERRITE

Carbon
100

Carbon steel
 B/W 9120 & 13940 – iron – FCC – Carbon as
interstitial – AUSTENITE

 Size of interstitial carbon > iron atom - in both these
stc’s
Distortion of the Fe atoms

 However in BCC stc, these atoms are easily held
because of the less densely packed Fe atoms
SOLID SOLUTION
101

Carbon steel
 All C steels - single phase austenite – > temp
 Cooling of austenite
 Slow cooling
 Rapid cooling (Quenching)

Slow cooling
solid state transformation at 7230 c – PEARLITE
Alternating lamellae of FERRITE & IRON CARBIDE
(CEMENTITE)
harder & rigid ferrite or austenite
102

Carbon steel
Rapid cooling (Quenching)
solid state transformation – Body centred
tetragonal stc – MARTENSITE

Fe atoms are highly distorted
- hard ,strong but brittle alloy

103

Carbon steel
Why this happens
Slow cooling allow C atoms to precipitate out –
intermediate cementite stc

Quenching – C atoms cannot escape & are
trapped within the ‘frozen’ austenite stc

104

Stainless Steel
 Approx 12% - 30% Cr – stainless steel

Why is it called so
Cr Fe
Thin ,transparent adherent layer of CrO2 - at
oxidizing temp – room temp

Protective layer – barrier to O and corrosive
agents – ‘Stainless’
105

Stainless Steel
 Classification
1. American iron & steel institute (AISI)
2. Unified number system (UNI)
3. German standards (DIN)

No’s range from 300 – 502
No’s having ‘L’ signify low carbon content

106

Stainless Steel
 3 major types are present
Ferretic SS Martensitic SS Austenitic SS
400 series Share 400 series 300 series
Good corrosion Have high strength & Most corrosion
resistance , < strength hardness resistant

Not hardenable by Can be heat treated Contain approx
heat treatment or cold 18 – 20 % Cr
work 8 – 12% Ni
18-8 steel
Industrial purposes Surgical and cutting Type 302 & 304
instruments Orthodontic wires and
bands
107

Stainless Steel
Other elements
 Nickel – stabilizes the crystal into a
homogenous austenitic phase
 adversely affect the corrosion resistance.
 Other elements like Mb, Mn , Cu are added to
in steels used for implants

108

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
corrosionwww.indiandentalacademy.com
resistance. 109

Stainless Steel
Austenitic steels more preferable :-
1. Greater ductility and ability to undergo more cold
work without breaking.
2. Substantial strengthening during cold work.
(Cannot be strengthened by heat treatment).
Strengthening effect is due partial conversion to
martensite)
3. Easy to weld
4. Easily overcome sensitization
5. Ease in forming.
110

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
 Manufacture of one piece brackets (eg Bioline ‘low
nickel’ brackets)
111

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.

112

Properties of Stainless Steel
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).

113

Clinically
Loop - activated to a very small extent so as to
achieve optimal force
 Once deactivated by only a small amount
(0.1 mm) Force level will drop tremendously
 Not physiologic
 More activations
114

 Difficult to engage a steel wire into a severely
mal-aligned tooth
 bracket to pops out,
 pain.
 Overcome by using thinner wires, which have
a lower stiffness.
 Fit poorly loss of control on the teeth.
115


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

116


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
117

3. High corrosion resistance

However the Ni content is the topic of concern

carciongenic, mutagenic, cytotoxic

118

Sensitization
 During soldering or welding, 400 - 900 oc
 Reduces the corrosion resistance
-Sensitization.

 Diffusion of Chromium carbide towards the
carbon rich areas (usually the grain
boundaries)
119

Stabilization – methods to overcome sensitiztion
 One or two elements that form carbide precipitates
more easily than Chromium are added
 Egtitanium, tantalum or niobium
 Expensive – not used for orthodontic wires

Routinely
 Lower carbon content – no carbide precipitates
are formed
 Use of low fusing solders
 Minimizing time and area of soldering

120

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.
121

 Beginners found it difficult to use the highest
tensile wires
 H D Kesling – US - Grading system
 Late 1950s, the grades available were –
 Regular
 Regular plus
 Special
 Special plus
122


 Newer grades were introduced after the 70s.
 Premium, premium +, supreme
 Disadv-

 Brittle.

 Softening , loss of high tensile
properties
123


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.

124


125

 Imp during manufacturing processes
 Wire is subjected to plastic deformation during
Straightening processes
 Prestrain in a particular direction.
 Yield strength for bending in the opposite direction will
decrease.
 Premium wire  special plus or special wire
126

Spinner straightening
 Itis mechanical process of straightening
resistant materials in the cold drawn
condition.
 The wire is pulled through rotating bronze
rollers that torsionally twist it into straight
condition.
 Disadv:
 Decreases yield strength
 Creates rougher surface
127

Pulse straightening
 Special method
 Placed in special machines that permits
high tensile wires to be straightened.
 Advantages:
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.

128

Zero Stress Relaxation
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.

dislocation movement takes place at the atomic level
Atoms try to revert back to stable positions

Property of a wire to give constant light force, when
subjected to external forces (like occlusal forces) –
zero stress relaxation.

129

clinically
 springs.
 To avoid relaxation in the wire’s working stress
Diameter of coil : Diameter of wire = 4
 High tensile wires - smaller diameter of wires 
smaller diameter springs (like the mini springs)

130

Twelftree, Cocks and Sims (AJO 1977)
 Premium plus, Premium and Special plus
wires showed minimal stress relaxation.
 Special,

 Remanit,

 Yellow Elgiloy,

 Unisil.

131

Studies on Australian wires
Author Property Result Other wires
Twelftree et al Stress relaxation Special +, Unisil –
premium, increased
premium -No SR curvature of the
over period of 3 coil shape
days
Special +
maintained it’s
original coil
shape

Barrowes (1982) Working range 0.016 Special + Standard ss,
had greater However nitinol,
working range TMA &
than other ss multistranded
wires had much
greater range
132

Author Property Result Other wires
Hazel , Rohan & Stress relaxation Wilcock wires Dentaraum
West (1984) have greater SS
% of force Elgiloy
remaining
after 28 days
Jyotindra kumar Working range Better Remanium,
(1982) recovery Co ax, Nitinol,
TMA

Skaria (1991) Strength , stiffness and Superior in Spinner
range Pulse straightened
Co- eff of friction straightened
wires
Stress relaxation
133

 Anuradha Acharya (2000) –
compared the Australian high tensile wires with
newly introduced high tensile wires
TP orthodontics –
Premier - Special
Premier + - Special Plus
Bowflex – Premium

Ortho organizers –
super + - between Special plus and Premium

134


Conclusion
 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. 135

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


sensitization

136

Small stress applied with the plier beaks

Crack propagation

Fracture of wire

137

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.
138


139

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.

140

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.
141

Multistranded Wires
 2 or more wires of smaller diameter are twisted
together/coiled around a core wire.
 Individual diameter - 0.0165 or 0.0178
final diameter – 0.016" – 0.025",
rectangular or round
 On bending  individual strands slip over each other
and the core wire, making bending easy. (elastic
limit) 142

Multi stranded wires

Co-axial

Twisted wire

www.indiandentalacademy.com Multi braided
143

Multistranded Wires – general
considerations
Implies that the wire delivers
lighter forces per unit activation
over a greater distance

strength – distortion + fracture

Twisting of wires

Result - high elastic modulus wire
behaving like a low stiffness wire144

Multistranded Wires
Elastic properties of multistranded archwires depend
on –
1. Material parameters – Modulus of elasticity

2. Geometric factors – wire dimension

3. Constants:
 Number of strands coiled
 The distance from the neutral axis to the outer
most fiber of a strand
 Plane of bending
 Poisson’s ratio
145

Multistranded Wires –
geometric factors

Typical geometry of a simple multistranded wire.
 wire of diameter D
 three wire strands, each of diameter d.
 The axial distance which a wire strand traverses per rotation equals
l*.
 The helix angle, a, which a wire strand makes with the normal to the
wire axis may be described in terms of d, D, and l* 146

Geometry of three common wire
configurations.

Neutral axis - the line that results from the
intersection of any wire cross section with
the neutral surface which is neither under
tension or compression

POISSON’s ratio (v)- ratio of the tensile strain in the x
& y co ordinates in a xyz coordinate system

x V = €x/ €z or €y/ €z
z
y
147

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
148

Multistranded Wires

Kusy ( AJO-DO 1984)

 Compared the elastic properties of triple
stranded SS wire (3 X 0.008 = 0.0175
Wildcat from GAC) with SS, NiTi & β -Ti

149

Results
 Stiffness was comparable to 0.010 SS wire
but strength was 20% higher & stiffness 25%
more
 Stiffness was comparable to 0.016 NiTi but
much lower than any TMA wire
 The multstranded wire did not resemble the
0.018 wire in any way except for the size and
& bracket relation

150

 . Nomogram comparing triple-stranded 0.0175 inch
arch round arch wires with those of NiTi and β-Ti
151

Multistranded Wires

Kusy (AJO-DO 2002)
 Interaction between individual strands was
negligible.
 Range Triple stranded Ξ Co-axial (six
stranded) Ξ single strand SS
 Stiffness & strength varied

152

Multistranded Wires

153

154

Cobalt Chromium

 1950s the Elgin Watch
“The heart that never breaks”

 Rocky Mountain Orthodontics - Elgiloy
 CoCr alloys - stellite alloys
 superior resistance to corrosion, comparable to
that of gold alloys.
155

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.
156

Cobalt Chromium properties
 Strength and formability modified by heat
treatment.
 Before heat treatment - highly formable and
can be easily shaped.
 Heat treated.
 Strength 
 Formability 

157

Cobalt Chromium
 Heat treated at 482oc for 7 to 12 mins
-Precipitation hardening
  ultimate tensile strength of the alloy, without
hampering the resiliency.

 Afterheat treatment, elgiloy has elastic
properties similar to steel.

158

Cobalt Chromium

159

Cobalt Chromium
various
tempers

Red – hard & resilient

green – semi-resilient

Yellow – slightly less
formable but ductile

Blue – soft & formable
160

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.

After heat treatment 
 Blue and yellow ≡ normal steel wire

 Green and red tempers ≡ higher grade steel

161

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
 Greater resistance to fatigue and distortion
 longer function as a resilient spring

162

Cobalt Chromium
 Kusy et al (AJO 2001)

 Evaluated round , rectangular ,square Cs wires of
sizes ranging from 14 mils to 21 x 25 mils of the 4
tempers available

 They evaluated the yield strength, ultimate tensile
strength , ductility and elastic modulus

163

164

Cobalt Chromium
1. The elastic modulus did not vary appreciably  edgewise or
ribbon-wise configurations.
2. Round wire had significantly higher ductility than square or
rectangular wires
3. The modulus of elasticity was independent of the temper of
the wire
4. The yield strength . ultimate tensile strength & ductilty -
differed from diff cross sectional areas and tempers

 Diff tempers – diff mechanical properties – care during
manufacturing

165

166

Corrosion

A chemical or electrochemical process in
which a solid , usually metal is attacked by an
environmental agent, resulting in partial or
complete dissolution

167

Corrosion
Nickel -
1. Carcinogenic,

2. mutagenic,

3. cytotoxic and

4. allergenic.

 Stainless steels, Co-Cr-Ni alloys and NiTi
are - rich in Ni

168

Corrosion
Placement in the oral cavity

wires implants

alloy is free to react with surrounded by a
the environment. connective tissue
capsule

169

Corrosion
 Stainless steel- Ni austenite stabilizer.
Loosely bond - slow release
 Passivating film  traces of Fe ,Ni and Mo.
 Aqueous environment
 inner oxide layer
 outer hydroxide layer.
 CrO2 is not as efficient as TiO2 in resisting corrosion
some Ni release
 Improper handling  sensitization
170

Corrosion types
1. Uniform attack
2. Pitting corrosion
3. Crevice corrosion/Gasket corrosion
4. Galvanic corrosion
5. Intergranular corrosion
6. Fretting corrosion
7. Microbiological corrosion
8. Stress corrosion
171

Corrosion
Uniform attack –
 entire wire reacts with the environment,
 hydroxides or organometallic compounds
 detectable after a large amount of metal is
dissolved.

172

Corrosion
Pitting Corrosion –
 Type identified in brackets and wires
 manufacturing defects - sites of easy attack
 Maybe seen before insertion into oral cavity

173

Corrosion
Pitting corrosion

Stainless Steel

NiTi

Scanning Electron microscope
174

Corrosion
Crevice corrosion or gasket corrosion -
 Application of non-metallic parts on metal in an
corrosive environment
 Eg - ligatures
 Plaque build up  depletion of O2 - disturbance in
the regeneration of the passivating layer
 Crevice depth - 2-5 mm
 High amount of metals can be dissolved in the
mouth. 175

Corrosion
Galvanic /Electrochemical Corrosion
Two metals are joined
or
The same metal – diff type of treatment (soldering etc)
oxidation and dissolution
 difference in the reactivity 
Galvanic cell.
 
Less Reactive More Reactive
(Cathode) (Anode) less noble metal
176

Corrosion
Intergranular corrosion
 Sensitization - ppt of CrC
 Corrosion – dissolution of Cr carbide rather than dissolution of
metal

Fretting corrosion
 Areas of load - Wire and brackets interface
 Friction + Pressure 
surface destruction + rupture of the oxide layer

  Debris get deposited at grain boundaries, grain structure is
disturbed.

177

Corrosion
Microbiologically influenced corrosion
 Matasa

 Microbiological attack on adhesives
 Enzymatic activity and degradation of
composites
 Craters at the base of brackets

178

Micro-0rganisms on various dental
materials

179

Corrosion
Stress corrosion
 Similar to galvanic corrosion

 Various stresses of tension and compression
– electrochemical potential
 Specific sites act as anodes and cathodes

180

Corrosion
Corrosion Fatigue:
 Cyclic stressing of a wire

 Resistance to fracture decreases

 Accelerated in a corrosive medium such as
saliva

181

Effects of sterilization on
tensile strength – AO 1993
 0.016ss, NiTi and β-Ti were evaluated
 3 common sterilization methods were used
 Autoclave
 Dry heat
 Ethylene oxide
 1-5cycles
 Universal testing machine – INSTRON

182

Results
Sterilization Wires
method
SS TMA NiTi

Dry heat No significant 1cycle 1 cycle
Change NSC – 5 NSC – 5
cycles cycles

Autoclave following 1 or NSC 1 cycle
5 cycles NSC – 5
cycles

Ethylene In any NSC NSC
oxide sterilization
method

Applying archwires
Stage Wires Reason

I aligning Multistranded SS Great range and light
NiTi forces are reqd

II stage Β-Ti , larger size NiTi , Increased formability,
SS – if sliding springback , range
and modest forces per
mechanics is needed
unit activation are
needed
III stage SS , preferably More stability & less
rectangular tooth movement reqd
184

References
 Proffit – Contemporary orthodontics

 Graber vanarsdall – orthodontics – current principles
and techniques

 Kusy & Greenberg. Effects of composition and cress
section on the elastic properties of orthodontic
wires. Angle Orthod 1981;51:325-341

 Kapila & Sachdeva. Mechanical properties and
clinical applications of orthodontic wires. AJO
89;96:100-109.
185

 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.

 Stannard, Gau, Hanna. Comparative friction of
orthodontic wires under dry and wet conditions. AJO
86;89:485-491

 Burstone. Variable modulus orthodontics. AJO 81;
80:1-16

 Kusy. A review of contemporary archwires: Their
properties and characteristics. Angle orthodontist
97;67:197-208
186

 Ingram,Gipe, Smith. Comparative range of
orthodontic wires AJO 1986;90:296-307

 Tidy.
Frictional forces in fixed appliances.
AJO 89; 96:249-54

 Twelftree,Cocks, Sims. Tensile properties of
Orthodontic wires. AJO 89;72:682-687

 Kusy and Dilley. Elastic property ratios of a
triple stranded stainless steel archwire. AJO
84;86:177-188
187

References
 Arthur
J Wilcock. JCO interviews. JCO
1988;22:484-489

 Frank and Nikolai. A comparative study of
frictional resistance between orthodontic
brackets and archwires. AJO 80;78:593-609

 Arthur Wilcock. Applied materials engineering
for orthodontic wires. Aust. Orthod J.
1989;11:22-29.

189

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