orthodontics wires 1

1

Orthodontic wires-I

Orthodontic wires-I

Dr.Meenakshi Vishwanath

2

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

3

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

4

Introduction

Orthodontics involves correction of the position of teeth –requiring moving teeth.

Forces and Moments

Optimum orthodontic tooth movement- light continuous force.

5

Introduction The challenge – Appliance which produces forces that are

neither too great nor variable.

Different materials and type of wires introduced to provide forces.

6

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.

7

Evolution of Materials Angle listed few materials appropriate for work:

Strips of wire of precious metals. Wood Rubber Vulcanite Piano wire Silk thread

8

Evolution of Materials 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

9

Evolution of Materials 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).

10

Evolution of Materials 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.

11

Evolution of Materials2. 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.

12

Evolution of Materials 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 & resilientgreen – semi-resilientYellow – slightly less formable but

ductileBlue – soft & formable

13

Evolution of Materials

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.

14

Evolution of Materials

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.

15

Evolution of Materials 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)

16

Evolution of MaterialsVariable – 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.

17

Evolution of Materials 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

18

Evolution of Materials3. 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

19

Evolution of Materialsβ 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

20

Evolution of Materials 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%

21

Evolution of Materials β III- Ravindra Nanda (2000-2001)

• Bendable,inc. force-low deflection

• Ni free

• Versatility of steel with memory of NiTi.

22

Evolution of MaterialsFiber 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

23

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 .

24


Atom - smallest piece of an element that keeps its chemical properties.

Element - substance that cannot be broken down by chemical reactions.

25


Electrons – orbit around nucleus.

Floating in shells of diff energy levels

Electrons form the basis of bonds

26


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).

27

Basic Properties of Materials Cohesive forces-atoms held together.

Interatomic bonds

Primary Secondary Ionic Hydrogen Covalent Van der Waals Metallic

forces

28

Basic Properties of Materials Ionic-mutual attraction between positive and

negative ions-gypsum, phosphate based cements.

Covalent-2 valence electrons are shared by adjacent atoms-dental resins.

29

Basic Properties of Materials 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”

30

Basic Properties of Materials Electrons free to move

Electrical and thermal conductivity

Ductility and malleability -electrons adjust to deformation

31


IONIC BOND METALLIC BOND

Ionic bond Metallic bond

32

Basic Properties of Materials Materials broadly subdivided into 2 categories - Atomic arrangement

Crystalline structure Non-crystalline structureRegularly spaced Possess short range

config-space lattice. atomic order. Anisotropic –diff in Isotropic-prop of materialmechanical prop due remains same in all directional arrangement directions. of atoms. Amorphous

33

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

34

Basic Properties of Materials 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.

35


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.

36

Basic Properties of MaterialsThere 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.

37

Basic Properties of Materials2.Body centered cubic:

Fe-below 910°C &above 1400°C. Cr &Ti above 880°C.

38

Basic Properties of Materials3.Hexagonal close packed:

Co & Ti below 880°C

39

Basic Properties of Materials 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)

40

Basic Properties of Materials 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).

41


42

Basic Properties of Materials 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.

43

Basic Properties of Materials 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.

44

Basic Properties of Materialsb.Vacancies:

2.Line defects: Dislocations along a line. Plastic deformations of metals occurs –motion of dislocations.

These are empty atom sites.

45

Basic Properties of Materials Edge dislocation

Sufficiently large force- bonds broken and new bonds formed.

Slip plane + Slip direction = Slip system

46

Basic Properties of Materials 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.

47


Elastic deformation

Plastic deformation

Greater stress - fracture

48

Basic Properties of Materials 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

49

Basic Properties of Materials 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)

50

Basic Properties of Materials 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

51

Basic Properties of Materials 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.

52

Basic Properties of Materials 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.

53

Basic Properties of Materials 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)

54

Basic Properties of Materials 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

55


Grain growth - minimizes the grain boundary area.

•Coarse grains

56

Basic Properties of Materials Before Annealing

Recovery – Relief of stresses

Recrystallization – New grains from severely cold worked areas

Grain Growth – large crystal “eat up” small ones

57

Basic Properties of MaterialsPolymorphism Metals and alloys exist as more than one type of

structure

Transition from one to the other-reversible- Allotropy

Steel and NiTi

58


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

59

Basic Properties of Materials 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.

60


61

Basic Properties of Materials Austenite (FCC)

slow cooling rapid cooling

Mixture of: Tempering Martensite (BCT) Ferrite(BCC) distorted lattice-

& Pearlite hard & brittle

Cementite(Fe3C)

62

Basic Properties of Materials 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

63

Basic Properties of Materials Bain distortion

• Transformations occur without chemical change or diffusion

• Result-crystallographic reln b/w parent and new phase

• Rearrangement of atoms-minor movements

64

Evolution of Materials 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

65

Basic Properties of Materials Metallic bond-properties

Crystals & lattices

Imperfections

Edge dislocations, Twinning

Cold working

Annealing

Polymorphism

Bain distortion

66

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.

67

Making an orthodontic wire 4 steps in wire production 1. Melting

2.The Ingot

3.Rolling

4.Drawimg

68

Making an orthodontic wire 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.

69

Making an orthodontic wire -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 producedo Cooling and shrinking –interior cools

late -Ingot – trimmed

Important to control microstructure at this Important to control microstructure at this

stage – basis of its physical properties and stage – basis of its physical properties and

mechanical performancemechanical performance

70

Making an orthodontic wire 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

71

Making an orthodontic wire -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

72

Making an orthodontic wire 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

73

Making an orthodontic wire - 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)

74

Making an orthodontic wire 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

75

Making an orthodontic wire 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.

77

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)

78

Mechanical properties Material can be stressed in 4 ways-

• Compression

• Tensile

• Shear

• Complex force systems

79

Mechanical properties 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

80


Universal testing machine

81


82


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.

83

Mechanical properties 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.

84

Elastic properties Stress-Strain relationship (ductile material)

85

Elastic properties

STRAIN

STR

ES

S

Elastic portion

Wire returns back to original dimension when stress is removed

(Hooke’s law)(Hooke’s law)

86

Elastic properties

0.1%

stre

ss

strain

Elastic limit

Proportional limit Yield point

87

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.

88

Elastic properties

Ultimate tensile strength Fracture point

stre

ss

strain

Plastic deformation

89

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)

90

Elastic properties

Slope α Stiffness Stiffness α 1

Springiness

stre

ss

strain

91

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

92

Elastic properties

93

Elastic properties

Springback deflection

forc

e

Range

YP

Point of arbitrary clinical loading

94

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.

95

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

resi

lien

cy

form

ab

ilit

y

YP

PL

stre

ss

strain

97

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

99

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 st

ress

strain

Toughness

103

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.

105

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.

106

Requirements of an ideal arch wire Robert P.Kusy- 1997 (AO) 1. Esthetics2. Stiffness3. Strength4. Range5. Springbac

k6. Formabilit

y

7.Resiliency8.Coefficient of

friction9.Biohostability10.Biocompatibility11.Weldability

107

Requirements of an ideal arch wire 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

Requirements of an ideal arch wire 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

Requirements of an ideal arch wire

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

Requirements of an ideal arch wire Range

•Distance to which an archwire can be activated

• Distance wire bends elastically before permanent deformation.

•Measured in millimeters.

112

Requirements of an ideal arch wire 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

Requirements of an ideal arch wire 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

Requirements of an ideal arch wire 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

Requirements of an ideal arch wire 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

Requirements of an ideal arch wire 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

Effects of wire cross-section 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

Effects of wire cross-section Rectangular wire

The principle is same In torsion more shear stress rather than bending

stress in encounteredHowever 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

Effects of wire cross-section 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

Effects of wire cross-section 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

Effects of wire cross-sectionShape Moment of

InertiaRatio to stiffness of round wire

Пd4

641

s4

121.7

b3h12

1.7 b3hd4

131

Effects of wire cross-section 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

Effects of wire cross-section

133


0

500

1000

1500

2000

2500

3000

3500

Sti

ffn

es

s n

um

be

r (B

urs

ton

e)

14 16 18 20 22 16x16 18x18 21x21 16x22 22x16 18x25 25x18 21x25 25x21 215x28 28x215

Wire dimension

Relative stiffness

134

Effects of wire cross-section 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

Effects of wire cross-section 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)

142

Effects of material

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

145

Nomograms

146

Nomograms

147

Clinical implications Balance between stiffness, strength &

range

Vary - material ,cross-section or length as the situation demands.

148

Clinical implications 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

Clinical implications 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

Clinical implications

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

Clinical implications 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

Clinical implications Fixed appliance

The length of wire between brackets can be

increased

Loops, or Smaller brackets,

or Special bracket designs –Mini-unitwin

bracket,Delta

153

Clinical implications 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

154

Clinical implications

155

Clinical implicationsStage 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

Clinical implicationsStage 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

Clinical implicationsA 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 propertiesNickel -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 corrosion1. 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.

165

Physical properties

166

Physical properties4.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 properties5.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 properties7.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

170

Physical properties

171

Physical properties8.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.

400-900oC-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

Basic Properties of Materials Austenite (FCC)

slow cooling rapid cooling

Mixture of: Tempering Martensite (BCT) Ferrite(BCC) distorted lattice-

& Pearlite hard & brittle

Cementite(Fe3C)

189

Stainless steel

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.

195

Stainless steel

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 steel3.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

High Tensile Australian Wires 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

High Tensile Australian Wires 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

High Tensile Australian Wires

•Higher working range-

E (same) But inc. YS

Range=YS/E

•Higher resiliency

ResilαYS2/E

•Zero stress relaxation

•Reduced formability

212

High Tensile Australian WiresZero 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

High Tensile Australian Wires 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

High Tensile Australian Wires 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

High Tensile Australian Wires 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

High Tensile Australian Wires 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

High Tensile Australian Wires Pulse straightening Placed in special machines that permits

high tensile wires to be straightened.

This method :Permits the straightening of high tensile wires1. Does not reduce the yield strength of the wire2. Results in a smoother wire, hence less wire –

bracket friction.

223

High Tensile Australian Wires 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

High Tensile Australian Wires 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

High Tensile Australian WiresTwelftree, 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

High Tensile Australian Wires 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

High Tensile Australian Wires 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

High Tensile Australian Wires 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 Tensile Australian Wires 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 propagationDislocation 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

High Tensile Australian WiresWays 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

High Tensile Australian Wires4. 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 fingersSpools 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

Multi braided

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 wiresElastic 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 wiresHelical 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 :-

245

Multistranded wires

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

knEIK – load/support constantP – applied forceL – length of the beamK – helical spring shape factorn- no of strandsE – modulus of elasticityI – moment of inertia

249

Multistranded wiresKusy (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

251

Multistranded wires

252

Multistranded wires

253

Multistranded wiresIngram, 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 stiffnessUnlike 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.

256

Multistranded wires

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- 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

. Heating above 650oC

partial annealing, and softening of the wire

Optimum heat treatment dark straw color of the

wire or temperature indicating paste

262

Cobalt chromium1958-1961-4 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 workingHeat 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.

271

Cobalt chromium

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

orthodontics wires 1

Health & Medicine

evolution of materials

abundance of materials

materials appropriate

different materials

orthodontic wires

type of wires

strips of wire

ligature wire