Materials in Orthodontics

Question 1

Which material

Answer 1

Most appropriate material

Don’t be a cheapskate

Question 2

Common materials

Answer 2

PMMA
Stainless steel
Nickel titanium 
Adhesives/cements
Plaster
Alginate

Question 3

BIocompatibility

Answer 3

Safety of patient
Nickel - found in some alloys used in ortho - 25% females allergic to it
Latex -
Estrogenicity of resin - some resins leak compounds and have an oestrogen like effect

Question 4

PMMA

Answer 4

Vinyl polymer

Free radical vinyl polymerisation from methyl methacrylate

Question 5

Types of PMMA

Answer 5

Heat cure - stronger and less free flow
Self-cure - chemically similar but contains activator
0.1-5% release of monomers and additives from base plate

Question 6

PMMA caution

Answer 6

All components are allergenic to some degree 
MMA is most allergenic 
Preventing allergy 
Use heat cure
Store appliance in water for several hours
Use light cure acrylic
Gloves
Ventilation 
Down-draught extraction

Question 7

Wire

Answer 7

Wires which apply force needed to move teeth

Question 8

Lower modulus of wire

Answer 8

More elasticity

Question 9

Forces - 3 types

Answer 9

Tensile forces
Compressive
Shear

Question 10

Tensile

Answer 10

Elongation in direction of load applied

Question 11

Compressive

Answer 11

Contraction in direction of load

Question 12

Shear force

Answer 12

Sliding displacement of one side of a specimen or twisting round axis

Question 13

Mechanical properties are generally assessed by

Answer 13

Tensile, bending and torsional tests

Question 14

Optimum characteristics of wire

Answer 14

Large springback
Low stiffness
Good formability
High stored energy
Biocompatibility and environmental stability
Low surface friction
Capacity to be welded or soldered to auxiliary supports

Question 15

Stainless steel wire

Answer 15

Popular since intro to orthodontics 
Formability 
Biocompatibility 
Environmental stability 
Stiffness
Resilience
Low cost

Question 16

r =
d =
l =
k =

Answer 16

radius of wire
deflection of wire
length of spring
stiffness of wire (young’s modulus)

Question 17

Co-Cr wire

Answer 17

Manipulated in softened state

Heat treatment results in similar properties to stainless steel

Question 18

NiTi

Answer 18

Good springback

Poor formability and join ability

Question 19

Beta-titanium provides

Answer 19

adequate springback
average stiffness
good formability
weldable

Question 20

Multi-strand wire

Answer 20

High springback
Low stiffness compared to stainless steel
Cheap substitute for NiTi

Question 21

Elastic properties of wires
Strength
Stiffness
Range

Answer 21

Stiffness x range
Quality of being strong - tensile strength
Quality of being rigid and not easily bent
Range is distance wire travels before permanent deformation

Question 22

Stiffness =

Answer 22

Gradient of strength-range graph
E x I
change of shape of same material changes stiffness

Question 23

Rectangular cross section vs round cross section

Answer 23

For a round cross-section:
I = [π X (diameter)4] /64

For a rectangular cross-section: I = [base X (height)3] /12

Wires round in cross section fit LOOSELY in brackets
Used for initial stages and only tilt teeth

Question 24

Rectangular wires used in

Answer 24

second stage of movement and engage the bracket much more firmly such that a torque force is placed on the tooth
Torque acts on long axis of tooth
Root moves into angle parallel with masticatory forces

Question 25

Strength is the measure of the force a material can withstand before the material permanently deforms. Strength may be viewed in these three ways

Answer 25

1 Proportional Limit
the point at which any permanent deformation first
occurs.
2 Yield Strength
the point at which 0.1% deformation is measured.
3 Ultimate Tensile Strength
the maximum load that the wire can sustain.

Question 26

Elastic properties

Answer 26

Strength
Stiffness
Range

Question 27

Which two properties can be determines from stress strain curve

Answer 27

Resilience
Resilience is the area under the curve out to the proportional limit. Resilience represents the energy capacity of the material that is a combination of the strength and stiffness.

Formability

Formability is the amount of permanent deformation that a material can withstand before breaking.

Question 28

Shape memory

Answer 28

• Shape memory effect describes the process of restoring the original shape of a plastically deformed sample by heating it.
• This is a result of a crystalline phase change known as “thermoelastic martensitic transformation”.
• The shape memory effect is repeatable.

Question 29

Bonding brackets

Answer 29

Rely on mechanical retention to both enamel and bracket base

Question 30

Adhesion

Answer 30

Force of attraction between the molecules or atoms on two different surfaces as they are brought into contact

Question 31

Adhesion 5 types

Answer 31

Mechanical adhesion
Two materials may be mechanically interlocked. Sewing forms a large scale mechanical bond, velcro forms one on a medium scale, and some textile adhesives form one at a small scale.
• Chemical Adhesion
Two materials may form a compound at the join. The strongest joins are where atoms
of the two materials swap (ionic bonding) or share (covalent bonding) outer electrons.
• Dispersive Adhesion
Also known as adsorption. Two materials may be held together by van der Waals
forces.
• Electrostatic Adhesion
Some conducting materials may pass electrons to form a difference in electrical charge at the join. This results in a structure similar to a capacitor and creates an attractive electrostatic force between the materials. The electrons are passed if one conducting material binds its electrons less strongly than the other does.
• Diffusive Adhesion
This may occur when the molecules of both materials are mobile and soluble in each other. It is also the mechanism involved in sintering. When metal or ceramic powders are pressed together and heated, atoms diffuse from one particle to the next. This joins the particles into one.

Question 32

Adhesives

Answer 32

Bracket failure