materials in ortho

Question 1

What are commonly used materials?

Answer 1

PMMA (polymethylmethacrylate)
SS (stainless steel)
NiTi (nickel titanium)
Adhesives/cements
Plaster
Alginate

Question 2

Why is biocompatibility important?

Answer 2

Safety of pt
Lots of nickel and latex allergies
Estrogenicity of resin (may have adverse effects for men)

Question 3

What is PMMA?

Answer 3

Vinyl polymer, made by free radical vinyl polymerisation from the monomer methyl methacrylate

Heat cured (for baseplates)
Self cured (also for baseplate, contains activator- dimethyl-p-toluidine)

0.1-5% residual monomer and additives released from base plate- allergy

In past research- carcinogen and embryotoxic concerns, not anymore

Question 4

How can you prevent an allergy to PMMA?

Answer 4

1. Use heat cure- less free monomer
2. Store appliance in water for several hours prior fit
3. Use light cured ‘acrylic’

Question 5

What are the most common PMMA allergens for dental professionals?

Answer 5

1. MMA
2. Dibenzoyl peroxide
3. EGDMA (ethylene glycol dimethacrylate)- cross linking agent

Question 6

How can you minimise the exposure to PMMA allergens?

Answer 6

1. Wear gloves
2. Ventilation
3. Down-draught extraction

Question 7

What is austenitic stainless steel wire?

Answer 7

17-20% Cr
8-12% Ni
0.15% C

Modulus of elasticity- 160-180GPa
Yield strength- 1100-1500(MPa)^a

For removable and fixed appliances

Question 8

What is elgiloy Co/Cr/Ni wire?

Answer 8

40% Co
20% Cr
15% Ni
15.8% Fe
7% Mo
2% Mn
0.15% C
0.04% Be

Modulus of elasticity- 160-190GPa
Yield strength- 830-1000(MPa)^a

For crozat and fixed appliances- has to be heat treated

Question 9

What is Beta-titanium (TMA) wire?

Answer 9

77.8% Ti
11.3% Mo
6.6% Zr
4.3% Sn

Modulus of elasticity- 62-69GPa
Yield strength- 690-970(MPa)^a

For fixed appliances

Question 10

What is NiTi wire?

Answer 10

55% Ni
45% Ti

Modulus of elasticity- 34GPa
Yield strength- 210-410(MPa)^a

For fixed appliances

Question 11

What are the three main forces in ortho?

Answer 11

Tensile- elongation in direction of load applied

Compressive- contraction in direction of load applied

Shear- sliding displacement of one side of a specimen or twisting around its axis (torsion)

Question 12

How are mechanical properties of wires assessed?

Answer 12

Tensile, bending and torsional tests

Provides basis for comparison, doesn’t reflect behaviour

Question 13

What are the optimum characteristics of wire?

Answer 13

Large spring back
Low stiffness
Good formability
High stored energy
Biocompatible
Environmentally stable
Low surface friction
Capability to be welded/soldered to auxiliaries

Question 14

What is Hookes Law?

Answer 14

For relatively small deformations of an object, the displacement is directly proportional to the deforming force/load

So the object returns to its original shape/size upon removal of load

F=kx

Stress is proportional to strain

Related to youngs modulus

Question 15

What is bulk modulus?

Answer 15

Aka incompressiblity

Measure of ability of a substance to withstand changes in volume when under compression on all sides

Question 16

What is shear modulus?

Answer 16

Aka rigidity

Shear stiffness of a material, ration of stress to strain

Question 17

What are SS wires popular?

Answer 17

Good formability, biocompatibility, stability, stiffness, resilience
Low cost

Question 18

What is the force and deflection of SS springs?

Answer 18

F=kdr^4/l^3

r= radius
l= length
d= deflection
k= stiffness (youngs modulus)

Question 19

Are CoCr wires used?

Answer 19

Manipulated into softened state and then heat treated to a certain stiffness- difficult

Similar properties to SS

Question 20

Are NiTi wires used?

Answer 20

Good spring back and low stiffness

BUT

Poor formability and joinability

Know due to ‘shape memory’

Question 21

Are beta titanium wires used?

Answer 21

Adequate spring back
Average stiffness
Good formability
Can weld to auxiliaries

Question 22

What is a multistrand wire?

Answer 22

High spring back and low stiffness compared to SS
Difficult to manipulate

Can be used for bonded retainers
Cheap substitute for NiTi

Question 23

What are elastic properties of wires?

Answer 23

Strength=stiffness x range

Range is distance wire travels before permanent deformation

Question 24

Why is there rectangular and round cross sections?

Answer 24

Round- fit loosely in bracket, for initial stages, only TILT teeth, root doesn’t move (only passively dragged)

Rectangular- engage bracket more, second stage, provides TORQUE force (acts on long axis so root moves into angle parallel w masticatory forces)

Question 25

What is resilience?

Answer 25

The area under the curve out to the proportional limit
Represents energy capacity of material that is a combination of strength and stiffness

Question 26

What is formability?

Answer 26

Amount of permanent deformation that a material can withstand before breaking

Question 27

What is shape memory?

Answer 27

Restore original shape of a plastically deformed sample by heating

Thermoplastic martensitic transformation (crystalline phase change)

Repeatable

Question 28

What is the constancy of stress?

Answer 28

NiTi- unloading curve the stays flat over large strains

Applies constant force over a broad treatment time and tooth position

Having a hot drink- might feel the wire creep

Question 29

What is adhesion?

Answer 29

The force of attraction between the molecules or atoms on two different surfaces as they’re brought into contact

Question 30

What adhesion do brackets rely on?

Answer 30

Mechanical

Etch tooth surface, base has mesh surface

Question 31

What adhesives are used to bond brackets?

Answer 31

4 groups
Chemical/light cured

Some may prevent demineralisation as contain fluoride

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