materials in ortho Flashcards

1
Q

What are commonly used materials?

A

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

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2
Q

Why is biocompatibility important?

A

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

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3
Q

What is PMMA?

A

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

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4
Q

How can you prevent an allergy to PMMA?

A

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

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5
Q

What are the most common PMMA allergens for dental professionals?

A

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

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6
Q

How can you minimise the exposure to PMMA allergens?

A

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

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7
Q

What is austenitic stainless steel wire?

A

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

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8
Q

What is elgiloy Co/Cr/Ni wire?

A

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

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9
Q

What is Beta-titanium (TMA) wire?

A

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

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10
Q

What is NiTi wire?

A

55% Ni
45% Ti

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

For fixed appliances

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11
Q

What are the three main forces in ortho?

A

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)

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12
Q

How are mechanical properties of wires assessed?

A

Tensile, bending and torsional tests

Provides basis for comparison, doesn’t reflect behaviour

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13
Q

What are the optimum characteristics of wire?

A

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

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14
Q

What is Hookes Law?

A

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

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15
Q

What is bulk modulus?

A

Aka incompressiblity

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

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16
Q

What is shear modulus?

A

Aka rigidity

Shear stiffness of a material, ration of stress to strain

17
Q

What are SS wires popular?

A

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

18
Q

What is the force and deflection of SS springs?

A

F=kdr^4/l^3

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

19
Q

Are CoCr wires used?

A

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

Similar properties to SS

20
Q

Are NiTi wires used?

A

Good spring back and low stiffness

BUT

Poor formability and joinability

Know due to ‘shape memory’

21
Q

Are beta titanium wires used?

A

Adequate spring back
Average stiffness
Good formability
Can weld to auxiliaries

22
Q

What is a multistrand wire?

A

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

Can be used for bonded retainers
Cheap substitute for NiTi

23
Q

What are elastic properties of wires?

A

Strength=stiffness x range

Range is distance wire travels before permanent deformation

24
Q

Why is there rectangular and round cross sections?

A

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)

25
What is resilience?
The area under the curve out to the proportional limit Represents energy capacity of material that is a combination of strength and stiffness
26
What is formability?
Amount of permanent deformation that a material can withstand before breaking
27
What is shape memory?
Restore original shape of a plastically deformed sample by heating Thermoplastic martensitic transformation (crystalline phase change) Repeatable
28
What is the constancy of stress?
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
29
What is adhesion?
The force of attraction between the molecules or atoms on two different surfaces as they’re brought into contact
30
What adhesion do brackets rely on?
Mechanical Etch tooth surface, base has mesh surface
31
What adhesives are used to bond brackets?
4 groups Chemical/light cured Some may prevent demineralisation as contain fluoride All v similar