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Orthodontics > Materials in orthodontics > Flashcards

Materials in orthodontics Flashcards

1
Q

Which materials are commonly used in orthodontics?

A 
Polymethylmethacrylate (PMMA)
Stainless steel 
NiTi
Adhesives/cements
Plaster 
Alginate
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2
Q

What are the biocompatibility concerns with materials?

A

Safety of the patient
Nickel
Latex
Estrogenicity of the resin

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3
Q

What is PMMA?

A

A vinyl poylmer made by free radical vinyl polymerisation from the monomer methyl methacrylate

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4
Q

What is heat-cured and self-cured PMMA used for?

A

Baseplates

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5
Q

How is self-cured different?

A

Contains an activator (dimethyl-p-toluidine)

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6
Q

What is the problem with PMMA as a material (in terms of patient safety)?

A

Depending on the type used, there can be 0.1%-5% residual monomer and additiives that can be released from base plate
Historically been a link between cacinogenic and embryotoxic potency of MMA
All PMMA components are allergenic to some extent

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7
Q

How can an allergy to PMMA be prevented?

A

Use heat cured PMMA
Store appliance in water for several hours prior to fit
Use light cured acrylic

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8
Q

Which are the most common parts for occupationally exposed to the professional?

A

MMA, Dibenzoyl peroxide and cross-linking agent EGDMA

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9
Q

How can the dental professionals minimise exposure?

A

Wear gloves
Ventilation
Use down-draught extraction

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10
Q

What is Austenitic SS used for and what is it made up of?

What are its properties?

A 
Cr 17-20%
Ni 8-12%
C 0.15%
Removable and fixed applicances 
High modulus of elasticity
High yield strength (MPa)
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11
Q

What is Eligiloy made up of?

What are its properties?

A

Mainly Co/Cr/Ni
High modulus of elasticity
Not as strong as SS

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12
Q

What is Eligiloy used for?

A

Crozat applicances and fixed appliances

Has to be heat treated - not used often

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13
Q

What is TMA made up of (betal-titanium)?
Properties
Uses

A

Ti/Mo/Zr/Sn
Lower modulus of elasticity
not as strong
Fixed appliances

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14
Q

What is NiTi made up of ?

Properties?

A

55% Ni, 45% Ti
Lower elsastic modulus (doesnt need a lot of force to move = more bendy)
Lower yield strength MPa

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15
Q

What is a tensile force?

A

Causes elongation in the direction of the load applied

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16
Q

A compressive force

A

Compressive force - contraction in the direction of the load applied

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17
Q

A shear force?

A

Caused either sliding displacement of one side or a twisting around the axis

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18
Q

What are the mechanical properties of the wires assessed by?

A

Tensile
Bending
and torsional tests

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19
Q

What are the optimum characteristics of wire?

A

Large springback
Low stiffness
Good formability
High stored energy
Biocompatibility and environmental stability
Low surface fricition
The capability to be welded or soldered to auxillaries

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20
Q

What was the composition of the first SS?

A

12.8% chronium, 0.24% carbon

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21
Q

What are SS’s popular?

A 
Formability
Biocompatibility and environmental stability 
Siffness
Resilience
Low cost
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22
Q

…

A

F= kdr^4/L^3

r- radius of wire
d - deflection of the wire
L - length of spring
k - stiffness of thewire

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23
Q

How can Co-Cr wires be used?

A

Can be manipulated in a softened state and subjected to heat treatment
Heat treatment results in wire with properties similar to those of SS

24
Q

What are the advantages of Nitinol wires NiTi?

A

Have good springback and low stiffness

Cuts down treatment time

25
Q

What are the problems with Nitinol wires?

A

Poor formability and joinability

Shaping is more difficult

26
Q

What are the advantages of beta-titanium?

A

Provides a combination of adequate springback
Average stiffness
Good formability
Can be welded to auxillaries

27
Q

How do beta- titanium work?

A

Freeze and bed or then will stay in position or heat up

28
Q

What is the problem with beta-titanium?

A

Very heat dependent

29
Q

What are the advantages of multi-strand wire and what can they be used for?

A

High spring-back and low stiffness when compared to SS

Can be used as cheap substitue to NiTi and for bonded retainers

30
Q

How do orthodontists ttend to look at the elasic properties of wires?

A

Strength = stiffness x range
Strength: quality or sate of being strong relating to tensile strength
Stiffness: the quality of being rigid, not easily bent
Range: the distance the wire travels before permanent deformation

31
Q

What happens to the stiffness if you change the shape of the wire?

A

Change in shape leads to change in stiffness

32
Q

What is meant by adhesion?

A

The forces of attraction between the molecules or atoms on 2 different surfaces as they are brough into contact

33
Q

What are the different types of adhesion?

A 
Mechanical adhesion
Chemical adhesion
Dispersive adhesion 
Electrostatic adhesion 
Diffusive adhesion
34
Q

What do changes in shape and size affect?

A

the stiffness of wire

35
Q

What are wires that are round in cross-section used for?

A

Fit loosely into the brackets and are used for inital stages and only can tilt the teeth
They do not move the root - the root is only dragged passively into an approximate position in the arch

36
Q

What are the rectangular wires used for?

A

In the second stage of movement to engage the bracket more firmly - can then get a torque force on the tooth
Torque acts on the long axis of the tooth such that the root moves into an angle parallel with masticatory forces

37
Q

What is strength?

A

the measure of force a material can withstand before the material permanently deforms

38
Q

What are the 3 ways strength can be viewed?

A

Proportional limit: point at which permanent deformation first occurs
Yield strength: the point at 0.1% deformation is measured
Ultimate tensile strength: the maximum load that the wire can sustain

39
Q

How is stiffness represented on a force-deflection graph?

A

It is proportional to the slope of the linear portion of the graph
This ranges from 0 to the proportional limit, the steeper the graph, the stiffer the material

40
Q

What is the range?

A

The deflection the material will encounter before any permanent deformation occurs fro 0-the proportional limit

41
Q

What happend beyond the propotional limit?

A

the mateial will bend but it will not return to its original shape

42
Q

What is the failure point?

A

Where the material breaks

43
Q

What unit is range measured in?

What unit strength measured in?

A

Range: length
Strength: force

44
Q

What is the slope in a stress-strsin curve?

A

Elastic modulus E and is proportional to stiffness

45
Q

What other characteristics can be determined from stress-strain curve? (apart from stress and strain)

A

Resilience - area under the curve, represents the energy capacity of the material that is a combination of strength and stiffness
Formability - the amount of permanent deformatino that a material can withstand before breaking

46
Q

What are the 2 terms used to describe the behavior of NiTi?

A

Shape memory and Super-elasticity

47
Q

What is meant by shape memory of NiTi?

A

Restoring it’s original shape of plastically deformed smaple by heating it
This is a result of crystaline phase - thermoelastic martensitic transformation
The shape memory effect is repeatable

48
Q

What is meant by the superelasticity of NiTi?

A

Response to an applied stress, caused by a phase transformation between the austenitic and martensitic phases of a crystal
Less energy is needed to stress-induce and deform martensite -(low temps) (in nitinol) than to deform the austenite (high teps) by conventional mechanisms.
Superelasticity assumes a reversible response to stress caused by a phase transformation

49
Q

Nitol shape memory

A

Shape Memory Alloys, such as Nickel Titanium, undergo a phase transformation in their crystal structure when cooled from the stronger, high temperature form (Austenite) to the weaker, low temperature form (Martensite).

50
Q

What are superelastic alloys?

How are they different to shape-memory alloys?

A

belong to the larger family of shape-memory alloys. When mechanically loaded, a superelastic alloy deforms reversibly to very high strains by the creation of a stress-induced phase.
When the load is removed, the new phase becomes unstable and the material regains its original shape. Unlike shape-memory alloys, no change in temperature is needed for the alloy to recover its initial shape.

51
Q

What is the constancy of stress of nitinol wires?

A

Super-elastic nitinol wires have an unloading curve that stays flat over large strains - can apply a constant stress over a wide range of strain.
They move teeth applying a constant force over broad treatment time and tooth position
Loading = unloading: returns to almost the same shape

52
Q

What to successful bonding agents in orthodontics rely on?

A

Mechanical retention to both the enamel and bracket base

53
Q

What is meant by adhesion?

A

The force of attraction between the molecules or atoms on 2 different surfaces as they are brought into contact

54
Q

What is chemical adhesion?

A

2 materials form a compound at the joint, strongest joins are where atoms of the 2 materials swap or share outer electrons

55
Q

What is diffusive adhesion?

A

Occurs when the molecules of both materials are mobile and soluble in each other
Is the mechanism involved in sintering
When metal or ceramic powders are pressed together and heated, atoms diffuse from one particle to the next
Joins the particles into one

56
Q

What can adhesives contain?

A

fluoride to prevent demineralisation around the bracket

Orthodontics (16 decks)

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