Materials in orthodontics

Question 1

List the common ortho materials

Answer 1

PMMA
Stainless steel
Nickel titanium
Adhesives/cements
Plaster
Alginate

Question 2

What is PMMA?

Answer 2

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

Question 3

Types of PMMA and their uses?

Answer 3

Heat cured - used for baseplates

Self cured - baseplates, contains activator (dimethyl-p-toluidine)

Question 4

Negatives of PMMA?

Answer 4

Historical link between carcinogenic and embryotoxic potency

All components are allergenic

Question 5

How to prevent allergy to PMMA?

Answer 5

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

Question 6

What are the most common allergens of PMMA?

Answer 6

MMA, dibenzoyl peroxide and EGDMA

Question 7

How to minimise PMMA exposure?

Answer 7

Gloves
Ventilation
Down-draught extraction

Question 8

What do wires do?

Answer 8

Apply force = move teeth

Question 9

When are austenitic stainless steel wires used?

Answer 9

Removable and fixed appliances

Question 10

When are elgiloy Co/Cr/Ni wires used?

Answer 10

Crozat appliances and fixed appliances - it has to be heat treated

Question 11

When are β-titanium wires used?

Answer 11

Fixed appliances

Question 12

When are NiTi wires used?

Answer 12

Fixed appliances

Question 13

What does a:
Tensile force
Compressive force
Shear force
cause?

Answer 13

Tensile = elongation in the direction of load applied
Compressive = contraction in the direction of the load applied
Shear = sliding displacement of 1 side of a specimen or twisting around its axis

Question 14

What assesses the mechanical properties of a wire?

Answer 14

Tensile
Bending
Torsional tests

Question 15

Optimum characteristics of wire?

Answer 15

large springback,
• low stiffness,
• good formability,
• high stored energy,
• biocompatibility and environmental stability,
• low surface friction, and
• the capability to be welded or soldered to auxiliaries

Question 16

Why are stainless steel wires popular?

Answer 16

formability,
• biocompatibility and environmental stability,
• stiffness,
• resilience, and
• low cost.

Question 17

How to work out force and deflection of stainless steel springs?

Answer 17

F = k d r4/l3
r = radius of the wire
d = deflection of the wire
l = length of the spring
k = stiffness of the wire (Young’s Modulus)

Question 18

How to work with Co-Cr wire?

Answer 18

Manipulate in softened state and subjected to heat treatment = properties similar to stainless steel

Question 19

Pros and cons of nitinol (NiTi)?

Answer 19

Pros = good springback and low stiffness
Cons = poor formability and joinability

Question 20

Pros of beta-titanium?

Answer 20

Adequate springback
Good formability
Average stiffness
Can be welded to auxiliaries

Question 21

Pros of multi-strand wire? When are they used?

Answer 21

High spring back and low stiffness

Cheap substitute for NiTi and for bonded retainers

Question 22

How do orthodontists look at elastic properties of wires?

Answer 22

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

Question 23

List the orthodontic wires from highest to lowest in elasticity

Answer 23

Highest = austenitic stainless steel
Elgiloy Co/Cr/Ni
Beta-titanoim
NiTi

Question 24

What is stiffness equal to?

Answer 24

The product of the elastic modulus of the material times its area moment of inertia = change shape = change stiffness

Question 25

What is the inertia of a round cross section? In a rectangular cross section?

Answer 25

``` Round: I = [π X (diameter)4] /64 Fit loosely in brackets Used for initial stages Tilt teeth Do not move root - root is dragged along passively through bone into an approximate position in arch ``` Rectangular: I = [base X (height)3] /12 Used in 2nd stage of movement Engage bracket more firmly = torque force on tooth Torque acts on long axis of tooth = root moves into an angle parallel with masticatory forces

Question 26

Define strength

Answer 26

Measure of the force a material can withstand before | the material permanently deforms

Question 27

List the 3 ways strength may be viewed

Answer 27

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 28

Define stiffness

Answer 28

Is the slope of the strength-range graph and is equal to the product of the elastic modulus of the material (E) times its area moment of inertia (I). Steeper slope means stiffer material.

Question 29

Define range

Answer 29

Deflection of material before permanent deformation occurs From zero to proportional limit Failure point - where it breaks

Question 30

Define resilience

Answer 30

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

Question 31

Define formability

Answer 31

Amount of permanent deformation that a material can withstand before breaking

Question 32

Features of nickel titanium wire?

Answer 32

= Family of inter-metallic materials = contains a nearly equal mixture of nickel and titanium Other elements added to adjust or "tune" the properties. "Shape Memory" and "Superelasticity".

Question 33

What is shape memory?

Answer 33

Restores original shapes by heating it

Question 34

What is superelasticity?

Answer 34

Martensite in Nitinol can be stress induced if stress is applied in the temperature range above Aƒ(austenite finish temperature). • Less energy is needed to stress-induce and deform martensite than to deform the austenite by conventional mechanisms. Shows the type of deformational behaviour

Question 35

What do nitinol archwires do?

Answer 35

Move the teeth by applying a constant force for long treatment time and tooth position - apply a constant stress over a wide range

Question 36

What do the most successful bonding agents used in ortho rely on?

Answer 36

Mechanical retention to the enamel and bracket base

Question 37

Define adhesion

Answer 37

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

Question 38

Define mechanical adhesion

Answer 38

2 materials may be mechanically interlocked Sewing forms a large scale mechanical bond Velcro forms medium scale bond Some textile adhesives form small scale bond

Question 39

Define chemical adhesion

Answer 39

2 materials may form a compound at the join | Strongest joins are where atoms of the two materials swap (ionic) or share (covalent) outer electrons

Question 40

Define disperse adhesion

Answer 40

Aka adsorption | 2 materials may be held together by van der Waals forces

Question 41

Define electrostatic adhesion

Answer 41

Some conducting materials may pass electrons to form a difference in electrical charge at the join Forms structure that has attractive electrostatic force between the materials Electrons are passed if one conducting materials binds its electrons less strongly than the other does

Question 42

Define diffuse adhesion

Answer 42

Occurs when molecules of both materials are mobile and soluble in each other Mechanism involved in sintering Metal or ceramic powders are pressed together and heated, atoms diffuse from one particle to the next – joins the particles into one