Materials in orthodontics Flashcards
List the common ortho materials
PMMA Stainless steel Nickel titanium Adhesives/cements Plaster Alginate
What is PMMA?
Vinyl polymer, made by free radical vinyl polymerisation from the monomer methyl methacrylate
Types of PMMA and their uses?
Heat cured - used for baseplates
Self cured - baseplates, contains activator (dimethyl-p-toluidine)
Negatives of PMMA?
Historical link between carcinogenic and embryotoxic potency
All components are allergenic
How to prevent allergy to PMMA?
Use heat cure PMMA
Store appliance in water for several hours prior to fit
Use light cured acrylic
What are the most common allergens of PMMA?
MMA, dibenzoyl peroxide and EGDMA
How to minimise PMMA exposure?
Gloves
Ventilation
Down-draught extraction
What do wires do?
Apply force = move teeth
When are austenitic stainless steel wires used?
Removable and fixed appliances
When are elgiloy Co/Cr/Ni wires used?
Crozat appliances and fixed appliances - it has to be heat treated
When are β-titanium wires used?
Fixed appliances
When are NiTi wires used?
Fixed appliances
What does a: Tensile force Compressive force Shear force cause?
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
What assesses the mechanical properties of a wire?
Tensile
Bending
Torsional tests
Optimum characteristics of wire?
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
Why are stainless steel wires popular?
formability, • biocompatibility and environmental stability, • stiffness, • resilience, and • low cost.
How to work out force and deflection of stainless steel springs?
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)
How to work with Co-Cr wire?
Manipulate in softened state and subjected to heat treatment = properties similar to stainless steel
Pros and cons of nitinol (NiTi)?
Pros = good springback and low stiffness Cons = poor formability and joinability
Pros of beta-titanium?
Adequate springback
Good formability
Average stiffness
Can be welded to auxiliaries
Pros of multi-strand wire? When are they used?
High spring back and low stiffness
Cheap substitute for NiTi and for bonded retainers
How do orthodontists look at elastic properties of wires?
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
List the orthodontic wires from highest to lowest in elasticity
Highest = austenitic stainless steel
Elgiloy Co/Cr/Ni
Beta-titanoim
NiTi
What is stiffness equal to?
The product of the elastic modulus of the material times its area moment of inertia = change shape = change stiffness
What is the inertia of a round cross section? In a rectangular cross section?
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
Define strength
Measure of the force a material can withstand before
the material permanently deforms
List the 3 ways strength may be viewed
- Proportional Limit =
the point at which any permanent deformation first occurs. - Yield Strength =
the point at which 0.1% deformation is measured. - Ultimate Tensile Strength = the maximum load that the wire can sustain.
Define stiffness
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.
Define range
Deflection of material before permanent deformation occurs
From zero to proportional limit
Failure point - where it breaks
Define resilience
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
Define formability
Amount of permanent deformation that a material can withstand before breaking
Features of nickel titanium wire?
= 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”.
What is shape memory?
Restores original shapes by heating it
What is superelasticity?
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
What do nitinol archwires do?
Move the teeth by applying a constant force for long treatment time and tooth position - apply a constant stress over a wide range
What do the most successful bonding agents used in ortho rely on?
Mechanical retention to the enamel and bracket base
Define adhesion
The force of attraction between the
molecules or atoms on two different
surfaces as they are brought into contact.
Define mechanical adhesion
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
Define chemical adhesion
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
Define disperse adhesion
Aka adsorption
2 materials may be held together by van der Waals forces
Define electrostatic adhesion
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
Define diffuse adhesion
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
