Materials Used in Ortho Flashcards
Commonly used materials in orthodontic?
PMMA Stainless steel Nickel titanium Adhesive/ cement Plasters Alginate
What materials may not be the most biocompatible?
Nickel/latex - allergy
Resin - estrogenicity of resin - mimic hormone
What is PMMA?
Polymethylmethacrylate
Made by free radical vinyl polyermisation from monomer methyl methacrylate
Difference between heat-cured and self-cured PMMA?
Self-cured similar to heat-cured but contains activator - dimethyl-p-toludine
What can be released from baseplate if PMMA used?
0.1-5% residual monomer and additives can be leached
Is PMMA allergenic?
Some components allergenic to some degree - cross-sensitisation within group of methacrylate compounds
How to prevent allergy?
Use heat-cured PMMA - minimise amount free monomer
Store appliance water prior fit
Use light cured acrylic
Minimisation of exposure- gloves/ ventilation
What are the properties of different wire?
See table in notes
What is tensile force?
Tensile force = elongation in direction of load applied
What is compressive force?
Contraction in direction of the load applied
What is shear force?
Sliding displacement of one side or a twisting around its axis
How are the mechanical properties of wire assessed?
Tensile/ bending/ torsional test
Test don’t necessarily reflect the behaviour of wire under clinical condition
What are the optimum characteristics of wire?
Large spring back Low stiffness Good formability High stored energy Biocompatible Low surface friction Capability to be welded/ soldered to auxiliaries
Characteristics of stainless steel?
Formability Biocompatible Stiff Resilience Lowe cost
Properties of CoCr wire?
Can be manipulated into softened state then subject to heat tx
Heat tx result wire w/ properties similar SS
Properties of NiTi wires?
Good spring back and low stiffness
Properties of beta-titanium wire?
Combination adequate springback, average stiffness, good formability and can be welded to auxiliaries
Properties of multi-stranded wire?
High spring back and low stiffness compared solid SS wires
Used as cheap substitute NiTi and for bonded retainers
What is strength?
Strength = stiffness x range
Strength is the quality of state of being strong, relating to tensile strength
What is stiffness?
The quality of being rigid, not easily bent
With change in shape - you will change stiffness
What is equation for inertia?
I = [pie x (diameter/4)]/64
What is the equation for a rectangular cross section
I = [base x (height/3)]/12
What are round wires used for?
In cross-section fit loosely in bracket and are used for initial stages - don’t move root - dragged passively through bone into approximate position in arch
What are rectangular wires used for?
Second stage of movement
Engage bracket firmly - allows torque to be placed on tooth
How does torque move tooth?
Act on long-axis of tooth - root moves into an angle parallel with masticatory forces
What are the 3 ways strength can be views?
- Proportional limit - the point at which permanent deformation fist occurs
- Yield strength - point at which 0.1% deformation is measured
- Ultimate tensile strength - maximum load wire can sustain
What are the 2 characteristics determined from stress-stain curve?
- Resilience
- Area under the curve out to proportional limit
- Represent energy capacity of material that is combination of strength and stiffness - Formability
- Amount of permanent deformation material can withstand before breaking
NiTi wire composition?
Equal mixture nickel and titanium
Shape memory and super-elasticity
What is shape memory?
Process of restoring the original shape of plastically deformed sample by heating it
Crystalline phase change known as thermoelastic martensitic transformation
Properties of adhesive for bonding brackets?
Successful agent that relies on mechanical retention to both enamel and bracket
Chemical or light cured
Some adhesive can prevent early decay due to F-
