materials in ortho Flashcards
What are commonly used materials?
PMMA (polymethylmethacrylate)
SS (stainless steel)
NiTi (nickel titanium)
Adhesives/cements
Plaster
Alginate
Why is biocompatibility important?
Safety of pt
Lots of nickel and latex allergies
Estrogenicity of resin (may have adverse effects for men)
What is PMMA?
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
How can you prevent an allergy to PMMA?
1. Use heat cure- less free monomer
2. Store appliance in water for several hours prior fit
3. Use light cured ‘acrylic’
What are the most common PMMA allergens for dental professionals?
1. MMA
2. Dibenzoyl peroxide
3. EGDMA (ethylene glycol dimethacrylate)- cross linking agent
How can you minimise the exposure to PMMA allergens?
1. Wear gloves
2. Ventilation
3. Down-draught extraction
What is austenitic stainless steel wire?
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
What is elgiloy Co/Cr/Ni wire?
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
What is Beta-titanium (TMA) wire?
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
What is NiTi wire?
55% Ni
45% Ti
Modulus of elasticity- 34GPa
Yield strength- 210-410(MPa)^a
For fixed appliances
What are the three main forces in ortho?
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)
How are mechanical properties of wires assessed?
Tensile, bending and torsional tests
Provides basis for comparison, doesn’t reflect behaviour
What are the optimum characteristics of wire?
Large spring back
Low stiffness
Good formability
High stored energy
Biocompatible
Environmentally stable
Low surface friction
Capability to be welded/soldered to auxiliaries
What is Hookes Law?
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
What is bulk modulus?
Aka incompressiblity
Measure of ability of a substance to withstand changes in volume when under compression on all sides
What is shear modulus?
Aka rigidity
Shear stiffness of a material, ration of stress to strain
What are SS wires popular?
Good formability, biocompatibility, stability, stiffness, resilience
Low cost
What is the force and deflection of SS springs?
F=kdr^4/l^3
r= radius
l= length
d= deflection
k= stiffness (youngs modulus)
Are CoCr wires used?
Manipulated into softened state and then heat treated to a certain stiffness- difficult
Similar properties to SS
Are NiTi wires used?
Good spring back and low stiffness
BUT
Poor formability and joinability
Know due to ‘shape memory’
Are beta titanium wires used?
Adequate spring back
Average stiffness
Good formability
Can weld to auxiliaries
What is a multistrand wire?
High spring back and low stiffness compared to SS
Difficult to manipulate
Can be used for bonded retainers
Cheap substitute for NiTi
What are elastic properties of wires?
Strength=stiffness x range
Range is distance wire travels before permanent deformation
Why is there rectangular and round cross sections?
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)
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
What is formability?
Amount of permanent deformation that a material can withstand before breaking
What is shape memory?
Restore original shape of a plastically deformed sample by heating
Thermoplastic martensitic transformation (crystalline phase change)
Repeatable
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
What is adhesion?
The force of attraction between the molecules or atoms on two different surfaces as they’re brought into contact
What adhesion do brackets rely on?
Mechanical
Etch tooth surface, base has mesh surface
What adhesives are used to bond brackets?
4 groups
Chemical/light cured
Some may prevent demineralisation as contain fluoride
All v similar
