Materials in Orthodontics Flashcards
Which material
Most appropriate material
Don’t be a cheapskate
Common materials
PMMA Stainless steel Nickel titanium Adhesives/cements Plaster Alginate
BIocompatibility
Safety of patient
Nickel - found in some alloys used in ortho - 25% females allergic to it
Latex -
Estrogenicity of resin - some resins leak compounds and have an oestrogen like effect
PMMA
Vinyl polymer
Free radical vinyl polymerisation from methyl methacrylate
Types of PMMA
Heat cure - stronger and less free flow
Self-cure - chemically similar but contains activator
0.1-5% release of monomers and additives from base plate
PMMA caution
All components are allergenic to some degree MMA is most allergenic Preventing allergy Use heat cure Store appliance in water for several hours Use light cure acrylic Gloves Ventilation Down-draught extraction
Wire
Wires which apply force needed to move teeth
Lower modulus of wire
More elasticity
Forces - 3 types
Tensile forces
Compressive
Shear
Tensile
Elongation in direction of load applied
Compressive
Contraction in direction of load
Shear force
Sliding displacement of one side of a specimen or twisting round axis
Mechanical properties are generally assessed by
Tensile, bending and torsional tests
Optimum characteristics of wire
Large springback
Low stiffness
Good formability
High stored energy
Biocompatibility and environmental stability
Low surface friction
Capacity to be welded or soldered to auxiliary supports
Stainless steel wire
Popular since intro to orthodontics Formability Biocompatibility Environmental stability Stiffness Resilience Low cost
r =
d =
l =
k =
radius of wire
deflection of wire
length of spring
stiffness of wire (young’s modulus)
Co-Cr wire
Manipulated in softened state
Heat treatment results in similar properties to stainless steel
NiTi
Good springback
Poor formability and join ability
Beta-titanium provides
adequate springback
average stiffness
good formability
weldable
Multi-strand wire
High springback
Low stiffness compared to stainless steel
Cheap substitute for NiTi
Elastic properties of wires
Strength
Stiffness
Range
Stiffness x range
Quality of being strong - tensile strength
Quality of being rigid and not easily bent
Range is distance wire travels before permanent deformation
Stiffness =
Gradient of strength-range graph
E x I
change of shape of same material changes stiffness
Rectangular cross section vs round cross section
For a round cross-section:
I = [π X (diameter)4] /64
For a rectangular cross-section: I = [base X (height)3] /12
Wires round in cross section fit LOOSELY in brackets
Used for initial stages and only tilt teeth
Rectangular wires used in
second stage of movement and engage the bracket much more firmly such that a torque force is placed on the tooth
Torque acts on long axis of tooth
Root moves into angle parallel with masticatory forces
Strength is the measure of the force a material can withstand before the material permanently deforms. Strength may be viewed in these three ways
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.
Elastic properties
Strength
Stiffness
Range
Which two properties can be determines from stress strain curve
Resilience
Resilience is the area under the curve out to the proportional limit. Resilience represents the energy capacity of the material that is a combination of the strength and stiffness.
Formability
Formability is the amount of permanent deformation that a material can withstand before breaking.
Shape memory
- Shape memory effect describes the process of restoring the original shape of a plastically deformed sample by heating it.
- This is a result of a crystalline phase change known as “thermoelastic martensitic transformation”.
- The shape memory effect is repeatable.
Bonding brackets
Rely on mechanical retention to both enamel and bracket base
Adhesion
Force of attraction between the molecules or atoms on two different surfaces as they are brought into contact
Adhesion 5 types
Mechanical adhesion
Two materials may be mechanically interlocked. Sewing forms a large scale mechanical bond, velcro forms one on a medium scale, and some textile adhesives form one at a small scale.
• Chemical Adhesion
Two materials may form a compound at the join. The strongest joins are where atoms
of the two materials swap (ionic bonding) or share (covalent bonding) outer electrons.
• Dispersive Adhesion
Also known as adsorption. Two materials may be held together by van der Waals
forces.
• Electrostatic Adhesion
Some conducting materials may pass electrons to form a difference in electrical charge at the join. This results in a structure similar to a capacitor and creates an attractive electrostatic force between the materials. The electrons are passed if one conducting material binds its electrons less strongly than the other does.
• Diffusive Adhesion
This may occur when the molecules of both materials are mobile and soluble in each other. It is also the mechanism involved in sintering. When metal or ceramic powders are pressed together and heated, atoms diffuse from one particle to the next. This joins the particles into one.
Adhesives
Bracket failure
