Materials in orthodontics Flashcards
Which material?
Use the most appropriate material for the
procedure that you are doing.
Don’t be a cheapskate
–cheap materials are often cheap for a reason
Commonly used material
• (poly)methylmethacrylate (PMMA) Stainless steel Nickel titanium (NiTi) Adhesives/cements Plaster Alginate - most important
Biocompatibility
• Safety of pt • Nickel -around 25% females allergic • Latex • Estrogenicity of resin -some compounds have oestrogen-like effect in body
PMMA
PMMA is a vinyl polymer, made by free radical
vinyl polymerization from the monomer methyl
methacrylate
LEARN STRUCTURE
Curing PMMA
Heat cured –used for baseplates Self cured –also used for baseplates -chemically very similar to heat cured PMMA but contains an activator (dimethylp-toluidine) • Depending on which type of PMMA used there can be between 0.1% and 5% residual monomer & additives can be released from baseplate
PMMA dangers
Historically there have been studies that report a possible link between the
carcinogenic and embryotoxic potency of MMA but more recent studies have not documented this
-long-term studies on people exposed to PMMA through
their occupation demonstrated no
increased frequency in tumors
It must be recognised that all components
of PMMA are allergenic to some degree
-there can be cross-sensitisations within
group of methacrylate compounds with
MMA being the most significant allergen
for most pts
Preventing allergy to PMMA
• Use heat cure PMMA
• Store appliance in water for several hours
prior to fit
• Use light cured ‘acrylic’
Most common allergens for occupationally exposed dental professionals
MMA, dibenzoyl peroxide and the crosslinking
agent EGDMA (ethylene glycol
dimethacrylate)
Minimisation of PMMA exposure
- Wear gloves
- Ventilation
- Use down-draught extraction
Wire
Wires which apply the force needed to
move teeth are central to the practice of
orthodontics. Altering any aspect of the
wire changes its properties and mode of action
Types of wire used in orthodontics
Austenitic stainless steel
Elgiloy Co/Cr/Ni
Beta-titanium (TMA)
NiTi
Composition of Austenitic stainless steel
17-20% Cr
8-12% Ni
0.15% C (max)
Austenitic stainless steel
- modulus of elasticity (GPa)
- yield strength (MPa)^a
- application
160-180
1100-1500
Removable and fixed appliances
Composition of Co/Cr/Ni Elgiloy
40% Co, 20% Cr, 15% Ni, 15.8% Fe, 7% Mo, 2% Mn, 0.15% C, 0.04% Be
Elgiloy Co/Cr/Ni
- modulus of elasticity (GPa)
- yield strength (MPa)^a
- application
160-190
830-1000
Crozat appliances and fixed appliances
-has to be heat treated (not really used very often)
Beta-titanium (TMA) composition
77.8% Ti,
11.3% Mo,
6.6% Zr, 4.3%
Sn
Beta-titanium (TMA)
- modulus of elasticity (GPa)
- yield strength (MPa)^a
- application
62-69
690-970
Fixed appliances
NiTi composition
55% Ni, 45% Ti
shape memory wire
High modulus of elasticity
Stiffer, less bendy
NiTi
- modulus of elasticity (GPa)
- yield strength (MPa)^a
- application
34
210-410
Fixed appliances
Forces (DIAGRAM)
Tensile
Compressive
Shearing
Tensile force
A tensile force causes elongation in the
direction of load applied
Compressive force
A compressive force causes a contraction
in the direction of the load applied
Shear force
A shear force causes either a sliding
displacement of one side of a specimen or a twisting around its axis (torsion)
Mechanical properties of wires generally assessed by
• tensile,
• bending, and
• torsional tests
-although characteristics determined by these tests do not necessarily reflect behaviour of wires under clinical conditions, they provide basis for comparison of wires
Optimum characteristics of wire
- large springback,
- low stiffness,
- good formability,
- high stored energy,
- biocompatibility and environmental stability,
- low surface friction
- capability to be welded or soldered to auxiliaries
Stainless steel
Harry Brearley 1913: 12.8% chromium, 0.24% C
Stainless steel wire
• Stainless steel wires have remained popular since their
introduction to orthodontics because of their:
• formability,
• biocompatibility and environmental stability,
• stiffness,
• resilience, and
• low cost
Force and deflection of stainless steel springs
F = (k d r^4) / I^3 r is radius of wire d is deflection of wire l is length of spring k is stiffness of wire (Young's modulus)
Hooke’s lawh
the strain in a solid is proportional to the applied stress within the elastic limit of that solid
Co-Cr wire
Can be manipulated in softened state and then subjected to heat treatment
–>wire with properties similar to stainless steel
Occasionally for pre-made clasps for RPDs
Occasionally for bonded retainers
NiTi pros
Good springback
Low stiffness
NiTi cons
Poor formability and ‘joinability’
Beta-titanium properties
Beta-titanium wires provide a combination of adequate
springback, average stiffness, good formability, and can
be welded to auxiliaries
Multi-strand wire properties
Compared with stainless steel:
-high spring-back
-low stiffness
Cheap substitute for NiTi and for bonded retainers
Elastic properties of wires
• Strength = Stiffness X Range
• Strength is the quality or state of being strong, relating to
tensile strength
• Stiffness is the quality of being rigid; not easily bent.
• Range is the distance a wire travels before permanent
deformation
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). The value, I, is geometry dependent. Thus, with a change
in shape of the same material you will change the stiffness
Rectangular cross-section vs round cross-section theory
For a round cross-section:
I = [π X (diameter)4
] /64
For a rectangular cross-section:
I = [base X (height)3
] / 12
• If you were to double diameter/ height of object, you will cause 16x
increase in moment of inertia (I) of circle and an 8x increase in moment
of inertia (I) of rectangle
-these result in a corresponding 16 and 8x
increases in stiffness
• With influence of the moment of inertia, you can see how changes in shape and
changes in size will affect stiffness
Rectangular cross-section vs round cross-section in the mouth
Wires that are round in cross-section fit
loosely in the brackets and are used for
initial stages and only TILT teeth. They do not move the root, the root is dragged along passively through the bone into an
approximate position in the arch
Rectangular wires are used in the 2nd
stage of movement and engage the
bracket much more firmly such that a
torque force is placed on the tooth.
-this torque acts on the long-axis of the
tooth such that the root moves into an
angle parallel with masticatory forces
Strength
Strength is the measure of the force a material can withstand before
the material permanently deforms
Ways to view strength (GRAPH)
- 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
Stiffness
Stiffness is proportional to the slope of the
linear portion of the graph of the force-deflection curve of a material. The linear portion ranges from zero to the
proportional limit. The steeper the slope, the stiffer the material.
Range
Range is the deflection the material will encounter before any permanent deformation occurs - from zero to the proportional limit.
Beyond the proportional limit, the material will bend, but it will not
return to its original shape. There is, however, a limit to the amount
of bending beyond the proportional limit to which you can bend a material - the failure point is were it breaks
Range units
Length
Strength unit
Force
Ways to measure internal stresses and external strains
Force-deflection curve
Slope of stress-train curve
The elastic modulus
-proportional to stiffness
Resilience (GRAPH)
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 (GRAPH)
Formability is the amount of permanent
deformation that a material can withstand before
breaking
Force-deflection curve
Stainless steel
Beta-titanium
Nickel titanium
GRAPH
Nickel titanium
Nickel Titanium wire (Nitinol, an acronym for Nickel Titanium Naval Ordnance Laboratory) is a
family of inter-metallic materials, which contain a
nearly equal mixture of nickel and titanium.
Other elements can be added to adjust or “tune” the material properties. Nitinol exhibits unique
behaviour. The two terms used to describe this
behaviour are “Shape Memory” and “Superelasticity”
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
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
Superelasticity and shape memory
• The shape memory effect is a very close to the phenomena of
superelasticity (also called”pseudoelasticity”).
• Superelasticity assumes a reversible response to stress caused by a
phase transformation.
• Superelasticity (or pseudoelasticity,) shows us the type of
deformational behaviour, traditionally an elastic one.
• Shape memory denotes the possibility of a body to return to its
original configuration, unlike superelastic materials it can do this
without applying temperature
Constancy of stress
Super-elastic Nitinol has an unloading curve that stays flat over large strains, i.e. Nitinol archwires can apply
a constant stress over a wide range. Orthodontic archwires were the first medical application of superelastic
Nitinol. Nitinol archwires “move with the teeth”, applying a constant force over a broad treatment time
and tooth position
Bonding brackets
Generally the most successful bonding
agents used in orthodontics rely on
mechanical retention to both the enamel
and bracket base
-GIC for cementing bands (bond to enamel and SS)
-composite with self-etching primer for brackets
Adhesion
The force of attraction between the molecules or atoms on two different
surfaces as they are brought into contact
Types of adhesion
Mechanical Chemical Dispersive Electrostatic Diffusive
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 attraction
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
Importance of adhesives in ortho
• Bracket failure increases the time spent in surgery for
repairs and the overall treatment time. At present
orthodontists can choose between four groups of
adhesives which may be set with a chemical reaction or
light.
• Some adhesives may prevent early decay around
brackets because they contain fluoride.
• There is only weak, unreliable evidence that one
adhesive may possibly be better than another.
• There is no clear evidence on which to make a clinical
decision of the type of adhesive to use
A patient needs to have the springiness of their arch wire increased eight-fold. How will you do this? Note-the arch wire is composed of a Nickel Titanium alloy.
a. Increase the diameter of the arch wire four fold.
b. Switch to an austenitic Stainless Steel arch wire of the same diameter and length.
c. Reduce the diameter of the arch wire two fold.
d. Increase the length of the arch wire two fold
D - increase the length of the arch wire two fold
