ortho wire Flashcards
Determine how much stress a material can withstand before it starts permanent deformation.
yield strength
Flexibility or stiffness
Basic property of the material, gives its fundamental characteristics of flexibility.
Modulus of Elasticity
First permanent deformation
Point at which material starts to deform permanently.
Elastic Proportional Limit
Maximum load
Point of clinical loading
SPRINGBACK – maximum load a wire can sustain and still be clinically useful.
Ultimate Tensile Strength
Wire breaks
BRITTLE – (materials that do not strain much beyond the elastic limit
DUCTILE – (those that deform a great deal before breakage)
Fracture / Failure Point
Maximum force that can be obtained from a wire (S=SxR)
STRENGTH
Amount of force produced per activation
STIFFNESS
- Ability to apply lower force.
- Greater ease and accuracy in applying force.
- A more constant force over time as the wire deactivates.
LOW STIFFNESS SLOPE
- Distance wire will bend elastically before permanent (plastic) deformation occurs.
- Clinically measured to the ultimate tensile strength – spring back / working range
RANGE
- When an austenitic wire is placed in the mouth and deformed by forcing it into the misaligned brackets, the ___________ is induced.
- This transforms the austenitic alloy into a martensitic state, which as the teeth align, gradually reverses to the austenitic state.
Pseudoelastic effect
Martensitic active alloys are stable at room temperature, but when raised to mouth temperature, the material changes into an austenitic state, which exhibits shape memory.
Thermoelastic effect
The alloy, introduced in 1970 by Andreasen, is stabilized by introducing a certain amount of work hardening during processing and does not show true memory shape properties.
Martensitic stabilized alloy (e.g. Unitek’s Original Nitinol)
- “Active” means that it exhibits the shape memory, in this case of the pseudoelastic type, the shape memory effect being induced by stress distorting the arch wire in malaligned teeth.
- Examples of superelastic NiTi are Titanol from Forestadent and Nitinol SE from Unitek
Austenitic active alloy
The range of wire is the distance it will bend elastically before permanent deformation occurs. If the wire is deflected beyond its yield point, it will not return to its original shape.
Range of Deflection–Spring Back
- The shape-memory effect, exhibited by the more recent nickel titanium wires, has revolutionized the selection of wires for appropriate tooth movement.
- The wires manufactured for orthodontic purposes are composed of an alloy of nearly equal parts of nickel and titanium.
- The shape memory effect is brought about by a change in the internal crystal formation from the martensitic phase with a hexagonal crystal structure to or from the austenitic phase with a cuboid crystal structure.
- The shape in crystalline structure can be brought about by either:
- Stress, as in the pseudoelastic effect in the austenitic active alloy.
- Heat, as in the thermoelastic effect in the Martensitic active alloy where the transition temperature is between room and mouth temperatures.
Shape Memory Effect
The strength of a wire is important because it determines the maximum force it can deliver.
The above three properties are related by the formula:
Strength = Stiffness × Range
Strength of the Wire
This is the amount of permanent deformation a wire can withstand before it breaks.
Formability
Stainless steel can be soldered and welded but NiTi cannot.
Miura recently reported a method of soldering nickel titanium wires. TMA is weldable as described by Burrstone.
Solubility and Weldability
The laboratory understanding of friction is not relevant to the clinical situation because every time the patient bites together, the tooth is liable to move a small distance in all three planes of space.
More important is the concept that the two components, bracket and wire, may damage each other as they moved across their surfaces.
Friction
Any material used for the construction of the wire must be stable in the oral environment. This has been one of the limitations of the esthetics.
Environmental Stability
Gold alloy were used in the manufacture of orthodontic arch wire.
It was intensively used before 1940. Gold and gold alloy arch wire exhibit excellent formability, environmental stability and biocompatibility.
The main drawbacks of these arch wires include high-cost, low-spring back and low-yield.
GOLD
Stainless steel was introduced by Wilkinson in 1929. Stainless steel arch wire exhibits adequate strength, high resilience, formability, high stiffness, biocompatibility and are economical.
BASIC COMPONENTS
18% Cr, 8% Ni, 0.2% C, 71% Fe
Cold working (hardens steel)
Annealing (softens steel)
Fully annealed (makes the steel dead soft)
The drawback of these arch wires includes high modulus of elasticity; more frequent activations are required to maintain the same force level.
STAINLESS STEEL
Similar to SS
Secret formula of Wilcock in Australia
Key wire to Begg Technique
Excellent working wire
20% stiffer than SS (more stiff – more brittle
AUSTRALIAN WIRE
Nickel titanium alloy, also known by nitinol (Nickel Titanium Naval Ordinance Laboratory) was invented by William R Buchler at Naval Ordinance Laboratory.
The main advantage of this alloy over others is the high elasticity and shape-back memory.
The drawback of these arch wires is that they cannot be welded or soldered and cannot receive bends or loops or helices.
NICKEL TITANIUM ALLOYS
Andreasen 1978: NITINOl (Nickel Titanium Naval Ordinance Lab)
Poor formability – can’t be bent into loops / stops
First Generation Ni-Ti
Shape memory (wire remembers its original shape)
Super elasticity (force stays fairly constant during wire deformation)
Second Generation Ni-Ti
Variable temperature activates.
27°C colder state, constant activation, rapid tooth movement
35°C recommended temperature, milder activation, lower force
40°C intermittent activation, mild forces, high canines
Soft, pliable wire for earlier bracket engagement
Enhanced torque control for faster treatment.
Momentary forces applied to teeth early in treatment for reduced chair time
Temperature Activated Ni-Ti
