Orthodontic Wires

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NITINOL

·        Invented in early 60’s by Wiliam F Buc-hler

·        Developed for the space program (Ni,nickel; Ti, titanium; NOL, Naval Ordnance Laboratory)

Composition

·        Nickel-55%

·        Titanium-45%

·        Have two remarkable properties - shape memory and superelasticity.

·        Exist in more than one form or crystal structure.

·        The martensite  form exists  at lower temperatures

·        Austenite form at higher temperatures

·        Shape memory and superelasticity are related to phase transitions within the NiTi alloy between the martensitic and austenitic forms that occur at relatively low transition temperature.

·        Shape memory refers to the ability of the material to "remember" its original shape after being plastically deformed while in the martensitic form.

·        In a typical application,a certain shape is set while the alloy is maintained at an elevated temperature, above the martensite-austenite transition temperature.

·        When the alloy is cooled below the transition temperature, it can be plastically deformed, but when it is heated again the original shape is restored –thermoelasticity

·        Nitinol was marketed in the late 1970s for orthodontic use in a stabilized martensitic form, with no application of phase transition effect

·        Nitinol is exceptionally springy and quite strong but has poor formability

·        Other martensitic alloys marketed later (Orthonol)have similar strength and springiness to Nitinol but better formability

·        In the late 1980s new nickel-titanium wires with an active austenitic grain structure appeared.

·        These wires exhibit the other remarkable property of NiTi alloys-superelasticity which is manifested by very large reversible strains and a non-elastic stress- strain or force deflection curve – A- NiTi wires

·        Phase transition in grain structure from austenite to martensite, in response not to a temperature change but to applied force.

·        The transformation is a mechanical analogue to the thermally-induced shape memory effect.

·        The austenitic alloy undergoes a transition in internal structure in response to stress without requiring a significant temperature change (which is possible because for these materials, the transition temperature is very close to room temperature)

Nitinol is basically is of two types:

 1. Thermal nitinol

 2. Elastic nitinol

Thermal Ni-Ti Alloy

     Initially, composed of equal parts of nickel and titanium.

Composition

1.      Nickel-54-55%

2.      Titanium-43-44%

3.      Cobalt-1.6-3%

·        Thermal nitinol shows shape memory in the martensitic phase.

·        These archwires are formed to the desired shape in the martensite form and they go through the transition temperature range (TIR) to the austenite grain structure

·        In the austhetetic grain structure it is deformed to confirm to the irregularities in the arch form; taking the wire through TIR again will result in its original shape in the martensitic form.

·        The wires with austenitic finish temperatures less than 37 degree centigrade exhibit superelasticity.

VARIOUS FORM OF NITI ALLOY WIRE AVAILABLE COMMERCIALLY

Elastic Ni- Ti

·        Elastic Ni-Ti alloy is used in the martensitic phase maintains its high elasticity and flexibility.

·        This wire also exhibits  lighter continuous forces on deformation

 Advantageous properties

1. High spring back

2. High stored energy

3. High elasticity.

Disadvantageous properties

1. High friction as compared to stainless steel.

2. Low stiffness cannot be used at the completion stages of orthodontic treatment.

3. Fractures easily if bent over a sharp edge.

4. Very limited bending is possible.

5. Cannot be welded or soldered.

6. Expensive as compared to stainless steel wires.

Copper Ni-Ti Alloys

       This alloy was developed by Or Rohit Sachdeva and Miyasaki in 1994.

Composition

v Titanium--42.99%

v Nickel-49.87%

v Chrornium-0.50%

v Copper – 5.64%

·      The stress induced martensite is responsible for the superelastic characteristic of Ni- Ti alloys.

·      Martensite transformation is also temperature dependent.

·      The stability of the martensite and/or austenite phase at a given temperature is based upon the transformation temperature of the alloy.

·      The most important marker is the materials Austenitic finish (Af)temperature.

·      The addition of copper

1.      decreases the difference between loading and unloading forces causing delivery of more constant forces for small activation

2.      increases surface smoothness making the surface roughnes

3.      Control the transformation temperature of the alloy.

·        To exploit superelasticity to its fullest potential, the working temperature of the orthodontic appliance should be greater than the Af temperature.

·        Four types of copper Ni- Ti alloys were developed

 Type I

       - At 15°C

       - not used clinically

        - exerts very high forces.

 Type 11

       - At 27°C

       - normally used in patients with average pain        tolerance

       - periodontium should be healthy

        - constant force is generated

Type III

            - At 35°C

            - Used in patients with a low threshold

            - Normal to slightly compromised periodontium

            - Used only where low forces are desired.

Type IV

    - At 40°C

    - Used in patients with low pain threshold

    - Where tooth movement is to be slow

    - Intermittent forces are generated

    - Used as an initial aligning archwire

Advantages of copper Ni- Ti alloys

 1. More resistant to permanent deformation

 2. Better springback as compared to other Ni-Ti alloys

 3. More constant forces are exerted over small activations.

CHROME-COBALT

·        Also known as Elgiloy.

·        Have properties similar to those of stainless steel but can be supplied in the softer and more formable state and then could be hardened by heat treatment.

·        This process increases the strength of the wire

Composition

1.      CobaIt--40%

2.      Chromium-20%

3.      Nickel-15%

4.      Iron-15.4%

5.      Molybdenum--7%

6.      Manganese--2%

7.      Beryllium--0.04%

8.      Others--0.05%

This alloy is manufactured in four tempers, depending on the amounts of cold work:

1.      Blue-soft and easy to bend

2.      Yellow-ductile

3.      Green-semi-resilient

4.      Red-resilient

·        The wires made from this alloy are generally supplied in the ductile form, allowing them to be easily deformed and shaped into appliances.

·        These are then heat treated to increase their strength

·        The standard heat treatment involves heating to 483 degrees centigrade for 7 to 12 minutes.

·        Low temperature heat treatment causes a phase change and stress relief.

·        Heating to 1100-1200 degrees centigrade and quenching can soften the wire

 Advantageous properties

1. Excellent tarnish and corrosion resistance.

2. Greater resistance to fatigue than stainless steel.

3. Greater resistance to distortion.

4. Good formability.

5. Functionally remains active for longer duration if used as a resilient spring

Disadvantageous properties

1. Has to be heat treated.

2. Soldering is demanding. A low fusing solder has to be used. These wires should be soldered with a silver solder in the presence of a fluoride flux or can be joined by spot welding.

3. The modulus of elasticity is high causing higher forces to be delivered for similar activations as stainless steel wires.

BETA TITANIUM OR TMA WIRE OR CAN WIRE

       Introduced into orthodontics by Jon Goldberg and Burstone in 1981

 Composition

1.      Titanium-79%

2.      Molybdenum-ll%

3.      Zirconium-6%

4.      Tin-4%

•     At room temperature the metal is stable in alpha phase and HCP lattice

•     At high temperature above 883 degree, the metal dearranges into BCC.

•     In these wires the metastable BCC structure of titanium is retained at room temperature by using a variety of alloying additives like molybdenum,vanadium and/or chromium.

Distinctive features

v good springback

v low force delivery levels,

v good formability

v weldability.

·      The process of ion implantation for the surface treatment of these wires decreases the frictional forces produced by these wires.

·      The absence of nickel makes these wires useful in patients allergic to nickel.

·      Ideal for situations where forces less than stainless steel and more than Ni- Ti alloy are required.

Adoantageous properties

1. High springback.

2. High formability.

3. Low modulus of elasticity.

4. Low load deflection rate.

5. Low stiffness.

6. Environmentally stable.

7. Excellent corrosion resistance.

8 .Can be joined by electrical resistance welding

Disadvantageous properties

1. More friction than stain less steel or chrome-cobalt alloys. The friction can be decreased using the ion implantation method where by titanium oxide and nitride are deposited on the wire to produce a smoother finish.

2. Become brittle on overheating

TOOTH COLOURED WIRES

COMPOSITE/COATED WIRES

·        These wires are made of a combination of materials coated one on top of another.

·        The coating fibers include fiberglass and ararnid.

·        The candidate resins include polycarbonate and polyethylene terephthalate glycol.

·        For each fiber/resin system, there is a heating or working range where the material can be formed or shaped without any degradation in its properties

 OPTIFLEX ARCHWIRE

·        These are composed of a silicon dioxide core which provides the force or resiliency to the wire.

·        These wires are available in the round as well as rectangular cross- sections and

·        are tooth colored, i.e. are more esthetic than other metaI alloy wires.

·        A silicon dioxide core which provide force

·        A silicon resin midddle layer – protects the core from moisture and adds strength

·        Nylon outer layer- stain resistance that prevents damage to the wire and a further increase in its stength

·        These wires provide light continuous forces and are used during the initial aligning phase of orthodontic treatment.

·        To prevent permanent deformation sharp bends should be avoided during ligation to brackets.

REFERENCE

1.      Contemporary orthodontics – proffit

2.      Textbook of orthodontics – Gurkeerat Singh

3.      Textbook of orthodontics – S Gowri Sankar