Stainless steel in orthodontics

stainless steel in orthodontics

1. STAINLESS STEEL IN ORTHODONTICS ISHAN GROVER M.D.S. 1 ST YEAR

2. INTRODUCTION STAINLESS STEEL IS A MAJOR ALLOY USED IN DENTISTRY TODAY. STAINLESS STEEL WAS DESCOVERED BY RESEARCHERS OF BRITAIN AND GERMANY IN 1913. FIRST INTRODUCED IN ORTHODONTICS BY WILKINSON IN 1929. IN 1930 ANGLE USED IT TO MAKE LIGATURE WIRES. By 1937 the value of Stainless steel as an orthodontic material had been confirmed. THE METALLURGY OF S.S. IS CLOSELY RELATED TO THE IRON-CARBON SYSTEM . STEEL IS AN ALLOY OF IRON AND CARBON WHERE CARBON CONTENT IS NOT MORE THAN 0.2 %. STAINLESS STEEL CARBON-STEEL CHROMIUM (12-30%) by weight PASSIVATION : Cr FORMS A THIN, TRANSPARENT AND ADHERENT LAYER OF Cr2O3 WHICH PROVIDES A BARRIER AND PREVENTS CORROSION OF THE UNDERLYING ALLOY.

3. TYPE OF STAINLESS STEEL CHROMIUM NICKEL CARBON FERRITIC (BCC) 11.5-27% 0% 0.20% (MAX.) AUSTENITIC (FCC) 16-26% 7-22% 0.25% (MAX.) MARTENSITIC (BCT) 11.5-17% 0-25% 0.15-1.20% COMPOSITION SILICON, PHOSPHORUS, SULFUR, MANGANESE, TANTALUM, NIOBIUM MAY BE PRESENT IN SMALL AMOUNTS. THE BALANCE IS IRON

4. Chromium: A thin transparent, tough, impervious oxide layer of Chromium oxide forms on the surface of the alloy when subjected to room air.- “passivating film effect”. Increases hardness, tensile strength and proportional limit. Nickel: Increases strength. Increases tarnish and corrosion resistance. FUNCTIONS OF CONSTITUENT METALS

5. Cobalt: Increases tarnish and corrosion resistance Decreases hardness. Manganese: Scavenger for sulphur. Increases hardness during quenching. Silicon: Deoxidiser and scavenger. Titanium: Inhibits the precipitation of Chromium carbide

6. TYPES OF STAINLESS STEEL Ferritic (BCC) AISI-400 Stable between room temperature and 912 degrees. Carbon has low solubility in this structure (max. of 0.2% at 723 degrees). The alloy can not be hardened by heat treatment. Little application in Dentistry.

7. Martensitic (BCT) AISI-400 If austenite is cooled rapidly (Quenched) it undergoes spontaneous transformation to a Body Centred Tetragonal. The lattice is highly distorted and strained resulting in a hard, strong and brittle alloy. Martensite decomposes into ferrite and carbide. Decomposition is accelerated by appropriate heat treatment to reduce hardness but this is counter balanced by increased toughness. This process is known as Tempering Increased strength and hardness – used for surgical and cutting instruments.  Least Corrosion resistance.

8. Austenitic (FCC) AISI-302,304 Most corrosion resistant of all types of stainless steel. Formed between 912 – 1394 degrees. 18-8 stainless steel : 18% Chromium, 8% Nickel and 0.15%(302) 0r 0.08%(304) Carbon Austenite is preferable to Ferritic because of greater ductility, ability to undergo more cold work without fracture. Increased strength during cold working. Readily overcomes sensitisation. When austenite is allowed to cool slowly to room temperature it forms Iron Carbide and Ferrite. The iron carbide compound is called cementite and the solid solution of ferrite along with cementite is called pearlite.

9. Austenitic type Chromium Nickel Carbon Molybdenum Type 302 17% to 19% 8% to 10% 0.15% (max) - Type 304 18% to 20% 8% to 12% 0.08% (max) - Type 316L ( low carbon) 16% to 18% 10% to 14% 0.03% (max) 2% to 3% AISI CLASSIFICATION OF AUSTINITIC S.S The  American Iron and Steel Institute  ( AISI ) is an association of North American steel producers. Its development was in response to the need for a cooperative agency in the iron and steel industry for collecting and disseminating statistics and information, carrying on investigations, providing a forum for the discussion of problems and advancing the interests of the industry

10. Austenitic stainless steel is preferable to ferritic stainless steel Greater ductility and ability to undergo more cold work without fracturing. Substantial strengthening during cold working ( some transform into a martensite phase ) Greater ease of welding Ability to overcome sensitization Less critical grain growth Comparative ease in forming

11. Acc. to ADA (specification no 32) Type 1 (low-resilience) Type 2 (high-resilience) Based on carbon content (0.8%) Hypo-eutectoid Hyper-eutectoid

12. ANNEALING The effects associated with cold working such as strain hardening, low ductility and distorted grains - reversed by simply heating the metal. This process is called annealing. The more severe the cold working the more rapidly the effects can be reversed by annealing. Austenitic SS – nonmagnetic, ferromagnetic qualities develop as a result of cold-working during fabrication, due to formation of ferrite and martensite forms. This magnetism is eliminated by a full anneal. The temperature of this transformation is termed the Curie temperature, which is around 700° C.

13. SENSITIZATION (weld decay) Austenitic stainless steel looses its resistance to corrosion if it is heated between 400 to 900 degree C. Decrease in corrosion resistance is caused by precipitation of chromium-iron carbide at grain boundaries. Small carbon atoms rapidly diffuse to grain boundary region, combines with chromium & iron atoms in solid solution and form (CrFe)4C Its formation is most rapid at 650 degree C, below this temperature, the diffusion rate for carbon is slower. Decomposition of (CrFe)4C occurs at high temperature. Corrosion resistance is reduced in regions adjacent to grain boundaries in which the chromium level is depleted below that necessary for protection ( approx. 12%) Stainless steel becomes susceptible to intergranular corrosion and partial disintegration of the weakened alloy may result. STABILIZATION This is the process by which carbon is made unavailable for the sensitizing reaction. This is done at the time the alloy is manufactured by either keeping the carbon content exceptionally low or by adding other metals such as Titanium that tie it up in different compounds.

14. COLD WORKING Cold forming stainless steel is generally different to plain carbon (mild) steels, primarily because stainless steels are stronger, harder and more ductile, work harden more rapidly and must maintain their inherent corrosion resistance When stainless steel is cold worked , carbide is precipitated along slip planes , so chromium is dispersed throughout rather than conc. at the boundaries. More protection from corrosion.

15. HEAT TREATMENT OF STAINLESS STEEL ALLOY The physical properties of orthodontic stainless steel wires improve by heat treatment at low temperatures between 750° C to 820° C for ten minutes and at a lower temperature of 250° C for twenty minutes. By heat treatment residual stresses are removed.

16. COMMON CAUSES OF CORROSION OF S.S. A common cause of the corrosion of stainless steel is the incorporation of bits of carbon steel or similar metal in its surface. For example, if a stainless steel wire is manipulated carelessly with a carbon steel pliers, it is possible that some of the carbon steel from the plier may become embedded in the stainless steel. Or if the stainless steel appliance is abraded or cut with a carbon steel bur or similar steel tool, some of the carbon steel from the tool may also become embedded in the stainless steel. Such a situation results in an electrochemical cell that may cause considerable corrosion in vivo. Soldered joints in orthodontic appliance can also form galvanic couples in vivo. Austenitic stainless steel are susceptible to attack by solutions containing Chloride. Chlorine containing cleansers should not be used to clean removable appliance fabricated from stainless steel.

17. ALLOY ELASTIC MODULUS (GPa) YIELD STRENGTH (GPa) TENSILE STRENGTH (GPa) NO. OF 90 DEGREE COLD BENDS WITHOUT FRACTURE STAINLESS STEEL 179 1.6 2.1 5 CO-CR-NI 184 1.4 1.7 8 NI-TI 41 0.43 1.5 2 BETA-TI 72 0.93 1.3 4 MECHANICAL PROPERTIES OF ORTHODONTIC WIRES

18. Characteristics of Clinical relevance Spring back (maximum elastic deflection): The extent to which the range recovers upon deactivation of an activated arch wire. A measure of how far a wire can be deformed without causing permanent deformation or exceeding the limits of the material. Higher the spring back, grater the working range and lesser are the requirements of frequent activations. Stainless steel has a spring back lesser than Nickel-titanium or beta titanium. Resilience: The capacity of a material to absorb energy when the material is elastically deformed. It is measured by the area under the stress strain curve. Stiffness: Amount of force required to produce a specific amount of deformation.  

19. Formability: High formability provides the ability to bend a wire into desired configurations such as loops coils and stops without fracturing the wire. Biocompatibility: Includes resistance to corrosion and tissue tolerance to elements in the wire.   Environmental stability: Maintenance of desirable properties of the wire for extended periods of time after manufacture. This ensures a predictable behavior of the wire when in use.   Joinability: This is the ability to attach auxiliaries to orthodontic wires by welding or soldering provides an additional advantage when incorporating modifications to the appliance.

20. Working range and flexibility: The distance a wire will bend elastically before permanent deformation occurs. Flexibility is the measure of the amount at which the wire can be strained without undergoing plastic deformation. Stress relaxation: When a wire has been deformed and held in a fixed position the stress may diminish with time even though the total strain may remain constant.   Friction: Space closure and canine retraction in continuous arch wire technique involves a relative motion of bracket over wire, excessive amount of wire friction may result in loss of anchorage or binding accompanied by little or no tooth movement. The preferred wire material for moving a tooth relative to the wire would be one that produces the least amount of friction at the bracket wire interface.

21. SOLDERING It is a process of joining two metals by the use of an intermediate alloy which has a lower melting point. Soldering temperature – 620 to 665 C. Ideally silver solders are used- alloy of silver, copper, zinc to which tin and indium are added to lower the fusion temperature and improve solderability. Needle like non luminous gas air flame is used. Thinner the diameter of the flame, less the metal surrounding the joint is annealed. The work is held 3mm beyond the tip of the blue cone in the reducing zone of the flame. Soldering should be observed in shadow against a black background so the temperature can be judged by the colour of the work. The colour should not exceed dull red. If possible the parts should be tag welded to hold them together.

22. The flux is applied and the heavier gauge is heated first. Flux should cover all the area and the metal should be allowed to flow around the joint. The work should be immediately quenched in water.   The flux used for soldering stainless steel contains fluoride to dissolve the passivating film formed by the chromium. The solder does not wet the metal when such a film is present. Potassium Fluoride is one of the active chemicals in this respect. Flux: Aids in removing the oxide coating so as to increase the flow. Dissolves any surface impurities. Reduces the melting point of the solder.

23. WELDING Joining of two or more metal pieces directly under pressure without introduction of an intermediary or a filler material. Spot welding is used to join various components in orthodontics. A large current is allowed to pass through a limited area on the overlapping metals to be welded. The resistance of the material to the flow of current produces intense localized heating and fusion of metals. The welded area becomes susceptible to corrosion due to Chromium carbide precipitation and loss of passivation. Increased weld area increases the strength.

24. Factors to be taken into account during soldering and welding As the annealing temperature of stainless steel falls within the soldering and welding temperature ranges, these procedure can lead to loss of working range and elasticity of the metal. Precautions: By using low fusing solders. Using low diameter needle like flame. Reducing the number of welding procedures and duration .

25. ADVANTAGES of S.S. High stiffness High yield strength High resilience Good formability Good environmental stability Good jointability Adequate springiness Biocompatible Corrosion resistant Economical

26. DISADVANTAGES Soldering is demanding Lower springback than NiTi High modulus of elasticity More frequent activations are required Heating to temperatures b/w 400-900 degrees cause release of Ni and Cr thereby decreasing corrosion resistance.

27. BIOCOMPATIBILITY OF S.S. CONSTITUENTS Nickel Hypersensitivity   Symptoms of toxicity : Rash, allergy and lung disorders. Ear infection and tinnitus. Nickel is one of the most common causes of allergic contact dermatitis and produces more allergic reactions than all other metals combined. Stainless steel contains Nickel and it can sometimes produce allergic reactions in the oral cavity.   Chromium toxicity Symptoms of toxicity : Depression and emotional disorders. Chromium is one of the major components of insulin. Chromium deficiency causes high blood sugar and diabetes.  

28. Cobalt toxicity   Symptoms of toxicity : Cobalt is carcinogenic and causes tumor. Sedation of the limbic nervous system, paralysis on the left side of the body, particularly the neck and shoulder. Nervous twitch, loss of control of body movement, clumsiness, chronic fatigue. Embolism and stroke, blockage in the carotid artery Numbness and tingling in the extremities. Pernicious anemia. Blood disorders and liver malfunction. Cobalt is a natural component of B12, therefore cobalt poisoning causes B12 deficiency.

29. NICKEL FREE STAINLESS STEEL In recent years a very new kind of nearly nickel free austenitic S.S was developed and introduced to the market. The steel is alloyed with 15-18% chromium, 3-4% molybdenum, 10-14% manganese, and about 0.9% nitrogen to compensate for nickel. This steel has got high corrosion resistance. The low nickel concentration results in the reduction of nickel allergy potential. Orthodontic wires under the name Manzanium (Scheu), or Noninium (Dentaurum) are already in the market. Unfortunately the melting and forming of this steel is very costly.

30. Australian Orthodontic arch wires Arthur J Wilcock developed an orthodontic arch wire for use in the Beggs technique . Unique characteristics are different from usual orthodontic arch wires. They are ultra high tensile austenitic stainless steel arch wires. The wires are highly resilient. When arch wire bends are incorporated and pinned to the teeth the stress generated within the wire generate a light force which is continuous in nature. Wire is resistant to permanent deformation and maintains it’s activation for maximum control of anchorage.

31. Manufacture: Spinner straightening and pulse straightening. Spinner straightening: The wire is passed through bronze rollers. The disadvantage of this process is resultant deformation. Pulse straightening : The wire is pulsed in a special machine which permits high tensile wires to be straightened. Types: Regular Regular plus Special Special plus S pecial plus pulse straightened Premium Premium plus Premium plus pulse straightened Supreme Supreme pulse straightened Wires are available as spools and as straight lengths.

32. BRAIDED AND TWISTED WIRES Small diameter wires are braided or twisted to form larger wires. Available in round and rectangular shape. Apply low forces for a given deflection when compared with solid stainless steel wire.

33. SOURCES 1.) PHILIPS’ SCIENCE OF DENTAL MATERIALS 12 TH EDITION 2.) CONTEMPORARY ORTHODONTICS – W.R. PROFFIT 5 TH EDITION 3.) TEXTBOOK OF ORTHODONTICS – GURKEERAT SINGH 3 RD EDITION 4.) APPLIED DENTAL MATERIALS – JOHN A. ANDERSON .