Stainless Steel in Orthodontics

stainless steel with

  • Recent advances in orthodontics wire alloys have resulted in a varied array of wires that exhibit a wide spectrum of properties. Up until the 1930’s the only orthodontic wire available were made of gold.

  • Austenitic stainless steel, with its greater strength, higher modules of elasticity, good resistance to corrosion and moderate cost, was introduced as an orthodontic wire in 1929 and shortly afterwards gained popularity over gold.
  • Since then several other alloys with desirable properties have been adopted in orthodontics. These include cobalt-chromium, nickel titanium, beta-titanium, and multi-stranded stainless steel wires.

Historical Background of Stainless steel :

  • 1903 to 1921 was the year that stainless steel was developed and perfected by Brearley of Sheffield and Becket of the U.S.
  • Stainless steel entered dentistry in 1919, being introduced at Krupp’s Dental Polyclinic in Germany by the Company’s dentists, F. Hauptmeyer. He first used it to make a prosthesis and called it Wipla ( Wie platin; in German, like platinum), the designation under which it is still used in Europe. Discovered by chance a few years before world war I, the new alloy allowed Germany to construct sophisticated chemical installations.
  • Angle used it in his last year (1930) as ligature wire. By 1937 the value of Stainless steel as an orthodontic material had been confirmed.
  • Physical and Chemical Properties
  • Stress:
  • “Stress is the internal reaction to the external force” Since both applied force and internal resistance distributed over a given area of the body, the stress in a structure is designated as force per unit area.
  • Stress = Force / Area
  •  Type of Stress :
  • Tension : Results in a body when it is subjected to two sets of forces that are directed away from each other in the same straight line.
  • Compression : Results in a body when it is subjected to two sets of forces in the same straight line and directed to each other .
  • Shear : Is a result of two forces directly parallel to each other.
  • Tensile Stress – is caused by load that tends to stretch or elongate a body.
  • Compressive stress – produced by a load that tends to compress the body.
  • Shear Stress – resist a twisting motion.
  • Strain :
  • If the stress produced is not sufficient to withstand the external force the body undergoes deformation.
  • Strain = Deformation/ ordinal length

Modulus Of Elasticity:

  • Elastic modulus describes the relative stiffness or rigidity of a materials which is measured by the slope of the elastic region of the stress strain graph.

 

 

  • Straight line represents reversible elastic deformation, because the stress remains below the proportional limit, the curved region represents plastic deformation.
  • By definition – modulus of elasticity is the ratio of stress to strain upto/less than the proportional limit.

Stiffness :

  • Stiffness is a measure of the amount of force required to produce a specific deformation in bending or torsion stiffness is proportional to the fourth power of the diameter of the wire.

 

 

Proportional Limit :

The greatest stress that may be produced in a material such that stress is directly proportional to strain.

Elastic limit.

Elastic limit is defined as the maximum stress that a material will withstand without permanent deformation.

Yield Strength:

Defined as the stress at which a material exhibits a limited deviation from proportionality of stress to strain,. i.e., it is a stress at which the material begins to function in plastic manner. At this stress a limited permanent strain as occurred in the material.

It is the practical indicator at which a deformation of 0.1% is measured. It is measured as stress units’ (gm/cm)2

Flexibility :

The maximal flexibility is defined as the strain that occurs when the material is stressed to its proportional limit.

Resilience

Defined as “ the amount of energy absorbed by a structure when it is stressed not to exceed its proportional limit”, i.e., Resilience represents energy storage capacity of the wire, which is a combination of strength and springiness.

 

Springiness Or Spring Back :

Spring back is a measure of how far a wire can be deflected without causing permanent deformation or exceeding the limits of the material. Clinically useful spring back occurs if the wire is deflected not beyond the yield point but no longer returns to its original shape. It can also be described as the elastic property exhibited by the wire between yield strength and the ultimate tensile strength of the wire.

 

Range:

Range is defined as the distance that the wire will bend elstically before permanent deformation occurs. It is measured in millimeters or other length units.

Strength : Stiffness x range

A combination of stiffness and range is the strength of the wire.

Formability :

It represents the amount of permanent bending of the wire which will tolerate deformation that a wire can withstand before failing.

 

 

 

Ductility –

  • It is a ability of a material to withstand permanent deformation under a tensile load without rupture.
  • A metal may be drawn readily into a wire and is said to be ductile.

Malleability

  • It is the ability of a material to withstand under compression, as in hammering or rolling into a sheet.
  • Gold is the most ductile and malleable metal and silver is second.
  • Lattices
  • The three dimensional network of lines that can be visualized to connect the atoms in undisturbed crystal is called a lattice.
  • In the 19th century Bravais showed that only a limited number of crystal lattices exist.
  • One of the simpler types ( and the most often encountered in orthodontic materials) is the cubic lattic, which includes the face centred cubic (fcc ) cell. – body centred cubic (bcc). Also commonly encountered are the monoclinic and the close – packed hexagonal lattic.
  • A – Body centred cubic
  • B – Face centred cubic
  • C – Hexagonal close pack
  • Cold working / strain hardening / work hardening
  • The process of plastically deforming a metal at a temperature below that at which it recrycaliizes new grains, which is usually one-third / one half times its absolute melting point temperature.
  • Cold working will increase the material strength and decreases the ductility.
  • Because of decreased ductility to prevent brekage a softening step    ( annealing) must be added to render the the distorted, cold-worked material strain free.
  • Effects of Annealing Cold worked Metal
  • The effects associated with cold working for e.g., strain hardening, decreased ductility, and distorted grain can be reversed simply by heating the metal to an appropriate elevated temperature. This process is called Annealing.
  • Annealing can take place in three successive stages
  • Recovery,
  • Re- crystallization
  • Grain growth.
  • Recovery
  • In the recovery stage the properties of the cold work metal begin to disappear before any significant changes are observed under microscopic examination. There is a very slight decrease in tensile strength and no change in ductility during recovery.
  • Recrystallization
  • This involves a radical change in the micro-structure. The old deformed grains disappear completely and are replaced by new strain-free grains.
  • After completion of recrystallization, the metal essentially attains its original soft and ductile condition.
  • Grain growth:
  • If the recrystallized metal is further annealed, grain growth occurs in such a way to minimize the grain boundary area, with large grains consuming small grains.
  • STAINLESS STEEL
  • Carbon steels:
  • Pig iron is a compound of iron with carbon silicon, sulphur, phosphorous and manganese. It is dark grey in colour due to graphite. The carbon content is 3 to 3.5%. It has poor strength and hardness.
  • Pig iron is cleared of impurities of iron like silicon, graphite, sulphur etc., to obtain wrought iron.
  • Plain carbon steels are iron-carbon binary alloys that contain less than approximately 2.1% carbon. The major classes of carbon steel are based on three possible crystal structures that can occur for the iron carbon alloys:-
  • Ferrite is a bcc phase, stable to temperature not exceeding 9120C
  • Austenite is an fcc phase, stable between 9120C and 13940C;
  • When a plain carbon steel contain 0.8% carbon is cooled slowly in the austenitic phase, it undergoes a solid-state transformation at 7230C. to yield a microstructural constituent referred as pearlite which consists of alternating fine scale lamellae of ferrite and iron carbide (Fe3C), called cementite / simply carbide. The Fe3C phase has an orthorhombic crystal structure and is much harder and more rigid than austenite / ferrite.
  • When a plain carbon steel contain 0.8% carbon is cooled rapidly (quenched), it undergoes spontaneous, diffusionless transformation of a body-centred tetragonial (bct) structure called martensite.
  • The arrangement of the iron atoms in martensite is highly distorted by the carbon atoms, resulting in a very hard, strong, brittle alloy.
  • The formation of martensite is an important strengthening mechanism for carbon steels. The cutting edge of carbon steel instruments are ordinarily martensitic because the high hardness of this structure allows the grinding of a sharp edge that is retained in use.
  • The hardness of carbon steel is reduced by the tempering process, this is balanced by the increased toughness that is of considerable practical important.
  • Classification of Stainless Steel (depending on the lattice)
  • Ferrtic
  • Mertenitic
  • Austenitic
  • Other
  • Duplex steels
  • PH steel
  • Cobalt containing steels
  • Maganese containing steels
  • Stainless steel:
  • When approximately 12% to 13% chromium is added to iron, the alloy is commonly called stainless steel. Elements other than iron, carbon and chromium may be present, resulting in a wide variation in composition and properties of  the stainless steel.
  • Composition ( weight %age ) of three types of stainless steel :
  • Function of each component
  • Carbon : provides strength.
  • Chromium : passivating property.
  • Nickel : stabilizes at lower temperature into a homogeneous and corrosion resistant austenitic phase.
  • Silicon : improves resistance to oxidation and carbonization.
  • Sulfur : allows easy machining of wrought parts.
  • Phosphorous : allows use of lower temperature for sintering.
  • Manganese : is used as replacement for nickel to stablize the austenite.
  • Ferritic stainless steel:
  • These alloys are designated as American Iron and Steel Institute (AISI) series 400 stainless steel. This series number is shared with the martensitic stainless steel.
  • These stainless steel provide good corrosion resistance at low cost.
  • These stainless steels are not readily work-hardenable, these stainless steels have numerous industrial uses; they have little application in dentistry.
  • Martensitic stainless steel:
  • Martensitic stainless steel share the AISI series 400 deisgnation with the ferritic stainless steel.
  • They can be heat treated in the same manner as plain carbon steel, with similar results.
  • Because of their high strength and hardness, martensitic stainless steels are used for surgical and cutting instruments.
  • Austenitic stainless steels:
  • The austenitic stainless steels are the most corrosion resistant alloys of the three major types and are the stainless steels used for:-
  • Orthodontic wires
  • Endodontic instruments
  • Crowns in pediatric dentistry
  • The austenitic structure for the AISI series 300 stainless steel is achieved by the addition of Nickel to the iron-chromium – carbon composition.
  • Austenitic types of stainless steel:
  • Both 302 and 304 stainless steels are often called as 18-8 stainless steel and or the most commonly used in orthodontic stainless steel wires and bands.
  • Type 316L( low carbon) stainless steel is used for implants and also for manufacturing brackets. This alloy contains  (wt %)
  • 16-18% chromium
  • 10-14% Nickel- ( stabilize the austenitic phase )
  • 2-3% molybdenum ( provides protection from crevice and pitting corrosion)
  •  0.03% (max) carbon.
  • The “L” designation refers to the lower carbon content
  • Austenitic stainless steel is preferable to ferritic stainless steel for dental application because in addition to reasonable cost, it processes the following excellent combination of properties.
  • 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
  • CORROSION RESISTANCE AND PROPERTIES OF AUSTENITIC STAINLESS STEEL:
  • The resistance of stainless steel to tarnish and corrosion is associated with the passivating effect of chromium.
  • A very thin, transparent, adherent layer of Cr2O3 forms on the surface of stainless steel when it is exposed to oxidizing atmosphere such as room air.
  • This protective layer provides a barrier to oxygen diffusion and other corrosion environments and prevents further corrosion of the underlying alloy.
  • Sensitization:-
  • Austenitic stainless steel may lose its resistance to corrosion if it is heated between approximately 4000C and 9000C, the exact temperature depending on its carbon content. Such temperature are within the range used by the orthodontics for soldering and welding.
  • The decrease in corrosion resistance is caused by the precipitation of chromium-iron carbide at the grain boundaries at these high temperatures.
  • The small carbon atoms rapidly diffuse to the grain boundary region to combine with the chromium and iron atoms to form (CrFe)4C, resulting in loss of the corrosion resistance provided by chromium. This is called as sensitization.
  • Corrosion resistance is reduced in regions adjacent to the grain boundaries in which the chromium level is depleted below that necessary for protection (approximately 12%). The stainless steel becomes susceptible to intergranular corrosion, and partial disintegration of the weakened alloy may result.
  • Stabilization:-
  • Two methods can be used to minimize sensitization when austenitic stainless steel is heated into this problematic elevated temperature range.
  • Reduce the carbon content of the steel to an extent that such carbide precipitation cannot occur, but this remedy is not economically feasible.
  • By stabilization if the stainless steel in which some element is introduced that precipitates as a carbide in preference to chromium. Titanium is commonly used. If titanium is introduced six times the carbon content, the precipitation of chromium carbide be inhibited at soldering temperatures. These are called stabilized stainless steel.
  • Mechanical properties of
  • four types of orthodontic wires
  • Duplex steels:-
  • Duplex steels consists of an assembly of both austenite and ferrite grains Besides iron these steels contain molybdenum, chromium, and they have lower nickel content. As a result of the ferrite content, these steels ( as opposed to austenitic ones ) are attracted by magnets.
  • Their duplex structure results in improved toughness and ductility compared to ferritic steels. While their yield strength is more than twice that of similar austenitic stainless steels.
  • They also are highly stress-corrosion resistant.
  • Combining a lower nickel content with superior mechanical properties, duplex steel has been used for the manufacture of one-piece brackets.
  • Precipitation – Hardenable (PH) steel:-
  • Unlike most stainless steel, the PH steels can be hardened by heat treatment. The process actually is an aging treatment, which promotes the precipitation of some elements purposely added. Because of its high tensile strength, PH stainless steel is widely used for “mini” brackets.
  • Cobalt containing alloys:
  • Several cobalt containing alloys are commonly used in orthodontics, both for wires and brackets.
  • Some, such as Eligiloy and Flexiloy, still contain a large proportion a nickel.
  • Others, however are almost nickel free, having been developed to replace their allergenic counterparts.
  • Although both types are used to make archwires, nickel free steels are used primarily to manufacture attachments.
  • These alloys generally are corrosion resistant.
  • Manganese – containing steels:-
  • Known as an “austenitizing” element, manganese acts by interstitially solubilizing  the really” austenitizing” element nitrogen, thus replacing nickel. Unfortunately, when used in a high proportion, manganese increases the alloys susceptibility to corrosion.
  • STAINLESS STEEL ORTHODONTIC WIRES
  • Classification
  • Manufacturing of orthodontic wires
  • Ideal Requirements of Orthodontic wire
  • Multistranded stainless steel wire
  • Australian orthodontic archwires:
  • Wires used in orthodontics may be classified in three ways.
  • By design/cross section
  • Round
  • Rectangualr
  • Square
  • Double cross sectioned ( wonder arches )
  • By Diameter
  • 0.016”
  • 0.018”
  • 0.020” etc
  • By Alloy
  • Stainless steel
  • NiTi
  • Elgiloy
  • TMA
  • a – Ti
  • B – Ti
  • Polymeric wires etc
  • Manufacturing of orthodontic wires
  • A round wire is made by drawing a cast alloy through a series of dies, with intermediate heat treatment to eliminate effects of severe work-hardening between drawing steps.
  • Orthodontic wires with rectangular/ square cross-sectional are fabricated by rolling round wires, using a TURK’S head apparatus that consist of pair of rollers positioned at right angles.
  • Many accessory dental materials and instruments are fabricated from cast alloys that have been rolled to form sheet or rod, drawn into wires or tubing or forged (plastically deformed by die under compressive force, usually at an elevated temperature) in to a finished shape.
  • Whenever a casting is permanently deformed in any manner, it is considered as wrought metal.
  • Ideal Requirements of Orthodontic wire 
  • Stainless steel wires
  • In the 1940’s Austenitic stainless steel began to displace gold as the primary alloy for orthodontic wires.
  • The most commonly used types are 302 and 304 stainless steel, which contain approximately 18% chromium, 8% Nickel and carbon of 0.08%.  to 0.15 (max).
  • These alloys derive most of their strength from cold working and carbon interstitial hardening.
  • The microstructure demonstrates the typical ‘fibrous’ appearance associated with extensively strains.
  • The only heat treatment used with this wire are for stress relieving which is typically done at 850oF (4540C) for less than 10min.
  • Multi-stranded stainless steel wire:-
  • Flexibility of stainless steel wire can be increased by building up a strand of stainless steel wire around a core of 0.0065” wire along with 0.0055” wire used as wrap wires. This produces an overall diameter of approximately 0.0165”.
  • The strand of stainless steel wire is more flexible due to the contract slip between adjacent wrap wires and the core wire of the strand.
  • Multi-stranded stainless steel wire
  • When the strand if deflected the wires which are both under tension and torsion will slip with respect to the core wire and each other. If there is no plastic deformation wire returns to its normal position giving the elasticity to the strand of the wire.
  • Multi-stranded wires are available in round, rectangular, square cross sections.
  • Multi-stranded wire can be used as a substitute to the newer alloy wire considering the cost of nickel-titanium wire.
  • Some of the multi-stranded wire available are:
  • Dentaflex- Dentaurum is available in triple strand, co-auxial, six strand, and braided eight strand.
  • Twist flex-unitex
  • Force-9-Ormco
  • D-rect Ormco
  • Respond- Ormco
  • Australian orthodontic arch-wires:-
  • Arthur J. Wilcock of Whittlesea, Victoria, Australia originally developed orthodontic arch wire to meet        Dr. Begg’s needs for use in Begg technique.
  • This wire is austenitic stainless steel round, that has been heat treated and cold drawn to its proper diameter from a round wire of a larger diameter, in order to give it the required property of resiliency, toughness  and tensile strength.
  • Available in variety of diameter sizes, grades of resiliency, coiled/ in straight lengths. Straight lengths are not considered to be as resilient as the coiled wire due to the straightening process.
  • The wire produced has certain unique characteristics different from usual stainless steel wire.
  • It is an ultra high tensile austenitic stainless steel arch wire.
  • The wire is resilient, ( energy storage capacity is good ) certain bends when incorporated into the arch form and pinned to the teeth become activated.
  • The wire has a unique property of zero stress relaxation. Zero relaxation allows the wire to maintain its force over a long period of time, yet resist permanent deformation from elastic load.
  • All these properties makes the wire very hard and brittle depending on the resiliency the wire have been graded and color coded for the use.
  • Regular grade  (white label):-
  • Lowest grade and easiest to bend used for practice bending/ forming auxiliaries.
  • Regular plus grade ( green label )
  • Relatively easy to form yet more resilient than regular grade used for auxillaries when more pressure and resistance to deformation is required.
  • Special grade ( black label )
  • Highly resilient yet can be formed into intricate shapes with little danger of breakage used as starting arches mostly.
  • Special plus grade ( orange label )
  • Hardness and resiliency of the wire are excellent for maintaining anchorage and reducing deep overbite must be bent with caution.
  • Extra special plus grade ( blue label ):
  • Highly resilient and hard, difficult to bend and subjected to fracture.
  • With the demand from the orthodontic faculty for harder and harder wires, even higher grades, premium and premium plus wires were developed.
  • In early 1980’s an even higher grade wire which is commercially available as supreme was produced by A.J.Wilcook. This wire is available in 0.008”, 0.009”, 0.010” and 0.011”.
  • This wires were initially used for alignment in lingual orthodontics where brackets are close together. The flexibility of supreme wire is comparable to that of nickel titanium wires and has the added advantage of good formability.
  • The low and medium grade wires exhibit better forming as they are subjected to less work hardening and hence are more ductile.
  • But it was the premium grade wire which was gaining popularity, that posed a challenge. Commercial companies which manufactured preformed arches were not only ready to use premium grades because of its increased chance of breakage.
  • Till then the wire were straightened by what is called as spinner straightening process.
  • Spinner straightening is a mechanical process in which resistant materials are straightened, usually in the cold hard drawn condition. The wire is pulled though high speed rotating bronze rollers which torsionally twist the wire into a straightened condition. This can result in permanent deformation.
  • Presently the premium and supreme wires are straightened by a process called pulse straightening.
  • Pulse straightening:- The wire is pulsed in a special machine which permits high tensile wires to be straightened and also of lower diameter than possible with earlier with spinner process. The yield strength is not altered and the surface has a smoother finish and resultant lower friction, without structural deformation and altering the physical properties.
  • Because of their ability to generate continuous low forces the supreme wires are routinely being used for making Torquing auxiliaries, and up-righting springs.
  • The more resilient premium wires are being used as base arch wires. The combination of these two wires reduces the complication in third stage of Begg’s mehcanotherapy.
  • Conclusion:-
  • In the last few decades, a variety of new wire alloys has been introduced into orthodontics.
  • These wires demonstrate a wide spectrum of mechanical properties and have added to the verstaility of orthodontic treatment.
  • Appropriate use of all the available wire types may enhance patient comfort and reduce chairside time and duration of treatment.
  • The restricted use of only stainless steel wire to treat an entire case from start to finish therefore may be indicated only in relatively few patients. It may be beneficial instead to exploit the desirable qualities of a particular wire type that is specifically selected to satisfy the demands of the presenting clinical situation.
  • This, in turn, would provide the most optimal and efficient treatment results.