By Dr. Claude Matasa

4. Most used alloys

_____4.1 Stainless steels
_____4.1.1 Classification
_____Carbon or common steels are mixtures of iron and carbon, while stainless steels comprise at least 11% chromium, addition that renders them corrosion resistant. The American Iron and Steel Institute (AISI), the Unified Number System (UNS) and the German standards (DIN) distinguish three different classes of stainless steels, all deriving from the basic structure of iron examined above: ferritic, austenitic, and martensitic. Ferritic stainless steel is the softest and least corrosion resistant; while ferrite is found at room temperature, austenite is normally stable at high temperatures. Martensite is generated when the steel is cooled or hardened. Classified by the American Institute for Steel and Iron (AISI), stainless steels bear designations corresponding to these three basic types of stainless steel.
_____Ferritic stainless steels. Their name reflects a structure that resembles that of iron at room temperature, the difference being in that a relative large proportion of iron atoms in the unit cells are substituted for chromium. The ferritic steels contain 12-18% chromium with up to 0.5% nickel, the other elements (aluminum, molybdenum, carbon, nitrogen) being found in small amounts. Their properties cannot be improved by heat treatment and they rely on mechanical working for their strength.
_____The “super-ferritics” have a content in chromium from 19% to 30% and have a very small content of carbon. Resistant to chlorides, such steels are used by Pyramid Orthodontics in the US and by Forestadent in Germany. The lack in nickel is compensated by an increased amount of chromium and molybdenum, with some additional cobalt (24-26%Cr, 4-5% Mo and 3% Co).
_____Austenitic steels. The steels commencing their AISI number with the figure 3 are all austenitic and are characterized by the existence of a single phase that presents advantages from the chemical point of view as it precludes a galvanic corrosion between phases. The lower the number, the less non-ferrous (expensive) metals: the letter L signifies a lower content in carbon.
_____The austenitic structure can be achieved at room temperature only by adding elements like nickel that act as “dissolving” agents. Due to their action, such elements are called “austenizing”. In recent times, the austenizing of stainless steels has been made possible also by the addition of other elements, notably nitrogen and manganese. The lack of polarity of the austenitic steels leads to an unexpected property: these are non-magnetic, thus different from all the other steels.
_____Their mechanical strength is average and can be significantly changed due to the generation (deliberate or not) of specific phases and/or to the addition of other elements. An inadequate heat treatment of these steels can generate other, less desirable phases, their exposure to soldering, welding, brazing and thermal recycling has to be controlled. These steels cannot be hardened by heat treatment and must rely for their mechanical properties on mechanical working. This means that any thermal joining treatment will reduce the mechanical properties in the ‘heat affected zone’. Unlike common and low-alloyed steels, austenitic stainless steels become softened, instead of hardened, if heated at elevated temperatures (1400o C or 1900 o F) followed by rapid cooling at room temperature. A strange behavior is also that, in spite of the large content in Fe and Ni, both ferromagnetic elements, austenitic steels are hardly magnetic. They can be, however, slightly magnetized if subjected to cold working. Until a decade ago, almost all the attachments used in orthodontics were made of austenitic steels.
_____AISI 303. While its use in the mouth is seldom disclosed and even less accepted, it is the workhorse of many smaller companies due to the ease in milling and automatic cutting. Its corrosion resistance is the lowest among the austenitic steels used in orthodontics due to it high content in sulfur and carbon. Used by Ormco for the “Diamond”® brackets.
_____AISI 304. Most brackets are based upon this austenitic steel1, which constitutes a practical balance between the easy to process AISI 303 and the difficult to mill AISI 316 L. This is achieved through a rather small content of sulfur and carbon. Good cold formability.
_____AISI 304 L. An improved variation of AISI 304 by having less carbon, this steel is a transition to the higher in quality 316 grades. Withstands the processes which involve heating (welding, brazing, thermal recycling) better than AISI 304. Along with AISI 304, it is also a workhorse of the orthodontic attachment industry as it exhibits good cold formability
_____AISI 316. Having molybdenum in it, it withstands better chlorides and non-oxidizing acids. Has a good cold formability, but it is difficult to mill. While less expensive, it is less preferred than its counterpart, AISI 316 L, because of its poorer corrosion resistance.
_____AISI 316 L. It has similar properties with 316; having the lowest C content, it is the most corrosion resistant austenitic steel. Both 316 and 316 L have been used by “A”-Co. for their “Standard Twin”®, “Comfort”® and “Attract”® brackets.
_____AISI 317. The highest content in alloying metals in the series, leads to the highest aqueous corrosion resistance. Its relatively high carbon content doesn’t make it too vulnerable to intergranular corrosion due to an increased content in molybdenum. Difficult to mill; used in Strite™ brackets.
_____Carpenter’s 18-18 Plus is an austenitic steel which contains 18% Cr and 18% Mn, the last element being needed to absorb interstitially some 1% nitrogen. As it does not contain nickel, it has been preferred for bracket manufacture by Scheu Dental and Dentaurum
_____Martensitic steels. Classified within the 400 series, these steels have a higher content in carbon and theoretically should be used only for short contacts with the oral environment due to their low corrosion resistance. Their main use was for instruments which exhibit sharp or hard edges. Their microstructure reflects the “martensitic effect”, i.e. a phase trans-formation from within, or diffusionless. The difference between austenite and martensite is the fundamental arrangement of the atoms in the metal unit cells, and by the position of the extraneous, solute elements within them (carbon and nitrogen). While the metal becomes more resistant to plastic deformation (the yield strength increases), it is at the same time less workable and brittle (the elongation decreases). If improperly treated, martensite may degrade into ferrite and a small amount of metal carbides.
_____Having a lower corrosion resistance due to the existence of carbides in their structure, martensitic steels are used in few “mini” brackets and especially for instruments which exhibit sharp or hard edges. Unlike the austenitic ones, the martensitic steels are ferromagnetic and hardenable by heat treatments.
_____Precipitation-hardenable steels. Classified with the prefix PH within the 600 series, these steels can be hardened by heat. The process is actually an aging treatment which promotes the precipitation of some metals purposely added. The process is called in metallurgy Precipitation Hardening.
_____The resulting submicroscopic particles harden the matrix by hindering the progress of dislocations, i.e. the grains movement produced by stress. Acting like obstacles in the way of the grain dislocations, these particles prevent an excessive sliding of the steel layers, i.e. oppose resistance to deformation. Such steels can be obtained by inducing some solute metals (Al, Cu, Nb) to precipitate out of the matrix metal as fine particles, which in turn stimulate the formation of fine grains after cooling. Chemically inter-combined metals (inter-metallic compounds) have superior qualities in higher temperatures and load bearing applications. This toughness doesn’t come without a price: the last stainless steel is more corrosion prone, as a result of its poorer homogeneity. Such microstructures attract higher mechanical properties without lowering corrosion resistance. These steels can have either a martensitic or an austenitic structure.
_____AISI 630. Known more as 17-4 PH, this is a formerly austenitic steel in which has taken place the precipitation hardening phenomenon leading to a limited cold formability. It can be successfully milled despite its high mechanical strength. Heat treated in demanding conditions renders it vulnerable to other, uncontrollable heat exposures like welding (intergranular corrosion). Used by Ormco to mill its “Mini-Diamond” ® brackets and by Unitek for sintering/injection molding its “Miniaturized”® brackets.
_____AISI 631, more known as 17-7 PH, is a martensitic (magnetic) precipitation hardenable steel, which can be soft and formable only when annealed. Its higher tensile strength and corrosion resistance are the price paid for a poor ductility and formability. Has been formerly used by Ormco for its discontinued Edgelock® brackets.
_____Duplex. Balanced to contain a mixture of austenite, these steels are sensitive to heat as this leads to the forming of a brittle phase called sigma and to hydrogen embrittlement which lower their toughness and corrosion resistance is low in spite of a high content in chromium and molybdenum. To avoid such phenomena, the “super-ferritic steels” like S-31803 have to be cooled rapidly, which is not always possible when handling massive parts, like the bars or tubes from which brackets are cut.
_____Due to its low content in nickel, the steel has been preferred for the manufacture of one-piece brackets by CEOSA, Madrid (Bioline® & Low nickel”®).

_____4.1. 2 Stainless steels in orthodontics
_____Manufacturers are or have used almost all the above steels according to their goals. Thus, if destined to be machined, and to be acceptable for low cost and moderate resistance to corrosion, the common choice is the austenitic
_____AISI 304. For higher cost and good chemical resistance is AISI 316L (the letter L indicating a low content in carbon, necessary to prevent sensitization and corrosion attack). For a lower cost (inexpensive machining), AISI 303 is selected against its lower corrosion resistance because it can be easily machined.
_____For a lesser corrosion resistance but for a higher hardness, mandatory in the manufacture of “mini” brackets, the precipitation hardened steel PH 17-4 is all too often selected. For the low or no nickel steels which have more chromium (“superferritics”), as well as for the other alloys that are hard to machine, the preferred manufacturing method is powder metallurgy.
_____The first stainless steel of commercial interest was the so called “18-8” (18 % Cr, 8% Ni), launched by Germans after World War I under the name V2A and Wipla (Wie Platin, i.e. like platinum, in German). As shown in Fig. 4.1, a series of steels followed during the next decades. While purification from deleterious elements was once a difficult task, starting with the ‘70 s when the argon-oxygen decarburization process was widely applied, precise control of each individual element has been achieved. Interestingly, some of the elements purposely added to stainless steels have a deleterious effect on their pitting corrosion, as shown in Fig. 4.2. Recent years have seen a real revolution in the field as a gradual switch from the alloys containing nickel has occurred, as shown in Fig. 4.3
_____While Angle’s German silver contained up to 18% Ni, the typical stainless steels used between the two world wars had less than 10%. In contrast, today’s precipitation hardened steels used extensively today for the “mini” brackets have only up to 5%. Recently, due to the recommendations against the use of such steels in orthodontics made by the German Bundesgesundheitsamt (Federal Dept. of Health)2 , all German manufacturers have started the production of nickel free brackets using newer alloys. The same policy has been extended even to arch wires. Thus Dentaurum has launched arch wires made of Remanium® (containing 29%Cr, 4.5%Mo, 0.5% Si, 0.3%Mn) and Noninium® (containing 18% Cr, 18% Mn, 2% Mo, 1% N, max. 0.2%Ni).
_____The composition of several alloys with less than 40% nickel used today is shown in Table 4.1.
_____The replacement of nickel is not an easy task. Indeed, this metal is a valuable component of stainless steels as it allows the existence at room temperature of an otherwise (thermodynamically) unstable solid solution of metals (austenite). In its absence, this homogeneous solid solution (no phases) is stable only at higher temperatures. Even in the presence of nickel, the reheating or intense cold-working of austenitic steels may lead to a conversion in other phases. This phenomenon can be understood when the vicinity of these phases in the stainless steels used in orthodontics is considered, as it can be observed in Fig. 4.4 where only the iron corner of the ternary system Fe-Cr-Ni is represented.
_____4.1.3. Properties
_____Introduced in orthodontics in the ‘30s, stainless steels are today the profession’s most used alloys. In them, chromium is an essential ingredient because it protects iron from corrosion, particularly rust. It bonds with oxygen to form a layer on the surface of the metal. As a difference from the oxides that form on carbon (common) steel, the chromium oxide layer does not allow oxygen to penetrate and “rust” the metal, Fig. 4.5.
_____If the chromium oxide is removed in various circumstances, the most common being chlorides attack, stainless steel can rust, Fig. 4.6..
_____Aside from chromium which renders the steels “stainless” and gives them a pleasant, silvery appearance, additional metals are nickel (up to 14%), manganese (up to 18%), molybdenum (up to 5%), as well as small amounts of copper, niobium, aluminum and, recently, cobalt. Some of the non-metals present are useful (nitrogen), others are harmful (phosphorus), while a third category (carbon, sulfur), can be either useful or harmful according to the purpose. Below is a list with the properties various elements impart to the stainless steel attachments.
_____Chromium. Responsible with the “stainless effect”, i.e. the formation on the alloy’s surface of a protective Cr oxide layer. All stainless steels have to have at least 11% Cr. It increases the pitting and crevice-corrosion resistance. Has a high affinity for carbon, giving a carbide that nucleates at the grain boundaries, rendering it prone to intergranular attack.
_____Carbon. Combines with chromium, depleting the austenitic phase in this essential element (sensitization), and thus increases intergranular corrosion, especially in steels that have been heat-exposed or welded. The lower, the better. It is important because it imparts (if up to 1%) a higher tensile strength.
_____Nickel. Stabilizes the austenitic (single phase) structure at room temperature. To allow this phenomenon to occur, it has to be at least 6%, the actual the limit for the AISI 300 series. It extends the stress-corrosion cracking as well as the corrosion resistance to chlorides and low oxidizing acids. It hardens sintered alloys but increases their shrinkage in molds
_____Manganese. Performs like nickel, but has only half of its austenizing power and doesn’t contribute enough to corrosion resistance. It is, however, less costly.
_____Silicon. If kept at low concentrations, it improves resistance to oxidation and carburization at elevated temperatures as well as corrosion resistance in some media.
_____Sulfur and Phosphorous. Austenitic Cr-Ni stainless steels are more difficult to machine than low alloy steels. The intentional addition of S to .015% leads to an easier manufacture of wrought parts, in the detriment of corrosion resistance. P reduces the temperature needed for sintering, while S is usually added to sintered alloys to render them easy to cut.
_____Aluminum. Improves the resistance to oxidation and carburization, behaving thus like silicon. Like Cu and Nb, it is a precipitation hardening additive in age-hardenable stainless steels
_____Copper. At low concentrations, it improves corrosion resistance to sulfuric acid and promotes age hardening in some of the precipitation hardening stainless steels, where it reduces the precipitation of deleterious secondary phases. Known however to reduce the resistance to seawater (chlorides). It decreases the hardening rate of austenitic stainless steels and thus improves cold working characteristics. Preferred in powder metallurgy because it increases the strength of sintered parts and compensates for the shrinkage which takes place.
_____Molybdenum. Increases the resistance of austenitic steels to pitting and crevice corrosion caused by chlorides and acids. As it preferentially combines with carbon to give carbide that can precipitate at the grain boundaries, it may also weaken the steel. While in sintered parts it increases hardness, in other alloys (age-hardenable), it decreases their resistance to intergranular attack.
_____Niobium. Minimizes the undesirable formation of chromium carbide, generating instead its own carbide, NbC. Due to this sacrificial behavior; it saves enough chromium to protect the metal reducing sensitization and its consequence, poor corrosion resistance
_____4.1. 4. Changes affecting properties
_____The homogeneous structure of austenitic steels is, as shown above, unstable, being relatively easy to alter by heating. This phenomenon, which occurs both by heating the steel in the undesired range and by slowly cooling it from higher temperatures, often affects the orthodontic attachments which occur during steel bars manufacture, bracket welding, brazing, sterilizing and recycling.1,2
_____Steel sensitization. As shown, the most corrosion resistant stainless steels are not only thermodynamically unstable at lower temperatures, but also undergo the phenomenon known as sensitization. While per se the phenomenon is rather physical, it leads to the leaching of heavy metals. The phenomenon has been thus described3 : “It is well known that austenitic steels are metallurgically unstable when heated in the temperature range of 350-800o C (650-1500o F). After heating in this temperature range, they become subject to severe attack at the grain boundaries by even mild corrosive media. This attack is referred to as intergranular corrosion, and it is so severe that the steel literally disintegrates into separate grains, losing substantially all its properties”. According to more recent data4 , heating at an even lower temperature range, 315o to 870o C (600o to 1600o F) causes the associated increased susceptibility to corrosion.
_____Both chromium and niobium carbide precipitates limit grains coherence diminishing steels’ toughness, as shown in Fig, 4.7 However, having a higher affinity for carbon than chromium, niobium saves the latter element from depletion and thus improves corrosion resistance.
_____Despite their apparent integrity, tools and devices made of sensitized steels can easily break. Not only the formed chromium carbide which forms at the grains border is soluble and leaches, but the alloy becomes poorer in chromium, its protection. Aside from being fragile, sensitized stainless steel devices may thus become as corrosion susceptible as if made of carbon steel. Inadvertently pre-existing, or purposely added, sulfur and sulfides generate film which separate grains weakening the alloy’s strength.
_____Corrosion susceptibility. Usually passive, i.e. showing indifference toward attackers, stainless steels are active as soon as the chromium oxide has been removed, becoming even more susceptible to corrosion than copper, aluminum or brass. This is illustrated in Fig. 3.3 in which stainless steels are represented with two boxes, the more intense blue indicating their active status.
_____The main attackers of stainless steels are the chlorides, both from the ingested salty foods and saliva, as the latter may contain over 500 mg/liter5 . ”Chlorides can penetrate and destroy the passivity that is responsible for the corrosion resistance of stainless steels, and the corrosion engineer should resist every attempt to use stainless steels in environments containing chlorides... Stainless steels, such as types 304 and 316, are not resistant to muriatic (hydrochloric) acid at any concentration and temperature.”6 In addition to chlorides, aggressive are also the organic acids that result from food decomposition and the sulfurated compounds found in saliva.
_____An urban mouth-breather inhales in two hours approximately a cubic meter with a potential sulfur dioxide intake of up to 2.3 mg.7

_____There are several types of corrosion among which the most common in the oral environment are the crevice, the intergranular and the pitting corrosion. Uniform corrosion, resembling the peeling of an onion and shown in Fig. 4.10, is overcome by the other forms of corrosion: the image has been taken after a bracket’s exposure to hydrochloric acid.
_____Pitting corrosion is rather common but its impact is limited only to the attachment’s surface, as shown in Fig. 4.11.
_____Crevice corrosion occurs whenever the attachment’s surfaces have been hidden from the environment, see Fig. 4.12, or if covered with a permeable or detachable layer or object of plastic, as shown in Fig. 4.13 and 4.14.
_____There are no relevant images showing the effect of intergranular corrosion, as the phenomenon weakens internally the coherence of the grains, without visible, missing material. Its influence can be shown, however, if a sensitized bracket is subjected to a solution of nitric acid. An oxidizer, this acid will promote the passivation (formation of protective chromium oxide) on the grains, but it will also dissolve the chromium carbide film which keeps together part of these. Fig. 4.15 shows a sensitized bracket after it has been exposed to nitric acid. Parts of the bracket and the mesh that have been heavily cold worked (i.e. becoming more resistant) can still be seen, while the rest of the bracket has been transformed in grains.
_____Another type of corrosion often encountered is galvanic in which an alloy is dissolved when in contact with a “nobler” one. In Fig. 4.16 is shown a base after the bracket was torn off due to the dissolution of the brazing alloy. In contrast, in Fig. 4.17 part of the stainless steel is missing due to its contact with a gold brazing. As the steel used for the bracket’s foil is seldom the same as the one used for the mesh, galvanic corrosion leads to their detachment, Fig. 4.18.

_____The presence of the chemical attackers is sometimes compounded by the existence of a variety of microorganisms like the sulfate-reducing Bacteroides corrodens, or the acid-producing Streptococcus mutans, both known to attack dental alloys in the mouth8 . Fig. 4.19 shows a stainless steel part heavily attacked by such microorganisms9 , while Fig. 4.20 the base of a direct bonding bracket exhibiting typical dents caused by microbial influenced corrosion. 10, 11
_____4.1.5 Potential health hazards
_____While not threatening, enamel staining has been attributed to the exposure of the metal to high temperatures.12 Such stains, the color of which corresponds to chromium+3 salts, are caused by pitting.
_____Stainless steel corrosion products are considered culprits in a wide range of cases, from allergies to inflammations around implants and prostheses13 and alterations of the spleen cellular populations (with immuno-toxicological consequences due to lymphocyte depletion). The discharge of nickel ions, a strong immunologic sensitizer,14 is known to result in hypersensitivity, contact dermatitis, asthma, and cytotoxicity 15-20 .
_____Studies made at the Dermatology Clinic of the University of Erlangen (Germany) have shown a positive reaction to Ni of 16.9% (females 23.8%), of 7.1% to Co and 6.6% for Cr. 21 While some studies have indicated that 9% of females and 1% of males are considered to be sensitive to Ni, other studies have shown that for females, the percentage can be as high as 28% or even 31.9%22 . The rate of incidence is growing rapidly in some countries23 and the age group ranging from 10 to 20 years, corresponding to the common period for orthodontic treatment, shows the highest frequency of first-time symptoms of nickel 24 it is very likely that the incidence is even higher, as in some instances the parts affected are so remote that a correlation with the orthodontic attachments is not made. It is likely that due to some early works in the orthodontic literature that have drawn an early alarm signal regarding the problems raised by the bracket corrosion,1, 25, 26 we have now “no nickel” and titanium brackets.
_____In contrast to the claims that the effect of the nickel and chromium leaching from orthodontic attachments is significant, it is contended that the related amount only seldom leads to allergies, 27 as it is below the average dietary intake. In support of this stands the blood analy- sis of orthodontic patients subjected to ortho- dotic treatment that has shown no significant or consistent increase in the blood level of nickel.28-30 According to this study, 40 orthodontists reporting on allergic reactions have observed extra oral reactions only in less than half (45%) of these patients. A review of the literature indicates that an allergic response to stainless steel is rare, although nickel is a common allergen and is encountered continually in daily life.
_____While the above may be true for a group using certain attachments, the badly corroded samples that are found in many instances in the brackets and expansion screws sent for recycling show that heavy metals releases could well exceed the sporadic tests mentioned.31 Fact is that among the two million patients treated in North America, between two to four thousand have nickel or chromium sensitivity32,33 . At least one of five hundred patients is allergic to nickel, and one of every three to six thousand to chromium34-36
_____A potential treat to the integrity of metal attachments that may affect patients is recycling, an activity at least a hundred twenty years old (J. J. Ravenscroft Patrick, 1884). Today, the recycling of orthodontic attachments has spread all over the world: statistics show that in the US and Great Britain, one of three37 and one of two38 orthodontists, respectively, recycle their attachments. _____The reuse of “single use only” devices extends to by far more potentially dangerous devices: while the orthodontic brackets and bands are considered by FDA Class I (low chance to generate problems), implantable generators, cardiac pacemakers, hemodialysis filters, guide wires, pacing electrodes, angioplasty balloons, are all Class III. Bracket and bands recycling can be performed by using heat to volatilize the polymeric matrix and electro-polish the metal to bring back its shine, or by dissolving the adhesive following burnishing, a process used by all orthodontic device manufacturers. While the first process sensitizes the steel, increases its corrosion susceptibility and removes metal, the other does not involve heat, except for decontamination. American Association of Orthodontists president, Dr. Donald Poulton, has shown that the processed attachments are “safe and effective.”39 Indeed, whereas all recycling methods alter somewhat the attachment’s dimensions as shown in Fig. 4.21, electro-polishing removes metal while burnishing by high-energy centrifugal tumbling compresses and hardens its surface. Indeed, a compressed external layer resists crack initiation and propagation and is more resistant to corrosion and fatigue.40 The thorough inspection, to which the attachments are subjected, eliminates these which are unfit to be reused41 : the average dimensional changes incurred from this process are smaller than those existing within the new samples42 .

_____References
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2. Legierungen in der zahnartzliche Therapie, Bundes-gesundheitsamt (BGA), Heimlich KG, 1993, Germany
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4. Department of Mechanical Engineering, Nanjing University of Chemical Technology, http://httd.njuct.edu.cn/MatWeb/material/m_ssaust.htm
5. McCann HC, Inorganic Components of Salivary Secretions, in: Art and Science of Dental Caries Research, Harris RS editor, Academic Press, NY, 1968; 55
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10. Matasa CG, La corrosion des verrous: un defi pour l’orthodontiste, Actualites Odonto-Stomatologiques (Paris, France) September, nr. 187, p.401-409, 1994
11. Matasa CG, Nichtrostende Edelstahle und Direkt- Bonding- Brackets. III Mikro-biologisches Verhalten-auch der Adhasive, Informationen aus Orthodontie und Kieferorthopadie (Heidelberg, Germany) 1993; 25(3): 269-285
12. Ceen RF, Gwinnett AJ, Indelible iatrogenic staining of enamel following debonding, J. CIin.Orthod.14:713-5,1980; Park HY, Shearer TR, In vitro release of nickel and chromium from simulated orthodontic appliances, Am. J. Orthod. Dentofac Orthop1986; 84: 156-9
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21. Burrows D. Hypersensitivity to mercury, nickel and chromium in relation to dental materials. Int Dent J 1986; 36: 30-34
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27. Schriver WR, Shereff RH, Domnitz JM, Swintak EF, Civjan S. Allergic response to stainless steel wire. Oral Surg Oral Med Oral Pathol 1976;42:578-81
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29. Bishara, SE, Barrett RD, Moustafa I, Selim MI, Biodegradation of orthodontic appliances. Part II. Changes in the blood level of nickel, Am. J. Orthod. Dentofac Orthop 1993; 103: 115-119
30. Menezes LM, Campos LC, Quintao CC, Bolognese AM. Hypersensitivity to metals in orthodontics. Am J Orthod Dentofacial Orthop. 2004; 126: 58-64
31. Matasa GC, Orthodontic attachment corrosion susceptibilities, J Clin Orthod 1995; 29(1): 16-23
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39. American Association of Orthodontists Annual Report 1997-1998, in: AAO, The Bulletin, September/October 1998; 16(5), Insert
40.www.nasatech.com/Briefs/Aug02/LEW17188.html. Last accessed August 2004
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42. Matasa CG, Orthodontic recycling at the cross-roads, J. Clin. Orthod. 2003; 37(3): 133- 139

_____4.2. Titanium alloys
_____4.2.1 Classification
_____Titanium is far from being a rare metal, as it is the fourth most abundant structural metal in the earth’s crust, and the ninth industrial metal. Twenty times more prevalent in the earth’s crust than chromium and thirty times than nickel, titanium exhibits an illusory abundance. Indeed, the energy needed to produce sponge-titanium from its ores is almost double than that needed for the energo-phage aluminum. “Pure” titanium has low strength, but is more corrosion resistant and less expensive. At room temperature it has a hexagonal (alpha) structure that transforms to a BCC structure (beta) form when heated above 882°C. Like in the case of stainless steel examined above, the addition of alloying elements to titanium influences its transformation temperature. In many instances it retains its BCC structure at room temperature, thus producing alloys that contain both alpha and beta phases. As a result, there are four groups of titanium alloys: alpha alloys, near alpha alloys, alpha + beta alloys and beta alloys. Orthodontics prefers the last group as it joins corrosion resistance to heat stability (sterilization, brazing, welding).
_____The general classification of titanium alloys recommended by the American Society for Testing and Materials (ASTM) simply lists the quantities of the principal alloying additions; thus “Ti-8-1-1” contains 8% aluminum, 1% molybdenum and 1% vanadium; and “Ti-6-4” or Ti6Al4V means 6% aluminum and 4% vanadium. Used to make brackets by Dentaurum, this alloy has an alpha-beta structure and accounts for more than 50% of all the titanium manufactured in the world. It is available in wrought, cast and powder metallurgy forms, with wrought accounting for more than 95% of the market. The medicinal grade is “extra-low interstitial” (ELI) grade, (i.e. with a very low content in oxygen, nitrogen and carbon). It is used mostly to make brackets.
_____The relative amounts of alpha and beta phases in any particular alloy have a significant effect on the properties of that material in terms of tensile strength, ductility, creep properties, weldability and ease of formability. It is common practice in the metallurgical industry to refer to titanium alloys by their structure, hence alpha, alpha-beta, and beta alloys. The alpha group contains most importantly aluminum and tin.
_____They can also contain molybdenum, zirconium, nitrogen, vanadium, columbium, tantalum, and silicon. Alpha alloys are not suitable for heat treatment in contrast with the alpha-beta group that can be strengthened by heat treatment. Stable at high temperatures, the beta alloys such as TMA (Titanium Molybdenum Alloy) are weldable and allow loops. Such an alloy, containing 11.5% Mo, 6 Zr and 4.5 Sn and known as Beta III (brand name taken also by Unitek’s Ti wires), has excellent cold work ability, heat treat ability and mechanical properties.
In Table 4.2 are shown alloys which contain up to 25% titanium, the nickel titanium alloys not being included.
_____4.2.2 Titanium alloys in orthodontics
_____Having a good spring back, good formability, withstanding twice the deflection of stainless steel before plastic deformation and being weldable, beta titanium wires are used for tooth alignment, space closure, torque control, arches with “T’ loops as well as for normal or reverse curve of Spee (Unitek/3M, Beta III®; Ormco, TMA®; Class One, Freedom®). Free of nickel and widely tested in implants, titanium and several of its alloys are highly corrosion resistant and well tolerated by the human body. Because of light weight, high strength to weight ratio, low modulus of elasticity, and excellent corrosion resistance, titanium and some of its alloys are important in medicine1 . With the additional advantages of excellent biocompatibility, good local spot weldability, and easy shaping and finishing by a number of mechanical and electrochemical processes, titanium alloys are finding uses in dental applications, such as implants and restorative castings.2 In prosthodontics there is a growing trend to use titanium as an economical and biocompatible replacement for existing alloys for fixed and removable prostheses.3
_____Due to a lack of interest in bio-compatible materials, American manufacturers have allowed Dentaurum, a German company, to corner most of the orthodontic uses of this metal.4, 5 As a result of a tighter control on the metals that generate risks, titanium and titanium-based alloys find an increasing use in the manufacture of stronger and corrosion resistant brackets
_____4.2.3 Properties
_____Titanium is almost as passive (or chemically inert) as gold and platinum due to a complex titanium-oxide film consisting mainly of TiO2, TiO and Ti2O3 which protect the metal the same way as Cr2O3 and Al2O3 protect stainless steel and aluminum, respectively.
_____Titanium attachments exhibit a high re-sistance to hydrochloric acid and their biocompatibility has been thoroughly tested. Among these alloys, the so called ASTM Grade 7 titanium, i.e. titanium plus 0.12 to 0.25% palladium, is known to be the most resistant metal commercially available, being able to service in HCl, H2SO4 and in their hot, low pH salt solutions.
_____To substantiate once more that the oral environment can be very aggressive, titanium alloys used for wires are attacked by fluoride mouth washes. Thus, while TiNb was found resistant, beta titanium (TMA) corrodes strongly in the presence of the additive stannous fluoride6 .
Titanium alloys exhibit an array of strengths, some twice as great as the others, Table 4.3. Important for the manufacture of “mini” brackets, this strength can be as high, or even higher than that of other alloys, as their related hardness shows, Table 4.4.

_____A white metal with a slight charcoal gray hue, titanium has a color which is darker and deeper than silver or white gold. Being made out of a spongy material and not from cast or wrought bar, orthodontic attachments are rougher and do not reflect enough light to be really shiny. Attempts to cover the metal with opaque oxides are under way: although titanium dioxide is white; to date the attempts to build on attachments such an adherent layer have failed. The metal’s wetting by adhesives is acceptable without being affected by the opaque agents mentioned. Although up to date no titanium mesh can be welded/brazed to brackets, the bonds can exhibit shear strength between 6 and 8 N/mm2, i.e. at the optimal level.7
_____4.2.4. Changes affecting properties
_____The transition from the BCC (beta titanium) to CPH/hexagonal (alpha titanium) and back, or the behavior of the mixed phases is not accompanied, as in stainless steel, by health risks. Indeed, the properties of each phase can be properly controlled in most cases. It is, however, important to remember that when heated, titanium and its alloys react readily with hydrogen, oxygen and nitrogen. These elements which are frequently found in small quantities in titanium form interstitial solid solutions in which the solute atoms are located in the interstices between the titanium atoms. While oxygen, nitrogen and carbon are alpha stabilizers, hydrogen dissolves preferentially in the beta phase and has negligible solubility in alpha and is thus classified as a beta stabilizer. Hydrogen is further distinguished from the other interstitial elements by the fact that its diffusion rate in titanium is rapid at elevated temperatures. As in NiTi, oxygen renders titanium alloys stiffer: from risks point of view, the presence of titanium dioxide does not constitute a health problem.
_____Titanium brackets are used in orthodontic patients with an allergy to nickel and other specific substances. In recent studies, the corrosive properties of fluoride-containing toothpastes with different pH values were investigated. The present study tested in vivo how the surfaces of titanium brackets react to the corrosive influence of acidic fluoride-containing toothpaste during orthodontic treatment. Molar bands were placed on 18 orthodontic patients. In the same patients, titanium brackets were bonded on the left quadrants and stainless steel brackets on the right quadrants of the upper and lower arches. Fifteen patients used a gel containing soluble tin fluoride (pH 3.2), whereas3 used fluoride-free toothpaste. The brackets were removed for evaluation by light microscopy and scanning microscopy 5.5 to 7.0 months and 7.5 to 17 months after bonding. The quality and quantity of elements present were measured by scanning microscopy. Macroscopic evaluation showed the matte gray color of titanium brackets dominating over the silver gleam of the steel brackets.8 The plaque accumulation on titanium brackets is high because of the very rough surface. Pitting and crevices were observed in only 3 of the 165 brackets tested. The present in vivo investigation confirms the results of in vitro studies, but the changes are so minor that titanium brackets can safely be used for up to 18 months. Wing surfaces should be improved by modifying the manufacturing process.
_____4.2.5. Potential health hazards
_____Being noble may have its drawbacks: while titanium alloys do not alter during clinical treatments, in the presence of an electrolyte (saliva), their vicinity with other metals may dissolve the latter. Another related phenomenon (galvanism) that changes this time the titanium alloys themselves is disalloying, a phenomenon in which less noble metals leach out of their alloys with nobler ones (thus, aluminum is known to leach from the most used titanium alloy, Ti6Al4V). An in vitro study of the corrosion characteristics of the commercially available arch wires has shown that the beta-Ti alloy wire (Ti-Mo-Zr) shows a reduced susceptibility to both general and localized corrosion, being the most biocompatible.9,10 Fluoride-based rinses, however, have been shown to attack the protective layer of titanium oxide, leading to the pitting corrosion of the substrate.11
_____Titanium brackets and wires are currently recycled without any formal complaints to have been ever filed: as the alloy is highly corrosion resistant and stable at high temperatures, the related devices are not affected. Being difficult to polish and having rougher surfaces, the shade of the attachments is, however, darker.

_____References
1. Brunette DM, Tengvall DM, Textor M, Titanium in Medicine: Material Science, Surface Science, Engineering, Biological Responses and Medical Applications, Springer, NY, 2001
2. Lautenschlager EP, Monaghan P, Titanium and titanium alloys as dental materials, Int Dent J. 1993; 43(3): 245-53.)
3. Wang RR, Fenton A. Titanium for prosthodontic applications: a review of the literature. Quintessence Int. 1996; 27(6): 401-8.
4. Sachdeva R, US. Patent 5,203,804, ’93
5. Sachdeva RCL, Oshida Y, US Patent 5,232,361, ‘93.
6. Schiff N, Grosgogeat B, Lissac M, Dalard F , Influence of fluoridated mouthwashes on corrosion resistance of orthodontics wires. Biomaterials. 2004; 25(19): 4535-42
7. Akin-Nergiz N, Nergiz I, Platzer U, Behlfelt K, Scherfestigkeit von Titanbrackets in Abhangigkeit von Beschichtungsverfahren, Fortschr. Kieferorthop. 1995; 56: 49-55
8. Harzer W, Schroter A, Gedrange T, Muschter F. Sensitivity of titanium brackets to the corrosive influence of fluoride-containing toothpaste and tea, Angle Orthod. 2001; 71(4): 318-23.
9. Menezes LM, Campos LC, Quintao CC, Bolognese AM. Hypersensitivity to metals in orthodontics Am J Orthod Dentofacial Orthop. 2004; 126: 58-64
10. Yonekura Y, Endo K, Iijima M, Ohno H, Mizoguchi I, In vitro corrosion characteristics of commercially available orthodontic wires, Dent Mater J. 2004; 23(2): 197-202
11. Schiff N, Grosgogeat B, Lissac M, Dalard F, Influence of fluoride content and pH on the corrosion resistance of titanium and its alloys. Biomaterials. 2002; 23(9): 1995-2002

_____4.3 Nickel-Titanium alloys
_____4.3.1 Classification and structure
_____NiTi is a family of alloys with properties that depend on composition (ternary additions) and treatment. For orthodontic wires, the main difference between these alloys resides in the height and width of their transition temperature range (TTR).
_____Considered two metals in one, NiTi may behave either closer to steel or to rubber, or even to both of these at the same time, switching from a structure to another, see Fig. 4.23. In relation to the human body’s temperature, their TTR classifies NiTi wires as martensitic (soft, flexible), austenitic (stiff) and heat-activated or plateau/austenitic (switching from soft to stiff). Nitinol, a variety made by work-hardening process, has excellent springbuck, but no shape-memory or super elasticity.
_____The many brand names refer to shape, strands, treatment and composition. The latter, based upon the addition of minute amounts of other metals, plays a major role in controlling TTR. In heat-activated orthodontic wires, this interval has to be very narrow: Ormco claims to be able to pre-establish a transformation temperature range of ±2ºC with its Copper Ni-Ti.
_____4.3.2. Properties
_____In the absence of other additional elements, NiTi’s martensite transforms in austenite at 275o C (525o F). To be useful, however, heat-activated orthodontic wires should have the transition temperature matching that of the oral environment. Making the problem even more difficult is the fact that the applied stresses have also an impact, as both plastic and elastic deformations further rise the martensitic transformation. Technology has, however, solved the problem by adding other elements and treatment. Both Ni and Ti have several valences, and generate, in addition to NiTi, combinations such as Ti2Ni3 and Ti2Ni that can easily transform in each other, segregating in the process an excess of one metal or the other. This explains the mobility of the alloy to pass from one form into another. The propensity for transformation, however, occurs only around the equiatomic Ni-Ti ratio: shape-memory alloys are selected within the range 49.7 to 50.7 atomic percents. An explanation for the dual nature of NiTi alloys is based upon the fact that nickel metal crystallizes in the FCC system, while titanium in the hexagonal one; subjected to stress or variation of temperature, a NiTi equiatomic mixture has to decide which way to go, often being forced to alternate paths.
_____When stressed, all metals are subjected first to a slight displacement of their atoms (Hookian elasticity): if the stress grows larger, the atom planes slide against each other and their return to the initial form becomes impossible. In the case of twinning, the atoms continue to be connected to each other through weak valences (electron pairs) but the bonds are this time longer and directed at another angle, as shown in Fig. 4.23 which depicts the transformation of NiTi alloys.
_____The twining does not limit to a single plane, but continues through all the crystal. The cubic cells become monoclinic: a redistribution of their atoms within the unit cells renders the metal more stable. As the bonds between the atoms of the vicinal unit cells are not disrupted, this allows coherence and ability to switch structures. In a specific arrangement compared to a harmonica, a twinned plane is followed by another one in the opposite direction. The representation of the transition starts with austenite, 1; due to some perturbation, few unit cells may become tilt (monoclinic), marking the start of martensite, Ms, 2. As the resulting cells are more stable if these accommodate with similar cells, a zigzag lattice is generated, as in Mf. 3. The important feature of this new structure is its capability of “detwinninig” without returning to the cubic lattice of austenite. Mf.4.
_____Under stress, or if properly heated, this structure allows NiTi alloys to expand, without plastic deformation, several times more than any other alloy known today. As a result, super elastic wires reversibly deform eight times more under a stress which is four times smaller, this property being maintained within a 2-8% elongation range. The most superficial advantage of super elastic alloys is that up to 11% spring back or elasticity is realized, as compared with 0.5% in the most commonly used medical material, stainless steel.
_____The mobile, coherent interface between unit cells allows the alloy not only to be easily deformed, but also to return to its previous, more stable form. The phenomenon is known as “shape memory”: along with super elasticity (known also as pseudo plasticity), these is the result of the “twinning-detwinninig” process. As a difference from the boiling, melting points and other constants that are precise, the temperature interval at which the switch from a structure occurs within a range of temperatures, TTR. When heating an alloy, the temperatures within this range do not match these observed when cooling: the lag between these temperatures is called hysteresis. For most of their uses in medicine, NiTi alloys are made to have the TTR at temperatures close to that of the human body: without any addition, NiTi’s hysteresis may expand up to 60oC, as shown in Fig. 4.24.
_____As in pure NiTi, martensite transforms in austenite at temperatures as high as 275o C (525o F), it became important to have the transition temperature matching that of the oral environment. A supplementary problem was that the applied stresses have also an impact, as both plastic and elastic deformations raise Ms. By adding small amounts of other metals such as copper, aluminum zirconium cobalt chromium and even iron, it is possible not only to bring TTR within the mouth temperatures, but also to narrow it within few degrees.1-4
_____4.3.3 Nickel-Titanium alloys in orthodontics
_____Using in his patent the drawing shown in Fig. 4.25, Andreasen opened a new era introducing NiTi in orthodontic arch wires. While he rightfully claimed its useful “shape-memory” properties, it took, however, years till the alloy was refined enough to straighten teeth as a response to the temperature changes in the mouth.5, 6
_____Based upon the basic, early findings on shape-memory/super elastic phenomena, 7-10 new applications have developed fast. Indeed, NiTi can replace all stainless steel wires except the hard, finishing ones. In addition, it can participate under a form or another into a multitude of attachments (Unitek’s SmartClip® brackets, Orec’s spring for Speed® brackets, Arndt’s palatal and expansion arches and other, complex removable appliances). Among others, the super elastic and shape memory properties of nickel titanium have been used to provide light continuous forces over a considerable range of activation,11 in arch wires,12 palatal expanders,13 and open and closed-coil springs.14 As it may take volumes to describe them, some basic developments unrelated to specific applications are worth mentioning.
_____A differentiated thermal treatment along the arch wire has been made possible to reduce the chair time by improving its general performance.15, 16 By varying the radius along the arches or wires, it is possible to obtain more adequate and efficient configurations17 . Multi-strand, braided arch wires made of these alloys have been found to be more resilient, i.e. return faster and more accurately to the desired shape.18, 19 To shape them at will, NiTi wires can be either heated in a deformable stainless steel tube/mold20 , or in several stages.21 Interesting to mention is that instead of complicated furnaces, the direct application of an electrical current can be successfully used to give the wires the desired form.
_____Super elastic NiTi wires have made the object of a comprehensive review12 , while a study has compared TTR for several wires only to find from 25 to 82o C (77-180o F).22 Two other studies have focused on the influence of temperature: 23, 24 interestingly, both dispel the claim that the slopes of the loading and unloading plateau values can be significantly different from a brand to another, when using the same alloy. Indeed, as there are not too many possibilities to obtain the sought-after austenitic plateau of the heat-activated NiTi wires, it is normal for the various competitive brands to fall on very similar recipes.
_____4.3. 4. Changes affecting properties
_____NiTi materials are sensitive to oxygen, corrosive agents and heat-or cold-work. Dissolved interstitial elements (i.e. those having small atoms such as O, N, C) cannot substitute the larger ones of the heavy metals, but can disrupt the matrices instead, interfering with its reversibility. Thus, oxygen forms a Ti4Ni2Ox inclusion that lowers the alloys elasticity: as a result levels are controlled to under 500 parts per million (ppm). Even so, its influence is great: at 150 ppm of oxygen, a stochiometric (equiatomic) alloy has its Ms at 45o C. At 450 ppm, its Ms will be only -5o C. While the phenomenon has not yet been described, it is possible that orthodontic NiTi wires stored in heat and dry conditions would suffer changes in their properties. This may well happen also during manufacture and while the metal is heated during the forming process. In addition to changes in “memory”, oxygen causes the metal to become susceptible to the pitting and crevice attack of halides such as table salt solutions25 and fluoride rinses. 26, 27 Nitrogen, used for the protection and hardening of NiTi alloys (Ion Guard® by GAS), behaves the same way, its effect being additive to that of oxygen.28
_____While coated with a layer of titanium oxide, NiTi items are attacked in vitro by standard accelerated corrosion agents29 (1% lactic acid and 0.9% sodium chloride solutions) and in vivo by the oral environment.30, 31 In addition to common pitting that damages/roughens the wire’s surface, hydrogen embrittlement (due to the absorption of the gas generated by galvanism and the related water electrolysis) has been found as reason for wire fracture in the mouth.32 As the immersion time increases, the metal released decreases. 33
_____Although the recycling of NiTi wires increases pitting, 34 it was reported that by comparing this effect with other types of wires that were not subjected to the treatment, its clinical effect was insignificant. 35
_____4.3.5. Potential health hazards
_____Their ability to store energy leads to the need to prevent heavier NiTi bent arch wires to reach their TTR too fast, see Fig. 4.26, as this could lead to their violent expansion that may harm the adjacent tissues.
_____NiTi items are covered by a protective, thin layer of titanium dioxide, 36 a substance with a moderate toxicity.37 However, if attacked with fluoride rinses, nickel may be released. 38 In addition, some interactions can generate galvanic corrosion. Cases in point are stainless steel brackets joining nickel titanium, or of titanium or NiTi brackets joining stainless steel wires. Due to disalloying, a form of galvanism which occurs also in the oral environment, NiTi-based devices are prone to become richer in Ti and poorer in Ni, the latter being carried out by saliva.39
_____The effect of the small amount of nickel released is controversial: while an allergy associated with a transpalatal arch appliance, 40 an increase in salivary polyamines41 was reported. A recommendation to search for alternatives in nickel sensitive persons42 has stirred recently an exchange of letters to the editor.43, 44 Considering such reactions occasional, David Tidy has protested the recommendation to deny to many the benefit of using NiTi wires.
_____Both NiTi brackets and wire devices can be recycled, as long as the temperature to which the devices are subjected is below that of the transition to a stable phase. While recycling increases the surface roughness and friction coefficients, this had a limited clinical significance. 45
_____References
1. Andreasen GF, Montagano L, Krell D. An investigation of linear dimensional changes as a function of temperature in a 0.010 inch Co55-substituted annealed nitinol alloy wire, Am J Orthop. Dentofac Orthop. 1982; 82: 469-72
2. Andreasen GF, Wass K, Chan KC. A review of super-elastic and thermodynamic nitinol wire. Quintessence Int. 1985; 9: 623-6.
3. Hurst CL, Duncanson MG, Nanda RS, Angolkar PV, Shape-memory phenomenon of Ni-Ti wires, 1990; 90: 72-76
4 Andreasen GF, Barrettt RD. An evaluation of cobalt-substituted nitinol wire in orthodontics. Am J Orthop. Dentofac Orthop 1973; 63: 462-70.
5. Andreasen GF, US Patent 4,037,324 ’77
6. Andreasen GF, Alignment of teeth using a 0.019 inch thermal nitinol wire with a transition temperature range between 31° C. and 45° C. Am J Orthop. Dentofac Orthop 1980; 78: 528-537
8. Andreasen GF, T.B. Hilleman, An evaluation of Co55 substituted nitinol wire for use in orthodontics. J Am Dent.Assoc. 1971; 82: 1373- 1375
9. Andreasen GF, USP 4,037,324, 1977
10. Burstone CJ, USP 4,197,643, 1978
11. Miura F, Magi M, Ohura Y, Karribe M. The super elastic Japanese NiTi alloy wire for use in orthodontics. Part III: studies on the Japanese NiTi alloy coil spring. Am J Orthod Dentofac Orthop 1988; 94: 89-96.
12. Waters NE. Super elastic nickel-titanium wires. Br J Orthod 1992; 19: 319-22.
13. Arndt WV. Nickel titanium palatal expander. J Clin Orthod 1993; 27: 129-37
14. Manhartsberger C, Seidenbusch W. Force delivery of NiTi coil springs. Am J Orthod Dentofac Orthop 1996; 109: 8-21.
15. GAC, USP 5,017,133, 1990;
16. Furukawa Electric Co., USP 5,102,333, 1991
17. Berendt CJ, USP 4,818,226, 1988
18. Ormco, USP 5,018,969, 1991
19. Karabin RJ, USP 5,080,584, 1991
20. GAC, USP 5,092,941, 1991
21. Yoneyama T, Doi H, Hamanaka H, Yamamoto M , T. Kuroda T, Bending properties
and transformation temperatures of heat treated NI-Ti alloy wire for orthodontic appliances, J. Biomed. Materials Res. 1993; 27: 399-402
22. Hurst CL, Duncanson MG, Nanda RS, Angolkar PV, Shape-memory phenomenon of Ni-Ti wires, Am J Orthop. Dentofac Orthop. 1990; 90: 72-76
23. Toner RIM, Waters NE, The characteristics of super elastic Ni-Ti wires in three-point bending. I. The effect of temperature, Europ. J. Orthod. 1994; 16: 409-419
24. Toner RIM, Waters NE, The characteristics of super elastic Ni-Ti wires in three-point bending, II, Intra-batch variation, Europ. J. Orthod. 1994; 16: 421-425
25. Fukuzuka T, On the beneficial effect of the titanium oxide film formed by thermal oxidation, Titanium’80 Science and Technology, The Metallurgical Society of AIME, 1980: 2781-2792
26. Yokoyama K, Kaneko K, Ogawa T, Moriyama K, Asaoka K, Sakai J, Hydrogen embrittlement of work-hardened Ni-Ti alloy in fluoride solutions. Biomaterials 2004; 26(1): 101-108
27. Yokoyama K, Kaneko K, Moriyama K, Asaoka K, Sakai J, Nagumo M, Hydrogen embrittlement of Ni-Ti super elastic alloy in fluoride solution. J Biomed Mater Res. 2003; 65A (2): 182-7
28. Duerig TW, Pelton AR, Ti-Ni shape memory alloys, in Titanium alloys, Boyer R, Welsch G, Colling EW, editors, ASM International, Materials Park, OH; 1994:1035
29. Iijima M, Endo K, Ohno H, Yonekura Y, Mizoguchi I. Corrosion behavior and surface structure of orthodontic Ni-Ti alloy wires, Dent Mater J. 2001; 20(1): 103-13
30. Eliades T, Zinelis S, Papadopoulos MA, Eliades G, Athanasiou AE Nickel content of as-received and retrieved NiTi and stainless steel arch wires: assessing the nickel release hypothesis. Angle Orthod. 2004; 74(2): 151-4
31. Eliades T, Pratsinis H, Kletsas D, Eliades G, Makou M, Characterization and cytotoxicity of ions released from stainless steel and nickel-titanium orthodontic alloys. Am J Orthod Dentofacial Orthop. 2004; 125(1): 24-9
32. Yokoyama K, Hamada K, Moriyama K, Asaoka K, Degradation and fracture of Ni-Ti super elastic wire in an oral cavity, Biomaterials. 2001; 22(16): 2257-62
33. Schwaninger B, Sarkar NK, Foster BE, Effect of long-term immersion corrosion on the flexural properties of nitinol. Am J Orthod Dentofac Orthop 1982; 82: 45-49
33. Hwang CJ, Shin JS, Cha JY, Metal release from simulated fixed orthodontic appliances. Am J Orthod Dentofac Orthop. 2001; 120(4): 383-91
34. Kapila S, Reichhold GW, Anderson RS, Watanabe LG, Effects of clinical recycling on mechanical properties of nickel-titanium alloy wires. Am J Orthod Dentofacial Orthop. 1991; 100(5): 428-35
35. Lee SH, Chang YI, Effects of recycling on the mechanical properties and the surface topography of nickel-titanium alloy wires. Am J Orthod Dentofacial Orthop. 2001; 120(6): 654-63
36. Firstov GS, Vitchev RG, Kumar H, Blanpain B, Van Humbeeck J. Surface oxidation of NiTi shape memory alloy, Biomaterials. 2002; 23(24): 4863-71
37. Bernard BK, Osheroff MR, Hofmann A, Mennear JH, Toxicology and carcinogenesis studies of dietary titanium dioxide-coated mica in male and female Fischer 344 rats, J Toxicol Environ Health. 1990; 29(4): 417-29
38. Grosgogeat B, Pernier C, Schiff N, Comte V, Huet A, Biocompatibility and resistance to corrosion of orthodontic wires, Orthod Fr. 2003; 74(1):115-21
39. Sarkar NK, Redmond W, Schwaninger B, Goldberg AJ. The chloride corrosion behavior of four orthodontic wires. J Oral Rehabil 1983; 10: 121-8
40. Counts AL, Miller MA, Khakhria ML, Strange S, Nickel allergy associated with a transpalatal arch appliance, J Orofac Orthop. 2002; 63(6): 509-15
41. Venza M, Visalli M, Ruggeri P, Cicciu D, Teti D. Age-related salivary polyamine increase in adolescents wearing orthodontic Ni-Ti arch wires, Amino Acids. 2002; 22(2): 119-30
42. Rahilly G, Price N, Nickel allergy and orthodontics, J. Orthod; 30(2): 171–4
43. Rahilly G, Price N, Letter to the Editor, Journal of Orthodontics, Vol. 31, 2004, 71
44. Tidy D, Letter to the Editor, Journal of Orthodontics, Vol. 31, 2004, 71
45. Sung Ho Lee SH, Young Il Chang YI, Effects of recycling on the mechanical properties and the surface topography of nickel-titanium alloy wires, Am J Orthod Dentofacial Orthop 2001; 120: 654-63)

_____4.4. Chromium cobalt alloys
_____4.4.1. Classification
_____Along with stainless steels and nickel titanium, chromium cobalt (Co-Cr) alloys are the basic components of modern orthodontic wires.1 At present, the different types of orthodontic arch wires contain 15% to 54% Ni, 20% to 30% Cr and 40% to 60% Co.2, 3 Similar alloys that contain besides chromium and cobalt also tungsten or molybdenum are known under the trade name Stellites® (Deloro Co, Canada): among these Stellite 6® is the most common and its composition is shown in Table 4.5.
_____An alloy that contains Co (40%), Cr (20%), Ni (15%), Fe (16%), Mo (7%), Mn (2%), Be (0.4%) and C (.04%) has been successfully used for over half a century for orthodontic purposes under the name of Elgiloy® by Rocky Mountain Orthodontics. In its attempt to develop wires for specific applications, this company provides four tempers in an increasing order of resilience. Blue Elgiloy® is the softest and can be bent easily with fingers or pliers: its heat treatment increases resistance to deformation. It is recommended for edgewise and lingual arches, retainers and removable, as well as whenever considerable bending, soldering, or welding is required. Yellow Elgiloy® is more resilient: being relatively ductile it can be bent with relative ease. Heat-treatments increase both properties. Green Elgiloy ® is even more resilient than yellow Elgiloy® and can be shaped with fingers or pliers before heat treatment. The most resilient Elgiloy® is marked red and provides high spring qualities: it is not recommended for heat treatment.
_____4.4.2. Place in orthodontics
_____With the exception of red temper Elgiloy®, no heat-treated Co-Cr wires have a smaller springbuck than stainless steel wires of comparable sizes, but this property can be improved by adequate heat treatment. Indeed, cobalt-chromium alloys can be delivered to the practitioner in different degrees of hardening, or tempers, and can be further hardened by heat treatment. Their strengths and propensity for plastic deformation increases as the tempers proceeded from “blue” temper level to the “red” levels, allowing the clinician to select the amount of formability desired.
_____The advantages of Co-Cr wires over stainless steel wires include greater resistance to fatigue and distortion, and a longer function as a resilient spring. In most other respects, the mechanical properties of Co-Cr wires are very similar to those of stainless steel wires. Therefore, stainless steel wires may be used instead of Co-Cr wires of the same size in clinical situations in which heat-hardening capability and added torsional strength of Co-Cr wires are not required. In contrast, Co-Cr wires, due to the different tempers that are available, permit the practitioner to choose an alloy with different amounts of formability. After acquiring the desired shape, the wire can be heat-treated to decrease the formability while its elasticity modulus increases. In doing so, the ability of the wire to store and deliver more energy is enhanced allowing the force to be delivered over a greater distance and for a longer time.
_____This higher force delivered by Co-Cr wires translates, however, into faster rates of mesial movement of posterior teeth, thus placing greater demands on intra- and extra oral anchorage. Indeed, undesirable movements are more likely to occur if the wires are made of Co-Cr. These would deliver considerably higher forces than if made of beta-titanium and even higher than if made of NiTi for the same activation.4, 5
_____4.4.3 Properties
_____Wires made of Stellites and cobalt-chromium alloys in general have good formability and can be bent into many configurations. Due to the high content in chromium, these alloys are highly resistant to corrosion.6 Compared to Nitinol, Co-Cr alloys maintain better their passivity in NaCl solutions.7
_____Their strength, resistance to wear, and galling and seizing over a wide temperature range, has led to a wide array of applications. Thus, tungsten-containing Stellites are used to make saw teeth, blades, valve parts, pump plungers, knives, shafts, erosion shields, rotors and bearings. Molybdenum-containing Co-Cr alloys are commonly used in surgical tools and in dental cast requiring exceptional strength, Fig. 4.26
_____4.4.4 Changes affecting properties
_____Even at room temperature, Co-Cr tempered wires have to be carefully manipulated when using pliers, as if tempered, these withstand only minimal cold-working. Increasing the temperature as low as it is needed for soldering, Co-Cr wires will lose in yield and tensile strengths: above 1200° F (749° C), annealing takes place. If subjected to further heat treatments, the high temper wires become brittle due to a precipitation-hardening process.
_____Theoretically, for each temper, the elastic strength should further increase without changing the wire stiffness. This does not frequently happen, as clinicians are not following the recommended time-temperature scheme.
_____The heat treatment of Co-Cr arch wires has also an impact on their corrosion susceptibility: subjected to immersion in a salt solution, their metal release starts from ~ 500oC and becomes 15 to 60 times higher than the normal, lowest values. The study’s authors caution practitioners to take this in account when applying heat to orthodontic wires.8
_____4.4.5 Potential health hazards
_____Due to their chromium oxide protective layer, Co-Cr alloys are relatively passive; moreover, patients known to be allergic to both nickel and cobalt show a mild response when subjected to patches of Co-Cr cast alloys.9 A guinea pig maximization test showed that chromium and cobalt do not cross-react.10 It is not often clear whether chromium or other metals have caused the allergic skin reactions elicited by dental metals. Thus, it has been reported that a patient has developed a generalized eczematous dermatitis caused by allergy to chromium liberated from a metal dental plate.11 In another report, a woman had severe dermatitis and allergic reactions to several metals; she recovered only after removal of a cast chrome-cobalt partial denture.12 The risks raised by the two metals are uneven, chromium being by far more dangerous.
_____Chromium is a human carcinogen according to the World Health Organization. In the US, the Department of Health and Human Services (DHHS) has determined that certain chromium (VI) compounds are known to cause cancer in humans. In general, these compounds are persistent, bioaccumulative and toxic. According to the Agency for Toxic Substances and Disease Registry (ATSDR),13 skin contacts with certain chromium (VI) compounds can cause skin ulcers. Some people are extremely sensitive to chromium (III), and especially to chromium (VI). Allergic reactions consisting of severe redness and swelling of the skin have been noted.
_____Most of the above hazards refer, however, to hexavalent chromium salts that are unlikely to form at the contact alloy-tissues. However, allergic contact dermatitis caused by chromium salts has been known since 1925 and is common. In industrialized countries, chromium salts have been one of the most common skin sensitizers.14, 15
_____The allergic reactions consist in severe redness and swelling of the skin and are more common in men than in women, a difference that probably depends on the pattern of employment. The causes of allergy to chromium salts vary from country to country, depending on the industry and the chemical environment.16
_____Cobalt has not been classified for carcinogenicity, and it can harm only in mega doses generating polycythemia, enlargement of thyroid gland and enlargement of the heart leading to congestive heart failure. When leaching from dental appliances, it may raise the blood serum level which in turn can cause long-term cardiovascular disease.17 In addition, cobalt leaching from fixed orthodontic appliances has been found to induce DNA damage in oral mucosa cells.18

_____References
1. Kusy RP, A review of contemporary arch wires: their properties and characteristics. Angle Orthod 1997; 67: 197-208
2. Hwang CJ, Shin JS, Cha JY. Metal release from simulated fixed orthodontic appliances. Am J Orthod Dentofacial Orthop 2001; 120: 383-391
3. Agaoglu G, Arun T, Izgu B, Yarat A. Nickel and chromium levels in the saliva and serum of patients with fixed orthodontic appliances. Angle Orthod 2001; 71: 375-379
4. Kusy RP, Mims L, Whitley JQ, Mechanical characteristics of various tempers of as-received cobalt-chromium arch wires Am J Orthod. Dentofacial Orthop 2001, 119: 274-91
5. Kapila S, Sachdeva R, Mechanical properties and clinical applications of orthodontic wires. Am J Orthod Dentofacial Orthop 1989; 96: 100-109
6. www.osti.gov/bridge.product.biblio.jsp?osti_
id=766657
7. Sarkar NK, Redmond W, Schwaninger B, Goldberg AJ. The chloride corrosion behavior of four orthodontic wires. J Oral Rehabil 1983; 10: 121-8.
8. Gjerdet NR, Hero H. Metal release from heat-treated orthodontic arch wires. Acta Odontol Scand 1987; 45:409-14
9. Magnusson B, Bergman M, Bergman B, Soremark R. Nickel allergy and nickel-containing dental alloys. Scand J Dent Res 1982; 90:163-7
10. Liden C, Wahlberg JE. Cross-reactivity to metal compounds studied in guinea pigs induced with chromate or cobalt. Acta Derm-Venereol (Stockholm) 1994: 74: 341
11. Hubler WR Jr, Hubler WR Sr. Dermatitis from a chromium dental plate. Contact Dermatitis 1983: 9: 377.
12. Brendlinger DL, Tarsitano JJ. Generalized dermatitis due to sensitivity to a chrome- cobalt removable partial denture. J Am Dent Assoc 1970: 81: 392.
13. http://www.atsdr.cdc.gov/tfacts7.html
14. Cronin E. Contact Dermatitis, Churchill Livingstone, Edinburgh 1980: 287
15. Burrows D, Adams RM. Metals. In: Adams RM, Occupational skin disease, 2nd ed, WB Saunders Co, 1990: 349
16. http://www.chromium-asoc.com/publications/References#References
17. Dong H, Nagamatsu Y, Chen KK, Tajima K, Kakigawa H, Shi S, Kozono Y. Corrosion behavior of dental alloys in various types of electrolyzed water. Dent Mater J. 003; 22(4): 482-93.
18. Faccioni F, Franceschetti P, Cerpelloni M, Fracasso ME, In vivo study on metal release from fixed orthodontic appliances and DNA damage in oral mucosa cells, Am J Orthod Dentofacial Orthop 2003; 124: 687-693

_____4.5. Brazing alloys
_____4.5.1 Classification
_____While their weight participation in the treatment is minor, the alloys and the fluxes used to join metal substrates are in fact the “Achilles’ heel” of many orthodontic devices. The related joints can have voids and pores, or detach, causing wounds. The brazing alloy can leach heavy metals or entrap toxic flux compounds: if it excessively wets the substrates, it may interfere with the treatment’s accuracy. Brazing joins base metals by using a filler metal that melts completely above 450o C (840o F): if the filler metal melts at lower temperatures, the joining is called soldering.
_____Leading to poorer joints and being more corrosion susceptible, soldering will not be discussed.
_____The selection of the brazing alloys depends upon the degree of oxidation resistance and corrosion resistance required. Destined to withstand mouth’s highly corrosive environment, the brazing alloys used in orthodontics are based upon noble metals such as silver, gold and palladium. Whilst joints with the noble metal alloys may be made also by using a flux and torch heating, industrial applications require a furnace and vacuum or inert atmospheres.
_____Brazing alloys based upon silver and gold are presented in Table 4.6 along with their classification according to the American Welding Society.1 For dental brazing, ISO’s committee TC 106/SC 2 (Dentistry) has its own standards.2
_____Flux pastes are critical to both the brazing and soldering process because they interfere with the formation of oxides that form fast when heating the metals to be joined. In industry, it is claimed that brazing metal without flux is like painting over rust. As it withstands oxidation, gold brazing does not need flux.
_____4.4.2 Brazing alloys in orthodontics
_____No other method of joining orthodontic metal devices has the advantages of brazing. Thus, welding joins metals by melting and fusing them together, usually with the addition of a welding filler metal. The joints produced are strong, usually as strong as the metals joined or even stronger. To fuse the metals, a concentrated, high heat has to be applied directly to the joint area. Because welding heat is intense, it sensitizes stainless steel and changes the structure of titanium and its alloys. Typically localized, its heat is pinpointed and therefore impractical to apply it uniformly over a broader area. Used for years to attach bases to brackets, spot welding has practically vanished today being replaced by brazing due both to concerns about miniaturization as well as to an article by Maijer and Smith showing its drawbacks.3
_____Adhesive bonding and soldering may give permanent bonds, but generally neither can offer the strength of a brazed joint – strength equal to or even greater than that of the base metals themselves. In addition, these cannot produce joints that offer long term resistance to temperatures above 200°F (93°C) such as those used in sterilization.
_____It is alleged that if Cromwell hadn’t been killed by a pebble in his kidney, the history of the world would not have been the same. On another scale, in the tiny world of direct bonding appliances, a similar role may have been played by brazing. In removable attachments, it has made possible a variety of attachments; in fixed appliances, a large array of brackets. Even when it failed to give good results, it forced manufacturers to venture in the non-brazed, mono-block (or one piece) appliance that will, sooner or later, dominate the market. Besieged by the high cost and the assembly mistakes generated by human labor, brazed attachment users have to face corrosion problems: indeed, silver-based brazing fillers are corroded by oral fluids. As the later phenomenon occurs rather seldom, gold-brazed appliances are being preferred. Among the manufacturers who are using gold filler alloys for brazing are Ormco (for their Diamond® and Mini-Diamond® brackets), and Rocky Mountain (for Taurus® and Mini-Taurus® and GAC (for Micro Arch™).
_____4.5.3 Properties
_____Brazing fillers and brazing per se are very much related: one cannot be satisfactory unless the other works well. Brazing generates strong and ductile joints between dissimilar materials and it is easy to perform as one operation at relatively low temperatures. On stainless steels, it is possible to develop a joint whose tensile strength is 130,000 pounds per square inch (896.3 MPa). Such joints are able to withstand considerable shock and vibration. After the joint is completed, it seldom requires grinding, filing or other mechanical finishing which reduces the possibility of warping, overheating or melting the metals being joined.
_____Most brazing filler alloys do not have a single melting temperature or melting point; instead they have a melting range. The parts must be joined without melting the base metals, while the filler metal has to be completely melted. The filler metal must wet the base metal surfaces and be drawn in the joint by capillary action: their contact angle with various substrates, metallic or ceramic, can be found in specialized books.4,5 As in contact with the substrate it tends to solidify, the filler has to be maintained liquid to wet the substrates while exerting a capillary action: consequently, the brazing temperature range must be maintained for a given period of time.
To achieve a good brazing, the parts have to be clean and protected against oxide formation. This can be performed by an inert atmosphere or by a flux which combine or dissolve the oxides formed on the hot substrates. Common fluxes are resins containing boric acid or borates that combine with the oxides abundantly formed on the hot metal surfaces, or salts releasing hydrochloric or hydrofluoric acids such as ZnCl2 , NH4Cl, and KHF2.
_____4.5.4. Changes affecting properties
_____Exhibiting an inhomogeneous structure, the alloys destined to join metal parts easily suffer changes because these are made of several elements that lead not only to galvanic corrosion between the substrates, but also to a depletion within themselves in the less noble metals. In addition, the brazed area may entrap flux or have voids that induce crevice corrosion. The operation has its own problems:
_____Insufficient wetting. All stainless steel alloys are difficult to wet by the filler metal because of their chromium content. The friendly film of chromium oxide that protects these steels from rusting becomes in this instance an enemy. In Table 4.713 are shown the affinities of various brazing alloys to stainless steel.
_____Brazing stainless steel requires special care like fluxes, purified hydrogen atmospheres or even vacuum, conditions that are not easy to achieve and maintain. If these requirements are not met, the filler will not wet the base metal enough, leaving gaps.
_____Excessive wetting. Lack of proper control may lead to the brazing alloy spreading around the joint, e.g. by obturating slots the Broussard or vertical slots or clogging the edgewise slot as shown in Fig. 4.27
_____Incorrect clearance. A too wide or too narrow distance between the base metal parts to be joined may lead to weak bonds; in the first case, this is because the rather weak metal, the filler, will ultimately determine the joint strength. In the second case, there may not be enough brazing where needed. Clearances have therefore to be both uniform and accurate, and this becomes particularly difficult to implement whenever matching minute, compound curved surfaces, such as in Fig. 4.28.
_____Improper temperature. The substrates have to be kept at least at 425o C (800o F). Even if exposed for a short time to these temperatures, stress cracking and intergranular attack of the base metal are likely to occur.6 Such conditions are met during the thermal recycling of brackets, which are 45 minutes at 454o C.7 In such instances, both brazing and the microstructure of the base metal are affected, becoming weaker and corrosion prone.
_____Diffusion. To be effective, a brazing filler metal must adhere to the surface of the base metal without undesirable diffusion into the last one. This would lead to its dilution, base metal erosion or formation of brittle compounds.8 The brazing layer has to be applied and remain there only where it is supposed to be. Its spreading should be carefully controlled by the manufacturer by selecting the proper filler metal as well as by applying the minimum of filler metal. This doesn’t frequently happen, and often the practitioner has to face unsightly brazing spots on the bracket.
_____Poor positioning. The result of a brazing can be unacceptable due to the improper positioning of the parts to be joined, or to their unintended movement. This occurs with mesh-based brackets that are liable to human error, Fig. 4.29. The resulting deviations may impair their clinical performance, and by not matching correctly the compound curves involved, the clearance may not be acceptable for a good bond.
_____Residual flux. Subjected to high temperatures, the reacted filler generates dark brown stains around the brazing perimeter. This becomes visible when gold brazing alloys are used, i.e. with GAC’s Micro Arch®, Ormco’s Diamond®, Mini-Diamond® and Mini-Wick® series9 as well as with Rocky Mountain’s Mini-Taurus® brackets.10 When processing large parts, manufacturers prevent the spread of brazing by using “stop-off” barriers, which are commonly alumina or graphite slurries painted over the protected areas. This becomes very difficult especially with minute brackets. Interestingly, if thoroughly cleaned, the layer of flux is removed and yellow stains (gold’s color) appear.
_____Disalloying. Known also as “selective leaching”, the phenomenon is well known both in industry11 and dentistry, where the alloying elements accompanying gold dissolve in saliva, leaving behind the noble metal.12
_____Corrosion susceptibility. The corrosion susceptibility of the silver-based brazing found on stainless steel brackets occurs because only few alloys lend themselves to corrosion-resistant joints: whenever the brazing alloy is less noble than stainless steel, it will dissolve, while when it is nobler, it will lead to the attack of the steel, as shown in Fig.4.30 and 4.31.
_____4.5.5 Health hazards
_____While the swallowing of a poorly brazed, detached bracket may not constitute a problem,14 the prolonged exposure to heavy metals is. The released metals may originate from the brazing, Fig. 4.30, or from the attachment’s stainless steel body, Fig. 4.31, depending which one is less noble.13 When a poorly allied silver brazing is used, it will dissolve as stainless steel is nobler. When gold-based fillers are used, stainless steel may dissolve (see the “Galvanic Series”, Fig 3.3).
_____While silver and gold have a low toxicity, most of the brazing alloys containing them are not. To protect against stainless steel’s sensitization and propensity toward intergranular corrosion, it is necessary to braze it within a short time and at lower temperatures.14 Thus, although the deleterious health effects of nickel are known, it is added to gold brazing fillers not because it is less expensive, but because at 20% weight it lowers the first’s melting temperature (1065o C) with about 100o C.15 (BAu-4 in Table 4.6)
_____While cadmium is banned in the US, it may still be used to braze attachments in other countries, as it has particular advantages as it lowers the working temperature required and provides good flow properties. Known to be toxic, it is rapidly released even when present in small amounts:16 its effects vary from acute and chronic exposure in both humans and animals to kidney damage.
_____Not many clinicians are brazing their own attachments: those who do it are exposed to fluxes releasing highly irritant fumes. While in small quantities, their residues are difficult to remove, leaching in time. Brazed and especially soldered areas have been reported to discolor during clinical use.17 If mouth rinses are used, the rate of deterioration increases.18
_____References
1. www.techstreet.com/info/aws.tmpl. Accessed August 2004
2. ISO Technical committee 106/SC 2, Dentistry, Brazing materials, ISO 9333: 1990
3. Maijer R, Smith DC, Variables influencing the bond strength of metal orthodontic bracket bases, Am J Orthod Dentofac Orthop. 1981; 70: 20-34
4. Iida T, Guthrie RIL, The Physical Properties of Liquid Metals. Oxford University Press, Oxford, 1988: 49
5. Allen BC, in: Liquid Metals - Chemistry and Physics, Beer SZ, ed., Marcel Dekker, NY, 1972: 161-212
6. Schwartz MM, Metal Joining Manual, Mc. Graw-Hill, NY 1989
7. Buchman DJL, Effects of recycling on metallic direct-bond orthodontic brackets, Am J Orthod Dentofac Orthop. 1980; 77: 654-668
8. Welding Handbook, VIIth edition vol. 2, Brazing and Soldering, Kearns WH ed., Am. Welding Soc., Miami 1989
9. Ormco Print No. 070-5051, 1991
10. Rocky Mountain, Prints P-512 (F5M), 1990; P-515 (S5M), 1991
11. ASM Handbook, vol.13, Corrosion, 9th ed., Materials Park, OH, 1992: 131-32
12. Brugirard J, Study of the electrochemical behavior of gold dental alloys, J. Dent. Res. 1973; 52 (4): 82- 95
13. Willingham JA, Brazing fillers, Stainless Steel Ind. 1980; 8 (46): 17-30
14. Bowers SA, Recycled brackets, Am J Orthod Dentofac Orthop 1993; 103: 194-6
15. CRC Handbook of Chemistry and Physics, 59th ed., CRC Press, Boca Raton, FL: F172; 1978
16. Berge M, Gjerdet NR, Corrosion of silver soldered orthodontic wires, Acta Odontologica Scand. 1982, 40: 75
17. Mjor AI, Orthodontic solders, in: Dental materials: biological properties and clinical evaluations, CRC Press, Boca Raton FL, 1990
18. Mueller HJ, Some considerations regarding the degradational interactions between mouth rinses and silver-soldered joints. Am J Orthod Dentofac Orthop 1982; 81; 140- 14