By Dr. Claude Matasa

3. Metals used in devices

_____3.1. General characteristics.
_____This adage is well illustrated by metals: light or heavy, most of these are indispensable for good health. Modern dentistry deals with many, and even the ultimate solution, the least dangerous, gold, has been found to lead to allergies. In a study performed in Norway, of almost 300 patients in their forties who complained about muscle and joint pain, fatigue and memory problems and who were patch-tested for material-related ailments, 23% were positive to gold, 28% to nickel, 14% to cobalt, 9% to palladium and only 6% to mercury.1 In many countries, toxic metals are the number one environmental health threat to children.
_____In the US, orthodontists are ironically called by their colleagues “wire-benders”. This may be rightfully so, as some 70% of the devices orthodontists use are made of metals, the largest material-related expense of an office. Part of these devices are for a short, temporary use, while others have to withstand almost without property loss an environment considered highly aggressive. Interestingly, the very definition of metals include activity: “those elements that, when in solution in a pure state, carry a positive charge and seek the negative pole, in an electric cell”.

_____3.2. Properties and health hazards
_____Orthodontic devices that do not work properly and biomaterials that leach can be foreseen, with few exceptions, from their composition, structure and general properties as shown in Tables 3.1 and 3.2. Aside their mechanical performances are listed their Pitting Resistance Equivalent (PRE) values, a rough indicator for their corrosion resistance, i.e. their capability to provide service without harming. PRE can be calculated from the contents in Cr, Mo and N; the higher the value, the less susceptible to pitting corrosion the alloy is. The values listed are for the annealed (soft) state.
_____Properties are dictated by structure, especially by the number and position of each metal’s electrons. The more they are able to share electrons, the stronger they are. Iron, nickel, chromium and manganese, most common components of the alloys used in orthodontics, belong to the group known as transition elements, see Fig. 3.1.
_____In Fig. 3.2, are represented the energy levels (shells) of the iron and calcium atoms: most of the transitional metal properties are due to the electrons located in the d orbitals, in contrast with calcium in which these orbitals are empty. Forming at least one ion with a partially filled sub shell of d electrons, transition elements exhibit a wide variety of oxidation states. Calcium ions typically don’t lose more than two electrons, whereas transition metals can lose up to nine, leading to a variety of complex compounds. Benefic when it comes to improve adhesion, this property can lead to sensitivity, allergies and immune disorders, as shown below. In most cases, metal atoms either share electrons among them or lose electrons to various anions (Cl-, SO4, etc.) becoming soluble (cations). Their propensity to react is given by the difference in electro-negativity they exhibit that ranges from 0 to 4 on the Pauling scale, Fig. 3.1. A strongly electronegative element, like fluorine, has an electro-negativity of 4 while weakly electronegative elements, such as lithium, have values close to 1. Elements in the top right of the periodic table have a higher electro-negativity; bonds between atoms with a large electro-negativity difference are usually considered to be ionic, while values between 2.0 and 0.4 are considered polar covalent. Values below 0.4 are considered non-polar covalent bonds. Although some metals can combine with themselves, most of them (with the exception of gold and the platinum family that have all electro-negativity above 2.2) are heavily attacked in by nonmetals such as chlorine and oxygen. Despite their lower electro negativity, some metals (chromium, aluminum, titanium) exhibit resistance to chemical attacks as the last gas generates on an impervious layer of oxide that protects them. The above considerations should provide enough bases for understanding the galvanic scale of metals in sea water, where platinum is the noblest and magnesium is the least, Fig. 3.3. As there are studies made on artificial saliva, the environment chosen was sea water. As it can be seen, depending of the existence or absence of its chromium oxide protective layer, stainless steels can be either “noble” (passive) or reactive, respectively.

_____The ability of the transition elements to combine with organic molecules can be understood by returning to Fig. 3.2 where the electrons are shown as red circles. If empty, the orbitals are represented as white circles. If vacant but ready to accept electrons from other atoms, these orbitals are represented with pink circles. In combinations, the bonds made by receiving electrons from nonmetals (ligands) lead to a special type of bond known as coordinative. In chelates (chela, claw in Greek), the metal atoms are so tightly attached by the “sequestrants”(see Fig. 3.4) to which they donate and at the same time from which they accept electrons, that its ions may lose their functional identity. Bonds between calcium and EDTA, shown in Fig. 3.5, are used in adhesives that contain “4-Meta” that enable these to attach to dentin and metals (crowns, amalgam).
_____While not having his own electrons in its 3d orbitals, calcium can host in there electrons from other compounds.
_____The above notions will be used to explain later why an instrument or device may be as good as the materials it is made of. Stainless steel pliers that break in your hand and direct-bonding brackets that detach from their bases are a non-event. Titanium is practically biologically inert, and so are, in a lesser degree, some of its alloys; however, it is difficult render its aspect attractive.
_____The currently used “noble” brazing alloys may join well, but may also cause the galvanic corrosion of the substrates. In contrast, less noble brazing alloys often dissolve, leading to attachment’s detachment. Aside the loss in mechanical properties, even more important are those related to the patient’s health
_____Device- caused problems.
_____The attachments and devices commonly used can generate both mechanics and chemistry related problems. While most mechanic-related incidents can usually be obvious, those involving chemistry-related and physiologic responses are insidious and complex. Most mechanical defects are easy to detect and can be effortlessly corrected, leading most of the time only to delays and annoyance. Examples quite often encountered are the brackets without slots, see Fig. 3.6, with clogged slots, Fig. 3.7, or without appropriate tie-wings, Fig. 3.8.
_____In other instances, however, defects such as improper marking, Fig. 3.9 or uneven slot depth, Fig. 3.10, and especially wrong torque, Fig. 3.11 and Fig. 3.12, may bring havoc in the treatment.

_____The problems related to chemistry and tissue response are more complex, most of the times being unexpected and difficult to correct. In acute poisoning, large excesses of metal ions can cause disruption of membrane and mitochondrial function and the generation of free radicals. This leads to generalized clinical effects that include weakness and malaise.
_____An example where the effects of nickel allergy did not show in predictable areas such as around the mouth, Fig. 3.13, but behind the knees, Fig. 3.14. In the process, the metal-combined cells are regarded as “foreign” by the immune system that attempts to destroy them. As shown above, our essential amino acids act as ligands, combining with the transitional metal ions through their reactive -COOH and -NH2 groups. Some tissue components, such as the amino acid cysteine, as well as enzymes, are rich in -SH groups that react with metals leading to changes in the protein’s structure. The modified molecules loose their ability to function properly, which leads to the malfunction or death of the affected cells.
_____An additional, topical problem in orthodontics resides in the fact that the bulk of the attachments impede upon the free exchange with the oral environment, leading to white spots, decalcification and caries.2 In Seattle, of a sample consisting of 21 children who have received an early orthodontic treatment, six complained about a broken wire or bracket, another six about gingivitis and three about staining and calcification.3
_____As the enamel’s hydroxy-apatite structure behaves like an ion-exchanger4 , it may bond not only to the desirable ions of fluorides, but acquire those of the undesirable heavy metals. This was confirmed by the incidence and the area where the white spots occur: both suggest a strong relationship with the rate of salivary flow and the related remineralization. It is logic therefore to predict an increased risk of decalcification or caries when conventional fixed systems are used, instead of removable appliances.5
_____The mechanical aspects are compounded by the fact that orthodontics devices are made of adhesives and sealants that not only hinder the desirable exchanges, but also act against the tissues. Among these, as a difference from the derivatives of the light metals, which are part of our diet (sodium, calcium, potassium), an increased exposure to heavy metals constitute a risk. Leaving for further and detailed discussion some specific alloys, the general afflictions associated to the metals used in orthodontics are listed below along with a parenthesis showing where these metals are or were used.
_____Cadmium –Headaches, dizziness, loss of appetite and weight loss have also been reported and the liver, kidneys and bone marrow may be injured by the presence of metal. Continued exposure to lower levels of cadmium has resulted in chronic poisoning characterized by irreversible lung damage and kidney damage. A single, high level exposure to cadmium can cause severe lung irritation that may be fatal. Used in some brazing alloys, cadmium is a suspected human carcinogen.
_____Chromium – In some subjects, chromium compounds act as allergens and may cause dermatitis and may also produce pulmonary sensitization. Chromic acid and chromates have a direct corrosive effect on the skin and the mucous membranes of the upper respiratory tract. Although rare, there may be the possibility of skin and pulmonary sensitization. There is sufficient evidence for carcinogenicity to animals and possible mutageneicity for chromium (VI) compounds: ingesting large amounts can cause stomach upsets and ulcers, convulsions, kidney and liver damage, and even death. Several studies have shown that chromium (VI) compounds can increase the risk of lung cancer. The World Health Organization (WHO) has determined that chromium (VI) is a human carcinogen.
_____The Department of Health and Human Services (PHS) has determined that certain chromium (VI) compounds are known to cause cancer in humans. Skin contact with certain chromium (VI) compounds can cause skin ulcers. Some people are extremely sensitive to chromium (VI) or chromium (III). Allergic reactions consisting of severe redness and swelling of the skin have been noted. (Used in all stainless steel devices.)
_____Cobalt – Cobalt has been reported as causing hyper sensitization-type dermatitis in individuals who are susceptible. Animal studies have shown that particulate cobalt is an acutely irritating substance. Exposure to high levels of cobalt can result in lung and heart effects and dermatitis. Liver and kidney effects have also been observed in animals exposed to high levels of cobalt. Cancer lesions appeared when cobalt was placed directly into the muscle or under the skin. Based on the laboratory animal data, the International Agency for Research on Cancer (IARC) has determined that cobalt and cobalt compounds are possibly carcinogenic to humans. (Used in some stainless steels devices and wires).
_____Copper –While industrial dermatitis has not been reported, keratinization of the hands and the soles of the feet have been reported. (Used in brazing alloys).
_____Manganese – Known to block calcium channels, it can lead to chronic intoxication and dopamine depletion. This latter condition duplicates almost all the symptoms of Parkinson’s disease, and is treated with some success using typical anti-Parkinson drugs. Chronic manganese poisoning is not a fatal disease although it is extremely disabling. Some individuals may be hyper-susceptible to manganese. (Used in some stainless steel devices).
_____Molybdenum – While this metal can be toxic via inter-peritoneal and subcutaneous routes, molybdenum is in generally considered to exhibit a low order of toxicity.
_____Nickel – The most common ailment arising from nickel or its compounds is an allergic dermatitis known as nickel-itch, which usually occurs when the skin is moist. Generally, nickel and most salts of nickel do not cause systemic poisoning, but nickel has been identified as a suspected carcinogen. The most common adverse health effect of nickel in humans is an allergic reaction. People can become sensitive to nickel when jewelry or other things containing it are in direct contact with the skin. Once a person is sensitized to nickel, further contact with it will produce a reaction. Used in most stainless steel devices and in some wires, it often generates a skin rash at the site of contact.
_____Silver – Chronic occupational exposure to silver results in argyria, a permanent pigmentation (gray to purple) of the skin and eyes. (Used in brazing alloys)...
_____3. 3. Structure and composition
_____Orthodontics uses heavy metals in its devices and a light metal, calcium as a base for chemical bonding. Metals are giant structures of atoms held together by metallic bonds that dictate their melting and boiling points. Their strength depends on the number of electrons that each atom delocalizes into the sea of delocalized electrons that surround the nuclei as well as on the way the atoms are packed in lattices. Metals are good conductors of heat that is picked up as additional kinetic energy by the electrons and then transferred throughout the rest of the metal.
_____Most metals are malleable (can be beaten into sheets) and ductile (can be pulled out into wires) as their atoms can easily roll over on each other into new positions without breaking the metallic bond. The changing of the metal’s shape without cracking or breaking it is possible because the metallic bonds are strong but not directed between particular ions. In metals, the atoms are not lose, but arranged according to their specific properties in lattices made out of repeat unit cells having various shapes, Fig. 3.15.
_____Only crystalline materials have periodic structures as non-crystalline materials (glass, polymers) lack periodicity. There are a limited number of unit cell types: if we represent the atoms as spheres, the most common are the face centered cubes (FCC) and body centered cubes (BCC).
_____Iron, a basic metal in most attachments, has at room temperature the last unit cell type. This structure, known as austenite, undergoes while it cools an important transformation, Fig. 3.16.
_____Heated between 910o C (1670o F) and 1394o C (2541o F) iron’s structure transforms to FCC. Cooled under these temperatures, it returns to a BCC structure. At high temperatures, iron’s FCC unit cell center is free, prone to be occupied by other, extraneous atoms. Such an atom is carbon, element that in small amounts transforms iron in steel. At higher temperatures, carbon occupies iron’s FCC vacant cell’s center, when the alloy is cooled, it is displaced and pushed out toward an extremity, Fig. 3.17.
_____While the new unit cell is still body centered, it is no more cubic, as the entrapped carbon atom lengthens its shape in just one direction. This lengthened cell structure is known as martensite, and is characterized by an increased strength, duplicating the situation of a soft, overfilled suit case. This transformation plays a major role in steels, alloys that contain iron and small quantities of carbon. Interesting to note, “martensitic transformations” takes place from within the materials, without any addition or subtraction. Such transformation occurs also in plastics, ceramics and even in living microorganisms.
_____Stainless steels and some nickel titanium alloys exhibit similar structures and undergo the same transformations as carbon steels. In stainless steels, the difference is that in their unit cells some of the iron atoms have been replaced by chromium, while in nickel titanium, iron is replaced with Ni and Ti atoms. This FCC structure, stable at higher temperatures, can easily host oxygen or carbon; when cooled, the generated BCC structure leads to a tough material, see Fig. 3.18. This is a reversal of the softer, martensitic structure found in the absence of extraneous, smaller atoms.
_____As a difference from austenite’s homogeneity, which renders it non-magnetic, martensite exhibits a polarization that makes its structure uneven and ferromagnetic. To take advantage of their remarkable homogeneity and corrosion resistance, austenitic stainless steels are rendered stable at lower temperatures by adding “austenizing” elements such as nickel or nitrogen (the latter along manganese).
_____Interestingly, due to the increasing demand for “mini” brackets (which have thinner slot walls); manufacturers prefer today the strong martensitic steels to the corrosion resistant austenitic ones.
_____Another important transformation is twinning. At small stresses, the metal’s layers of atoms will start to roll over each other, Fig. 3.19. If the stress is released, they will fall back to their original positions.
_____Under these circumstances, the metal is said to be elastic. If a larger stress is put on, the atoms roll over each other into a new position, and the metal is permanently changed. In some cases, small stresses move the atoms only a fraction of an inter-atomic space, leading to a rearrangement of the lattice structure called twinning. Bearing the name “McBain transformation,” it is a particular case of the martensitic transformations encountered especially in minerals.
_____The old lattice arrangement is continued by a similar one that starts this time under a different angle. Instead of continuing a face of one of the unit cells of the old lattice, the atoms in the new lattice continues the cells’ diagonal plane, Fig. 3.20. With shape memory alloys such as NiTi, instead of a single change of direction, the twinning can be multiple, being generated by stress or change of temperature within the “Transition temperature range.” As in steel, the phase found at higher temperatures is called austenite, while the one stable at lower temperatures, martensite. In steels, this transformation from one structure to the other is irreversible while in shape memory alloys, it can go indefinitely both ways.
_____In grains, groups of atoms are bound together in a regular geometric pattern. When cooling a melt metal, the atoms within each growing grain are lined up in a specific pattern leading to specific structure known as crystals. In their growth, each grain eventually impinges on others and forms an interface where the atomic orientations are different, see Fig. 3.21.
_____Isolated, single metal crystals cannot be obtained except in special conditions; used as “whiskers” to reinforce composites, these are the metal’s toughest form. While these mismatch areas (grain borders), see Fig. 3.22, may reduce strength, overall most mechanical properties improve as the size of the grains decreases. The reason is that the rolling of layers of atoms over each other (known as slip) is indeed hindered by grain boundaries, because there the rows of atoms cannot line up properly. In other words, the more grain boundaries there are (and the smaller the individual crystal grains), the harder the metal becomes. A similar situation takes place when fine particles (added metals) are added; this is actually the recipe for the type of stainless steels most used for “mini” brackets. Among these, the most used (e.g. Ormco Mini Diamond) is PH 17-4 (Precipitation Hardening, AISI 630).
_____However, because in the mismatch areas the atoms are not in sufficient contact with each other, metals tend to fracture along the grain boundaries.

_____Increasing the number of grain boundaries not only makes the metal harder, but also it makes it more brittle. Heating a metal tends to shake the atoms into a more regular arrangement that decreases the number of grain boundaries and thus renders the metal softer.
_____Cold working produces small grains, therefore makes a metal harder. To restore its workability, it is necessary to reheat it. Using magnification, the individual grains and areas of mismatch can be observed if the metal surfaces have been finely polished and attacked with various reagents, see Fig. 3.23.
_____As there is no metal device that is made of a single type of atoms, it is common practice to disturb their regular arrangement by inserting atoms of a slightly different size into the structure. Alloys such as brass (a mixture of copper and zinc) are indeed harder than the original metals because the irregularity in the structure helps to stop rows of atoms from slipping over each other. Smaller atoms such as nitrogen or carbon lead to similar results, as it is the case with the Ion Guard™ (GAC) arch wires. Evidencing grain size, composition and boundaries, metallography provides important information on any piece of metal as it will be shown below.
_____Phases, impurities and precipitates.
_____Solute, large atoms such as Cr, Ni replace or substitute for iron atoms in stainless steels. In contrast, the elements oxygen, nitrogen, carbon and hydrogen, which are frequently present in steels, form interstitial solid solutions in which the solute atoms are located in the “holes” or interstices between the steels’ atoms. As the insertion of extraneous atoms in a certain lattice has a beneficial effect, hindering the sliding against each other of the metal layers (i.e. resisting plastic deformation), all devices are made of alloys. In some instances, the proportion of the additional elements is small, in other large enough to lead to distinct, separate phases. Due to poor affinity, such mixtures remain immiscible in both liquid and solid-state. A phase is defined as a homogeneous part or aggregation of material that differs from another part due to difference in structure, composition, or both. The difference in structures takes place at the interface between adjacent or surrounding phases. In an alloy with several phases, these can be seen under magnification as separate islands; when attacked with acids, these of one metal are dissolved more than the other. In contrast, stainless steels exhibit a single phase: under acid attack, the dissolution takes place unit cell by unit cell. As shown in Fig.3.24, the ratio between dissolved iron, chromium, and nickel corresponds to that existing in the steel attacked.
_____While metal atoms usually play a beneficial role by forming particles that hinder the matrix’s layer from sliding, some nonmetals act conversely. Thus, manganese sulfides and serenades as well as sulfur and phosphorus form thin layers at the interface between the grains, interfering with the metal’s cohesion6 . As a result, before the advances that took place half a century ago (argon-oxygen decarburization), all steels were weaker. As the machining of hard materials is expensive, some manufacturers have chosen to add sulfur to steel. The price paid for an increased corrosion susceptibility are in this case higher machining speeds, lower power consumption and savings on blades In Germany, resulfurized free-machining stainless steels are not allowed for dental use7 .
_____3.4. Attachment manufacture vs. quality
The metals used today to manufacture brackets and bands are stainless steels and titanium. In addition, for arch wires, cobalt-chromium and nickel-titanium alloys8 are used.
_____The most sophisticated and complex attachments are the direct bonding brackets, for which three methods of manufacture are. The oldestmethod of manufacture is based upon the milling (or machining) metal bars to brackets, and then brazing to them mesh laminated to foil, Fig. 3.25. The process is simplified if round bars are used as raw material: while still on the lathe, it is feasible to cut grooves into the pad, as shown in Fig. 3.26.
_____Recent methods comprise a turret in which a computer-controlled cutting tools assembly (CAM) can machine a tube to generate intricate rings that have numerically controlled dimensions (CNC). The rings are then cut to brackets having intricate surfaces that are finished in a series of tumblers. While a decade ago only softer alloys could be machined, even harder cutters allow the processing of very hard steels such as AISI 316 or 18-18 Plus. In general, the metal resulted is dense, and the method leads to good brackets.
_____High precision micro-casting uses an improved version of the millennia–old “lost wax” process. The molten steel is forced under a reducing atmosphere in a mold made of a fine refractory powder that was previously filled with a polymer pattern shaped like a tree, as shown in Fig. 3.30a. The brackets or tubes resulted which can be seen at the end of each of the sprues “branches”, Fig. 3.30b, represent only a fraction of the volume of metal processed. If improperly controlled, the exposure to heat may damage the metal’s structure. As cast slots are not accurate, these have to be cut in another operation. As in some designs a base has to be added through brazing, it is easy to see that in the future the method will not be competitive.

_____Metal injection molding (MIM) is, at least economically, the most promising method to produce large quantities of brackets. Metal powder as fine as less than 5 microns is mixed with lubricants (stearic acid, its salts, synthetic waxes) and thermoplastic polymers (polyethylene), heated and then injected into metal mold as shown in Fig. 3.31.
_____This process is different from plastic injection as the typical MIM feedstock is ten times less viscous and more thermally conductive.9 In other words, the simultaneous “flowing and freezing” in the mold has to be very closely monitored, as distortions, cracks, voids and short shots can easily develop. The “green” that results has to be hard enough to withstand the subsequent operations, the first being the lubricant removal or “debinding”. Since any carbon-containing deposit has an adverse effect on corrosion resistance, the organic materials “burn-off” (425o C, 800o F) has either to be total, or in combination with solvent extraction. The next step is the sintering of the fine particles, i.e. their heating near and below the melting point of the alloy. The coalescence of the particles is carried out at 1150-1260o C or 2100-2300o F, in either vacuum or under an inert atmosphere (nitrogen or dissociated ammonia). Both batch and continuous mesh belt furnaces are used. During cooling, carbides and nitrides tend to precipitate, lowering the corrosion resistance (intergranular corrosion); as a result, quenching is often used.
_____As the "green” is 20% larger than the bracket, Fig. 3.32, it has to be strong enough to be further subjected to a „debinding” process at 425o C/800o F. In it, the bracket shrinks while the organic material is gradually removed (volatilized). After being subjected to a temperature somewhere under the alloy’s melting point (1150-1260o C/2100-3200o F), the metal particles coalesce (sinter). The rough bracket is then burnished to obtain shine. While it allows processing any type of alloy, it leads to rough surfaces and to a less dense metal (usually 96% of the machined ones). Being the most economic method and leading to fairly acceptable products, MIM will be the way of the future.
_____As both injection molding and casting use molds from which the bracket has to be easily removed, it is difficult to get bases with pre-formed undercuts. Intricately designed molds 10, 11 by themselves, no matter how sophisticated, cannot provide enough adhesive retention. These are usually provided later by cold work, i.e. by pressing or crushing protrusions purposely made on the base. By bending these toward the inside of the grooves12 , adhesive locks can be generated. Another method generates a multitude of posts rising from the base. Their crushing (or coining) leads to an aggregation of mushroom-shaped buttons.13
_____References
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8. Kusy RP, A review of contemporary archwires: their properties and characteristics. Angle Orthod 1997; 67: 197-208
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10. Viglietti J., U.S. Pat. 4,604,057, 1986;
11. Webb DE., Andrews L.A., U.S. Pat. 4,349,334, 1982
12. Miller FR., U.S. Pat. 4,165,561, 1979
13. Schmitt R., U.S.Pat. 5,267,854, 1993