Factors influencing your attachments

____Today, most orthodontic appliances are made from several stainless steels either by machining bars or tubes, or by using molds. (Both steels¹² and manufacturing procedures¹³ have been discussed in previous issues). During their manufacture and use, metals of various compositions and structures are subjected to a series of operations involving cold work, exposure to heat or to particular treatments. All these leave a mark on the attachments that can be traced well with the help of metallographic studies.
I. Composition
____Metals. Pure metals are always mechanically weaker than their alloys. Purposely added metal(s) can generate lattice alterations by substituting some of the matrix atoms, or form inert obstacles in the sliding path of its atomic planes. In both cases, the resulting friction renders the metal more resistant to deformation, Fig. 10. When the added metal(s) does not participate in the matrix, it remains as particles forming a dispersed phase, as shown in Fig. 11. Carbide inserts exhibit a similar structure to that of Ti-6Al-4V, but with larger and more numerous particles: the dispersed phase is tungsten carbide and the matrix is cobalt. Figure 12 illustrates a composite (the proportion of the added metal exceeds 10%) subjected to rolling (see below). Due to its fibrillar structure, the added metal becomes a reinforcement in this composite metal-metal.
Nonmetals. Smaller atoms, like C or N, can participate in the matrix and harden it by stressing the unit cells (as seen in our past issue). Surface nitriding is thus used to harden and at the same times seal NiTi arch wires (Ion Guard® by GAC).
____Nonmetals such as sulfur, phosphorus and other compounds (carbides) can weaken the matrix by occupying its interstices, by forming inclusions, Fig. 13, or by gathering at the grain boundaries, as shown in Fig. 14. In all cases, a decrease in coherence occurs. (Some manufacturers, who want to save energy and blades while machining brackets, use AISI 303 stainless steel, disregarding the ensuing increased corrosion susceptibility: its content in sulfur is five times that of both AISI 304 and 316 which are commonly used for the purpose).
____In some instances, the spaces between grains are filled with gases which are not absorbed. In metal injection the most recent method to make brackets, the metal powders are heated under isostatic pressure to a temperature just a few degrees below their melting point, when these coalesce and bond to each other. In time, these bonds become tighter and the pore volume shrinks, as shown in Fig. 15. At the end, as shown in the micrograph of a sintered metal fracture, these become spherical. The densities achievable are of commonly only 94-96 % of the wrought metal.
II. Cold work
Subjected to forces above their yield point, such as under stress or during machining, metals suffer plastic deformations. These occur through slip (when the layers of crystals are displaced with respect to the others, Fig. 10), twinning (when the slip planes change in direction, dividing the lattice in symmetrical parts differently oriented), or end as fracture. In a previous issue¹⁵, we have examined both twinning (and especially the multiple twinning occurring in NiTi wires). Less understood is, however, metal cracking, phenomenon which occurs under the influence of chemicals or environments. In Fig. 16 are presented examples of trans-granular and intergranular cracks in the austenitic stainless steels commonly used in bracket manufacturing. Also shown (Fig. 17) is the mechanism of intergranular corrosion which not only complements cracking, but commonly follows the sensitization of stainless steels (see below).
____Subjected to rolling, i.e. to a reduction in the sample's section, not only the added, non-alloyed metal suffers changes, as shown in Fig. 12, but also the grains of the matrix become longer and equiaxed, as shown in Fig. 19.
____Intense cold work reduces the size of the grains and strengthens the metal, rendering it more corrosion resistant. A highly deformed metal becomes not only harder, but at the same time quite brittle (as it is well exemplified by the Wilcock's Australian Elgiloy® wire which can be bent only with special care).
____Interestingly, cold work has been reported to increase the intergranular corrosion susceptibility of the AISI type 304 of steel at lower temperatures, while decreasing it at the elevated ones¹⁴. The mechanism, which may be shared also by other types of steel, is not well understood.
III. Heat treatment.
___ Elevated temperatures cause grains to unite and grow, a process which can be stopped by quenching, i.e. subjecting the metal to low temperatures. As mutual solubility decreases with temperature, a common procedure to increase steel strength is to add other metals (Al, Mo, Ti, Nb) which are soluble at higher temperatures but precipitate as microscopic particles when the metal is cooled. As show in Fig. 10, these hinder the sliding of the atomic planes opposing resistance to deformation. The procedure is currently used to make "Precipitation Hardenable" steels (such as PH17-4, currently used to make "mini" brackets).
____While carbon is needed to give iron the strength of stainless steel (see our previous issue), it is also a liability. Indeed, instead of remaining "trapped" in the lattice's unit cells, within a certain temperature range it combines with chromium giving a carbide which diffuses at the grains boundaries, as shown in Fig. 14. "It is well known that austenitic steels are metallurgically unstable when heated in the temperature range of 350-800^oC (650-1500^oF). 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"¹⁶.
____In an interesting experiment, a bar of 18-8 stainless steel having 0.08% carbon has been severely cold-worked first and then had one of its ends immersed in cold water while the other was heated to almost fusion. The bar had thus one end cold worked, the middle heated in the carbide precipitation range and the other end annealed. Subjected to a chloride solution, only the middle portion was noticeably attacked¹⁷.

Bracket microstructure revealed

Influence of composition. Stainless steels not only rust, but also can be heavily attacked even by what we and drink. While their elemental composition cannot be assessed from microstructures, it is obvious that a solid solution, such as that exhibited by the austenitic stainless steels, will lead to a better resistance than a structure involving several phases. Galvanic corrosion doesn't only apply to the joints of different metals, but takes place even at a microscopic scale, i.e. between grains having a slight difference in electric potential.
____In our previous articles^18-20 we have discussed the behavior of different brackets in a dilute solution of hydrochloric acid.
Influence of surface finishing. Rough surfaces not only offer a larger area to attack, but also can host in their fissures and pockets an environment which is often more deterrent than the environment itself. The influence of surface finishing will be, however, discussed in a future issue.
Influence of cold work. A combined direct bonding bracket (bracket, brazing, foil and mesh) was subjected to the uniform attack of a diluted (10%) solution of hydrochloric acid. Its most delicate part, the mesh, proved to be the most resistant part, as a result of the higher amount of cold work involved in its making. Second in resistance was the foil: the relatively massive bracket was found considerably thinned.
Influence of sensitization. Austenitic stainless steels ____exhibit a microstructure where the grain boundaries are difficult to see, as the micrograph of a Rocky Mountain Bioprogressive® bracket shows, Fig. 20. Subjected to temperatures in the sensitization range (due to brazing or thermal recycling), a chromium carbide film forms around part of the grains, Fig. 21 and 22, decreasing their coherence. At the same time, the grains become depleted in chromium, a metal of which oxide is responsible for rendering the steel "stainless".
Influence of cold work and heat treatment. We ____subjected a combined bracket to 450^oC, i.e. within the sensitization temperature range for half an hour, and then to concentrated nitric acid at 20^oC. An oxidizing agent, this acid is commonly used to passivate stainless steel which it does not attack. After exposure, the bracket was transformed in a fine, shiny powder, while the foil and the mesh., i.e. the most cold- worked parts, were recovered unscathed, Fig. 24.
Other precipitates. In some microstructures, such as these shown in Fig. 25 and 26, punctiform precipitates can be found. As these are not situated at the grain boundaries, these cannot be made of chromium carbide, as it was shown in Fig. 14, 22, and 23. As we found them in an AISI 303 steel (see above), a connection with its increased content in sulfur has been done. As sulfur does not form such precipitates, being quite soluble in the steel matrix, it is more likely that these are made of manganese sulfide or niobium carbide.
____While the first compound may cause this structure in Ormco's Diamond brackets, Fig. 25, the last one may be the cause for the dots found in Ormco's Mini Diamond® brackets which have less sulfur, but has niobium for added strength, Fig. 26.
Influence of grain size. ____In general, the larger the grain, the lower the mechanical strength. Cast brackets such as the one presented in Fig.27 are characterized by large grains: their mechanical strength, as we determined it by measuring microhardness, was the lowest⁷. In contrast, the brackets having fine grains, like the milled ones shown in Fig. 28 and 26, are stronger (for the latter, the mean value of the Vickers Hanemann is 335.5 kgf/mm² vs. 159,1⁷). In terms of tensile strength, this translates in about 158 vs. well under 100x 10³ psi.
Influence of drawing. More recently, brackets are made of tubes which are drawn, a commonly encountered form of cold work. Due to the trend toward less nickel, a known allergenic, steels such as 22% Cr, 5% Ni are preferred. The decreased amount of nickel, a known "austenizing" element, is responsible for the partial conversion of austenite (the phase stable at higher temperature) in ferrite. The two phases along with the elongated structure of the grains are shown in Fig. 29.
Influence of molding. ____As shown¹³, brackets can be made either by milling bars or tubes or by using molds. The processes using molds are casting, launched by "A"-Co. two decades ago, and injection molding, initiated by Ortho-Organizers almost a decade ago. As the first process is expensive and the second quite economic, it is easy to foresee that injection molding will replace casting.
____In both these processes, the metal is not cold worked, which leads to a less compact structure. In the case of casting, due to the cooling of the poured molten metal in the molds, contractions can take place, as is the gap shown in Fig. 30. Less extensive as size, but of normal occurrence are the pores in sintered brackets, as seen in Fig. 31.
Influence of brazing. Combined direct bonding are assembled with the help of brazing materials which are either gold- or silver based. Only few of the latter type have withstood the test of time, as in most instances these have allowed bracket-base separations. In Figure 32 the silver -brazing layer penetrates in some extent the steel in both the Microloc® base and the bracket, leading to a good bond.
Conclusions
____While the inspection of the attachment's surface is quite useful (as we will show in a future issue), it cannot provide information about what ultimately decides its mechanical strength and corrosion resistance. Faulty microstructures are often the culprit in many mechanic failures, as phenomena at a microscopic level determine the attachment's behavior. While the orthodontist does not have the means nor the necessary experience to makes such studies, the knowledge involved should help him understand and prevent failures as well as to make better selections.
____As in many other instances, the conclusion which can be drawn is that we live in an imperfect world: the brackets which are highly corrosion resistant, such as those made of AISI 316L (e. g. "A"-Co.'s Standard edgewise) are the least strong from the mechanical point of view. Likewise, those which are economic to manufacture (such as the sintered ones obtained by injection molding) are less dense, but can be made of stronger steels which are difficult to machine.
References
1. Khier SE, Brantley WA, Fournelle A, Properties of as- received and heat-treated stainless steel wires, Am J. Orthod. Dentofac Orthop 1988; Mar. 206-212
2. Donovan MT, Lin JJJ, Brantley WA, Conover JP, Weldability of beta titanium wires, Am J. Orthod. Dentofac Orthop 1984; Mar. 207-216
3. Mueller HJ, Some considerations regarding the degradational interactions between moth rinses and silver-soldered joints, Am J. Orthod. Dentofac Orthop 1982 Feb :140-146
4. Grimsdottir, MR, Gjerdet NR, Hensten-Petersen A, Composition and in vitro corrosion of orthodontic appliances Am J. Orthod. Dentofac Orthop 1992 Jun; 525-532
5. Buchman DJL, Effects of recycling on metallic direct-bond orthodontic brackets, Am J. Orthod. Dentofac Orthop 1980; 77: 654-668
6.. Matasa CG, Orthodontic attachment corrosion susceptibilities J. Clin. Orthod. 1995; 29(1): 16-23
7. Matasa CG, Metal strength of direct bonding brackets Am. J. Orthod. Dentofac Orthop 1998; 113: 282-286
8. Kehl GL, The principles of metallographic laboratory practice, Mc Graw-Hill Ny 1949
9. Vander Woort GF, Metallography, principles and practice, Mc. Graw-Hill, NY 1984
10. ASM Handbook, Vol. 9 Metallography and microstructures ASM, Materials Park, OH 1985
11. Reaumur RAF, L'art de convertir le fer forge en acier, Paris, 1722 (from Martensite, Olson GB ed., ASM International, Materials Park, OH, 1992
12. Matasa CG, Bracket metal is not the same, Phoenix without ashes 1991; 4(3): 2-4
13. Matasa CG, Milling, casting or injection molding? The ortho-dontic materials insider 1996; 9(1): 1-7
14. Tedmon CS, Vermilyea DA, Corrosion 1971; 27(3): 104
15. Matasa CG, NiTi alloys, two metals in one. The orthodontic materials insider, 1997;10(1): 2-7
16. Franks R, in Corrosion Handbook, Uhlig HH editor, J. Wiley &Sons, NY 1948; 161
17. Aborn RH, Bain EC, Nature of the nickel chromium rustless steels, in The metallurgical evolution of stainless steels, Pickering FB, editor, American soc. for metals (now ASM, Materials Park, OH) 1979: 299-335
18. Matasa CG, Nicht rostende Edelstahle und Direkt - Bonding Brackets. II Chemishes Verhalten Informationen aus Orthodon-tie und Kieferorthopadie 1993; 25(2): 147-166
19. Matasa CG, La corrosion des verrous: un defi pour l'ortho-dontiste, Actualites Odonto-Stomatologiques 1994; 187: 401-409
20. Matasa CG, Orthodontic attachment corrosion susceptibilities, J. Clin. Orthod. 1995; 29(1): 16-23
21. Davis HE, Troxell GE, Wiskocil CT, The testing and inspection of engineering materials, 2nd ed., McGraw Hill, NY 1955

MANY THANKS TO THOSE WHO HAVE SUPPORTED US !

____Half a year ago, NBC’s TV “Dateline” had broadcast a report on orthodontic recycling entitled “What is hiding behind your children’s smile”. Six people filmed for a full day, invited by Ortho-Cycle Co. Unable to find bad things about our recycling, NBC decided to focus the clinicians who are using recycled attachments. The arguments were, in the words of the orthodontist who spoke up for recycling, Dr. Douglas D. Durbin, “the lowest form of attack”. The accusations were so full of deceptions, half truths, omissions and innuendos that it prompted us to respond by writing in this newsletter an article entitled “Move over, Machiavelli !!! What NBC knew, and didn’t tell you”.
____In the months which followed, we have received many related telephone calls and letters. Quite a few praised our activity, others our stand, and some both. We were surprised to see that many clinicians took the attacks to themselves, even though they were not recycling their attachments. While “Another attempt to tell us what to do and what to think” was the most common leitmotif, the second was“Keep on doing your good job”.
____We cannot assess the impact the mentioned broadcast had on the profession. We can positively state, however, that it had a negative PR-impact on the relations between clinicians and some of their suppliers, and made us unwilling heroes in the process. Thanks again!.