___.At the 99th AAO Annual Meeting in San Diego, CA, more companies than ever before exhibited gold-looking attachments. Aside from coating brackets, gold and nitride layers have now been applied to a variety of arch wires and tubes using different procedures. Assessing the trend, we have presented there the first poster analyzing the variation in corrosion susceptibility of these not yet debated alterations of metal attachments. The poster aroused the interests of manufacturers, clinicians and scientists alike (see Fig. 1)
___.In what follows, we will present part of this study. To abide by AAO's conventions requirements, no brand names were used.
ABSTRACT
___.Several types of coated brackets were examined from the point of view of their corrosion susceptibility as well as their aspect after wear. Various attacking agents were used and their effect measured with the help of the hydrogen evolved from the attack, and the amount of metal leached. The attacking agents were 5% hydrochloric acid, 1% lactic acid (Japanese standard) and two solutions of lactic acid and sodium chloride (one of these being an ISO standard). The metals leached were analyzed quantitatively by a specialized labortory which determined nickel by spectrometry, as well in house by using both colorimetry and gel chromatography. While in the latter cases the metal determined was iron instead of nickel, in the particular case of stainless steel, this becomes possible due to structural reasons (the two elements are leached in a constant ratio). Gold-coated brackets, irrespective of the type tested, showed a significant increase in corrosion resistance, while these coated with titanium and zirconium nitrides, a rather limited one. While in some instances the methods used lead to divergent results, such do-it-yourself tests can help the clinician to disclose and avoid the use of allergenic attachments.
INTRODUCTION
___.Today's metallic orthodontic brackets are far from being perfect: while the stainless steel ones are corroded in the oral environment, the titanium-made are unaesthetic. In addition, both types exhibit a higher coefficient of friction than it is desirable. This has led to their coating with materials ranging from diamond-like, amorphous carbon, to inorganic opacifiers, or from gold to the nitrides of titanium (TiN) or zirconium (ZrN). Initially selected for their aesthetic look, these coatings are claimed to provide corrosion resistance and decrease friction.
___.Out of the available coatings, those based on gold can in most cases protect the attachments even if the film deposited is not impervious and homogeneous. Both titanium and zirconium nitride films, if the ratio nitrogen to metal is high enough, decrease the corrosion susceptibility of the attachments when compared with uncoated controls. However, if this ratio is lower than the stoichiometric one, i.e. if unreacted titanium is deposited on the steel surface instead of just its nitride, some galvanic corrosion may also occur.
___.While the electrolytic deposition of gold is claimed to have been practiced by the Precolombian civilizations, the Physical Vapor Deposition (PVD, same abbreviation as for Peripheral Vascular Disease, term common in medicine) of titanium and zirconium in films up to few micrometers is new and lesser known. Sketches of the principle is shown in Fig. 1 where the brackets are located in tumbler/cathode. A spin-off from the recent development in high-energy radiations, such as laser and electron-beams, PVD (Fig.2) is becoming more common, being used today to coat drill bits, watch and pen parts, lighters, knifes, plumbing fixtures, etc.
___.Generated in partial vacuum by sputtering a metal target (titanium or zirconium in our case) with the help of radiations, part of the metal’s ionized vapors react with nitrogen. The produced nitride that results is deposited on the substrate’s surface, in our case on the brackets placed on a convenient support. The resulting films are hard, resistant to abrasion and chemical attack and can take on a wide range of colors according to the ratio metal to nitrogen.
___.Thin TiN and ZrN structures are influenced by such deposition parameters as substrate composition and surface structure, nature of the source and deposition temperature, as well as composition and pressure of the nitrogen atmosphere. As a result, the structure of the films is often less perfect than that of the bulk material: smaller grains, higher dislocation concentrations and deviations from stoichiometry are typical. “Island films” can generate zones of alternative corrosion resistance. In addition, less-than-stoichiometric compositions, intentionally used to brighten the films, contain either unreacted Ti or Zr. As a result, these films may generate galvanic corrosion phenomena in contact with another metal (the substrate). Indeed, gold-brazed brackets are known to lead to a galvanic corrosion of the stainless steel substrate⁷, while those assembled with the help of a gold-nickel brazing, to a dealloying process. In both pheno- mena, nickel is leached⁸.
___.A comparison between the films' Vickers hardness and several substrates such as stainless steel, tungsten carbide (used in cutting inserts) and alumina is shown in Fig.3. In addition, these coatings are biocompatible or inert. At the 17^th Biomaterials Conference, May 1991, two studies have shown that “titanium implants, with or without TiN, elicit a comparable tissue response” (Mortimer & al.), and that “surface modifications using nitrides do not significantly effect the biological properties of metal implants” (Wang & al.).
POSSIBLE SIDE EFFECTS
___.Theoretically all the coatings mentioned should constitute a barrier against the leakage of heavy metals from orthodontic attachments. It is well known that both nickel and chromium are allergenic agents: some sources indicate that some 16.9% in males and 23.8%, in females¹, or even 28% and 31.9%⁵, respectively are allergic to nickel. Released from loosened prostheses, nickel has been found to cause necrotic phenomena which can be associated with the inflammatory reaction of the surrounding tissues^2-4. At the same time, nickel is an important factor in pathogenesis, even at very low concentrations⁵. Stainless steel corrosion products accumulate in the spleen, where they may cause histological alterations and induce changes in cellular populations, and in particular in lymphocyte production⁶.
I. CORROSION SUSCEPTIBILITY MATERIALS AND METHOD
___.a. Brackets and coatings.
___.Two types of new stainless steel, edgewise-type brackets were tested. The criterion used for selection was their high corrosion susceptibility of the metal used in their manufacture (such as AISI 303), that was determined in a previous study¹. The brackets marked (A1 and A2) were four part or combined, i.e. comprising bracket, brazing, foil and mesh, while in the other, one-piece or monoblock (B). After keeping controls, these brackets were sent to two specialized laboratories which provide attachment coatings both to clinicians and bracket manufacturers. In addition, a few titanium brackets were also sent. All the brackets were new and similar in size (upper laterals), but probably produced in different batches. Some of the brackets were gold electroplated, and some PVD coated using either a stoichiometric TiN and ZrN film, as well as with a less than stoichiometric TiN film. The last coating was tested under the designation NSTiN (non-stoichiometric, i.e. containing non-reacted Ti). The gold and nitride coatings are easy to differentiate: while the gold plating is applied over the whole bracket, in a tumbler, see Fig.1, the PVD coating is depo-sited only on its labial side.
___.All the brackets tested were subjected to a treatment common in metallurgy which has as its goal to degrease and equalize the different degree of passivity which these may have had. The attachments were placed in mesh containers and submersed in trichloroethylene. After drying, these were soaked for half an hour in an alkaline bath (5% sodium hydroxide, at 160-180^oF). Rinsed with water, these were again submersed for another half an hour in an oxidizing solution (nitric acid of 20% volume added with 3 oz./gallon sodium bichromate at 120-180oF).
___.b. Measurement of the hydrogen volume released by the attack with muriatic acid (HCl)
___.Attachments treated as shown were subjected to an accelerated corrosion test as already described1. Its principle is based upon the fact that most metals (Me), if attacked by an acid, generate a salt, while hydrogen is released:
___.2Me + 6HCl = 2MeCl₃ + 3H₂
___.According to the test, a single bracket is immersed in a bath containing a diluted hydrochloric acid solution, and atop of it is placed a graduated test tube filled with the bath liquid. In this way, the hydrogen generated from the brackets’ attack displaces the liquid and can be readily measured.
___.For a better visualization of the volume of hydrogen gathered at the top of the graduated test tube, Fig. 4, traces of an acid resistant dyestuff were added. The hydrochloric acid solution /distilled water volume ratio was 1:10, the acids’ initial concentration being between 35-38% (Fisher Scientific, “trace metal grade”, CAS 7647-01-0). The distilled water used was equally free of metals (CAS 7732-18-5). After its first use, this solution cannot be reused for testing, as metal impurities resulting from its previous use alter the speed of the attack.
___.c. Measurement of the hydrogen volume released by the attack with the concentrated ISO’s standard solution
___.The test was conducted in the same way as the previous one, with the difference that another attacking agent was used. Indeed, as the corrosive solution recommended by ISO standard for the testing of dental cast alloys is too dilute for accelerated corrosion purposes (0.1 mol lactic acid and 0.1 mol sodium chloride per liter of distilled water), a solution ten times more concentrated was used.
___.d. Measurement of the nickel released by diluted lactic acid attack
___.In contrast with the mentioned ISO Standard, which cannot be applied to brackets, the Japanese standard¹⁰ has been specifically designed for the purpose. It subjects a single bracket to 50 ml of 1% lactic acid in distilled water (the Japanese solution) for a week, at 37^oC. The test tubes with acid and brackets are hosted in a constant temperature bath, as shown in Fig. 5. The leached extract is then digested with nitric acid (68%, “trace metal grade”, Fisher Scientific, CAS 7697-37-3).
___.After partial evaporation, the acid solution was sent to have its nickel content determined by atomic absorption spectroscopy. Performed by Spectrum Laboratories (Ft. Lauderdale-Savannah), the test was done using a Perkin-Elmer spectrophotometer with a Zeeman furnace module. [While the Japanese test mentions the use of flame atomic absorption (FLAA), the detection limits achievable by using a graphite furnace (GFAA) are significantly lower. The latter is preferred in the US as it provides a more accurate quantitative determination of the traces of nickel, a “most wanted” chemical according to EPA].
___.e. Evaluation of acid’s color change due to corrosion products
___.While subjecting the brackets to a 1% lactic acid solution for a week, as required by the previous test, we noticed an obvious change in color of the liquid. To provide further evidence, different brackets were placed along with 5 ml acid solution in a porcelain plate, left for a week at room temperature and then photographed. The change can be strongly enhanced if a reagent which forms a colored compound with the released ions is added. Unfortunately, both the reagents commonly used to evidence nickel (dimethyl glyoxime and dithioxamide) develop their color only in an alkaline medium, and therefore cannot be used to determine the progress of an acid attack.
___.f. Gel chromatography assessment of the corrosion products
___.All stainless steels are attacked in various degrees by the acid solution recommended by the above standard: the differences lie in degree. In a liquid environment, as time elapses, the corrosion products quickly diffuse, and the solution becomes evenly colored. When a gel is used, the process can be considerably be slowed down. If a reagent which changes color with the corrosion product is also added, the test becomes gel chromatography, a widely used analytical method. The method makes it possible to differentiate various brackets by the amount of corrosion products as evidenced by the size of the stains extended around the samples. This method may be used as a do-it-yourself approximation of the ISO standard: indeed, the solution is the same, with the exception of the addition of an inert gelifying agent and of a very small amount of a reagent which changes color in contact with the corrosion products. To reduce evaporation during the several days needed for assessing the stain evolution, Petri dishes with covers were used.
RESULTS
___.1. Volume of hydrogen released by acid attack
___.The volume of hydrogen released by the attack with both acids (hydrochloric and ISO's) of three types of coated steel brackets A1, B and A2, as well as of few titanium coated samples are shown in Fig. 6 as well as less detailed in Fig. 7-9 (together four experiments, noted from Exp. I to IV). To compensate for the variations in time, acid concentration and temperature, the volume released was evaluated comparatively within each of the four batches examined. To compare them, we have assigned arbitrary values to the hydrogen released which varied from +++ (large volume of hydrogen released, i.e. nonresistant, N), to ++ (medium or moderate volume, M) and + (small to - practically none, resistant, R). As shown, a large volume of hydrogen released signifies a high corrosion susceptibility, while none, or a small volume, a good corrosion resistance. As titanium attachments, coated or uncoated, led to the release of insignificant volumes of hydrogen, their testing was discontinued. The result of the experiments where the brackets were attacked with muriatic acid and with the ISO’s solution are centralized in Fig. 6-9.
___.2. Nickel released according to the Japanese standard
___.The solutions leached from the different brackets when exposed to 1% lactic acid for a week at 37^oC were sent for atomic absorption analysis to Spectrum Laboratories. The results are presented in the summarizing Table V.
___.3. Color changes due to bracket attack
___.If a bracket is immersed for a week in 5 ml of the solution recommended by the Japanese Standard, the solution discolors, see Fig. 10. The effect is enhanced if a reagent which reacts with the corrosion products is added to this system, rendering thus possible the photocolorimetric analysis of the corrosion process.
___.5. Gel chromatography
___.In the reactive gel previously described, the stains which develop around each bracket expand with various speeds which are proportional with the leached metal concentration. ___.As shown in Fig 11-14, some of the stains continue to grow, occupying the whole surface of the dishes within three to four days, while others will remain almost stagnant, as the amount of corrosion produced is limited. As in the previous qualitative tests, we have assigned for each bracket arbitrary values to the magnitude of the effect. Thus, we noted with +++ a large stain (which signifies a higher corrosion susceptibility, i.e. nonresistance, N), with ++ a medium-sized one (showing moderate resistance, M), and with + a small one (indicating resistance, R). The stains formed around the differently coated brackets are shown in Fig 11-13 and the assessment of their size in Tables I-III. Table IV summarizes the results.
II. ASPECT
Coated brackets sent for cleaning or sold to Ortho- Cycle during the last two years were compared with new ones using magnifications varying between 3X and 30X. Some of these were also subjected to Ortho-Cycle's cleaning process and then reexamined under the same magnification and compared with new controls. Due to the many variables already mentioned regarding the bracket and the coatings, as well as to the differences between the experimental conditions, the following results are preliminary as well as qualitative rather than quantitative.
___.1. Coated brackets, after wear
___.The attachments received were examined for inte-grity, shine and hue, being compared with new controls. The TiN coated ones have a reddish hue which may be used to differentiate them from the gold coated ones (beside the fact that the last ones are coated all over). At a higher ratio Ti or Zr/ N, this hue becomes brighter, to a point where some ZrN coated brackets mimic quite well the gold plated ones. In time, however, the TiN coated ones became more reddish, due probably to the prolonged action of chlorides in the mouth (titanium trichloride is red).
___.The examination of worn gold plated brackets sometimes shows exfoliation, the soft layer being abraded to leave behind the silvery aspect of stainless steel, as shown in Fig. 14. A more common case is the dulling of the coating, Fig. 15. In other instances, the tie wings may become dark colored, probably due to galvanic corrosion, as shown in Fig. 16. In less frequent cases, a combination of galvanic and crevice corrosion may lead to the dissolution of the stainless steel (crevices), see Fig. 17.
___.2. Coated brackets, after cleaning.
___.Subjected to Ortho-Cycle’s proprietary cleaning process, most attachments changed color, as shown when examined under magnification and compared with both new controls and brackets sent to be recycled. Those which were less influenced were these coated with gold. Almost all the PVD coated ones changed color. Thus, the TiN coated ones became more reddish, while some of the ZrN coated attachments lost their film and returned to their original, stainless steel look.

DISCUSSION
___.There are no generally accepted corrosion tests for coated or uncoated orthodontic attachments. Their testing is important, as corrosion resistant coatings could allow the use of lower grades of steel and a subsequent reduction in the attachments’ price. Indeed, this could allow the use of brackets low in expensive metals such as Cr, Ni, Mo, or/and highly porous, as produced by metal injection molding, MIM. To determine the influence of the coatings studied, this preliminary study has used little known methods: hydrogen release, a Japanese standard, colorimetry and gel chromatography. While the first one has already been described1, the last two have never been tried. Less accurate, their main advantage resides in the fact that the latter are friendlier and less expensive (even of the do-it yourself type) than the Japanese one which uses atomic absorption spectroscopy.
___.In selecting the attachments to be coated, the general approach was based upon the idea that, if the coatings examined can reduce corrosion susceptibility, this could be best evidenced if applied on brackets known for their poor corrosion resistance. Another fact found was that there is a marked difference in the hydrogen released by brackets when attacked by acidic solutions.
___.The study has met difficulties due to the fact that in the US there are no regulations concerning bracket manufacture or alloy. As these can be changed at will, significant differences were found among attachments. Seemingly identical, these may have been made at different dates, in various batches and from dissimilar alloys. While such a testing may lead to less reliable results, the study may offer, however, a randomized view which may be preferable to a comparison limited to a single batch. The coatings themselves are only seldom homogeneous and uniform, and as the PVD coatings cover only the labial side of the bracket, these leave fully exposed to the attacking agents the large surface area of the base. The corrosion resistance of the brackets tested and their coated counterparts is summed up in Table V.
___.The tests based upon the hydrogen released. The acid solution based upon the ISO formula was milder and lead to different results than the diluted hydrochloric acid used. Indeed, cases when the attachment was resistant to this formula, but not when attacked with the last acid, are many, as seen in Fig. 9-12 and Table I. (In our opinion, the hydrochloric acid attack is more significant, as in the mouth attachments are less corroded by organic acids than by chlorides, known to be the stainless steels’ No.1 enemy). In this test, titanium attachments were practically not influenced by coatings, a fact which is important in view of the need to mask their unsightly appearance. While gold coated attachments were clearly more corrosion resistant, the PVD coated ones showed an improvement in corrosion resistance for Brand A1 only in the case of the hydrochloric acid attack. Brand B showed some improvement only against the attack by the ISO Standard solution; the rest remained almost the same.
___.Tests based upon the Japanese Standard. Compared to uncoated samples, both types of coating showed an improvement. The testing of a single bracket, as suggested by this standard, is prone to lead, however, to inconsistent results. This became obvious when Brand B was coated with titanium nitride: analytical errors or freak samples may lead to wrong conclusions. The coatings increased somewhat the corrosion resistance of both brands, especially when coated with gold.
___.Tests based upon gel chromatography. Examining Fig. 17 to 19 and the summarizing Table V, the results again shows that Brand B is by far less corrosion resistant than Brand A1, and some improvement in corrosion resistance may be expected when the attachments are coated with PVD films and especially with gold. While there is consistency between the behavior of the two brands and their gold coatings (except cases were the coating was found to be poor), there was not enough found in the case of the Ti and Zr nitrides.
CONCLUSIONS
___.Today, there are practically no restrictions applied to the manufacture of orthodontic attachments. The problems raised by the heavy metals these attachments can leach fully justifies their testing, in spite of the fact that practically there are no related standards in both Europe and America. Unfortunately, the results of this study are marred by a series of variables difficult to eliminate, and only qualitative instead of quantitative results can be obtained.
___.Among two opposite effects, a protection due to the addition of a corrosion resistant layer and the galvanic corrosion due to the contact of the metallic substrate with layers of metals or free metal-containing nitrides, the first seems to prevail. Gold-coated brackets, irrespective of the type tested, showed a significant increase in corrosion resistance, while these coated with titanium and zirconium nitrides, a rather limited one, partly due to the fact that their coating extends only on the bracket labial side.
___.Among the testing methods used, gel chromato-graphy, despite not showing consistent results when applied to PVD coatings, seems to be acceptable for testing the alloys brackets are made of. An inexpensive, do-it-yourself method, the test used can provide indications which otherwise are obtainable only with the help of atomic absorption spectroscopic analysis. Indeed, a single analysis of the nickel released, as recommended by two standards, is laborious and costs in average $100.
___.As in the case of ceramic brackets, where alumina crystals can stand forces greater than steel while brackets made of it are much more fragile, an increase in corrosion resistance of coated stainless steel brackets cannot just be presumed based upon the characteristics of the pure coatings, and has to be tested for each specific case.
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