___________________THE TRUTH...
____In our previous issue we have presented the influence of metal microstructure on the mechanical strength of the orthodontic attachments, stating that rough surfaces not only offer a larger area for chemical attack, but also can host, in their fissures and pockets, an environment which is often more deterrent than the surrounding oral one¹. In this issue, we will present evidence on the influence of their surface, as metallographic means reveal them. Unfortunately, there is little difficulty in finding culprits, as there is a steady trend toward a poorer quality.
___ Surface defects due to manufacture.
Powder sintering. To face a stiff competition (ten years ago, there were thirty manufacturers)², many direct bonding brackets are made today by injection molding. This process, based upon the sintering of metal powders (Fig.1), leads to attachments which have a rougher surface than the older ones which were either machined or cast³. If roughness is highly desirable in order to provide the base a larger surface area which helps bonding, it is undesirable on the labial side.
Grain separation. If exposed to an improper heat treatment, some of the stainless steel grains become separated by a film of chromium carbide, as examined in the first part of this series3 and as shown in Fig. 2.
____Both casting and injection molding lead to less finished slot surfaces than those obtained by milling, especially with diamond wheels. This can be readily seen in Fig. 3, where the difference in roughness of two brands is obvious.
____As a result, while some manufacturers sell their mold-made brackets as they result directly from the process; others include a supplementary operation - slot milling.
Poor finishing. Surface roughness or leveling is determined by the finishing procedure. The micro-peaks and -valleys exhibited are measured with a profilometer on specimens having specified sizes (see below), leading to diagrams similar to the one presented in Fig. 4. In Fig. 5 are shown the average roughness left after several finishing methods. Thus, precision casting leads to an almost rough surface as lathe cutting. Due to their size, orthodontic attachments cannot be fine-grounded or honed, but are either polished in tumblers or electro-polished.
____Neither of these procedures, however, can effectively affect the slot bottom. Indeed, the roughness of the slot bottom significantly increases friction by hindering the sliding of the arch wire. Thus, Kusy et al. have studied the surface roughness of orthodontic wires4. Very rough surfaces can cause high friction because of the contact and interlocking of peaks and valleys^5,6. While the apparent macroscopic contact area may be misleading⁷, we have been able to measure the real contribution of the peaks and valleys of the slot bottom to friction by using atomic force microscopy (AFM)⁸
Insufficient passivation. Stainless steel rusts: if for some reason, the thin, impervious and invisible layer which protects it is removed, attachments behave almost as if made of common steel. To provide proper protection, the manufacturer should subject stainless steel items to passivation. Over two hundreds of years ago, it was observed that iron is not attacked by concentrated nitric acid⁹: this passivity has been later attributed by Faraday to the formation of an oxide film¹⁰. While iron exhibits this property only in few instances, its alloys with over 11% chromium are protected in most situations. Known as stainless steels, these alloys form, if exposed to air or another oxidizing agent, a layer of chromium oxide.
____While stainless steels can get this film directly from their exposure to air, it often becomes necessary to generate a thicker film by purposely passivating them.This practice, recommended for resulfurized stainless steels such as AISI 30311 , it is not always performed - fact which leads to brackets which rust in the mouth (Fig. 6). This is particularly important whenever stainless steels are purposely added with nonmetals (e.g. sulfur in the steel AISI 303). The addition of sulfur aims at generating "free machining " steels: the disruption generated within the metal's microstructure facilitates milling. Indeed, the cutting of such alloys requires less energy and saves blades.
____Surface defects due to material.
Not all metals or ceramics lend themselves to provide homogeneous, smooth surfaces.
Steels. Aside from injection molded parts which we have examined, even some milled brackets exhibit impurities visible under a slight magnification (Fig. 7). Some steel appliances, while initially smooth and shiny, are readily attacked. This is rather common with the "Free-machining" grades mentioned as their sulfur and sulfides content generates discontinuities in the surface of the machined parts. Still accepted in the US, these steels are presently phased out in Germany¹².
____Titanium. Unfortunately, this highly biocompatible metal lacks shine and exhibits a comparatively rougher surface than stainless steel. The reason is that the raw material from which these are coming from, is not a compact and dense bar, as in the case of milled steel parts, but a sponge.
____Alumina vs. PS Zirconia. Due to its particularities, partially stabilized zirconia leads to smoother parts than alumina (synthetic sapphire), despite the use of molds in both cases. In Fig. 8 are shown two ceramic brackets exhibiting a marked difference in leveling despite the similitude in their manufacturing.

...THE CONSEQUENCES

____a. Inventory degradation
Corrosion susceptibility. The finishing of a metal surface strongly influences chemical attacks. In our last issue we examined stainless steel sensitization and its consequence, intergranular corrosion, as these damage the internal structure of the attachment. Other attacks occur, however, at the metal surface. ____Smooth surface, as well as freedom from surface imperfections, traces of scale and other foreign material greatly reduce the probability of corrosion. Rough surfaces catch impurities generating a microenvironment which initiates localized corrosive attacks. In the mouth, instruments used even for a limited time are, according to engineering manuals, subjected to a "true corrosive application"¹³: the above applies all the more to the orthodontic attachments which are used for years.
____The main attackers are the chlorides from both saliva (500mg Cl-/liter¹⁴) and food. "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 the AISI types 304 and 316 ( widely used in attachment manufacture, our note) are not resistant to muriatic (hydrochloric) acid at any concentration and temperature"¹⁵.
____To a lesser degree contributing to the attachment's corrosion are the organic acids and the sulfurated compounds found in saliva, as well as a number of microbes. We have explored the topic of attachment's corrosion susceptibility both in a previous issue of this Newsletter¹⁶, as well as in other journals^17-20The attacks start from non-homogeneities on the surface (microfissures, cracks, inclusions) and develop gradually to pits and then to crevices which go as far as perforating brackets (Fig. 9).
Biocorrosion. We have described the contribution of microbes to the corrosion of orthodontic attachments (microbial influenced corrosion ( MIC) both in a previous issue of our Newsletter²¹ as well as elsewhere^22-24. Microbes are known to prefer heat-affected zones (HAZ in industry) as the welded- or laminated interfaces (Fig. 10).
___b. Health impairment
Heavy metal leaching. The attachment's corrosion is important not only because it degrades the inventory or may delay the end of the treatment. The heavy metals leaked from the attack on the orthodontic attachments, especially nickel, can generate affections ranging from allergies (up to 30% in women) to tissue necrosis. This problem, presented in our past issues, has led both Germany and Japan to implement variations of the International Standard ISO 6871-2 (1996) which measures the nickel released following an attack with diluted lactic acid. Japan actually prohibits the import of direct bonding brackets releasing more than 0.2 mg/ml of the test solution.

Calculus deposition. Rough attachment surfaces may promote, in the absence of proper hygiene, the deposition of calcified deposits (dental calculus) (Fig. 11). Saliva is secreted at a CO₂ partial pressure about twenty times higher than that existing in the atmosphere²⁵, and as in stalactite formation, its release transforms the soluble calcium bicarbonate to the less soluble carbonate. The latter precipitates together with the less soluble phosphates, process induced by the same drop in acidity.

WHAT CAN BE DONE
Attachment's selection

Milled vs. mold-made. No metal attachment today is perfect. The practitioner should choose the one which best adapts to his patient if he, or especially she, happens to be allergic. It is important to note that both milled and cast attachments are condemned as far as price is concerned. Rougher, porous injection-molded, one-piece brackets will establish the market despite their roughness and porosity. The only hope is that manufacturers, under the pressure of both consumer and government organizations, will be forced to use only higher quality and costly steel powders to make orthodontic attachments. The clinician, in turn, could demand to have their slots diamond-cut, as well as a smooth labial face. The first demand is costly, while the second is quite difficult to do. Indeed, the commonly used tumbling in barrels with aqueous suspensions of fine abrasives will interfere with the base's bonding strength which is already one-piece brackets' Achille's heel. An answer could be, as applied successfully since 1976 at Ortho-Cycle, the one-sided electro-polishing of the attachments²⁶. We wonder, however, if any manufacturer could afford the dramatically increased cost brought by this intensive labor demanding operation which will be presented below...
Passivation. During the years, we wondered about the rust or scale found on many stainless steel appliances received from customers or sellers. If these would have been properly protected by a thick enough layer of chromium oxide, the situation would have been different. While we understand that is difficult to operate with chemicals in an office, we still believe it is worth maintaining the stainless steel armamentarium in proper shape.
____Both attachments and instruments should be first solvent degreased and then subjected to an alkaline soak cleaning (30 min. in 5% sodium hydroxide at 160-180^oF) followed by thorough water rinsing. As some orthodontic appliances may be made of the "free machining" steels, i.e. the sulfur bearing grades, it is desirable to add a supplementary step, the so called alkaline-acid-alkaline method (this neutralization is necessary as sulfur inclusions retain residual acid). The passivation per se is performed by immersing the degreased attachments for half an hour in 20 volume % nitric acid added with 3oz/gal sodium bichromate at 120-180^oF. After water rinse, these should be immersed for another half an hour in the alkaline bath used initially and then thoroughly water rinsed¹¹. The operation should preferably be performed once a year on all the stainless steel inventory: the effort will pay off in terms of not only maintaining the value of your inventory, but also in protecting the patient.
Electro-polishing. As a difference from abrasive polishing, which generates micro-scratches, the reversed electroplating achieves two desirable results at once. First, it removes the micro-peaks protruding from the surface (see Fig. 11) and then, due to the oxygen which is concomitantly released, passivates it. Second is the removal of tarnish, scales and scratches resulting in a high quality leveling of the surface, as shown in Fig. 12. A comparison between the steel's surface before and after electro-polishing is presented in Fig. 13 and 14.
____Before subjecting the stainless steel parts to electro-polishing, it is necessary to degrease them as shown above. A common, harmless electrolyte used for the purpose is made of 37 volumes of technical phosphoric acid (85%), 56 volumes glycerol and 7 volumes water¹¹. If the current is moderate (a battery charger could be successfully used as a DC source), and the temperature is high enough, the operation may take a few minutes. At Ortho-Cycle three operations occur simultaneously. A short electro-cleaning which removes oxides and scratches is followed by a flash electro-polishing to bring back the metal's shine. In addition to phosphoric acid, the electrolyte contains heavy-metal chelating agents. To preserve the beneficial roughness of the base, the electrolyte only wets the labial size of the attachment²⁵. During the above treatment, the attachments are subjected to an anodic passivation, when oxygen is released statu nascendi directly on the metal's surface.

THE TESTING

Passivation. Unlike the melting or boiling points, passivity is not an absolute property: the same metal can exhibit variable degrees, according to the thickness and homogeneity of its protective layer. According to ASTM A 380, the stainless steel part (attachment or especially instrument) should be wetted with a solution of copper sulfate in sulfuric acid. Wetness should maintained for six minutes. Trying this test on a multitude of brands of direct bonding brackets as resulted from our electro-cleaning and anodic passivation, none of these exhibited the copper deposition characteristic in activation (lack of passivation).
Surface roughness. Empirically, the simplest way to assess the roughness of an attachment is to observe its light reflection: its visible, labial side should be shiny and smooth, while the base should be grayish and rough.
____Scientifically, the tiny size and the complexity of direct bonding brackets limits, as in the case of corrosion susceptibility, the use of methods which were designed to be used on specimens having a certain (too large) size. Even for implant purposes, the surface roughness of both stainless steels and titanium sheets should have at least the length of 35mm, the width of 5mm and the thickness of 1mm have to be measured with the help of a profilometer type Taylor Hobson²⁷. ____The diagrams resulted for electro-polished stainless steel provide accurate measurements of the peaks and the valleys similar to the one already presented in Fig.5. In the quoted study, the maximum difference peak to valley found for electro-polished stainless steel was of 0.993 mm, lower than any of those found for titanium surfaces leveled with the help of several methods (anodized, alumina blasted, electro-polished and hand ground). Among these, electro-polishing gave the best leveling (1.255mm vs. 1.725mm for fine anodizing and 10.356mm for blasting)²⁷.
____Aside profilometry, roughness can be measured using techniques such as scanning electron microscopy (SEM) and reflected light interference contrast microscopy. Unfortunately, both these microscopic methods do not lead to such an eloquent depiction of roughness than profilometry. In Fig. 15 and 16 are compared SEM views of highly electro-polished surfaces of stainless steel and titanium.
____Aside from a rather subjective evaluation of the roughness of these two surfaces (which allow only to understand why stainless steel attachments will be always smoother than those made of titanium), there is a new technique which can provide a profilometric diagram along with easy to interpret images of the surfaces examined. As a difference from SEM, which has a wider range, atomic force microscopy (AFM) provides detailed measurements on surfaces ranging between interatomic distances to a tenth of a millimeter. The samples are analyzed both in contact mode (AFM), leading to a topographical ( tridimensional) map, as well as using lateral friction microscopy (LFM). In a poster presented at the 97th AAO annual meeting in Philadelphia28 as well as published elsewhere29, we have used this sophisticated technique to determine the roughness of worn vs. brand new ceramic brackets.
____Using the same technique, we examined both the roughness and the friction of brand new and used steel attachments, in an attempt to confirm once more what we found with the help of microscopes. This surprising fact is due to the clogging (and leveling) of the valleys of the bottom of the ceramic slots by metal debris The study has demonstrated that the roughness of the used ones (i.e. exposed to the treatment) is significantly lower than that of their steel counterparts resulting from the fretting of the arch wires. Indeed, it is obvious³⁰ that recycled brackets favor the sliding mechanism. In Fig. 17 and 18 are shown the slot bottoms of two otherwise similar brackets, one being new and the other being used and recycled. In Fig.19-26, are shown, this time with the help of AFM and LFM, maps illustrating the same fact.
References
1. Matasa C.G, Metallography and you. I. (Dis) section. The Orthod.
Mater. Insider, 1998, 11(3): 1-8
2.Matasa CG, Direct bonding metallic brackets: Where are they heading ? Am. J. Orthod./D.O. 1992; 102: 552-560
3.Matasa CG, Milling, casting or injection molding? The Orthod.
Materials Insider, 1996; 9(1): 1-7
4. Kusy RP, Whitley JO, Mayhew MJ, Buckthal JE, Surface roughness
of orthodontic arch wires via laser spectroscopy, Angle Orthod.
1988; 58: 33-45
5.An engineers guide to friction, Defense Metals Information Center, Batelle Memorial Inst. Columbus OH 1970, DMIC Memorandum 246
6. Palmer F, Friction, Sci Amer. 1951: 184; 54-60
7. Krim J. Friction at the atomic scale, Sci. Amer. October 1996: 74-80
8. Matasa CG, Bracket slot friction examined through atomic force microscopy (Portuguese), Revista dental press de ortodontia 1997: 2(5): 60-74
9. Keir J, Phil. Trans. 1790; 80: 359
10.Faraday M, Experimental researches in electricity, vol. II, Univ. of London, 1844; 243
11. Sedricks AJ, Corrosion of stainless steels, IInd ed., Wiley-Interscience, NY, 1996: 127
12. Bundesgesundheitsamt, Legierungen in der zahnartzlichen Therapie, 1993, BGA, Hellmich KG, Germany
13. Kalish HJ, Corrosion of cemented carbides. In: Metals Handbook, vol. 13, "Corrosion", 9th ed., ASM , Materials Park, OH, 1987: 848
14. McCann HC, Inorganic components of salivary secretions, in: Art and Science of Dental Caries Research, R.S. Harris ed., Academic Press, NY 1968
15. Degnan, TF, Corrosion by hydrochloric acid, in: Metals Handbook, vol. 13, "Corrosion", 9th ed., ASM, Materials Park, OH, 1987: 1162
16. Matasa CG, Facts about bracket corrosion, Phoenix without ashes, 1992; 5(1): 1-6
17. Matasa CG, Orthodontic attachment corrosion susceptibilities,
J. Clin. Orthod. 1995; 29(1): 16-23
18. Matasa CG, La corrosion des verrous: un defi pour l'orthodontiste, Actualites Odonto-Stomatologiques, Paris, 1994 September, nr. 187: 401-409
19. Matasa CG, Nicht rostende Edelstahle und Direkt - Bonding Brackets. II. Chemishes Verhalten, Informationen aus Orthodontie und Kieferorthopadie, 1993; 25(2): 147-166;
20. Matasa CG, Materiales usados por ortodoncistas. Aceros. Journal of Orthopedics, Orthodontics and Pediatrics (Caracas, Venezuela)
1996; 1: 27-38
21. Matasa CG, Microbes feed on your stainless steel attachments, Phoenix Without Ashes, 1992; 5(4); 3-7
22.Matasa CG, Nichtrostende Edelstahle und Direkt- Bonding- Brackets. III. Mikro-biologisches Verhalten-auch der Adhasive. Informationen aus Orthodontie und Kiefer-orthopadie, 1993; 25(3): 269-285;
23Matasa CG, Microbial attack of orthodontic adhesives, Am. J. Orthod. / D. O. 1995; 108: 132-41
24. Matasa CG, Microbial Attack of Orthodontic Adhesives, Scientific poster exhibit at the AAO Annual Meeting in Orlando, FL, May 1994 25. Grant DA, Stern IB, Listgarten MA, Periodontics. Mosby, St. Louis, 1988
26. Matasa CG, You are forcing us to disclose, Mr. Horowitz, Phoenix without ashes, 1990; 3(4): 3-6
27.Ungersbock A, Rahn B, Methods to characterize the surface roughness of metallic implants, J. Mat. Sci.: Mat. in medicine 1994; 5: 434-440
28.Matasa CG, Bracket slot friction examined through atomic force microscopy (AFM), Scientific poster at the 97th American Assoc. of Orthodontists Annual Meeting, Philadelphia, May 1997
29. Matasa CG, The edgewise bracket versus friction, Phoenix Without Ashes, 1995, 8(1): 2-8
30. Matasa CG, Edgewise-Friktionsbracket-ligatur-Drahtbogen, Informationen Orthop. Kieferorthop., 1993; 3(3): 523-530

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