___.Florida’s “Election 2000” brought the world’s attention to the problems arising from misaligned Votomatic machines. These machines, derived from the original punchcard deve-loped from a concept introduced by political scientist and former government administrator Dr. J. P. Harris, were later patented and... repeatedly blamed. Orthodontics has also its own controversial patents: in this issue we will present two which are related to our holidays habits. Commentary on patents can be not only useful 1,2, but sometimes also entertaining.
___.The first patent1 is entitled “Method suitable for influencing the ingestion of food by humans via the mouth ca-vity”, and describes the means to reduce the capacity of the mouth with a view to reduce the body weight. As shown in Fig.1, which is not a poorly drawn Herbst appliance, the purpose is accomplished by limiting the maximum degree to which the lower jaw can be opened. According to the invention, a bar banded to a maxillary molar is provided with thickening ends which engages another similar bar with the help of a pivoting element. This second bar is connected to a banded mandibulary molar. If one attempts to open the mouth above the controllable, but imposed limit, pain is generated.
If this is not enough, a connected electric circuit will apply a voltage to a “sensitive part of the mouth cavity”.
___.The other patent, entitled “Oral appliance device and method for use thereof for appetite suppression”4 envisions a plastic device “which is substantially a U-shaped channel for receiving the upper teeth of the wearer and a roof portion that conforms to the roof of the wearer’s mouth. The roof portion includes an upwardly protruding rear tab portion that contacts the roof of the mouth substantially at the juncture between the soft and hard palates”. While the device leaves the tongue and lower jaw substantially free, it initiates a gag reflex and increases salivation, phenomena that lead to a suppression of the appetite.
___.Although bulimia and the related affections are very serious and people have had hard times observing a diet, one cannot but wonder -if the threat of continuously wearing those contraptions and get electrocuted- is not enough to make you to more conscencious, if not render you fit and slim...
References
1. Matasa CG, An insidious syndrome may threaten your practice: “Patentite”. Patents and orthodontics, materials and procedures, The Orthodontic Materials Insider, March 1999.
2. Matasa CG, Graber TM, Angle, the innovator, mechanical genius, and clinician, Am. J. Orthod. Dentofac. Orthop. 2000, 117: 444-452
3. Renders JB, Linders RW, U.S. Patent 6,138,679, 2000
4. Steer EL, US Patent 5,979,449, 1999

A little chemistry can help metalo-orthodontics !
In an ISO simplified corrosion test, a reagent detects leached nickel

INTRODUCTION
___.For over a century, there were no alternatives in teeth-straightening other than the use of various metallic attachments. Today, however, there is an alternative: InvisalignR. Limited for the time to adults, there is a chance that sooner or later its improvements will allow it to extend to others. As a result of the mass media campaign by Align Technologies, a company located in Santa Clara, California, orthodontists in the US are bombarded by present and future patients with related questions. While to date, the advantages of the system relate only to appearance, it is very likely that sooner or later the impact of braces on general health will raised.
___.In several past issues 1-7, as well as in various articles 8-11, we have presented not only the harmful effect of the heavy metals these release, but also attachments found partly dissolved in the mouth12-18. Among the metals released, nickel ranks first as importance, as it can lead from allergies to tissue necrosis and hepatic dysfunction. Due to such problems, both the European Community and the Ministry of Health and Welfare of Japan prohibit in the territories they control any dental attachments leaching nickel above some preestablished limits.
Methods to limit nickel release. During Angle’s time, the content in nickel in German silver, an alloy he strongly recommended to replace noble metals, was almost twenty percent. With the advent of the 18-8 stainless steel, the content dropped to more than half this value. In time, the nickel content of these further decreased to 4-5% such as in PH 17-4, used in most “mini” brackets, SAF 2205 (Low Ni) of CEOSA, Spain, or in some of these, even to almost zero (Mezanium^R by Scheu Dental, Ni-Free^R by Forestadent, Elite^R by Lancer International, Prestige^R by Pyramid Orthodontics, Noninium^R by Dentaurum).
___.In contrast, the preference for mesh-based brackets has led in recent years to the use of gold-containing brazing alloys which, in contact with the brackets’ stainless steel, occasionaly generates the galvanic dissolution of the latter. In this light, the introduction of one-piece attachments has been beneficial. This applies, of course only to the case where the stainless steel used is, in turn, resistant to corrosion (not all steelsare resistant, as some are sulfur-added to decrease both the energy needed for cutting and the blade consumption)
___.Another method attempted was the deposition of an impervious layer of another metal such as gold (Au), platinum (Pt), rhodium (Rh), alloys such as nickel-palladium (NiPd), or even of inert compounds such as titanium (NiT) and zirconium nitrides (ZrN), inorganic opacifiers or amorphous carbon. While the metal layers could be made to coat the brackets by electroplating, the other layers can be generated by chemical (CVD) or physical vapor deposition (PVD). The latter -made possible by the ionization of a metal target (Ti or Zr) with the help of an electron beam in a nitrogen-containing, rarefied atmosphere (plasma)- has led to gold-like looking brac-kets (to differentiate those from the really gold plated ones, one has simply to take a look at their bases, which remain uncoated).
Coatings critique
___.Along with chemical resistance, another sought- after pro-perty of the deposited layers is high hardness. Titanium nitride is widely used to convey the last property even to drill bits; in contrast, gold coatings are soft, as many worn brackets exhibit portions where the layer was exfoliated. Irrespective of their composition, most layers are removed after few weeks due to the fretting that occurs at the arch wire-slot interface. While these are only few microns deep, they provide good protection as long as these are still there. The above is demonstrated for carbon steel in Fig.1 and 2²¹.
___.Due to every day brushing with tooth pastes containing abrasive particles, modern attachments release a relatively high amount of harmful ions22, fact that happens even when coated. In addition, the complex geometry of the direct-bonding brackets leads more often than not to imperfect layers, which in turn generates alternations of metals of different nobility , i.e. to a galvanic cells where saliva constitutes the electrolyte. As a result, the least noble metal dissolves. Whenever the plated layer is made of gold or metals from the platinum family (Pt, Rh, Os, Pd), the bracket’s stainless steel may be attacked.
Nitride coatings are not too resistant to chemical attack12,17, as one can also observe from gold-looking car ornaments: while exposed to a less corrosive environment than the oral environment, these degrade relatively quickly.
Tests for nickel release. There are sophisticated and accurate methods to determine the amount of nickel released: two tests, one German and the other Japanese, use flame photometry and atomic absorption spectrometry. In the first test, a sheet of the steel to be used for dental casting is cut to specified dimensions and subjected to an attack by a 50 ml solution of 0.1 moles lactic acid and 0.1 moles sodium chloride for a week at 37oC19; in the second 20 , the sample is attacked in similar conditions by 50 ml of 1% lactic acid. The resulting solutions are then tested for nickel content. While the German method applies to metal samples, the Japanese one is currently used as an acceptance criteria for individual brackets: any of these which releases more than 0.2mg nickel is rejected.
___.In a recent issue of our newsletter¹², and in an AAO Poster¹⁷, we have shown that it is possible to detect the progress of the corrosion in a medium simulating the oral environment by following the iron released. The test is based upon the color given by Fe⁺⁺⁺ ions in the presence of a gel containing potassium ferrocyanide.
___.The advantage of this gel chromatography-based method resides in its simplicity, as well as in the fact that it allows one to watch the progress of the attack over time. The test can be used to show how the corrosion attacks a variety of attachments ranging from wires to brackets, or from headgears to expansion screws. Although the test doesn’t directly measure nickel, it allows to indirectly assess its presence. Indeed, the iron ions released are in direct proportion to those of nickel, in a ratio determined by the composition of the alloy used¹³.
Nickel direct testing. In analytical chemistry, two reagents are known to evidence nickel ions: dithiooxamide (or rubeanic acid) and dimethyl glyoxime²³.
___.About a century ago, Tschugaeff reported that nickel ions give, in the presence of the last reagent and ammonia, a complex compound having a specific pink color and the juxtaposed structure.
___.This discovery is applied today in analytical chemistry as well as in products destined to evidence nickel’s undesirable presence in jewelry. A kit based upon this reaction is sold for some $18 by Allerderm Co., P.O. Box 2070, Petaluma, CA 94953-2070.
___.According to the company,“for those with suspected or diagnosed nickel allergy, Allertest™ Ni is a safe, fast, and effective useful tool for avoiding contact with dermatitis causing nickel.‘The use of the dimethylglyoxime test for nickel is indispensable in the management of nickel dermatitis. All nickel-sensitive patients should be taught how to perform the test to help prevent further dermatitis. A substance that gives a negative result is unlikely to cause nickel dermatitis.’ (Fisher, Contact Dermatitis 1995).” (The efficiency of the test can be checked with the help of any silver-looking American coins, which are all plated with nickel).
___.According to Allerderm, stainless steel items do not free nickel ions detectable with the help of this kit. However, if the item is subjected to corrosion in a liquid or gel environment and then put in contact with the reagent in an appropriate medium, the pink will appear, and its intensity will be proportional with the amount of nickel released.
MATERIAL AND METHODS
The preparation of the lactic-sodium chloride gel (0.1 mole/l each in distilled water to which 50g Aerosil 200 from Degussa was added) has been described by us in a previous article¹³.
___.The reagent is prepared by dissolving 1g of dimethyl-glyoxime in 100ml of 98% ethylic alcohol. While the related literature²³ claims that it reacts with nickel in neutral, acetic acid and ammoniacal solutions, we have not obtained the proper spread of the stain except in the last environment. In Fig. 3 are shown two cases when the two headgears were tested in Petri dishes, a. after two days in the presence of commercial ammonia, and b. when these were tested in a neutral environment and a by far longer exposure (two weeks). ___.As one can see, in the first case the colored complex easily dissipated around the attacked zone, while in the second it clumped there.
___.The method used was the same as the one previously described¹³, with the difference that instead of adding from the start the color generating reagent, the corrosive gel based upon the ISO-recommended solution was first left to act on the attachments for a week. The action was then retarded the moment we added ammonia and the reagent which gives a pink color with nickel ions, i.e. the dimethylglyoxime solution. As mentioned, the addition of ammonia needed to evidence nickel dissolution does not allow to follow the dynamic of the corrosion.
___.The spread of the pink stain was photographed with the help of a digital Nikon Coolpix 950 camera. The processing of the images has been performed with the help of a computer and the Adobe Photoshop^R program.
All tests were conducted in Petri dishes; instead of potassium ferrocyanate (reagent which gives a blue color with iron) we used dimethylglyoxime. For this purpose, the accele-rated corrosion was retarded at the desired time by spraying the dishes with aqua ammonia and with the 1% solution of the reagent.
Expansion screws. The test has been applied to the same expansion screws used in our previous issue13 which were placed together in the same position. After three days of exposure at room temperature to a gel containing dimethylglyoxime, the dishes were photographed, Fig.7.
Direct bonding brackets. Five brackets of thirty-nine types, either new or used and reconditioned, were tested using the same conditions and photographed. After a week of exposure at room temperature, the dishes were sprayed with aqua ammonia and the solution of dimethylglyoxime, left overnight and photographed. The Petri dish without number was used as control.
___.Following the progress of the corrosion, it has been observed that some brackets belonging to the same type showed stains of different sizes and intensities, despite the fact that these were evenly degreased. This has lead to the hypothesis that either the brackets were different among themselves (batch, year of production?) or they were exposed to some surface modification. Consequently, previously ethyl acetate degreased Ormco Mini DiamondR brackets were subjected for few minutes to aqueous solutions of several known surface modifiers, and then to the accelerated corrosion test as described. As the results were encouraging, we tried a more advanced treatment. First, the attachments were degreased as shown, then pickled with an acqueous solution of hydrofluoric and nitric acid, rinsed and dried. After being subjected to passivation in a solution of chromic anhydride (CrO₃) in nitric acid, rinsed and dried, groups of ten brackets each were spread in Petri dishes containing the gel and a diluted solution of potassium ferrocyanate. We selected the latter reagent because despite it detects only iron, it is by far faster and more sensitive.

RESULTS
___.While the original, acid environment is not desirable to assess the progress of Ni⁺⁺ release, it becomes useful in the detection of the weak spots, i.e. where the attack is the stronger. This can be seen from Fig. 4, 5 and 6, all showing an aggregation of the pink complex especially where the parts are joined.
I. Expansion screws.
___.Due to the absence of ammonia, the red colored complex clumped on the screws rather than expanding. After a week, the Petri dishes containing the gel and the screws placed in the same position as in our previous test¹³ were photographed, Fig. 7. The lesser spots were given by these made by Dentaurum, Forestadent and a Czech company.
II. One piece, coated and self-engaging brackets. ___.After a week of exposure, the dishes were sprayed, left overnight and photographed as shown in Fig. 8. Their content comprises the number of the dish, the name of the manufacturer (where some origin, other than US, is shown) followed by a mark varying from (-) to (++). The mark (-) shows a low corrosion susceptibility, (+) a moderate one, while (++) indicates a high susceptibility as evidenced by an increased Ni leakage), while some didn’t (marked with -). To assess some coatings, uncoated controls were used in parallel.

___.One piece brackets (1-17): 1. Lancer, Sinterline, new (-); 2. Adenta, Econoline, new (+); 3. Ortho-Organizers, Single Bracket Bonding System, used (-); 4. Pyramid Orthod., Prestige, used (-); 5. Glenroe, cast (-); 6. Pyramid Orthod., Prestige, new (-); 7. Pumpa (Russian), new (-); 8. Unitek, Glance, new (-); 9. Technomed (German), new (-); 10. Chinese , new (-); 11. Unitek, Dynabond III, new (+); 12. Unitek, Miniature Twin, new (-); 13. Lancer, Elite Opti-Mim, new (-); 14. Dentaurum, Discovery, used (-); 15. CEOSA (Spain), Low Ni, new (-); 16. Advanced Ortho-dontics, Integra, new (-); 17. Unitek, Mini Unitwin, used, (+).
___.Varia. 18. Retainers, used (+); 19. Stops, used (-).
___.Self-engaging brackets. 20. Forestadent, Mobil-Lock, new (++); 21. Ormco, Twin Lock, new (++); 22. Ormco, Edgelock, new (++); 23. “A”-Co., Activa, new (++); 24. Orec, Speed, used (-); 25. American Orthodontics, Time, new (+); 26. “A”-Co., Damon, used (+).
___.Coated brackets & controls. 27. Unitek, Mini Victory, control, new (+); 28. Same, gold coated, new (+); 29. Advanced Orthodontics, Formula, control, new (+); 30. Same, nickel-palladium coated¹), new (+); 31. Same, recycled (+); 32. Altheis²), platinum coated, new (++); 33. Same, control, new (++); 34. Ormco, old type, NiCoTef³)coated, new (++); 35. Same, control, new (+); 36. Ormco Mini Diamond, Altheis²), gold coated, new (++); 37. Same, control, new (+); 38. Ormco Diamond, Altheis²), gold coated, new (+); 39. Same, control (+).
III. Brackets with mesh pads (multi-piece or combined)
___.Five brackets of forty types, new or used and reconditioned, were tested using the same conditions and photographed after ten days exposure at room temperature, Fig. 9. The marking is the same as before: 1. “A”-Co., Comfort, used (-); 2. Ormco, Vari Simplex, used (-); 3. Ormco, Mini-Vick, used (+); 4. Unitek, Chun-Hoon, used (-); 5. Rocky Mountain, Edgewise regular, medium single, new (-); 6. American Orthodontics, Mini Tweed, single (-); 7. Unitek, 4-Stage, used (-); 8. American Orthodontics, Channel Edge, used (+); 9. Lancer, CAT, used (+); 10. Rocky Mountain, Edgewise regular, medium single, used (-); 11. Rocky Mountain, Synergy, new (+); 12. Rocky Mountain, Mini Taurus, new (++); 13. Unitek, Unitwin, used (+); 14. Ortho-Organizers, Elite, used (+); 15. Unitek, Twin Torque, used (-); 16. Unitek, Victory Series, used (++); 17. TP Orthodontics, Straight Edge, used (+); 18. TP Orthodontics, Advantage, used (-); 19. “A”-Co., Attract, used (+); 20. “A”-Co., Standard Edgewise, used (-); 21. “A”-Co., Mini Twin, used (+); 22. Dentaurum, Ultra Mini Trim, used (+); 23. Dentaurum, Ultra Trim, used (+); 24. American Orthodontics, Master Series, used (-); 25. American Orthodontics, Triple Action, used (++); 26. GAC, Omni, used (-); 27. GAC, MicroArch, used (-); 28. Ormco, Diamond, used (+++); 29. Lancer, Natural Arch, used (+); 30. Ormco, Diamond, used (+); 31. Lancer, Sinterline, used (+); 32. Orec, Aardwark, used (++); 33. GAC, Shoulder, new (-); 34. GAC, Viazis, used (-); 35. TP Orthodontics, Tip Edge, used (-); 36. American Orthodontics, Mini Master Series, vertical slot, used (+++); 37. Forestadent, Mini-Mono, used (+); 38. TP Orthodontics, Begg, 256 series, used (+); 39. Unitek, Light Wire Begg, used (-); 40. Ormco, Begg, used (++).

IV. Chemically modified brackets
___.Differences in the size and intensity of the colored spots surrounding brackets of the same type can be seen in Fig. 9, #25 and #36. While stainless steels are protected against chemical attack due to a thin but impervious layer of Cr₂O₃, this protective layer can either be dissolved (chloride solutions are the main culprit), or rendered more resistant with the help of various compounds. Known as corrosion inhibitors, these belong to various classes among which, as far as we know, only silanes have been tried. This treatment was, however, limited to NiTi wires²⁴.
___.To test the influence of such surface activity modifiers, we used Mini Diamond^R upper laterals from Ormco, all reconditioned. These were placed in the dimethylglyoxime-added corrosive gel after being surface treated with two silanes made by Dow Chemicals Corp., Z-6030 and Z-6032, No-Rust (an emulsion containing tannic acid), benzotriazole and a Kenrich titanate (Titanate KR-44), Fig. 10. Observing that the corrosion susceptibility was actually deceased, we tried also other agents, as shown in Fig. 11. Among these were fatty amides by Armour, a quaternary ammonium salt (Maquat, by Mason Chemicals, Chicago), thiourea, and again benzo-triazole. In addition, we also tried a well known chelating agent (the tetra sodium salt of ethylene diethylamino tetraacetic acid, EDTA, which produces stable chelates with nickel²⁵), see juxtaposed formula. Attachments thus treated maintained the protection achieved even after two weeks of tumbling with a suspension of corn seeds in water.
DISCUSSION
___.There is an undeniable difference in heavy metal release between today’s orthodontic attachments, be these wires, headgears, brackets, retainers and brackets as evidenced by tests relying on Fe⁺⁺ or Ni⁺⁺ detection. Aside from signaling poorer alloys, the tests show that the parts most prone to attack are the joints (galvanic corrosion can take place even between various stainless steels, or even within parts made of the same one, if treated differently). Due to imperfect deposits, coated brackets often cannot offer the desired protection: actually, galvanism may enhance corrosion. One-piece brackets, if made of a corrosion resistant alloy, have less of a chance to release nickel than those combined or self engaging.
___.Both tests, i.e. the one detecting the release of Fe⁺⁺ and the other that of Ni⁺⁺, are dependable and user friendly. Instead of spending money for a sophisticated analysis involving atomic absorption spectrometry, the clinician can test for himself the harmful potential of any attachment using gels which can be provided free by Ortho-Cycle Co. just by asking for it, of course using the Insider’s subscriber’s letter head.
___.While the use of chelates in dentistry is restricted to bonding²⁵, these may also offer a never before attempted use, that of protecting the patient. Indeed, by treating the attachments’ surface prior to wear it is possible to reduce their corrosion susceptibility for at least two weeks (tests for longer periods are in progress).
___.Although our initial attempts to reduce the brackets’ corrosion susceptibility by using chelates in conditions which duplicate a single mouth-rinsing showed only a slight improvement, the research will continue. Rinsing and even swallowing harmless agents such as the sodium salts of EDTA is not harmful: in contrast, it would result not only in reducing the release of heavy metals, but also in blocking their action. Indeed, EDTA is approved by the FDA for blocking and removing lead and other heavy metals in afflictions caused by intoxicants or wounds, and is commonly added in drugs and... mayonaise. In addition, due to its propensity to combine with the metals which enter in the composition of various enzymes, EDTA acts also as a bactericide.
CONCLUSIONS
___.Stirred by the nice appearance offered by Plasto-Orthodontics, patients will demand more and more for a better way to improve their occlusions. Problems which were up to now left unanswered because there was no alternative, will start to surface more forcefully than ever before. Clinicians, who believe that Metalo-Orthodontics is the only way to correct most malocclusions, will have hard time to explain this to their constituency. This may become even more difficult in time as it has been found²⁷ that the newer generations are less and less resistant to the afflictions caused by leached heavy metals. While the use of titanium or tantalum may provide some answers, the properties of stainless steels attachments makes them difficult to replace. Consequently, proper selection, as well any treatment which may render them less harmful, may prove to become a must for the survival of Metalo-Ortho-dontics as we know it today.
References
1. Matasa CG, Facts about bracket corrosion. I. Phenomena and consequences, The Orthodontic Materials Insider, 1992; 5(1): 1-6
2. Matasa CG, Facts about bracket corrosion. II. Prevention, Idem, 1992; 5(2): 4-8
3. Matasa CG, Microbes feed on your stainless steel attachments, Idem, 1992; 5(4): 3-7
4. Matasa CG, One-piece brackets are here to stay! I, Idem, 1994; 7(1): 3-8; II, Idem, 1994; 7(2): 3-8
5. Matasa CG, Trends: Good bye Ni, Welcome Co, Mn, Idem, 1995; 8(4): 1-6
6. Matasa CG, Milling, casting, or injection molding? Idem, 1996; 9(1): 1-7
7. Matasa CG, Metallography and You. I. “Dissection”, Idem, 1998; 11(3): 1-8; II, Surface analysis, Idem, 1998; 11(4): 1-7
8. Matasa CG, Direct bonding metallic brackets: Where are they heading ? Am. J. Orthod.Dentofac Orthop 1992; 102: 552-560
9. Matasa CG, Nicht rostende Edelstahle und Direkt - Bonding Brackets. II Chemishes Verhalten (In German: Stainless steels and direct bonding brackets. II Chemical properties) Infor-mationen aus Orthodontie und Kieferorthopadie (Heidelberg, Germany), 1993; 25(2): 147-166
10. Matasa CG, Materiales usados por ortodoncistas. Aceros (In Spanish: Materials used by orthodontists: steels), Journal of Orthopedics, Orthodontics and Pediatrics (Caracas, Vene-zuela) 1996; 1: 27-38
11. Matasa CG, Nichtrostende Edelstahle und Direkt- Bonding- Brackets. III. Mikrobiologisches Verhalten-auch der Adhasive (In German: Stainless Steels and Direct-bonding Brackets. III. Microbiological properties.) Informationen aus Orthodontie und Kieferorthopadie (Heidelberg, Germany), 1993; 25(3): 269-285
12. Matasa CG, The wear and corrosion resistance of metal deposits, The Orthodontic Materials Insider, 1999; 12 (3): 1-8); Doc, I’m allergic to metals! Idem, 1999; 12(4): 4-8
13. Matasa CG, It’s just up to you to fight nickel allergies… Idem, 2000; 13(30: 1-7
14. Matasa CG, La corrosion des verrous: un defi pour l'orthodontiste (In French: Attachment corrosion: a chalenge for the orthodontist) Actualites Odonto-Stomatologiques (Paris, France) September, nr. 187, p.401-409, 1994
15. Matasa CG, Orthodontic attachment corrosion susceptibilities. J. Clin. Orthod. 1995; 29(1): 16-23
16. Matasa CG, Evaluacion de la corrosion en los aditamentos ortodoncicos" (In Spanish: Corrosion testing of orthodontic attachments) Rev. Venezolana de Ortodoncia (Caracas), 1998; 15 (1): 437-444
17. Matasa CG, Corrosion susceptibility of coated stainless steel direct bonding brackets, Poster at the 99th AAO Annual Session in San Diego, May 1999
18. Matasa CG, Biomaterials in orthodontics, Chapter in Orthodontics: Current Principles and Techniques, IIIrd ed., T.M. Graber, R. Vanarsdall, ed., C.V. Mosby, 2000, St. Louis, MO : 305-337
19. ISO Standard BSI, EN 6871-2: 1996
20. Standard 1985.3.30 No.294, Japan Min. of Health and Welfare
21. Iancau V, Nica D, Tratamente termice cu azot si carbon, Editura Tehnica, Bucharest, 1999
22. Wataha JC, Lockwood PE, Frazier KB, Khajotia SS, Effect of toothbrushing on elemental release from dental casting alloys, J Prosthodont 1999; 8(4): 245-51
23. Lewis CL, Ott WL, Analytical chemistry of nickel, Pergamon Press, Oxford, 1970
24. Endo K, Chemical modification of metallic implant surfaces. II. Corrosion resistance of a chemically modified NiTi alloy, Dent. Mater. J. 1995; 14(2): 199-210
25. Matasa CG, Ionites, complexes, chelates.. and you. The Orthodontic Materials Insider, 1995; 8 (3): 1-8
26. Martell AE, Calvin M, Chemistry of the metal chelate compunds, 1952, Prentice Hall, NY
27.Gjerdet NR, Clinical and biological aspects of orthodontic materials. In: Dental materials: Biological properties and clinical evaluations, Mjor IA, ed., 1995, CRC Press, Boca Raton, FL