I. Indirect tests
What Ni does, and where it goes. For years we have been discussing the problem^1-4, while the amount of literature devoted to it steadily grows. “In the 1980's, the incidence of allergies to nickel was about 10 percent,” claims Dr. David Cohen of the New York University School of Medicine. “By the mid-1990's, that number had increased almost 40 percent to 14.3 percent.” For a long time, nickel has been the source for post allergy is the most common of all contact allergies in the industrialized countries. Young people are more frequently affected than the elderly. The sensitivity to nickel-induced reactions is never inherited; it develops gradually by extensive skin contact with nickel-containing metals. Once sensitized, a person will normally remain so for the rest of his life. The symptoms developed can affect parts of the body (knees, buttocks) distant from the nickel contact^5,6. A known carcinogenic agent^7-9, nickel has been found to lead to tissue necrosis^10,11 altering both internal organs (spleen)¹² and muscles¹³. Cell exposure to carcinogenic nickel compounds induces many genes that are commonly expressed in cancer cells, but not in normal ones, and modify the lymphatic system¹⁴. Last but not least, nickel has recently been found to contribute to the Chronic Fatigue Syndrome and to altered immunity in general^15-17.
Preventive measures. To avoid or cure the above mentioned afflictions, several measures can be taken. Some of these are simple, such as the detection of the phenomena and the removal of the cause, followed by diets avoid nickel, which may exclude acid foods cooked in stainless steel utensils, canned foods, cocoa, chocolate, corn and even whole grain flour¹⁸.
___.On a broader plane, the European Union has recommended the restriction of the use of nickel in the manufacture of objects placed in direct and prolonged contact with skin, establishing a threshold of 0.5 microgram of nickel/cm2/week, to which only a small number of the nickel sensitive patients will react^19-22. To implement this Directive, groups of experts are currently at work in order to establish the bases for future European and international standards on tests.
Nickel in orthodontics. In dentistry, measures have already been taken to limit the nickel that is released from casting alloys²³. Inn orthodontics, the topic is still under debate: while some authors claim that the treatment may induce nickel sensitivity²⁴, others show that it doesn't increase the risk for nickel hypersensitivity²⁵, and does not induce such a reaction²⁶. Some authors limit carefully their statements: "Minor amounts of Ni could be of importance in cases of hypersensitivity; fixed orthodontic appliances do not seem to affect significantly the nickel and chromium concentration of saliva during the first month of treatment²⁷", "Biodegradation of orthodontic appliances during the initial five months of treatment did not result in significant or consistent increase in the blood level of nickel"²⁸ (the underlining is ours).
___.It is very likely that an unquestionable confirmation of nickel's harmful and far reaching effects on patients will bring havoc in the Metalo-Orthodontics practiced today. Prior to the 30's, precious metals were gradually displaced by Angle's German Silver, an alloy which had up to 20% Ni. Today, it is replaced by stainless steels which have up to half of that amount.
___.Today, under the pressure of both the EU and the clinicians, all German manufacturers are offering attachments with less than the standard 8% or even without nickel in them. As an alternative to metals, Invisalign and Essix suggest Plasto-Orthodontics, a treatment taking place in the absence of metals²⁹. It has been argued that the release is not related to the nickel content, but rather to the junction of various alloys³⁰. The manufacturer of a kit allowing the detection of Ni on various surfaces has claimed that stainless steel is safe for use by nickel sensitive patients³¹, as it doesn't give a color reaction with a nickel reagent.
Few basic notions. Often times, the differences between various alloys are misunderstood. Alloys can be homogeneous or multi-phasic: if homogeneous, as in the case of most stainless steels, these consist of a single phase when examined under a microscope. Made of several kinds of atoms (Fe, Cr, Ni, C) which are soluble one in another (solid solution), the phase is a lattice built from tiny unit cells in which each of these elements participate, and from which these can not be physically separated.
___.This helps understand some interesting properties of stainless steels. First, being homogeneous, solid solutions, they will better withstand a chemical attack. Second, a multi-phase alloy (such as Ni and gold, or a plated layer of nickel on a baser alloy) allows an easy migration/dissolution of one of its components, in our case nickel. In contrast , when a lattice such as that of stainless steel is destroyed (chemical attack), the leached elements can be found in the same proportion as in each unit cell and the alloy, respectively. In other words, it is not necessary to go extra lengths to measure the content of the Ni released by a certain stainless steel object: it suffices to measure that of Fe or Cr which are easier to assay and which are proportional with the nickel content.
___.Stainless steel can be readily attacked. While resistant to many chemicals due to the impervious layer of Cr₂O₃ which coats its surface, stainless steel can rust and be attacked the moment the mentioned layer is removed. The main culprit are the chlorides, which solubilize the oxide mentioned and expose the steel to various chemical attacks. If a source of oxygen is readily available, the layer is reformed: if not, such as in hidden places or under a plastic layer (crevice corrosion) the corrosion continues, as seen in Fig. 1. Another common source for corrosion is the joining with another metal (galvanism). If nobler than stainless steel, that metal will cause the steels' dissolution, as seen in Fig. 2 where a gold brazed bracket exhibits a missing part.
Indirect testing of the leached nickel
___.Due to corrosion, nickel ions are released from attachments in variable amounts. These ions can be detected directly and quantitatively with the help of flame photometry, an expensive test, or by comparing the size of the colored stain (gel chromatography) produced with dimethyl glyoxime or dithioxamide, as mentioned in our December 1999 issue. The results obtained with the help of these will be presented in our December 2000 issue.
___.Indirectly, the nickel released can be estimated from the reaction given by iron with reagents such as potassium ferrocyanide. As a difference from most alloys, which comprise several phases, no particular element is preferentially leached: the alloy is dissolved gradually, unit cell after another, as shown in Fig. 3, leading to constant ratios between, let say, iron and nickel. In other words, for each ten or fifteen atoms of iron leached, there is one of nickel, per the stainless steel's composition.
The measurement of the iron released has been used over a year ago^32,33 to compare different attachment brands.
___.Subjected to an attacking agent recommended by an ISO standard for dental alloys, the attachments generate around them spots due to the reaction between the iron ions and a reagent which changes color and is standardized in industry³⁴. While the size and color intensity of the spot are a function of the Fe⁺⁺⁺ released, these are, as shown above, proportional with the solubilized nickel.
___.This simple test has stirred interest in the readers of our newsletter, as it will be shown below.
1. Testing arch wires.
___.Performed at the New York University Kryser Dental Center by Dr. Leticia Boos under the supervision of Professor Dr. M. M. Kuftinec, several arch wires made by several manufacturers were subjected to a corroding gel, as seen in Fig. 4, prepared by Ortho-Cycle Co. and described in our December 1999 issue. Both the spread and the intensity of the spot were recorded and presented as diagrams, as seen in Fig. 5 and 6.
___.After being degreased with a solvent, ethyl acetate, the arch wires were properly immersed in the gel. To enhance contact, the gel was vibrated for a few seconds and then left for forty-eight hours at room temperature. Blue spots soon appeared around the samples. At the end of this desired period, the diameter of the spots formed was measured with a Boley Gauge and the results statistically processed and compared. The procedure was repeated three times, following the same steps.
___.Although the attacking agents were diluted, all arch wires generated stains after 48 h. The same arch wires from the same companies behaved the same way, showing consistency, but marked differences were found between the brands.
___.The arch wires from American Orthodontics produced the smallest spots, which lead to the conclusion that it leached less iron, and implicitly, less nickel. GAC arch wires produced the second smallest stains, followed by ORMCO, Rocky Mountain, Unitek and Ortho Organizers (Fig.5 and 6).
___.According to the NYU report, the method used in this study allows the selection of the stainless steel arch wires that leach the lowest amount of heavy metals. While it cannot provide the exact amount of nickel released, it can be used as a tool to compare and rank the arch wires among different companies and thus choose the one that would be less likely to produce an allergic reaction in sensitive patients.
2. Testing expansion screws.
___.At the invitation of Dr. Stavros Kiliaridis, Professor of Orthodontics at the Geneva University and Dr. Francis Pribula from the same city, we have evaluated a number of new expanion screws sent by the latter, using the same gel chromatography method previously described^2,3.
___.While there are dozens of expansion screws on the market, the principle on which these are based is simple, as shown in Fig. 7 where a drawing from Wallshein's U.S. Patent 3,832,778' 74 is reproduced to indicate the use of several alloys. Expansion screws manufacturers do not provide details concerning the alloys they use: in Table I are shown these scant manufacturers descriptions. In it, along with the German designation (DIN), we provided also the American one (AISI) as well the composition. On this occasion, we found a major mistake in the steels identification by both Dentaurum and Forestadent, i.e. a mismatch between the sulfur content and the steel designation. Interestingly, Lewa (Remchingen, Germany) specifies also the use of nonferrous alloys: the other two manufacturers claim that the whole device is made of stainless steels.
___.Using the patent's terminology, this classic orthodontic biassing device is comprised of two spaced elements (stainless steel alloy I) housing a rod having reversed threads (stainless steel alloy II) and one or two guiding pins (stainless steel alloy III) as well as a brazing material ( I) that is needed to consolidate the latter. Sometimes, an additional brazing or soldering material (Brazing alloy II) is used to connect the whole to arms which are in turn brazed to bands. While some of these alloys are in some instances the same, it is common to have the threaded rod made of a different alloy than the spaced elements. The intimate contact of these various alloys in the presence of an electrolyte such as saliva, as well as the bulk of the metals involved lead to a significant potential for corrosion and heavy ions release.
___.Bearing in mind that the weight of a single Hyrax screw (without arms) equals that of some 35 Ormco, Mini Diamond, brackets, it is only fair to assess that its potential to leach dangerous heavy metal ions is equivalent to that of a mouthful of the tinier attachments. The position of the screws in each of the gel-containing plastic trays shown below matches the one shown in Fig. 8: the progression of the attack (indicated by the size of the stain) in the beginning and after 24 and 48 h are shown in Fig. 9,10 and 11.
___.Analyzing the area of the spot (the color intensity was found to approximate the spot size, as witnessed also by the NY University study, see above), it became obvious that the two types of Fo bove results should be useful to the clinician who cannot use NiTi expanders or titanium expansion screws (Dentaurum) in his quest to protect his patient.
3. Testing new vs. recycled brackets.
___.It has been established that the maximum release of harmful ions (Ni⁺⁺, Cr⁺⁺⁺) occurs within the first weeks of wearing the attachments, after which there follows a decrease, until a plateau is reached35, 36. In other words, most of the harmful releases take place in the wearer of the brand new brackets, in contrast with the one who wears, years later, the recycled ones.
___.This makes sense in view of the known drop in activity of the crystallites which form the various sites: it is well known that after few hours of functioning, catalysts lose a significant portion of their active sites, which become inactive.
___.In an attempt to evidence a difference in corrosion susceptibility between new and recycled brackets, nine brands were tested in parallel, in groups of two: the new one in the left tray, and the recycled ones in the right one. The couples of trays, containing each twenty brackets are shown in Fig. 10 and 11, from left to right.
___.In the first row are Orec, Speed; Ormco, Mini Diamond and Unitek, Unitwin. In the row below, Unitek, Dynabond II; American Orthodontics, Mini Master series; "A"-Co., Standard Edgewise. In the last row, Unitek, Victory; Rocky Mountain, Triple Control; Ormco, Diamond.
___.Few minutes after their equally distanced placement in the gel-containing trays, several brands started to show a slight staining. After 24 h, the stains were evident in all cases, with some consistency within the trays containing the same brackets, but with obvious differences between brands. The latter cannot be traced to their size, which was in all cases about the same, as seen in Fig. 10. After 48 h, the difference became even more obvious, Fig. 11.
___.The brackets which were least attacked, i.e. these which have released the lowest amounts of heavy metals, as evidenced by the smaller spots around them, were Orec, Speed™, Ormco Mini Diamond™, American Orthodontics, Mini Master™ Series, and Ormco, Diamond™. The heaviest attack occurred on Unitek's Unitwin™ one-piece brackets. With the exception of the last brackets, it was obvious that the new ones do not release more harmful ions than the recycled ones. The test didn't confirm that newer brackets are worse than the recycled ones from the point of view of the metals leached. New tests should therefore be performed, taking care to eliminate all surface contamination which may alter the results.
___.Interestingly, this test also bears upon the brackets made of what German standards recommend to be banned³⁷, i.e. the so called "automatic" steels. In these steels, such as AISI 303, mistakenly identified as AISI 305 by the expansion screws manufacturers, see above, sulfur is added to save on cutting energy as well as on blades, as it is probably the case of the Unitwin™ attachments. (The difference in sulfur content can be seen from Table I).
___.Another notable fact is that, according to this test, Ormco's Diamond™ brackets (made of the inferior steel AISI 303) seem to be superior to "A"-Co. Standard Edgewise ones, made of the superior steel AISI 316L. Indeed, tests performed by immersing both types in diluted muriatic acid (5%) and measuring the hydrogen evolved show, in sharp contrast, that the latter are superior to the former³⁸.
Conclusions
___.While the conditions encountered in the mouth are different from those existing in the acid gel used in our tests, it is possible to rank stainless steel attachments as their potential to harm the patient. A simple immersion in a gel containing the accelerated corrosion medium standardized by ISO, to which a reagent for the iron released has been added, allows to disclose the attachments which release more nickel. The gel^1,2 can also be successfully used instead of the Ferroxyl test for iron³⁴, evidencing if the brackets' surface has been contaminated with iron (as well as Cr and Ni) particles. According to this test widespread in industry, the appearance of a blue stain within 15 seconds of application evidences a surface contamination. Actually,compared with the ASTM test, where the reagent is a solution of nitric acid and potassium ferrocyanide in distilled water, our reagent is better, as the ferrocyanide contained in a gel clings better to metal surfaces.
___.Since the element evaluated is iron, and not nickel, the test is approximative. However, if one disregards the differences in C and S content, it is easy to see from Table I that in all the steels mentioned, the ratio Ni/Fe is almost the same. In other words, the test may be used to show how a certain attachment, be it arch wire, direct bonding bracket or expansion screw, would behave in the oral environment. Samples of this gel have been offered for free at the AAO Annual Conventions in 1999 and 2000: starting December 2000, we will offer samples of a gel for detecting nickel directly.
References
1. Matasa CG, Doc, I'm allergic to metals, The Orthod. Materials Insider, 1999;12(4): 4-8
2. Matasa CG, Shiny coatings are in, Ibidem, 1999; 12(3): 1-8
3. Matasa CG, Trends: Good bye Ni, welcome Co, Mn. Ibidem, 1995; 8(14); 1-6
4. Matasa CG, Facts about bracket corrosion, Ibidem, 1992; 5(1); 1-6
5. Gjerdet NR, Clinical and biological aspects of orthodontic materials, In: Dental materials: Biological properties and clinical evaluations, Mjor IA, CRC Press, Boca Raton, FL 1995
6. Council on Dental Materials, Instruments, and Equipment. Biological effects of Ni-containing dental alloys. J Am Dent Assn 1982;104:501-5.
7. Covington JS, McBride MA, Síagle WF Disney AL. Quantization of nickel and beryllium leakage from base metal casting alloys. J Prosthet Dent. 1985; 54(1)127-36.
8. Kelly JR, Rose TC. Non-precious alloys for use in fixed prosthodontics: a literature review. J Prosthet Dent. 1983; 49 (3): 363-9.
9. Menne I, Brandrup F, Thestrup-Pedersen K et al. Patch test reactivity to nickel alloys. Contact Dermatitis 1987; 16: 255-9.
10. Peters KP, Heese A, Allergie gegen Metale in der Zahn-heilkund, Tagung der DGKMZ (German Academy for Dentistry), Wiesbaden'95
11. Levy A, Hanau D, Foussereau J, Contact dermatitis in children, Contact Dermatitis, 1980: 6: 260-4
12. Tracana RB, Pereira ML, Abreu AM, Sousa JP, Carvalho GS, Stainless steel corrosion products cause alterations on mouse spleen cellular populations, J. Mater, Sci., Mater. in Medicine 1995: 56-61
13. Costa M, Klein CB, Nickel Carcinogenesis, Mutation, Epigenetics, Selection, Environmental Health Perspectives 1999; 107, (9); 40-45.
14. Eggleston DW. Effect of dental amalgam and nickel alloys on T- lymphocytes J Prosthet Dent 1984; 51(5): 617-23
15. Sterzl I, Procházková J, Hrdá P, Bártová J, Matucha P, Stejskal V, Mercury and nickel allergy: risk factors in fatigue and autoimmunity; Neuroendocrinology Letters; 20:221-228, 1999
16. Regland B, Nickel allergy, Second World Congress on Chronic Fatigue Syndrome and Related Disorders held in Brussels, 9-12 September, 1999
17. Marcusson JA, Lindh G, Evengard B, Chronic fatigue syndrome and nickel allergy, Contact Dermatitis 1999 May; 40(5) :269-72.
18. Internet: http://allergies.about.com/health/allergies/library/weekly
19. Amendment- Annex 1 to Directive 76/769/EEC
20. Menné T. Prevention of nickel allergy by regulation of specific exposures. Ann Clin Lab Sci 1996; 26: 133-8
21. Delescluse J, Dinet Y. Nickel allergy in Europe: The new European legislation. Dermatology 1994; 189 (suppl. 2): 56-7
22. Lidén C, Menné T, Burrows D, Development of European standards for the prevention of nickel allergy, Second Congress of the European Society of Contact Dermatitis, 6-8 October 1994, Barcelona, Spain
23. International Standards Organization (ISO) #6871-2, 1996
24. Bass JK, Fine H, Cisneros GJ, Nickel hypersensitivity in the patient Am. J. Orthod Dentofac Orthop. 1993; 103: 280-285
25. Kerosuo H, Kullaa A, Kerosuo E, Kanerva L, Hensten-Pettersen A, Nickel allergy in adolescents in relation to ortho-dontic treatment and piercing of ears, Am. J. Orthod Dentofac Orthop 1996; 109: 148-154
26. Janson GR, Dainesi EA,Consolaro A, Woodside DG, De Freitas MR, Nickel hypersensitivity reaction before, during, and after orthodontic therapy, Am. J. Orthod Dentofac Orthop 1998; 113: 655-660
27. Kerosuo H, Moe G, Hensten-Pettersen A, Salivary nickel and chromium in subjects with different types of fixed ortho-dontic appliances, Am. J. Orthod Dentofac Orthop. 1997; 111: 595-598
28. Bishara SE, BarrettRD, and Selim MI, Changes in blood level of nickel, Am. J. Orthod Dentofac Orthop 1993 103; 115-119
29. Matasa CG, The Congress laughs, sings and dances, The Orthod. Mater. Insider 2000; 13(2) 1-8
30. Grimsdottir MR, Gjerdet NR, and Hensten-Pettersen A, Corrosion of orthodontic alloys, Am. J. Orthod Dentofac Orthop 1992 :101 (525-532
31. Allertest Ni, Allerderm Laboratories, P.O. Box 2070, Petaluma, California, 94953-2070
32. Matasa CG, Corrosion susceptibility of coated direct bon-ding brackets, Poster at the 99th AAO Congress in San Diego, May 1999.
33. Matasa CG, Shiny coatings are in, The Orthod. Mater. Insider, 1999; 12(3) 1-8
34. ASTM Method A 380, Section 7.3.4
35. Ryhanen J, Biocompatibility evaluation of nickel-titanium shape memory metal alloy, Academy of Finland, Oulu, April 1999
36. Barrett RD, Bishara SE, Quinn, Biodegradation of orthodontic appliances. Part I. Biodegradation of nickel and chromium in vitro, Am. J. Orthod Dentofac Orthop. 1993;103:8-14
37. Bundesgesundheitsamt, Legierungen in der zahnartzlichen Therapie, 1993, BGA, Helmich,KG, Germany
38. Matasa CG, Orthodontic attachment corrosion susceptibilities, J. Clin. Orthod. 1995; 29(1): 16-23

YOU MAY INHERIT THE PLAST'ORTHODONTISTS !

___.With the advent of Invisalign's system, a number of practitioners have sold us both used and brand, excellent new direct bonding brackets made by top manufacturers. Let us know, using your letterhead, the brand, line, slot size and prescription you prefer, and we will send you, free of charge, a sampling of these which we sell for only $0.90 a piece, standard size, and $1.10, mini (Add 20¢ for power arms).

DO YOU REALLY KNOW THE RESINS YOU USE ?
Bis GMA (Part II)

___.In our March 2000 issue we have presented the first part of this series dedicated to the ubiquitous Bowen's resin, focusing on less known properties such as cytotoxicity, mutagenicity, carcinogenicity and estrogenicity. In addition, we have provided information on the resin hydrophilicity and ability to wet teeth, alone or added with other resins. The last property was related to the Hydrophilic-Lipophilic Balance (HLB), an empiric scale with the help of which one can determine in advance this important feature.
___.We continue this series with information about what happens if the adhesive, be it chemically or photochemically cured, is not subjected, before its application, to homogenization.
To mix, or not to mix?
___.Those who examine 3M Unitek's catalogues may notice that, after being boasted for years as "forming a strong base-to-enamel bond", the Unite No Mix adhesive used for direct bonding has been recently dropped. The advantages of a no-mix adhesive are obvious: less material to buy (only half of the resin), easier to apply. In some instances, the brushable activator in such systems is even claimed to act as a sealant (TP Orthodontics '98).
___.An analysis of the phenomena occurring during the polymerization indicate that the simple superposition of a polymerizable component over an enamel which has been wetted with a second, non-polymerizable, initiator-containing component (an ester, usually a plasticizer) cannot lead to a strong bond. Indeed, no polymerizable resin can be used to carry the initiator, as it will otherwise polymerize long before its normal use.
___.First, instead of relying on anchorages generated by the polymer tags which penetrate the enamel, the bonds in the no-mix systems involve capillaries and micro-fissures which are filled with a non-polymerizable liquid (a similar phenomenon occurs when the enamel has not been properly dried, case in which the use of "water-chasers", such as acetone, is recommended).
___.Second, the induced lack of homogeneity during the polymerization process leads to a variety of unevenly distributed molecular weights. Aside from the resultant uneven strength, the excess of the initiator in a certain portion of the curing adhesive will lead locally to shorter macromolecules, i.e. to a polymer poor in strength which is at the same time less resistant to water absorption and microorganisms.
___.Third, in both photochemically cured adhesives and two paste systems, there is little or no non-polymerizable liquid ingredient which lowers the resin's concentration. An additive of this type, dibutyl phthalate, while no more accepted as plasticizer in the EC community toys, is still used in some of the no mix systems. Such an ingredient not only cannot become part of the polymer, but it also interferes with the cross-linking of the resin, leading to a weaker and more water soluble oligomers (lower molecular weight polymers). In addition, such non-polymerizable components leach.
___.Two resin mix-systems may have, it is true, their own drawbacks: due to stirring, air may be included in the resulting paste (aside from voids, oxygen is a known polymerization inhibitor) and, if the operation lasts longer, the resin may start to thicken. This pales, however, when compared with the drawbacks enumerated for the no mix systems...