Enamel wetting and orthodontics
___.In our past issues^1-3 we have provided evidence that the antagonism of the water/oil type existing between stainless steel and resin-base adhesives can have an impact on bond strength. In this issue, we will focus on the similar antagonism existing between the water-loving, etched enamel and water-hating, resin-based adhesive, Fig. 1.
___.Indeed, stainless steel owes its protection to a layer of chromium oxide and the enamel to hydroxyapatite, both water-loving. In contrast, most resin-based adhesives are based upon bis GMA, a resin which exhibits a strong water -hating behavior². The needed wetting of the metal is lacking, as the bond is due exclusively to mechanical interlocking between the composite and the pad’s mesh or undercuts.
The interface adhesive/metal. The SEM analysis of both in vitro and in vivo bonded brackets1,² has shown continuous gaps between the composite and the mesh, Fig. 2.
___.The poor metal wetting and the resin-based adhesive’s inability to seal are amply demonstrated by the fact that oral fluids often penetrate at their interface, as shown in Fig. 3. This phenomenon leads not only to weak bonds, but also to the metal’s corrosion, as shown in Fig. 4. Along the fluids penetrating this interface are a variety of micro-organisms, some of which are specialized in attacking ferrous alloys, Fig. 5
The interface adhesive/enamel. Attacks due to the poor wetting take place on both the adhesive and the enamel. While the polymer in the adhesive can depolymerize from within due to inappropriate composition, Fig. 6, its decomposition (and actual consumption) can be generated by microorganisms, Fig.7.
___.Caries occurring under the adhesive were reported, see Fig. 8⁴, as were both white^5-8 and colored enamel spots. While the first, known caries precursors can heal (remineralization), the second are indelible. While attributed to the dispersion of chromium salts into the monomer of the bonding resin⁹, such spots, Fig, 9, are more likely due to the diffusion of the heavy metal ions leaching from a corroded attachment, the tooth structure acting as an ion exchanger.
Was wetting properly addressed? Years ago, Lee Pharmaceuticals was first to try (unsuccessfully) to launch a silane-based primer. Unfortunately, silanes are too sluggish to react in the short time needed. Later, addressing microbial attachment on appliances, Eliades et al. have measured the contact angle of various liquids on attachments¹⁰ (as a difference from its meaning in orthodontics, in physics the contact angle q is the angle measured between the liquid and the solid by drawing a tangent at the point of contact).
___.While Rossouv et al.¹¹, as well as Swift et al.¹² have addressed years ago the need for low viscosity bonding resins with high spreading potential, in the later years, the interest for improving bond strength has soared^13-31. Thus, at the 104th AAO Annual session in Orlando, May 2004, three posters focused on related phenomena^32-34.
___.To improve bonding and control etching, manufacturers recommend sealants or primers imported from dentistry. Instead phosphoric acid, these contain either its esters (Transbond Plus™, Unitek/3M, Monrovia, CA) or weaker, organic acids (ESPE Dental AG, Seefeld, Germany). To generate affinity between the resin and the enamel, another approach is the use of 2-hydroxyethyl methacrylate, a hydrophilic monomer (Ideal 1™, GAC International, Islandia, NY). To prolong the monomer’s shelf life, Transbond Plus Self Etching Primer keeps the latter separated from the etchant, Fig. 10.
___.Interestingly, a basic approach for wetting, i.e. the use of surfactants, seems to have been ignored despite its recognized importance, see Fig. 11. Indeed, as far as we know, there are no publications in orthodontics about the use of surface active agents to induce affinity between two antagonistic substrates, the water- loving enamel and the water-hating, resin-based adhesive.
Substrate wetting is key. Orthodontic literature indicates adhesive penetrations into the enamel of extracted teeth up to 100mm: no such depths could be reached on live teeth.
___.For demonstration purposes, we altered the images taken from literature and presented in Fig. 12 and 13 what we believe is the situation the orthodontist faces, that of working with live teeth whose capillaries are filled with water.
___.While live teeth can be rendered dry enough using liquids that boil/evaporate at lower temperatures carrying along a constant proportion of water, the so called azeotropes, shown in Table I, some may require special attention due to their toxicity.
___.Another way to improve wetting is to introduce additives. While the contact angle for a pure liquid on a pure solid is set by the interfacial tension of the pure liquid and solid, there are additives that will adsorb on the solid to reduce the contact angle as shown in Fig. 14, altering the interfacial tension and reducing the time required for liquid penetration.
___.In physics, the penetration of nonpolar fluids into a water-loving substrate (e.g. a hydrocarbon in concrete) is calculated with the formula^41,42:

___.where e₀ = penetration depth of capillary suction, q = contact angle, r = pore radius, h = dynamic viscosity, t₀ = test duration; s = constant. In other words, to improve wetting and reduce the penetration time it is important to reduce the contact angle for a liquid on a solid as this will increase cos q.
___.In a similar case, where sealing is used (monument & stone artwork protection), the depth of penetration is calculated again as a function of the contact angle, surface tension and viscosity of fluid and time of penetration⁴³.
Wetting in other fields. In resin-based adhesives, the decisive factor in the mechanical retention phenomenon is the extensive interlocking between the adhesive and the enamel³⁷. A higher penetration is often desirable, and the orthodontist may be helped by seeing the measures taken elsewhere. Without surfactants to allow oils to wet plants’ foliage, herbicides could not perform. Gasolines are added with special detergents to wet the otherwise water-lowing steel cylinders: this helps protecting the engine against corrosion while decreasing friction, Fig. 14.
___.Drilling muds would be lost due to their penetration in the earth layers, unless thicker (viscous) emulsions are used. Like in adhesives, the naturally hydrophilic additives of these muds (minerals) must be rendered lipophilic. Muds are, whenever possible, oil-based in order to limit their lost in the water-loving earth substrate.
___.In contrast, where a deep penetration of the resin is necessary to protect the inorganic substrate (on bridge decks and in monument preservation), the surfaces have to be rendered oil-loving and the resins as fluid as possible, Fig. 15
Which are the adequate surfactants? The agents that enhance the oil wetting of a water-loving substrate are not in the realm of the common, household surfactants, but rather in those used in gasolines. The difference can be understood with the help of the HLB (Hydrophilic-Lipophilic Balance) system, in which the agents are situated somewhere along a line which unites the extreme water-loving and water-hating substances, e.g. water and paraffin. Based upon this concept, it is possible to disclose not only the agents that can lead to solubilization or emulsification, but also those that can provide improved wetting and penetration into tight discontinuities Discussed in a previous issue of this newsletter³⁸, this scale devised by Griffin ^39,40 is widely used today in both science and industry, Fig. 15.
___.The HLB values are related to the contact angle (see above), to the dielectric constant, ionization potential and polarity of the substances involved... A small droplet of pure water, unaffected by gravitational forces, will, due to the balance of three interfacial tensions (between liquid and air, liquid and solid and air and solid), have a contact angle of 110° on hydrophobic surfaces, such as that of a hydrocarbon (paraffin). This angle will decrease with added surfactant, down to an optimum minimum of 0°, which means complete droplet spreading (perfect wetting).
___.A surfactant molecule generally has two parts: a nonpolar (designated as hydrophobic, lipophilic or oleophilic) hydrocarbon chain, and a polar (hydrophilic, lipophobic or oleophobic) group. These two structures in the same molecule form a special amphiphilic molecular structure which, due to its appurtenance to both phase, allow their interaction, Fig. 17.
___.Surfactants’ structure is polar (hydrophilic or water loving) at one end and nonpolar (lipophilic/hydrophobic or water hating) at the other. Hydrophilic groups may be cationic (organic amines -especially with three hydrocarbon chains attached to the nitrogen atom), anionic (fatty acids or sulfates with hydrocarbon chains) or nonionic (organic compounds with oxygen containing groups such as alcohols, esters and ethers). Hydrophobic or lipophilic (oleophilic) groups may be large, straight or branched chain hydrocarbons, cyclic hydrocarbons, aromatic hydrocarbons and/or combinations of them.
___.Silanes are not suitable for use as adhesives or sealants as their hydrolysis takes at least half a hour⁴⁴. Indeed, in order to be able to interact with the desired substrates, these have first to incur an alcoholysis as shown in Fig. 18, its speed varying with concentration, solution, and temperature.
References
1. Matasa CG, Do adhesives & sealants really seal the brackets’ pad ? I. Corrosion. The Orthodontic Materials Insider, 2002; 14 (4) : 5-8
2. Matasa CG, Do adhesives & sealants really seal the brackets’ pad ? II Surface tension. The Orthodontic Materials Insider, 2003; 15 (1): 4-8
3. Matasa CG, What is hidden behind your bracket’s mesh ?The Orthodontic Materials Insider, 2003; 15 (4): 1-7
4. Lehman R, Davidson, CL, Duijsters PPE, In vitro studies on susceptibility of enamel to caries attack after orthodontic bonding procedures, Am J Orthod. Dentofac Orthop. 1981; 79: 61-72
5. Ogaard B. Prevalence of white spot lesions in 19-year olds: a study on untreated and orthodontically treated persons 5 years after treatment. Am J Orthod. Dentofac Orthop.1989; 96: 423-7.
6. Gorelick L, Geiger AM, Gwinnett AJ. Incidence of white spot formation after bonding and banding. Am J Orthod. Dentofac Orthop. 1982; 81: 93-8.
7. Mizrahi E. Surface distribution of enamel opacities following orthodontic treatment. Am J Orthod. Dentofac Orthop. 1983; 84: 323-31.
8. Årtun J, Brobakken BO. Prevalence of carious white spots after orthodontic treatment with multibonded appliances. Eur J Orthod 1986; 8: 229-34
9. Ceen, R. F., and Gwinnett, A. J.: Indelible iatrogenic staining of enamel following debonding, J. Clin. Orthod. 1980; 15: 713-715
10. Eliades T, Eliades G, Brantley, WA, Microbial attachment on orthodontic appliances: I. Wettability and early pellicle formation on bracket materials, Am J Orthod.Dentofac Orthop. 1995; 108: 351-60
11. Joseph VP, Rossouw PE, Basson NJ, Some “sealants” seal -a scanning electron microscopy (SEM) investigation, Am J Orthod.Dentofac Orthop. 1994; 105: 362-8
12. Swift EJ, Jr., Triolo PT PT Jr., Barkmeier WW, Bird JL, Bound SJ. Effect of low viscosity resins on the performance of dental adhesives. Am.J. Dent. 1996; 9: 100-4
13. Miller RA. Laboratory and clinical evaluation of a self-etching primer. J Clin Orthod 2001; 35: 42-45.
14. Bishara SE, VonWald L, Laffoon JF, Warren JJ. Effect of a self-etch primer/adhesive on the shear bond strength of orthodontic brackets. Am J Orthod Dentofacial Orthop 200; 119: 621-624
15. Bishara SE, Ajlouni R, Laffoon JF, Warren JJ. Effect of a fluoride-releasing self-etch acidic primer on the shear bond strength of orthodontic brackets. Angle Orthod 2002; 72: 199-202
16. Manhart J, Hickel R. Esthetic compomer restoration in posterior teeth using a new all-in-one adhesive: case presentation, J Esthet Dent 1999; 11: 250-258
17. Fritz UB, Diedrich P, Finger WJ. Self-etching primers—an alternative to the conventional acid etch technique? J Orofac Orthop 200; 62: 238-245.
18. Littlewood SJ, Mitchell L, Greenwood DC, Bubb NL, Wood DJ. Investigation of a hydrophilic primer for orthodontic bonding: an in vitro study. J Orthod 2000; 27: 181-186.
19. Grandhi RK, Combe EC, Speidel TM. Shear bond strength of stainless steel orthodontic brackets with a moisture-insensitive primer. Am J Orthod Dentofacial Orthop 200; 119: 251-255
20. Dorminey JC, Dunn WJ,Taloumis LJ, Shear bond strength of orthodontic brackets bonded with a modified 1-step etchant-and-primer technique, Am J. Orthod Dentofac Orthop 2003; 124: 410-413
21. Yamada R, Hayakawa T, Kasai K. Effect of using self-etching primer for bonding orthodontic brackets, Angle Orthod 2002; 72: 558-564
22. Buyukyilmaz T, Usumez S, Karaman AI. Effect of self-etching primers on bond strength—are they reliable? Angle Orthod 2003; 73: 64-70
23. Torii Y, Itou K, Hikasa R, Iwata S, Nishitani Y. Enamel tensile bond strength and morphology of resin-enamel interface created by acid etching system with or without moisture and self-etching priming system, J Oral Rehabil 2002; 29: 528-533
24. Zeppieri IL, Chung CH, Mante FK, Effect of saliva on shear bond strength of an orthodontic adhesive used with moisture-insensitive and self-etching primers, Am J. Orthod Dentofac Orthop 2003; 124: 414-419
25. Bishara, SE, Gordan VV, VonWald L, Olson, ME, Effect of an acidic primer on shear bond strength of orthodontic brackets Am J. Orthod Dentofac Orthop 1998; 114: 243-247
26. Kimura T, Dunn,WJ, TaloumisLJ, Effect of fluoride varnish on the in vitro bond strength of orthodontic brackets using a self-etching primer system. Am J. Orthod Dentofac Orthop 2004; 125: 351-356
27. Bishara SE, Gordan VV,VonWald L, Jakobsen JR, Shear bond strength of composite, glass ionomer, and acidic primer adhesive systems Am J. Orthod Dentofac Orthop 1999; 115: 24 -28
28. Ireland AJ Knight, Sherriff M, An in vivo investigation into bond failure rates with a new self-etching primer system Am J. Orthod Dentofac Orthop 2003; 124: 323-326
29. Major, PW, Koehler JR, Manning KE, 24-hour shear bond strength of metal orthodontic brackets bonded to porcelain using various adhesion promoters, Am J. Orthod Dentofac Orthop 1995; 108: 322-329
30. Arnolda RW, Combe EC, Warford, Jr, JH, Bonding of stainless steel brackets to enamel with a new self-etching primer. Am J. Orthod Dentofac Orthop 2002; 122: 274-276
31. Cacciafesta V, Sfondrini MF, Baluga L, Scribante A, Klersy C, Use of a self-etching primer in combination with a resin-modified glass ionomer: Effect of water and saliva contamination on shear bond strength, Am J. Orthod Dentofac Orthop 2003; 124: 420-426
32. Horliana RF, Rodiguez G, Carvalho P, Bonfim R, Voigorito J, Shear bond strength with a novel “self-etching primer”, Poster at the 104th AAO Annual Session, Orlando, May 2004
33. Matasa CG, What happens behind the brackets mesh?Poster at the 104th AAO Annual Session, Orlando, May 2004
34. Miguel JAM, Calneto J, Effect of self-etching primer on shera bond strength of adhesive coated brackets. Poster at the 104th AAO Annual Session, Orlando, May 2004
35. Anusavice J K, Phillips’ Science of Dental Materials, 10th ed., 1996
36. Gwinnett A J, J. Am.Soc. Prevent. Dent. 1973; 3: 21
37. Diedrich P, Enamel alterations from bracket bonding and debonding, Am J. Orthod Dentofac Orthop 1981; 71: 500-522
38. Matasa CG, Do you really know the resins you use? Bis GMA. The Orthodontic Materials Insider, 2000; 13(1): 5-8
39. Griffin WC: Classification of Surface-Active Agents by 'HLB,' Journal of the Society of Cosmetic Chemists 1949; 1: 311
40. Griffin WC: "Calculation of HLB Values of Non-Ionic Surfactants," Journal of the Society of Cosmetic Chemists1954; 5: 259.
4. Reinhardt HW, Pfingstner A, Pore-size determination from penetration tests on concrete with n-decane, Otto Graf J. 2002; 13: 65-76
42. Reinhardt, H. W. (ed.): Penetration and permeability of concrete: barriers to organic and contaminating liquids. London: E&FN Spon, 1997
43. Dibenedeto A, Conservation of Historic Stone Buildings and Monuments, Committee on Conservation of Historic Stone Buildings and Monuments, National Materials Advisory Board, National Research Council, 1982, National Academy of Sciences.

PRELIMINARY SURFACTANT SCREENING

Introduction
___.As mentioned in the first article, to enable wetting, the surfactants have to be in a specific HLB range, with particularities in agreement with the substrates chosen, in our case one or both of the two parts of a commercial orthodontic sealant and a solid, hydrophilic substitute for enamel. Used as typical resins, sealants have the same composition as the adhesives, except the filler, as these allow a more direct observation of their wetting ability. The surfactants have been, with few exceptions, the same as used for improving the bonding of bracket bases¹. As it is widely accepted that resin-based adhesives bond to enamel only by mechanical interlocking (no chemical affinity)², several hydrophilic substrates were chosen, each having its specific properties. The surfactants were either added (as such or in solution) to the sealant, or added to a standard etching gel.
Materials and method
___.The sealant selected was Bonding resin, from Reliance (Itasca, IL): to observe their absorption, traces of oil-soluble dyestuff were added. The surface active agents were Miglyol 812 and Miglyol 840 (Sasol, Witten, Germany); Petro AA, Phospholan CS-121 and Armid 18 ( all from Akzo Nobel, Houston, TX 77053); Aerosol OT (Cytec, W. Paterson, NJ 07424); Emsorb 6915 (Monson Co., Leominster, MA 01453); Fluorad 4430, Flourad 4432 and Flourad 99 (all from 3M, St. Paul, MN 55144); Span 40 Spray and Span 8 (Uniqema, New Castle, DE 19720); Merpol A, Agent X-1417-46 and Stepanate SXS (Stepan Co., Northfield IL 60093); Surfinol 104 (Air Products, Allentown, PA 18196); Zonyl FSP,. Zonyl FSK and Zonyl FSO 100 (Dupont, Wilmington, DE 19898); 5. CD-2 Oil detergent (CD-2 Co., Chicago, IL 60638); Triton X-101(Dow Chem. Midland, MI 48642); Lauryl sulfate (Spectrum Chem. , Gardena, CA 90248) . With the exception of Fluorad 99, soluble in isopropanol, all the surfactants were dissolved in a ratio 1: 20 volume in butyl acetate. The surfactants and their solutions in appropriate solvents are shown in Fig. 1
___.The substrates chosen were humidified qualitative Whatman filter paper (RA Lamb, Apex, NC 27502), common, unflavored gelatine (Knox, Parsippany NJ 07054) and a cement made out of two parts “Pure diatomaceous earth” (Recreational Water Prod., Scottsdale, GA 30079) and one part of Surface bonding cement (Sakrete,WR Bonsal, Charlotte, NC 28224) from which the plastic fibers were removed. To allow an in depth observation of the resins’ penetration, plastic foils were inserted into the cement to allow an easy and controlled separation of the cement parts.
___.The etching gel was made out of a 35% phosphoric acid solution and fumed silica Aerosil 200 (Degussa, Parsippany, NJ 07054). To allow observation, traces of a dyestuff were added. Other. substrates were common ceramic tiles which were previously etched with hydrofluoric acid 49% (Puritan Pro., Bethlehem, PA 18017) and a gelatine gel made by dissolving gelatine powder in warm water. To observe affinity, after adding the surfactant added drops, the gelatin was spread with diatomaceous earth to absorb the excess resin and then brushed repeteadly till most of the stains detach.
___.The sealant/resin, as such or added with a surfactant, was added in drops over the substrates, the latter being used as such or previously treated with water or gel. After various periods, the stains were examined or photographed with a Nikon Coolpix 5000 digital camera.

Results
___.Drops of surfactant added resin laid on filter paper wetted with gel show stains exhibiting differences in size when compared with the un-added resin (control, marked with 0, in the middle of each paper).
___.The surfactant-added drops of resin placed in specific (marked) places on gelatine showed, after the resin excess was removed with the help of the diatomaceous earth powder, a marked difference in affinity toward the hydrophilic substrate, as shown in Fig. 3, where all the controls (un-added resin. row marked with 0) ) where whipped out.
___.The stains obtained by placing drops of colored, surfactant-added resin on an etched tile, Fig.4, as well as these obtained from drops of un-added resin on an etched tile which was previously smeared with various etching gels, each added with traces of surfactants, Fig.5, where quite different from the controls. While several controls were placed throughout the tile, an appropriate comparison can be made with the spot marked A-1.
___.Using instead of flat surfaces an absorbing cement like the one containing diatomaceous earth, the influence of the surfactant becomes more striking. Fig 5 shows that the surfactant-added resin pentrates not only deeper, but also in a more diffuse way.
___.Fig. 6 shows the cement penetration of the two parts of the sealant (colored differently) as well as that of the result of their addition. In Fig.7, equal drops combined parts of the sealant were placed simultaneously, but after a two minute waiting period, on the porous cement. The difference resides in the fact that on the left, the sealant was not surfactant added
Discussion and conclusions
___.Adhesion is a surface phenomenon, and wetting plays a major role in it. By adding surfactants to various substrates, it is possible to screen potential additives that may significantly improve bonding strength. Years ago, we were the first to use the etching of the metal bracket bases, as well as their silanation: today, major manufacturers are using these methods on their newest brands.
___.While the addition of surfactants may not be easily feasible on adhesives, it may surely bring an edge to advanced etching gels which leave behind, afterr rinsing and washing, a hardly water-soluble film that has an affinity for the oil-loving resins. An added advantage of surfactant addition is that the gels get even thicker as part of these becomes emulsified.
References
1. Matasa CG, Orthod. Insider 2003; 15 (1): 5-8
2. Diedrich P, Enamel alterations from bracket bonding and debonding,
Am J. Orthod Dentofac Orthop 1981; 71: 500-522
3. Siomka LV, Powers JM, .Am J. Orthod Dentofac Orthop 1985; 88: 135

Orthodontic recycling can be “safe & effective”

___.One of two orthodontists in the UK¹ and one in three in the US² are recycling their attachments. As a result of the savings they generate, there are today some thirty orthodontic recyclers spread all over the world,
___.Today, medical recycling, a trademark of modern times, has reached the status of a socially an environmentally acceptable activity. In the American Association of Orthodontists Annual Report for the years 1997-1998, it was clearly stated that recycled direct bonding brackets are both “safe and effective”.
___.In our recent article in J. Clin Orthod3, where we have shown the dimensional changes generated by the existing recycling methods, we have not included etching. Following two articles in Am. J. Orthod^4,5 which suggested that micro-etching (abrading) new brackets is not an effective mean of enhancing bond strengths in brackets, we have measured the dimensions of twenty worn brackets which were subjected to the “Microetcher II” (Danville Engineering, Inc., San Ramon, California, U.S.A.) The statistical result has been added to our previous study³ and is shown below. Ortho-Cycle’s method (DB) uses adhesive dissolution and burnishing instead of subjecting the steel to high temperatures or to metal removal.
___.Among recyclers, Ortho-Cycle Co. is the only one that, besides being accepted in the USA by FDA, has been globally certified by the Scandinavian Institute for Dental Materials (NIOM) to refurbish orthodontic attachments, see adjoining copies of the ISO 9001: 2000, as well as the latest, ISO 13485: 2003 and EN 46002: 1996. These certificates indicate that Ortho-Cycle has followed the quality system requirements for medical device manufacturers, being in addition CE-certified for the production of medical devices for dentistry in accordance with the European Economic Community (Council Directive 93/42).
References
1. Coley-Smith A, Rock WP, Bracket Recycling—Who Does What? Brit. J. Orthod. 1997; 24(2): 172-4
2. Survey, J. Clin. Orthod. 1991: 25: 145-56
3. Matasa CG., Orthodontic recycling at the crossroads, J. Clin. Orthod. 2003; 37(3): 133-138
4. Grabouski, JK, Staley, RN, Jakobsen, JR, The effect of microetching on the bond strength of metal brackets when bonded to previously bonded teeth Am J Orthod Dentofacial Orthop 1998; 113:452-60
5. Maijer R, Smith DC, Am J Orthod Dentofacial Orthop 1986; 90: 198