___.Efforts to improve bonding strength focus rather on the bracket pad than on tooth treatment or adhesive. In recent years, the size of the bonding pad of the metallic direct bonding brackets has been reduced by 75% , and is still receding. In contrast, while the forces involved have remained the same or have even increased, tooth etching and the basic ingredients of adhesives do not differ from those used a half century ago. The latter are still a filled mixture of Bowen’s resin (bis GMA) diluted with a less viscous acrylate. In contrast, the bonding pad has evolved from perforated to mesh and then to grooves, dents or pegs. The demand for miniaturized brackets, along with orthodontists’ quest for reduced overhead and shorter chair time, has indeed put a strain on manufacturers (see figures and details next pages). To reduce the labor cost involved in brazing the mesh, Unitek shyly launched in the 80’s the machined bases (Dyna-LockR), joining the etched ones (Micro-Loc^R) made by GAC and the cast ones (Uni-Grid^R) by Ortho-Organizers. In the 90’s, brackets started to be made by injection molding. These processes opened the way not only to inexpensive, but also to mass produced direct bonding attachments that exhibited poor bonding strength-premature debonding. After an unsuccessful attempt to launch sintered (metal powder coated) bases, Orec had to return to mesh.
___.As practitioners became reluctant to experiment with the newer bases, most manufacturers limited themselves to the time-tested mesh: others, however, tried to improve the less expensive pads. Judging Dentaurum’s ads¹ and articles published in the specialty’s journals^2,3, their “IntelligentR” base not only equals, but also exceeds the performance of mesh bases. Similar claims were made for the base of the Time^R bracket⁴ sold by Adenta/American Orthodontics, as well as by Ortho-Organizers and Pyramid Orthodontics for their injection molded brackets. Responding to economic reasons, all the major manufacturers sell today, along with mesh-based brackets, attachments made using the last method.
___.In parallel, the quest for the best retentive mesh has continued in two directions: 1. the selection of the best performing type or combination, and 2. the finding of enhancing treatments. In the next pages, we have added our own measurements to the scarce information provided by manufacturers. In the Table, the older pads are shown first, along with specific data. The most important is the “mesh number”, i.e. the number of openings per lineal inch measured from the center of the wire to center of wire. Almost as important is the “wire diameter”, noted there with D. If too thin, it could break; if too thick, it limits the penetration of the adhesive, see the 40 mesh base Dyna-Bond I (Unitek I). A better penetration is facilitated by a high percentage of the “open area”. The size of the “aperture” (in m) also plays a role, as it can prohibit the passage of coarser particles (mesh = sieve).
___.Long considered to be the best, 100 mesh seems to be now in the second place, most manufacturers leaning toward a less dense mesh, a trend that is supported by our research, see next article. In time, both mesh and non-mesh bases have been treated by manufacturers in various ways, i.e. etched, sandblasted, polymer-coated or sprayed with fine particles of molten metal. Both etching and silanation were initiated by Ortho-Cycle Co., the first treatment being reported in 1985⁶ and the second before 1982⁷. Far from being over, the possibilities for improving bases retention still exist, as we will show later.
References
1. Brackets with mesh and laser structured bases: a comparison of bonding, Dentaurum, Print 989-532-21;. Rematitan titanium brackets, 1995, Dentaurum, Print 989-576-20
2. Alam RE, Sorel O, Cathelineau G, Morphologic comparison of the base surface of various metallic brackets, Rev. d’Orthop. Dento-Faciale, 1999; 33(2): 265-273;
3. Sorel O, Alam RE, Chagneau F, Cathelineau G, Alteration de l’email apres decollement in vitro d’attaches collees au vere ionomere modifie, L’Orthodontie Francaise, 2000; 71: 155-163
4 Heiser W, A new orthodontic philosophy, J. Clin. Orthod. 1998; 32(1): 1-10
5. Siomka LV, Powers JM, In vitro bond strength of treated direct bonding metal bases, Am.J. Orthod. D.O. 1985: 88: 133-136
6. Mascia VE, Chen SR, Shearing strengths of recycled direct-bonding brackets, Am.J. Orthod. D.O. 1982: 82: 211-216

DO ADHESIVES & SEALANTS REALLY SEAL THE
BRACKETS’ PAD? II. SURFACE TENSION

Introduction
___.Most adhesives and sealants used today are based upon bis GMA, a honey-like viscous monomer having a hydrophilic-lipophilic balance of +7.4¹. This value indicates a non-polar compound, far from the value +3.3 that indicates solubility in water.
___.Although there are formulations that render bis GMA derivatives more polar, i.e more water loving (e.g. through additions of hydroxy-ethyl methacrylate, HEMA), it is widely accepted that today’s orthodontic resin-based adhesives/sealants do not exhibit a chemical affinity for metals or teeth. Bonding is based exclusively upon mechanical interlocking, an phenomenon where hardened resin tags act as rivets. If bis GMA derivatives could, however, better wet and penetrate in the highly polar surfaces of the hydrated tooth enamel and the oxide-covered attachments, the resulting bond strength should improve. Today’s adhesives/sealants act like as oils, while stainless steel attchments as water (the latter property derives from the fact that such steels are covered with an impervious chromium oxide layer).
___.Some twelve years ago, we were the first to improve bond strength by etching² and silanating brackets3. As proven by the first study, this has lead to a higher bond strength. Today, both of these processes are used, among others, by the largest orthodontic attachments manufacturer, Ormco. While the silanation and the polymer coating used by TP Orthodontics (PrimeKote^R) are demanding procedures, the first requiring an almost mono-molecular layer and the second being based upon a proprietary formula, both lower somewhat the surface tension at the interface resin/metal. In what follows it is shown that better results can be obtained by treating the mesh with adequate surfactants, while using simpler means. The additives suggested are similar to those used to increase oil lubricity in cars’ engines or metal cutting.
Materials and Methods
___.Various AISI 304 stainless steel mesh samples obtained from McNichols, Tampa, FL 33630, were subjected to a thorough cleaning: the dissolution of oil contaminants with trichlorethylene was followed by the saponification of greases with a 10% sodium hydroxide solution in an ultrasonic cleaner. After abundant rinsing and drying, the mesh samples were subjected to several tests.
___.To allow a better observation of the rheological properties in which we were interested, instead of using a real, colorless adhesive, we simulated its properties by adding to bis GMA (sold as Rohamere 6662-0 by Röhm America, Piscataway, NJ 08855-0365) either a reddish filler, or a pigment. The filler used was the iron oxide CP-2240 Dark Orange (Ferro, Color Div., Cleveland OH 44101), and the pigment Naphthol AS, Coupling (Hunt, Statesville NC 28671). The latter was added just enough to render easy to observe the clear resin. In the iron oxide-added resin, which we will call Composite, the filler was added in proportion of 50-50% in weight, leading to a mass having a viscosity similar to that of adhesives, see Fig. 1. As two-part resin-based adhesives can exhibit an array of viscosities during their application, no efforts were made to measure the rheologic properties of the filled resin/Composite.
___.Out of some thirty surfactants tested, only half were promising: 1.Miglyol 812 (Sasol, Witten, Germany); 2. Aerosol OT (Cytec, W. Paterson, NJ 07424); 3. Emsorb 6915 (Monson Co., Leominster, MA 01453); 4. Fluorad 4430, 5. Flourad 4432 and 6. Flourad 99 (all from 3M, St. Paul, MN 55144); 7. Merpol A (Stepan Co., Northfield IL 60093); 8. Span 40 Spray, and 9. Span 8 (Uniqema, New Castle, DE 19720); 10. Surfinol 104 (Air Products, Allentown, PA 18196); 11. Zonyl FSP, 12. Zonyl FSK and 13. Zonyl FSO 100 (Dupont, Wilmington, DE 19898); 14. Agent X-1417-46 (Stepan); 15. CD-2 Oil detergent (CD-2 Co., Chicago, IL 60638). With the exception of Fluorad 99, soluble in isopropanol, all the surfactants were dissolved in a ratio 1: 20 volume in butyl acetate.

Bis GMA’s contact angle on stainless steel mesh. The test was performed by adding about 0.2ml pigment-colored bis GMA on a flat piece of 100 mesh, and recording the image with the help of a Hitachi VK-C-150 MOS Color Video Camera microscope (Edmund Scientific, Barrington, NJ 08007) connected through a Dazzle digital video creator (SCM Microsystems, Fremont, CA 94538) to a computer. To obtain the contact angle q, a protractor was applied on the recorded image.
Mesh penetration by colored bis GMA. Large drops of clear and later of pigment-colored bis GMA (Composite) were placed upon samples of stainless steel mesh measuring 5x3 inches, and observed and photographed for their ability to penetrate. The time elapsed since the drop was placed till half of it had penetrated the mesh, see Fig. 2, was recorded.
Mesh penetration by the surfactant-added bis GMA. Portions of about 1 cc pigment-colored bis GMA were added with various surfactants in a proportion 1 to 100 in weight, the mixture homogenized and the resulting compounds placed upon 100 mesh samples. To select the most active surfactants, the time till half of the resin’s volume penetrated the mesh was compared.
Surfactant-treated mesh penetration by bis GMA. Squares were scratched on a 4.5 x 4.5" 100-mesh sample with a tungsten carbide, and in each of these a different surfactant was applied, diluted 1% in butyl acetate. The next day, after the solvent evaporated, drops of colored bis GMA were added on each square, see Fig. 3 (a). After 12 h, the penetration of the mesh was photographed on both sides (b) and (c) with a Nikon Cool Pix 950 camera.
Surfactant-treated mesh penetration by the Composite. To duplicate the penetration of the Composite trough a bracket’s mesh, a simple system was improvised using PVC plumbing parts and a C battery. The latter was emptied and made to slide along a tube serving as guide directing its protrusion to lean on the center of an end cap, Fig.5. In the later, disks of filter paper were succesively placed upon the various 100 mesh disks tested that rested on a blob of Composite. Treated with various surfactants or not, both disks were carefully centered. The paper ones (Filter paper, Whatman, Fisher Scientific, Suwanee, GA 30024) measured 6.6 mm in diameter, while the one of stainless steel mesh 7.6 mm. Both were cut using common paper punches.
___.For ease of handling, the battery shell had a wire with handle brazed to, Fig. 4 and 5. For provide pressure, the battery shell was filled with enough lead foil so that the cylinder, the wire and the handle would weigh together 100g. To increase the pressure exerted, Fig. 6, two standard 100g weights could be added. As the weight caused the Composite to penetrate the mesh, it led to red marks on the paper, Fig.7. After a chosen time, the system was dismantled and the marks examined and compared.
Results
Bis GMA’s contact angle on stainless steel mesh. The contact angle q shown in Fig. 8 is well above 90o, indicating that bis GMA poorly wets the 100-mesh stainless steel sample.
Mesh penetration by colored bis GMA. As shown in Table I, the viscous liquid takes quite a variable time to flow through the mesh. The finer the latter, the longer it takes for the resin to penetrate, despite a similar open area (See Table, first article).
Mesh penetration by the surfactant-added bis GMA. Preliminary tests were used to determine: 1. if the surfactant could be added to the adhesive, 2. which is the most active surfactant. The tests indicated activity for the ones listed above from 1 to 15. The colored bis GMA, without other additives (Control) did not penetrate the 100-mesh sample even after 12 h.
Surfactant-treated mesh penetration by bis GMA. Instead of adding the surfactants to bis GMA, drops of the colored version of the latter were placed upon differently surfactant-treated squares of a 4.5 x 4.5" 100 mesh sample. Even after 12 h, the clean, the untreated 100-mesh was not penetrated, as shown in Fig. 3 (Control: #A-1, C-5, D-5 and E-5). Some of the surfactant-treated squares behaved the same way, while these indicated above with numbers from 1-7 allowed the penetration within minutes.
Surfactant-treated mesh penetration by the Composite. In a test where the Composite was forced to pass through differently treated mesh disks to leave marks on subjacent paper disks, Fig. 5, several parameters were examined:
Variation of the penetration with the force (weight). The mesh disks were pressed for 1 minute upon the Composite by weights varying between 100 and 300g. The marks left on paper by the control mesh disks (not treated) and by the mesh disks treated with the best performing surfactants (2 and 3 on the previous list), are shown in Fig. 9. The image was obtained by attaching the corresponding marked paper disks to a paper template, then scanned and recorded. The force exerted by 100 g was judged satisfactory, as it best allowed comparisons.
___.Variation of penetration in time. The same procedure was applied to determine the time when the most telling marks were obtained. The weight used throughout was 200g, and the surfactant chosen for testing, along the control, was # 2 in the list. Acceptable telling times were considered to be 1 to 2 min, Fig. 10.
___.Variation of penetration with # of mesh disks. Instead of using just one mesh disk, two 100-mesh disks were superimposed. A force of 200 g pressing for 2 min. practically did not generated marks on the paper disks even when the two mesh disks were treated with the most active surfactants. In contrast, the mesh disk in direct contact with the Composite was well wetted (Mesh 1, back, Fig.11). The second mesh disk, facing the firsts opposite site, was barely spotted.
___.Variation of the penetration with the surfactant. Using the parameters considered to be the most telling, i.e. forces of 100 and 200g as well at an exposure of 1 min., tests were performed mesh disks treated with surfactants. The numbers shown in both Fig. 12 and 13 correspond to these of the list of surfactants presented above. In addition to the disks of paper, the side of the mesh disk opposed to the Composite is also presented. While several surfactants favored a better mesh penetration than that allowed by the control (untreated mesh), the best results were given by Aerosol OT and Emsorb 6915 (#2 & 3). The first is a white solid-resembling wax (sodium dioctyl sulfosuccinate), while the second a yellowish, viscous liquid (an ester of a fatty acid with sorbitol, a sugar). None of them are toxic.
Discussion
___.While bis GMA is not the only resin used in sealants and adhesives, its weight and properties determine their behavior. Its diluents, lower molecular weight acrylates, lessen but do not alter substantially the phenomena involved. In the same vein is the addition of small quantities of a pigment/dye or the substitution of aluminum oxide (the most common filler) with another oxide.
___.Wetting and waterproofing are phenomena we encounter daily. If a fabric or paper is coated with a surfactant, it will absorb water. If coated with a waterproofing agent, such as our raincoats or glossy papers, it will repel it. Metals and teeth are hydrophilic, repelling oil-like liquids. The phenomenon of wetting or non-wetting of a solid by a liquid is evidenced by what is known as contact angle. A related large body of reliable data has been accumulated, and standards^5-7 use contact angles to evaluatesurface properties.
___.A drop of liquid on a solid may be considered as resting in equilibrium as it shape indicates a balance of three forces (the interfacial tensions between solid and liquid, that between solid and vapor and that between liquid and vapor). The angle within the liquid phase is known as the contact angle or wetting angle, and is formed between the tangent plane to the surface of the liquid and the tangent plane to the surface. The surface tension of the solid will favor spreading of the liquid, but this is opposed by the solid-liquid interfacial tension and the vector of the surface tension of the liquid in the plane of the solid surface. For a liquid to wet a solid, the surface energy of that solid must be able to overcome the surface tension of the liquid.
___.In the mesh-adhesive system, even the significantly more polar methyl methacrylate doesn’t wet the oxide-coated stainless steel, as primers are specifically designed for the purpose (Hysol Primer 2000, Dexter, Irning, Tx 75061). The 108^o contact angle exhibited by bis GMA shows that the mesh actually repels the liquid, as proven by drops that remain for many hours above a stainless steel 100 mesh. A proper primer not only helps wetting, inviting the adhesive to pass through the mesh, but, as advertised by Dexter, would “significantly enhance the bond strength while improving bond durability after exposure to harsh salt-water environments”.
___.As a difference from mesh silanation or polymer coating, none of which has a marked surface activity, both bracket bases and tooth enamel can be rendered more penetrable by adhesives or sealants by simply wiping the desired area with a solution of an active surfactant: after the solvent evaporates, it leaves behind a film enhancing the substrates’ wettability.
___.Some manufacturers claim (see Fig. 14a, taken from an ad) that the adhesive penetrates the mesh, and by setting there, generates a riveting effect. The crevice corrosion often found at the interface enamel-base (see our previous issue8), contradicts this. Unless pressed, the adhesive won’t pass even through one mesh layer, leaving behind an empty space, as seen, marked in red, in Fig. 14b. To claim that the adhesive penetrates three layers of mesh9, see Fig. 15, is wishful thinking: even GAC, the patent’s assignee, has reduced the layers to two in its “Supermesh” base.
___.It is strange that some clinical recommendations, referring to direct bonding, use terms such as “placement”, “sitting” and “positioning” instead of stressing the need to PRESS the bracket against the tooth, strong enough to force the oily adhesive to penetrate deep enough into both the water-loving substrates.
___.The finding that specific surfactants improve bonding is new and patentable. However, as in the case of etching and silanation, instead of exploiting the method for gain, we are giving it away, in the hope that it will better benefit the profession and the patient. While determining the nature of the added surfactants may be difficult, a treated attachment will be easy to spot: drop it on a water surface covered with Baby powder, (talcum), and the powder will be instantly repelled away.
Conclusions
___.Both sealants and resin-based adhesives are hydrophobic oils, while teeth (hydroxy apatite!) and oxide-covered metals are hydrophilic, repelling oils. In particular, the basic organic ingredient in most sealants and adhesives, bis GMA, exhibits very little propensity to wet the chromium oxide-protected stainless steels.
___.By adding suitable surface-active agents to the sealant, to the adhesive or to the etching gel, or by coating the metal surfaces or the teeth to be bonded with a film of appropriate surface-active agents, the penetration of the resins and the ensuing bond strength cannot but be enhanced.
___.While in the present study we did not try to correlate this property to actual bonding, or to improved tooth preparation, it is obvious that the potential is there. Clinical tests to confirm both hypotheses are under way: these could contribute to save chair time by reducing rebonding and limit microorganism attacks, crevice corrosion as well as other consequences of poor contacts between the joined parts.
References
1. Matasa CG, The Orthod. Mat. Insider 2000; 13(1): 5-8
2. Siomka LV, Powers JM, In vitro bond strength of treated direct bonding metal bases, Am. J. Orthod. D. O. 1985: 88: 133-136
3. Mascia VE, Chen SR, Shearing strengths of recycled direct-bonding brackets, Am. J. Orthod. D. O. 1982: 82: 211-216
4. Scarola BV, Thesis M.S., St.Louis Univ.,1986
5. ASTM D724 (1999) Standard Test Method for Surface Wettability of Paper (Angle-of-Contact Method)
6. ASTM D5725(1999) Standard Test Method for Surface Wettability and Absorbency of Sheeted Materials Using an Automated Contact Angle Tester
7. ASTM C813-90(1994) Standard Test Method for Hydrophobic Contamination on Glass by Contact Angle Measurement
8. Matasa CG, Do adhesive & sealants really seal the brackets pad? I. Corrosion. The Orthodontic Materials Insider 2002; 14 (4): 5-8
9. Iida E., Orthodontic apparatus for attachment to a tooth,
US Patent 4,889,485 ’89