Introduction An appeal for a grid of brackets’ strengths and weaknesses, launched last month by members of ESCO (Electronic Study Club for Orthodontics, a free Internet service operated and sponsored by the Department of Orthodontics, University of Illinois at Chicago, http://www.uic.edu/depts/dort/esco.html) demonstrates that orthodontists, despite the vast literature available, are unhappy with some of today’s attachments and related information.
___.As part of the sponsoring organization, I will present a different, rather “engineer-type” approach to bond strength testing. While there are attempts to test bonding strength in vivo,^{1, 2} in vitro researches remain key. For the researcher, the attempt to duplicate what happens in the mouth is quite frustrating, as there are too many variables involved in each of the parameters listed below. In addition, as long as one of the substrates (or surface to be bonded) continues to be bovine teeth, progress will be slow. Unfortunately, attempts to encourage substituting tooth substrates with ceramics^{3, 4} or metal⁵ (as early as in 1993 we suggested laminated stainless steel mesh) have not received acceptance.

Variables involved in testing: discussion

The substrate. Teeth are different in shape, profile, structure, and composition. While bovine teeth are the accepted choice, it is obvious that herbivores have a different diet, and all too often their teeth do not match that of humans, which in turn exhibit differences even among themselves.
The attachment. Be it bracket, band, cleat, or button, the attachment must present to the substrate (enamel) a retentive surface. Aside from the many types of mesh (examined in a previous article⁶), modern technology offers a variety of retentive elements, e.g., indentations, grooves, dents, micro-cracks, pores, locks, and protrusions (as such or coined, or making an angle with the base’s plane).
The adhesive. Glass ionomers, compomers and the like will not be discussed below, as they introduce a supplementary variable: chemical affinity. Relying only on mechanical interlocking, resin cements are the ones most used because of their strength. Their basic composition has remained almost the same as half a century ago; their most reactive part, the initiator, is unstable and has a limited shelf life even if refrigerated. Their inherent or accidentally acquired impurities might also play a role, speeding or slowing the setting.
The cure. Two-part adhesive systems fail to achieve the polymer’s full strength, either by insufficient mixing of the parts (most often encountered in the so called “no-mix” adhesives), or by their inappropriate ratio (an excess of initiator leads to too many short, and therefore weak, chains; whereas not enough leads to insufficient cross-linking). Both a too short light exposure and a too extended handling of the chemically activated adhesive will lead to lower bond strengths. Even though the mix may still flow, its increased viscosity might hinder its ability to penetrate both substrates (the Trommsdorff effect.)⁷ In addition, the photochemical cure has a major drawback: the attachment is often an obstacle that cannot be readily overcome. A “soft core”, i.e., an insufficiently polymerized portion found under the base of an opaque bracket, is the rule rather than the exception. In both systems, an even, thin “glue line” (which offers the maximum bond strength⁸) between the two substrates (tooth and bracket) is difficult to achieve.
___.As a result, neither the chemical nor the photochemical systems, as currently used, can achieve a uniform, ultimate cross-linking of the monomers contained in the resin cements.
The procedure. The phenomena taking place while bonding an oily adhesive (methacrylate monomers are almost insoluble in water) to hydrophilic substrates (teeth and brackets) are complex. Microscopic analysis of the cured adhesive at the bracket/enamel interface^{9, 10} shows not only a lack of affinity between the surfaces to be joined, but also unfilled spaces. The addition of hydrophilic monomers to the adhesive helps in this respect but, unfortunately, fails to provide the composite with a structure able to withstand the stresses to which brackets are commonly subjected.
___.Some differences in vivo bond strengths, often leading to failures, derive from aesthetic considerations. Not only does the amount of adhesive have to be sufficient, but its spread around the attachment must be controlled; quite often, the movement involved leads to a loss in strength.
The evaluation. The forces required to debond brackets have been assessed with the help of universal testing machines, using tensile, shear or “shear-peel” strength tests. Researchers who rely on “shear-peel” should be aware that this is a misnomer; the angle of pull is critical, and even a small deviation of the attachment jig adds peel, the lowest force necessary for debonding. For clinical situations, debonding forces should be between 6 and 8 MPa,¹¹ one megapascal being equivalent with 145 psi. Bond strength values above 10 MPa are considered dangerous in that they could cause enamel fracture during debonding.
___.Universal testing machines, irrespective of the brand and model, are not only large and expensive, but also demanding and difficult to handle. As a result, the number of attachments tested seldom exceeds 20. In addition, the resemblance of such tests to in vivo situations is slim because the complex interaction process in the oral cavity cannot be reproduced in vitro. In addition to other criticisms,^{12, 13} it has been found that in vitro bond strength cannot even be correlated with clinical failure indices, thus questioning the applicability of these tests to the clinical situation.¹⁴
__________________Ways to minimize the number of variables
___.The testing of any system involving many variables proves to be futile: to approach reality, the number of variables shown above must be reduced to just one per group.
The substrates. Assuming there is no chemical affinity between the water-hating adhesive and the tooth, the joining being based just upon mechanical interlocking), any chemically inert substitute can be used instead. In other words, it is unnecessary to substitute human teeth with irregular bovine teeth, no matter how well stored or fresh: to get and use them is difficult and time consuming. To allow a larger number of tests all over the world, the substrate has to be easy to make and duplicate in order to provide the same retention. An exception may be the case of photo-cure, when the light reflectivity of the substrate is sought to match that of teeth.
___.While acid-etched ceramics provide comparable results, it is difficult to achieve a standardized composition and degree of porosity. Better reliability is provided by a mesh-laminated, stainless steel plate, an approach we described 15 years ago.⁵ In contrast to ceramics, stainless steel mesh is standardized and not difficult to laminate on a solid metal sheet.
The attachment. Although some of the retentive elements of bracket bases vary from batch to batch, a larger number of tests using attachments made at different times offer a better representation. Any flat attachment can be used as being typical, as long as it is properly cleaned and seems identical.
The adhesive. As both the photo- and chemically activated systems exhibit liabilities (see above), a total cure (highest cross-linking) of the monomer is unlikely, as long as in-office conditions are respected. Every adhesive has a portion containing an initiator and it is reasonable to assume that by heating this thermodynamically unstable part, the saturation limit of the monomer’s cross-linking capability (highest bond strength) will be reached. Any sufficiently fluid, initiator-containing adhesive, as such or as part of a system is therefore adequate for comparing bond strengths..
The cure. Be it chemically or photo-chemically activated, our tests show that any initiator-containing component of an adhesive achieves cross-linking saturation at a temperature of around 100°C (212°F) if subjected for over 30 min. Charring or other degradations of the polymer in these conditions are highly unlikely phenomena.
The procedure. Maximum strength (cross-linking saturation of the polymer contained in the adhesive) can hardly be reached using common, in-office methods. In contrast, an advanced cure can be achieved by using controlled heat. This can be obtained by post-curing the bond using a common, temperature-controlled conventional oven, or a microwave oven. In the latter case, because it reflects energy, the metal does not get too hot, unless an arc is formed (contacts). While the oven’s magnetron tube might get somewhat more wear, our experience shows that its sporadic use has no adverse effects. Aside from the warming of the adjacent substrate and the brackets, the seemingly non-polar adhesive contains enough polarizing ingredients to heat up. Microwave heating of bonded brackets to a metallic, mesh-laminated plate ensures a more uniform distribution of heat for curing.
The evaluation. Universal testing machines are difficult to adapt for a limited, specific test. Tensional force is preferred not only because it is less damaging to the enamel,¹⁵ but because it provides more accurate data. Tension is easier to control and requires less force than shear,¹⁶ and as long as the strength and direction of the debonding force are properly controlled, the results are more reliable.
___.Universal testing machines are widely used for the purpose because they provide better force control. A study has shown, however, that crosshead speed variations do not influence debonding force measurements or the failure mode of brackets bonded to enamel with a composite adhesive.¹⁷ Any system permitting a controllable increase and speed in delivering the debonding force may therefore be preferable because it offers both ease in handling and a sufficient number of tests achievable in a limited time.
Conclusion
___.Replacing the bovine teeth substrate with a laminated stainless steel mesh, adding to the common office procedure an in vitro thermal post-cure, or directly heating the bonded brackets would eliminate some of the system variables. Replacing the complex universal testing machine with a simple, controlled scale system may lead to an easier and faster assessment of bond strength. Such an attempt is presented in the following article.
Reference
1. Hubert EARB, Almeida RR, Lascalla CA, Cotrim-Ferreira FA, Vellini-Ferreira F. Development of an adequate device to measure bracket debonding force in vivo. J Brasil Ortod. Ortopedia Facial 2001; 6: 227–33.
2. Prietsch JR, Spohr AM, Lima da Silva IN, Beck JPB, Oshima HMS. Development of a device to measure bracket debonding force in vivo. Eur J Orthod 2007; 29: 564-70.
3. Arici S, Minors CJ, Messer PF. Porous ceramic lamellae for orthodontic ceramic brackets. I. Fabrication and characterization. J Mat Science: Materials in Medicine 1997; 8: 441-46.
4. Matasa CG. Searching for the best direct-bonding brackets? The ARI concept can help to save you money! Orthod Mat Insider (formerly Phoenix without Ashes) 2001; 14(1): 1-7 (Visit the Internet at www.orthodonticmaterials.com, March 2001).
5. Matasa CG. The poor man’s tensile strength tester. Orthod Mat Insider (formerly Phoenix without Ashes) 1993; 6 (1): 6-7 (Visit the Internet, see above, March 2001)
6. Matasa CG. In search of a better bond. Orthod Mat Insider 2003; 15(1): 1-3 (Visit the Internet, see above, March 2003).
7. Matasa CG. A simple bond strength testing device and . . . the Tromsdorff effect. Orthod Mat Insider 2005; 17(2): 5-8 (Visit the Internet, see above, June 2005).
8. Matasa CG. Adhesion and its ten commandments. Am J Orthod Dentofac Orthop 1989; 95: 355-56
9. Matasa CG. What is hidden behind your bracket’s mesh? (I). Orthod Mat Insider 2003; 15(4):1-7 (Visit the Internet, see above, December 2003.
10. Matasa CG. What is hidden behind your bracket’s mesh? (II). Orthod Mat Insider 2004: 16(2):1-5 (Visit the Internet, see above, June 2004).
11. Reynolds IR. A review of direct orthodontic bonding. Br J Orthod 1975; 2: 171-78.
12. Fox NA, McCabe JF, Buckley JG. A critique of bond strength testing in orthodontics. Brit J Orthod 1994; 21: 33–43.
13. Van Noort R, Noroozi S, Howard IC, Cardew G. A critique of bond strength measurements. J Dent 1989: 17: 61-67.
14. Sunna S, Rock WP. Clinical performance of orthodontic brackets and adhesive systems: a randomized clinical trial. Brit J. Orthod 1998; 25: 283–87.
15. Valletta R, Prisco D, Santis R, Ambrosio L, Martina R. Evaluation of the debonding strength of orthodontic brackets using three different bonding systems. Eur J Orthod 2007; 29: 571-77.
16. Matasa CG. Modeling mechanic debonding with the help of the Velcro fastener. Scientific poster at the 108th AAO Annual Session, Denver, May 2008.
17. Klocke A, Kahl-Nieke B. Influence of cross-head speed in orthodontic bond strength testing. Dent Mater 2005; 21:139-44

Bond strength of various bracket base designs

Introduction
___.Studies having titles similar to that of this article1-4 have appeared and no doubt will continue to appear in the orthodontic literature. In general, they describe the bond strengths of up to 10 types of bracket bases bonded to bovine teeth, each time using a different adhesive or curing system. The number of attachments tested is small, the force used is shear or the misleading “peel/shear,” and the measuring instrument is a universal testing machine.
___.The following approach starts from the premise that resin adhesive bonding is based only on mechanical locking and that advanced polymerization of the adhesive can be obtained only by using heat. As force it uses tension, as delivered by an easy-to-duplicate, modified scale, and as a substrate, laminated stainless steel mesh. These simplifications allow the performance of more tests in less time, use less skilled effort, and circumvent the unevenness of adhesive curing. A theoretical presentation of the above was the object of the preceding article.
___.From a pool of 32 types of flat-based (for central and lateral incisors) brackets, we randomly chose 20 for testing the differences in bases’ structure according to the related manufacturing and design requirements, indicating each time the method [MIM: metal injection molding; PIM: polymer injection molding; CIM: ceramic injection molding; MEC: other fabrication; Mesh # (openings/linear inch): M # [40, 60, 80 or 100)]; SM: super mesh [2 layers]; SINT: sintered powder].
___.With the exception of 3M Unitek’s ceramic Transcend 6000 and Clarity and the polymeric brackets (CEOSA’s Polysulfone, Unitek’s Damon 3 [hybrid], and Ormco’s Spirit MB), which were all new, the rest were used and had the adhesive removed using Ortho-Cycle’s polymer dissolution method.
___.The brackets were bonded to two identical stainless steel substrates consisting of 15x15 cm plates, 2.3-mm thick to which was laminated an 80-mesh net. In all cases, the adhesive used was the initiator-containing paste of Rely-A-Bond (syringes of 3.5g Reliance Orthod. Itasca, IL).
___.Instead of using the current office method of curing the adhesive (two parts/chemically cured or photochemical), the brackets smeared with adhesive were pressed on the substrates and subjected to heat in a microwave oven. To avoid undesirable contacts or electric arcs, the substrates were placed on a rigid (thermoset) sponge. Tests made in advance under the same conditions but having different exposures have shown that the adhesive’s strength reaches cross-linking saturation (polymer’s highest cohesion, i.e. maximum strength) after an exposure of 30 min at medium heat. Measured with an infrared thermometer (Cen-Tech, Camarillo, CA), the temperature of the brackets and the substrate was around 90^oC. After a metal block. The brackets were then evaluated within a few hours for both adhesive remnants (in an attempt to use the adhesive remnant index, ARI) and bond strength, and the system photographed before (Fig 1) and after the test (Fig 5). The bond strength test was performed with the help of the modified scale shown in Figure 2. With the help of a counterweight connected through a wire, the brackets were pulled by inserting under the tie wings the debonding jigs (mostly 0.016-in wire loops) shown in Figure 3. In the case of ceramics and plastics, the wire had to be padded or coated to avoid breakage. In some instances, a special jig was used to not only lift, but also to hold the attachment by pressing it (Fig 4).
___.In the counterweight container (f), some 5 kg of 8-mm diameter stainless steel balls (h) were added manually at an average flow of 2.5 kg/min. If, after adding 5 kg, the bond did not break, the balls were evacuated through the outlet (g) and replaced with a lighter weight (e). The balls were added again and the operation repeated until the attachment detached. The tension forces needed to debond the brackets are shown in Fig. 6 both as such (in kilogram-force, 1 Kgf = 9.8 Newtons), and taking in account the base’s surface (in megapascals, 1MPa = 10.2 kilogram-force/cm2) . The bases’ configuration and detail (lower square, right), are shown in Fig. 9. In the circumstances described, the concept of ARI was only indicative, as in all instances the plate laminated with mesh retained almost all the adhesive, Fig. 10.as the continuous mesh network of the plate offers the adhesive a stronger support (Fig 11).
Discussion
___.This approach permits evaluating the bond strength of a large number of attachments in a short time and with less effort, opening the way for a grid of bracket strengths and weaknesses. Since some of the attachments couldn’t be properly removed (due to delamination or to tie wing breakage), Figure 5 shows the average debonding force for the attachments which could be tested (of which there were at least ten). Because of the large number of tests, no attempt to statistically evaluate the debonding forces was made.
___.While some of the variables plaguing the tests were avoided, a common problem with in-office curing, i.e. the inability to place the optimal amount of adhesive on each bracket base, continued to scatter the results. In addition, in contrast with in-office curing, it was unfeasible to remove the excess adhesive from the mesh-laminated substrate. As known, the ARI concept couldn’t be applied; in the case of the highest bond strengths, however, it allowed some correlation between the adhesive remnant’s lack of cohesion and the plate substrate’s mesh distortion.
___.Examination of the data in Figure 6 strikingly reveals that almost all the one-piece brackets (made by MIM) yielded better bonds than their mesh counterparts (M#). This is in sharp contrast with clinicians’ general perception (some manufacturers use this to provide non-mesh bases with mesh appearance). Some bond strengths were so high that it can be presumed that if Dentaurum’s laser-produced craters in its Discovery bracket (Dentaurum, Ispringen, Germany) (Fig 9, image 1) were extended over the entire surface of the base, the bracket would fracture the enamel. Owing to its more homogeneous structure, the mesh bracket exhibits less variation in bond strength than that of its non-mesh counterpart, which may be in some instances less dependable. Surprisingly, few nonmetallic brackets exhibit higher bonding strength than some of their metal counterparts (although some the latter also broke/delaminate in the process).
___.The high bonding strength exhibited by some brackets should be a warning for the future, as some may be on the brink of fracturing the enamel. While the in vitro conditions we used might not parallel the in vivo ones, and the tests should be taken only as indicative, attention should be paid to the excessive retention manufacturers sometimes provide to please customers. The unexpected bond strength of the polysulfone brackets raises the interesting question of whether the property rests with the base’s configuration as patented,⁵ or with the polymer’s surface affinity for acrylic adhesives.
___.Some of the features discussed were foreseen long ago, as shown below. Indeed, in articles we published 18 years ago in the Journal of Clinical Orthodontics⁶ and continued in the American Journal of Orthodontics and Dentofacial Orthopedics on the future of brackets,⁷ we concluded that“there will be one-piece attachments having built-in bases, vertical slots, or ball-ended power arms. New self-ligating systems will be found . . . Their bases will be retentive not only because of macro-undercuts, but also because of larger surface areas which could be produced by chemical treatments, providing micro roughness and coupling affinity.”
___.At that time, we also wrote in our newsletter^{8, 9}: “One-piece brackets, if properly selected and . . . checked, can satisfy the most sophisticated demands for less money. As shown in the first part of this series, one-piece brackets can even rise above the combined ones, as not having heterogeneous metal additions (brazing). From the accidental faults point of view, one-piece brackets compare again favorably with the combined ones as assembly is eliminated. ___.Their systematic faults are no different from those found on the combined brackets: on the contrary, as there are fewer parts to watch and assemble, the chances of picking a “lemon” is even slighter. Unfortunately, only a few exhibit strong enough undercuts to rival the mesh bases. The advent of newer technologies makes us believe that, despite a certain decline today (see statistics, page 8), one-piece brackets will be the preferred orthodontic attachments of tomorrow.”

References
1. Wang WN, Li CH, Chou TH, Wang DD, Lin LH, Lin CT. Bond strength of various bracket base designs. Am J Orthod Dentofacial Orthop 2004; 125: 65-70.
2. Knox J, Hubsch P, Jones ML, Middleton J. The influence of bracket base design on the strength of the bracket–cement interface. J Orthod 2000; 27: 249-54.
3. Bishara SE, Soliman MMA, Oonsombat, Laffoon JF. The effect of variation in mesh-base design on the shear bond strength of orthodontic brackets. Angle Orthod 2004; 74: 400–04.
4. Sharma-Sayal SK, Rossouw PE, Kulkarni GV, Titley KC. The influence of orthodontic bracket base design on shear bond strength. Am J Orthod Dentofacial Orthop 2003; 124: 74-82.
5. CEOSA, WO Patent 2006/136618, www.wipo.int/pctdb/en/wo.jsp?wo=2006136618.
6. Matasa CG. Flaws in bracket manufacturing. J Clin Orthod 1990; 24: 149-52.
7. Matasa CG. Direct bonding metallic brackets: where are they heading? Am J Orthod Dentofacial Orthop 1992; 102: 552-60.
8. Matasa CG. One piece brackets are here to stay! I. Manufacture, advantages, disadvantages. Be among the smart ones who ride with the wave. Orthod Mat Insider (formerly Phoenix without Ashes) 1994 (March); 7(1): 3-8 (or visit the Internet at www.OrthodonticMaterials.com/March 1994).
9. Matasa CG. One piece brackets are here to stay! II. But keep your eyes open! Orthod Mat Insider (formerly Phoenix without Ashes)1994; 7(2): 3-6 (or visit the Internet, see above, June 1994).