___.Half of the products that were launched ten years ago have since been discontinued. In contrast, for almost half a century orthodontists have used the same adhesives, sealants and restoratives, all of which have the same basic formula. The resins on which these are based have long been reported to be toxic, cytotoxic, mutagenic, carcinogenic and oestrogenic.*
___.Why are these still used? Because it is by far more comfortable and more profitable to change packaging, details and/or advertising than to spend effort and time to develop and test new formulas. Even if we stick with acrylics, there are hundreds of these to choose from. According to the FDA, it takes eight and a half years on average to study and test a new drug before the agency can approve it for the general public, and the early laboratory and animal testing, as well as later clinical trials using human subjects, used to cost some $359 million in 1993.
___.While new adhesives fare better than drugs, the necessary physical lab tests may require thousands of hours and specialized personnel. Complex equipment can do jobs that take months in minutes, but the reverse is also true. Raphael Bowen, DDS, who is not a chemist, found the formula discussed in a modest lab, using simple methods. Shouldn’t we learn from this?

Introduction
___.The American Journal of Orthodontics and Dentofacial Orthopedics reported the first use of universal testing machines to test orthodontic biomaterials in the 80’s.¹ Large and expensive, these instruments used in industry to measure loads and the associated test specimen deflections require specialized personnel who also have to be also familiar with the complex computer programs with which these interface. Designed for other purposes, the proper mounting/ inserting of tiny specimens is difficult and prone to error. In addition, their complexity leads to results which are seldom reported using the same conditions and units.
___.While during the last two decades the universal testing machines have contributed to improve biomaterials, the time and effort necessary to use these to evaluate a large number of samples are considerable. Besides other resins, the investigation should be directed toward finding beneficial additives or surfactants that can improve direct bonding, a topic which was recently presented in San Francisco.² Taking the last issue as an example, surface-active agents, key for bonding, can act both ways, some improving while others worsening the substrate’s wetting. On the market, there are literally thousands of them. Only Dow Chemicals sells tens of surfactants which are all Tergitols or Tritons, the apparent difference being just a code following these names... Types aside, the additive’s concentration plays a major role: if too diluted, surfactants may not exhibit the properties expected, while if too concentrated, these may behave like oils clogging the microfissures and pores, hindering the penetration of the adhesive.
___.After decades of trial and error during which we have suggested do-it yourself methods, we have now found a way to avoid, at least for the initial search, the cumbersome use of universal testing machines. We are so happy about it that we believe it could be constructive for others to know how our attempts to evaluate/measure the mechanical properties of brackets, ligatures and adhesives can lead to simple methods.
Evolution of... our evaluations
___.Throughhout the years, a number of university theses we know of have been inspired from our works on orthodontic biomaterials. While we believe that their number is greater (some may have not indicated the filiation of ideas), we will review them in the hope that this trend which we encourage will continue. In our opinion, the experience gathered by attempts to reduce an investigation to its core may lead not only to fundamental solutions, but also to a better understanding of the phenomena involved.

Optical systems. Borrowing a technique developed for universal testing machines, we embedded brackets in acrylics and then sectioned and micro-photographed these to demonstrate their manufacturing particularities or defects,^3-6 as well as the impact of subsequent treatments.⁷ Thus, by examining various brackets from different manufacturers, we found major differences in slot shape: some were too rough to allow the arch wire’s proper sliding; others had their slot bottom too round to enable adequate torque.⁸ Some of these preliminary tests were confirmed with the help of sophisticated equipment, the use of which was reduced, however, to a minimum. Thus, in order to evaluate the slot’s roughness and its consequence and friction, several metal and ceramic brackets were examined using Scanning Electron Microscopy, as well as Atomic Force Microscopy and Lateral Force Microscopy.⁹ By strongly magnifying sections of cast and injection-molded bracket to expose their retention systems, we found out that in some cases, their ability to mechanically interlock with the adhesive was far from being acceptable.¹⁰ For this purpose, randomly selected worn brackets, with the adhesive clogging the base, were acrylic-embedded and sectioned; after a low magnification screening, these were subjected elsewhere to scanning electron microscopy. The mesh bases showed both pores and even gaps at the interface metal-adhesive,¹¹ which are the results of a poor interfacial wetting. In addition to demonstrating a source of potential corrosion and poor bond strength, the above findings have led to other research having as a goal to show the influence of surfactants. In this research, which deals not with chemical affinities, but with mechanical interlocking, we substituted acrylic-mounted teeth, Fig. 1, with ceramic tiles, Fig. 2 to demonstrate a decrease in surface tension and a better interpenetration of the adhesive with the mesh^12-14 and the inorganic support.
___.Another study had as its goal the slot width variations exhibited by brackets from the same manufacturer, and a comparison between these variations and those found between the new and recycled ones. This study, performed first with modest means, was then contracted by Ortho-Cycle to be performed by a forensic expert at the Penn State University in Harrisburg with the help of the sophisticated SmartScope® system, a complex non-contact optical instrument with an accuracy of .004 mm in all three axes (OpticalGaging Products, Rochester, NY, USA 14621-4896). The study showed that the variability between the slot widths of new, similar brackets from the same manufacturer is greater than that between the new (Ormco) and their (Ortho-Cycle) recycled counterparts.¹⁵
___.Using simple means, we developed a direct contact, “geometric” method to measure brackets’ slot width: the tested bracket is secured in an adjustable clamp, which is in turn was tightened into a vise. In the bracket’s slot is inserted a straight wire in such a way that when its shorter end is wiggled horizontally (only within elasticity limits), the movement of the other end will cover a triangle, Fig. 3. The wider the slot, the larger the movement’s angle and the surface the wire covers. From the dimensions of the isoscelles triangle thus formed, the use of geometry or trigonometry allows to accurately measure the bracket’s “play”.^16,17
___.In contrast with the non-contact method described above, this “geometric” method measures the actual “play” as encountered in real cases. Interestingly, the results obtained confirmed both the measurements performed with the SmartScope®, as well as the small variability in slot width introduced by recycling.
Metallography. To evidence an alloy’s phases, grains and impurities, in most cases it is enough to embed the sample (brackets) in acrylic, section and polish the cut surface, and etch it for observations and microphotography. This allows evaluating in advance the attachments’ mechanical behavior and especially metal’s strength. As far as we know, we published here the first direct bonding bracket-revealing microstructures.¹⁸ Witnesses of the attachment’s composition, treatment and transformations, such information can help prevent both mechanical and chemical failures.
Hardness. To fight the opinion that stainless steel is just a common alloy (orthodontics use today at least ten different ones),¹⁹ the Rockwell “B” hardness of nine brands of direct bonding brackets have been evaluated and compared using the Ames “Model 1” portable hardness tester²⁰ (ElectroArc/Ames, Ann Arbor, MI 48105). These first attempts were later continued with specialized means, such as a microhardness tester, in fact a microscope provided with a diamond indenter.^21,22
Impact. One of the problems raised by the ceramic brackets, which were designed to duplicate the metallic ones, is their poor resistance to impact, we built an arrangement borrowed from the optical lenses industry. In it, a steel ball is directed to fall from a constant distance upon a properly mounted bracket. The test allows disclosing those which, by material or shape, are more prone to accidental breakage.²³
Bond strength. Adhesive retention by direct bonding brackets has been a recurrent preoccupation. Our first attempt²⁴ was to adapt for the purpose an instrument commonly used in the coating industry, the Elcometer (N. Gardner Co., Pompano Beach, FL 33060). The tester, “Model F106/1”, pulls a “dolly” which is first cemented on the coating to be tested. By tightening the spanner nut found on top of the instrument, the dolly subjected to tensile breaks a controlled portion of the coating allowing to read in kg/cm2 the force needed for the debonding. To measure bonding strength, the brackets were attached (as far as we know, the first time for the above purpose) to a stainless steel 100 mesh-laminated plates. The dolly was substituted for a clamp whose claws can be screw-tightened under the brackets tie-wings, Fig. 4 and then inserted in a seat found under the Elcometer.²⁴
___.A related test is the evaluation of both brackets and adhesives with the help of Artun’s Adhesive Remnant Index (ARI).²⁵ Various brackets were bonded on a hydrofluoric acid etched glazed tile and then subjected to peeling using a constant force given by keeping identical both the fulcrum and the effort arm length. By reading how much of the adhesive remains on the tile after debonding, or on the bracket’s base, it is possible to compare with other brands a certain attachment retentivity. In a similar approach we used a suitable lever (such as a sharp-ended spoon, on which rests a container which can be filled with measured volumes of water, Fig. 5). If applied to brackets attached to an etched ceramic tile (see above), these can be easily pried-off: All the other conditions remaining the same, the force necessary for debonding can be evaluated from the weight of the water collected. Using the same substrate and adhesive, this test has been applied to brackets (similar in pad surface) to determine which has the best retention,²⁶ and to adhesives to find the strongest ones: of course, in this case the bonding substrate and the brackets attached were identical.
Slot width, creep. An even simpler test can prevent the surprises offered by the brackets made of soft steels (source of the arch wire’s unwanted “play”). The base of the bracket is firmly held with a serrated plier in such a way that it will not hinder the movement of a steel blade (.018 or .022" thick) which is inserted deep into the slot. By strongly wiggling the latter against the slot’s walls, it pushes the metal wall well above its elasticity limit. If the tie-wing bends outward without breaking, the attachment is prone to undergo undesirable deformation. If, in contrast, the tie-wings break, the attachment is hard enough to offer better chances of withstanding the orthodontic treatment.²⁷
Elastomers’s strength. Our quest to measure the mechanical properties has been extended also to elastomers. A first test was to find the maximum elongation of these rings using a vise to which two pointy steel rods were brazed. After pulling the elastomer over these, the vise was slowly expanded till the ring broke, Fig. 7
___.To determine the ultimate tensile strength, a simple, automatic tensile tester, Fig. 8,²⁸ has been put together. The rings were slowly expanded between two hooks, one of this hanging firm and the other being attached to a container in which water was gradually added. At the time the elastomeric ring breaks due to the excessive weight, the metallic container falls on foam provided with an electrical contact, establishing thus a circuit. Acting on an electric valve (Bio-Chem, Boonton, NJ 07005), this stops instantly the water flow.
Friction. To quantify the friction that takes place between the bracket and the arch wire, we eluded the use of universal testing machines by devising a simple device which can measure the sliding taking place between certain brackets and wires and thus preclude inadequate systems. The core of the device used is actually a deadened coil spring balance, from which hangs a straight orthodontic wire tied to a weight. The deadening effect is generated by the insertion of a firmly supported direct bonding bracket in whose slot the wire can slide. These tests, performed either with or without having the wire ligated, allow the measurement of the friction exhibited by various types of brackets and wire, or both, as well as the influence of various types of ligatures.^29-34
Conclusions
___.Our generation seems to worship sophisticated equipment and methods, often willingly ignoring simple means that may give fast, if not more accurate, at least indicative results. While in most instances complex machines can perform in minutes what takes us days, the reverse is also true. In addition, advanced technology comes with a price which we may not be able to afford.
___.In medicine, the fragmentation of our health care system has led to an excessive amount of technologically sophisticated equipment. Every hospital wants to be on the cutting edge of technology and own expensive pieces of equipment, while few operators can really get the most out of these. As time passes, we are falling prey to an audience which is fascinated by advanced technology which should be an extension, and not a goal per se, as it is seems often to be the case.
___.Unless we also take into consideration simple means to solve our problems, we are condemned to be led by machines or abandon promising ideas due to lack of funds. We do not go as far as Alexis Carrel, Nobel Prize in Physiology or Medicine 1912, who deplored the intensive use of sophisticated diagnostic means, as he saw these as a departure from a more direct contact with the patient. We believe, however, that sophisticated equipment should be used wisely, and only after we have exhausted the simple means available. Simple means may not provide accurate measurements, but can be used to compare. But what are measurements, other than comparisons to established standards?
References
1.Alexandre P, Young J, Sandrik JL, Bowman D, Bond strength of three orthodontic adhesives, Am. J. Orthod. Dentofac Orthop 1981 Jun: 653-660
2. Matasa CG:“Surfactants can improve the resin sealing and bonding of hydrophilic substrates, Presentation at the 105th AAO Annual Meeting in San Francisco, May 2005
3.Matasa CG, Flaws in bracket manufacturing, J. Clinical Orthod. 1990; 24(3): 149-152; Fallas sistematicas en la fabricacion de los brackets de pegado directo, Ortodoncia (Buenos Aires), 1991; 56 (109): .5-10
4. Matasa CG. Defend yourself against faulty appliances. I. Faults due to poor manufacturing, J. Gen. Orthod. 1991; 2(4): 5-9
5. Matasa CG, Defend yourself against faulty appliances. II. Alterations produced by wear, debonding and poor reconditioning". J. Gen. Orthod. 1992; 3(2): 13-17
6. Matasa CG, Orthodontic brackets: Should the new ones be considered a standard ? Revue d'Orthopedie Dento Faciale (Paris), 2000; 34: 459-476
7. Matasa CG, Not all appliances are created equal, Am. J. Orthod. Dentofac Orthop 1988; 94: 168-169
8. Matasa CG, One-piece brackets are here to stay! II. Phoenix Without Ashes June 1994; 7(2): 3-7
9. Matasa CG, Bracket slot friction examined through atomic force microscopy, Revista Dental Press de Ortodontia e Ortopedia Maxilar (Maringa, PR, Brasil), 1997; 2: 60-75; Bracket slot friction examined through atomic force microscopy, The Orthodontic Materials Insider 1997; 10(2): 6-8; Metallography and you, Surface analysis. The Orthodontic Materials Insider 1998; 11(4); 1-7
10. Matasa CG, Milling, casting or injection molding, The Ortho-dontic Materials Insider 1996; 9(1): 1-7
11.Matasa CG, What is hidden behind your bracket’s mesh? I. The Orthodontic Materials Insider 2003; 15(4); 1-7
12. Matasa CG, What is hidden behind your bracket’s mesh? II. The Orthodontic Materials Insider 2004; 16(2); 1-5
13. Matasa CG, Do adhesive and sealants really seal the brackets’pad II. Surface tension. The Orthodontic Materials Insider 2003; 15(1): 4-8
14. Matasa CG, Preliminary surfactant screening, The Orthodontic Materials Insider 2004; 16(2); 6-7
15. Matasa CG, Haller MW, Jr, Are reconditioned attachments worse than the new ones? The Orthodontic Materials Insider 1998; 11(1): 2-4
16. Matasa CG, A new and simple method to evaluate slot size, The Orthodontic Materials Insider 2001; 13(3): 4-6
17. Matasa CG, A simple, do-it-yourself method to evaluate slot width tolerance. Oral Health and Dental Management in the Black Sea Countries, 2003; 2 (4): 52-56
18. Matasa CG, Metallography and you, The Orthodontic Materials Insider 1998; 11(3): 1-8
19. Matasa CG, Orthodontic biomaterials, in: Orthodontics, Current Principles & Techniques, Ed. Graber TM, Vanarsdall R, Vig, KWL, IVth ed., Elsevier/Mosby, St. Louis 2005
20. Matasa CG, Bracket metal is not the same, Phoenix Without Ashes (later The Orthodontic Materials Insider), Fall 1988: 3-5
21. Matasa CG, Microhardness, a tool to evaluate brackets, idem 1994; 7(3) 4-8
22. Matasa CG, Metal strength of direct bonding brackets, Am. J. Orthod. Dentofac Orthop. 1998; 113: 282-286
23. Matasa CG, Impact resistance of ceramic brackets according to ophthalmic lenses standards, Am. J. Orthod. Dentofac Orthop
1999; 115; 158-6; Ceramic brackets: properties never studied before, The Orthodontic Materials Insider 1997; 10(2): 1-5
24. Matasa CG, The poor man’s tensile strength tester, Phoenix Without Ashes April 1993:6(1): 6-7
25.Artun J, Bergland S, Clinical trials with crystal growth conditioning as an alternative to acid-etch enamel pretreatment, Am. J. Orthod. Dentofac Orthop. 1984; 85: 333-340
26. Matasa CG, The ARI concept can help to save you money! The Orthodontic Materials Insider 2001; 14(1): 1-7
27. Matasa CG, One-piece brackets are here to stay! Idem, 1994; 7(1): 3-7
28. Matasa CG, Can a simple evaluation of elastomeric ligatures be meaningful? The Orthodontic Materials Insider 2003; 15(2): 1-29 Matasa CG, A do-it-yourself friction testing, The Orthodontic Materials Insider 1995; 8(3): 1-7
30. Matasa CG, Weighing... friction. The Orthodontic Materials Insider 2001; 13(2): 1-8
31.Matasa CG, The friction between the edgewise brackets, ligations and arch wires, State of the art, Informationen aus Orthodontie und Kieferorthopadie (Heidelberg, Germany), 1995; 27 (4): 523-534: Rev. Espagnola Ortod. (Madrid, Spain) 1996: 26; 179-188
32. Matasa CG, The friction between the edgewise brackets, ligations and arch wires II. Do-it-yourself tests, Informationen aus Orthodontie und Kieferorthopadie 1995; 27 (4): 535-548, Rev. Espagnola Ortod. 1996: 26; 189-198.

A simple bond strength testing device and... the Trommsdorff effect

Introduction
___.Consider the adhesive’s setting as a function of time: at the beginning, we have an induction time. Following this, we have a rapid increase in rate of reaction. This autocatalytic sort of behavior is known as the Trommsdorff effect, or Gel Effect. While this phenomenon has been debated in dentistry, having as a result a trend toward composite’s “soft polymerization”, in American Journal of Orthodontics & Dentofacial Orthopedics we found only one mention of a highly viscous gel formation1 during polymerization. This is in sharp contrast with its importance: while the adhesive may still look fluid enough, it may have become a gel, a stage associated with a poor substrate penetration. When this happens, it may mislead the user of a chemically cured adhesive in his belief that he can still achieve an acceptable bond.
___.To evidence this not yet described phenomenon, we used peeling as a method for debonding and a simple device that allows performing a large number of tests.
___.Attachment debonding can be performed in several ways, as shown in Fig. 1.
___.Joints bonded with adhesives are generally stronger in compression, shear and tension than in peeling/tearing- it is much easier to break an adhesive joint by accessing an edge and peeling it away. If the tension load is of centre or is not normal to the joint, there is a tendency for peeling. Likewise, pure shearing, the most used method to test bonding strength in orthodontics, can be seldom achieved. The samples often tilt slightly at the moment of rupture and the resultant peel or cleavage loading leads to considerable lower strengths.²
___.As the shear force needed to debond brackets exceeds 10 MPa,³ i. e. requires a quite high weight delivery for a simple lab device, we have used instead peeling. According to 3M (Unitek Corp.)⁴, metal brackets are typically debonded by using a peeling or prying motion. The most used is the Armstrong’s Lift Off Debracketing Device (LODI)⁵, shown in Fig. 2.
___.Noticing that this device use, along with a .018" or .022"-thick blade inserted in the slot, allows most brackets to be reused, 3M has decided not to sell it anymore: fortunately, the patent expires this year. Before it, there were prior designs that used a prying action, Figs. 3⁶ and 4.⁷
___.According to the first 3M patent quoted, “peeling-type debonding methods are usually considered satisfactory for detaching brackets made of ductile materials such as metal. Debonding of such brackets often begins by fracturing the adhesive bond along one side of the bracket base, and then peeling or bending the base of the bracket so that the fracture propagates to remaining regions of the adhesive bond. In this manner, the debonding force is applied only to a relatively small, generally linear area at any particular point in time.”
___.Strangely enough, while most clinicians use peeling and in industry the adhesion of adhesives to a hard surface is quantified in the same way, we couldn’t find in American Journal of Orthodontics & Dentofacial Orthopedics or in Journal of Clinical Orthodontics any bond strength test using this method. The inappropriateness of the conventional orthodontic bond strength assessment protocols has also been noted by Eliades & Brantley.⁸

Materials and method
___.A simpler substitute for universal testing machines has to provide the force needed in a controlled manner and be able to stop delivering it when the breaking point has been reached. Aside simplicity, a comfortable mounting of the samples is a plus. These conditions are fulfilled by the device shown in Figs. 5 and 6, the “mechanical” version of the electrically controlled device shown in Fig. 2 of the preceding article.
___.Mainatined at a constant level, the water kept in the cylinder 4 flows in a container hanging from a wire which is part of a system of pulleys. The wire’s opposite end is placed under the tie wing of a bracket bonded to a support. At breaking point, the fallen container redirects the water flow into another container. The water weight (or volume) gathered at this point measures the debonding force. The plastic cup atop of the tested bracket, see Fig. 6, hinders bracket’s dissipation and allows its examination for ARI purposes.
___.While more accurate tests can be performed by allowing water to flow in the container till the bond breaking occurs, a “Go/No go” shortcut led to far faster evaluations: the weight of the hanging container was successively increased, and the brackets tested in series. Each of these breaking above a certain weight was marked in diagrams weight-time elapsed since the end of the mixing. The test supports were either 6x 6" stainless steel 100 mesh laminated plates or 8x 8" ceramic tiles that have been etched for ten minutes with 50% hydrofluoric acid. The brackets were all single laterals with vertical slots made by American Orthodontics which have an 80 mesh pad surface of 22 mm². The adhesive used in all instances was Phase II by Reliance, Itasca IL.
___.The tests were performed in the absence or in the presence of a cold slab (a marble piece of 8x 4x 3/4"). In both cases, the two parts were mixed for twenty seconds. Tiny portions were added to the bracket’s base enough to cause the adhesive flow freely around when the bracket was pressed against the substrate. At room temperature, the laps between two bondings were of 15 seconds, while in the case of the cold slab technique, of 30 seconds.
Results
___.All tests were performed on the etched ceramic tile shown in Fig. 2, preceding article respecting the series names and the bracket’s succession. For the experiments marked SS we have used a stainless steel mesh-laminated plate. Fig. 7 shows the debonding force for brackets that were bonded at room temperature at a 15 seconds interval, while Fig. 8 these when the mixed adhesive was kept on the cold slab and the brackets were bonded within 30 seconds.
Discussion
___.A first observation is that the bond strength between mesh & mesh can be almost as high as that between mesh & etched ceramic. While the above diagrams show, taken per bracket, erratic values, bond strength’s decrease in time is inescapable, shortening the handling time prescribed by the adhesive’s manufacturer. While in the tests described the adhesive may have still seemed fluid, its ability to bond has decreased. Indeed, after an induction period while the polymerization inhibitor contained in the monomer is depleted, the free radical reaction becomes autocatalytic, evolving from a pre-gel structure, in which the viscous flow behavior predominates over the elastic behavior, into a post-gel structure, in which the elastic behavior predominates over the viscous flow behavior. This rapid increase in molecular weight as conversion progresses is known as the Trommsdorff Effect or Gel Effect. With further polymerization, the semi-solid’s contraction and flow decrease, while stiffness and stress of the semi-solid increase. As this translates in a polymerization contraction, known to generate marginal and interfacial failures of bonded restorations,⁹ in dentistry there is a strong trend toward a “soft start” polymerization,¹⁰ i.e. postponing as much as possible gelation.
___.As the diagrams in Fig. 7 and 8 show, quite soon after the 20 seconds mixing, the adhesive’s bond strength diminishes, becoming significantly lower after in some 2 minutes at room temperature and in some 5 minutes if it is kept on a cold slab.
___.Etched ceramic tiles or mesh-laminated plates can successfully substitute acrylic mounted bovine teeth. Along with the simple device described, it helped avoid the use of a universal testing machine which translates in evaluations performed in days vs. weeks. As its flexibility allows a multitude of similar tests, it offers a chance for studies involving a large volume of work.
Conclusions
___.While an activated chemically cured adhesive may look fluid enough, its ability to penetrate the enamel or the bracket’s base may have dropped in time substantially.
___.Both stainless steel mesh-laminated plates and etched ceramic tiles can be successfuly used to study the various phenomena involved in bonding. Whenever the tests do not involve a chemical affinity, these can substitute the controversial acrylic-embedded bovine teeth. While widely used in practice, peeling, the easiest way to debond, can successfuly used for testing bracket’s bond strength. Making use of it in a do-it-yourself device, tests which may have taken weeks using complex machines have been performed within days. The method’s simplicity offers the possibility of multiple testing, allowing preliminary research in fields that required intensive efforts.

References
1. McNamara JA, Howe RP, Clinical management of the acrylic splint Herbst appliance, Am J Orthod Dentofacial Orthop 1988; 94: 142-9
2. Koehn GW, Behavior of adhesives in strength testing, in: Clark, Rutzler, Savage ,ed., Adhesion and Adhesives, Fundamentals and Practice, NY, Wiley: 120-126, 1954
3.Bishara SE, Gordan VV, VonWald L, Jakobsen JR. Effect of an acidic primer on shear bond strength of orthodontic brackets. Am J Orthod Dentofacial Orthop 1998;114:243-247
4. Georgakis EG, Christoff JD, Hand instrument for debonding orthodontic brackets, US Patent 6,474,988, 2002
5. Armstrong MM, Rogers JR, Houser, S, US Patent 4,553,932, 1985
6. Cusato AJ, Orthodontic tool, US Patent 3,986,265 1976
7. Kurz CH, Orthodontic tool, US Patent 4,248,587, 1981
8. Eliades T, Brantley WA.The inappropriateness of conventional orthodontic bond strength assessment protocols. Eur J Orthod. 2000; 22(1): 13-23.
9. Braga RR, Ferracane JL, Alternatives in polymerization contraction stress management, Crit Rev Oral Biol Med. 15(3):176-184 (2004)
10. Discacciati JAC et al, Effect of light intensity and irradiation time on the polymerization process of a dental composite resin, Mat. Res. (São Carlos ) 2004; 7(2): 100-105