Introduction
___.Until the mid 80’s, the debonding methods were satisfactory enough to provide few enamel fractures and a 50-50 chance of recovering a reusable bracket. With the advent of the ceramic brackets, new methods became necessary. Ceramic brackets are relatively hard and brittle: they do not bend or flex like metal ones during debonding.
___.To cope with the new situation, their debonding should occur by fracturing the adhesive bond in all areas at essentially the same time, rather than in a propagating manner, as when metal brackets are subjected to a peeling force.
___.Lifting or prying them is not desirable either, as the excessive stress may lead to an iatrogenic damage. Often difficult to see, fissures or fractures are all too often occurring when the tooth structure is weakened or has been previously damaged. Pulling on the tie-wings of a ceramic bracket is also not recommended because they could break off the bracket. Recovery of loose bracket fragments in the oral cavity is difficult because they are small and translucent or transparent.
US Patents issued
___.Unwilling to assume the risks of using of ceramic brackets, many clinicians are now avoiding them—despite the demand. In an attempt to recapture the market, several manu-facturers are patenting new debonding pliers. Some are still being debated or are not yet fully disclosed.1
___.In the accompanying table are shown, using a specific number, the images or principles of all the fully published US patents. Almost all have as assignees 3M Unitek or Ormco.
___.1. This instrument is designed for brackets having a portion of their base (along an edge) whose adhesion is reduced. Because the grip is exerted on a weak area, such as that between base and tie wing (Fig. 1), its use may be acceptable as long as one knows the key, i.e., where that edge is, before prying in that direction.
___.2. This instrument’s jaws move toward each other at the surface of the tooth, thus exerting a double cleavage on the adhesive. It is provided with suction and a shield to recover ceramic fragments.
___.3. To protect the ceramic, the instrument is soft (plastic). As in No. 1, its jaws engage the bracket between the base and the tie wing (Fig. 2), which is in most cases a recipe for attachment damage.
___.4. Designed for the debonding of a collapsible bracket (such as Clarity™, US Patent 5.439.379 ‘95), the instrument compresses its mesio-labial sides, Fig 2.
___.5. Pistol-like, inspired by # 8, the instrument’s jaws lift the bracket. 3M/Unitek’s debonding instrument 444-7701.
___.6. Exerts cleavage at the interface base-adhesive: the jaws are adjustable to tightly grip brackets having differing widths.
___.7. Wrench designed for bracket debonding, using torsion. Patented also as Design (US Patent #306.207). Engages the “strong portion” of the ceramic bracket’s base.
___.8. A basic instrument: a wire loop at the tip of this pistol-like instrument hooks one or both the tie wings (Fig. 4). To avoid bracket reuse by allowing the archwire slot to be damaged, 3M Unitek doesn’t want to sell, along with the instrument, a tenaculum (a slender sharp-pointed hook attached to a handle and used mainly in surgery) as recommended by the inventors. 3M Unitek’s Lift-off Debonding Instrument No. 444-761.
___.9. The head of this tool engages the top of the tooth while the tip of a spring-loaded catch fits under the edge of the bracket. Found suitable for lingual brackets and Bite Turbos. Ormco’s ETM No. 800-0431.
___.10. Another classic: One of the instrument jaws, which can be provided with a removable cap of molded plastic, engages the top of the tooth. The other jaw exerts a scraping or prying force on the adhesive, Fig.4.. Recommended by Bjorn Zachrisson²: “This quick and gentle technique leaves the brackets intact and fit for recycling”. The wedging effect of the sharp edge separates the enamel and adhesive surfaces. Orthopli’s 095, Dentronix’s straight, D231; angled, D230N; ORMCO’s 803-0104, 800-0345, -346 and 800-0348.
___.11. Along with #8 and #10, this plier is among the most used to debond metal brackets. The chisel-like wedge scrapes the adhesive without harming the enamel or the bracket. Sold in variations: ORMCO 803-0419, -2410, -0210, -348 and 9; 3M/Unitek 900-713, -717.
Stresses in debonding
___.The mechanic of solids recognizes the methods of stressing a joint shown in Fig.5. Debonding or delamination is a deliberate separation of a bonded joint, most often an interface failure of laminated composite materials. While load applications can be quite complicated, the simple modes (Fig 5) are:
___.-Tension, when the substrates are pulled apart by forces perpendicular to the plane of the adhesive,
___.-Shear, when one substrate is slid parallel to the other,
___.-Torsion, when the substrates are pushed together by forces perpendicular to the adhesive planes.
___.-Peel, when the force is concentrated along the zone of contact of the substrate to the adhesive the interface in both tension and shear. The contact zone can degenerate into a very small area, and the local tensile stresses that develop become very high even while the total peeling load is relatively small
___.-Cleavage, when peeling is aided with a wedge; the stress is concentrated at one end of a joint and occurs when a prying force, transmitted through a path of least resistance, is placed on the adhesive bond.
A misnomer with consequences
___.Widely used to test debonding, the shear-peel syntagm was not found during an on-line search.. Peel takes place in addition to the forces applied whenever these do not conform to the proper angle: there is neither “tension-peel” nor “torsion-peel”. Equating the terms shear to peel, or using shear-peel for both test and treatment is not only wrong, but also misleading, having an impact on the treatrment, as it will shown in the next article.
___.According to the American Society for Testing and Materials (ASTM), the British Standards Institute and several dictionaries³, shear is the mode of application of a force to a joint that acts in the plane of the bond. In other words, it is the state wherein the stress is tangential to a face of the material, i.e., is applied parallel to the cross-sectional area tested. In contrast, peel is the mode of application of a force to a joint in which the stress is concentrated at a boundary.³ In other words, the stress of pulling away marginally something from the surface: it is usually applied at right angles to the lap joint and it reaches a maximum at its ends.
___.The forces comprises into the syntagm “shear-peel” are so different that these cannot be put together except for indicating an attempt to approximate pure shear. In fact, shear-debonding is quite difficult to achieve in the lab and possible just by chance in the office. Force-specimen misalignment is quite common,⁴ and as it leads to the creation of moment arms, it has a negative impact on the accuracy of the results. Just a “parallelism” between the bracket’s point of contact and the pulling grip of the Universal Materials Testing Machine, as seen from one side only, is not enough.
Stresses involved in the US Patents described
___.The bracket debonding tools described next page use practically all the possible types of stresses, often combined.
___.Thus, the #1 plier engages the bracket between base and tie wing, taking advantage of the less adherent edge. Although risky in this particular case, its inventor recommends peeling. The jaws of plier #2 exert the double cleavage of the adhesive, followed by a “pivoting action” (torsion). The #3 plier has at least one plastic jaw that applies both an “unequal debonding tension force” (peeling) and “pivoting” (torsion). After the mesial and distal jaws of plier-like #4 engage the bracket, a “pivotal, or rocking, action” (torsion) is applied
___.The pistol-like instrument (#5) applies tension as well as a “rotational type of pulling force”, i.e., torsion. The adjustable jaws of instrument #6 engage the bracket mesio-distally, applying a :”quick pivotal movement”, i.e., torsion. Torsion is provided also by the wrench #7 also uses: its slotted ends fit over the mesial and distal sides of the bracket.
___.The by now-classic, pistol-like instrument shown at #8 exerts a peel force. Tool #9 has a head catch that engages the top of the tooth, and a spring-loaded one that fits under the edge of the bracket to exert peel. The jaws of plier #10 remove by cleavage both the bracket and the adhesive. One of the jaws of plier at #11 has a resilient surface, while the other jaw has a chisel portion with a sharp edge to generate a cleavage between the tooth and the onlay.
___.While other debonding-related comparisons^5-7 have been published, none has rated the stresses involved, nor their importance. In the following article, we will try to do it in an effort to protect both your patients and your attachments.
References
1. Farzin Nia F, US Patent Application Publications 2004/0219470 A1; Soo PP, 2006/0127835; Farzin Nia F, 2007/0122763
2. Zachrissen B, in: Orthodontics. Current Principles & Techniques, IV-th ed., Graber TM ed., Mosby 2005
3. http://www.npl.co.uk/materials/programmes/mms11/design/glossary.html. Accessed June 2007
4. Katona TR, Chen J. Engineering and experimental analyses of the tensile loads applied during strength testing of orthodontic brackets. Am. J Orthod. Dentofac Orthop 1994; 105: 543-51
5. Bishara SE, Fehr DE, Jakobsen JR, A comparative study of the debonding strengths of different ceramic brackets, enamel conditioners, and adhesives, Idem 1993; 104; 170-9
6. Bishara SE, Trulove TS, Comparisons of different debonding techniques for ceramic brackets: An in vitro study Part II. Findings and clinical implications. Idem, 1990; 98, 263-273
7. Bihara SE, Olsen ME, Von Wald L, Jakobsen JR, Comparison of the debonding characteristics of two innovative ceramic bracket designs. Idem, 1999; 116; 86-92

Modeling mechanic debonding with the help of the
Velcro™ fastener

Introduction
___.As shown in the preceding article, there seems to be confusion about the forces involved in orthodontic debonding. A search of the American Journal of Orthodontics and Dentofacial Orthopedics (AJODO) online archive between 1986 and June 2007turned up 190 articles containing the word bond in the title, and only 44 with debonding. The same exercise showed for the Angle Orthodontist (AO) for 1930 to 2007, 92 and 13, respectively, and for the Journal of Clinical Orthodontics (JCO) for 1967 to 2007, 243 and 21, respectively. Aside from showing that bonding is of major interest to practitioners,¹ this endeavor demonstrated that its reverse (debonding) is considered far less important. Searching similarly for the words shear bond and shear-peel, we found in the AJODO 100 titles with shear bond and 6 with “shear-peel”. In the AO there were 57 and 2, respectively, while in the JCO,¹³ and none, respectively.
___.Other terms used by some to describe the nature of the forces involved in orthodontic debonding, such as “pivotal or rocking action” for torsion, or “unequal debonding tension force” for peel, were also sporadically found. This may be a reason for the rather chaotic information found in the literature. According to a comprehensive review,² the mean bond strengths reported for different combinations of bracket, adhesive and enamel conditioner range from a minimum of 3.9 MPa (found to be adequate for most clinical needs) to a maximum of 29.4 MPa; as early as 1974 Retief3 reported that enamel fracture can occur with bond strengths as low as 13.5 MPa, a magnitude comparable to the linear tensile strength of enamel.
___.As recently as last month, Swartz⁴ reviewed the many pitfalls found in vitro studies, asking for more meaningful clinical research into orthodontic bonding, while De Castro⁵ pleaded for standardization of in vitro testing methods involving bracket debonding. Despite existing standards^6,7 and adaptations,^8,9 a review of the literature¹⁰ reveals a large variation in these methods, making the comparison of papers difficult and often impossible. Finite-element model calculations of the stress distribution taking place during debonding under tension, shear-peel and torsion show that these methods influence strength measurement and that the typical method of reporting test results cannot reflect such differences.¹¹
___.This multiple choice situation has extended from testing to clinical debonding, only one method actually being used both as a test and on patients.^12,13 The gap between the two has led to serious consequences, for example, the fact that 3 out of 100 teeth exhibited enamel fracture after debonding Transcend Series 2000 ceramic brackets (3M Unitek).¹⁴ This has prompted the American Association of Orthodontists to warn its members about the dangers involved¹⁵ and to advocate the use of hand instruments —pliers, scalers and chisels—over other debonding methods (ultrasound, laser, thermal, etc.), despite their being tedious, time-consuming and painful.
___.In an attempt to make this complex endeavor easier to understand and prevent the use of misnomers, we have tried to model bracket bonding and debonding using the Velcro system.
Basics
___.While their elasticity modules are widely different, the Velcro system™ (Fig.1) resembles orthodontic bonding in that what joins the parts in both cases is simply mechanical interlocking (no chemical interactivity involved). This can be seen by comparing them: in the latter, the resin/composite tags (Fig.2)¹⁶ penetrate the empty enamel prisms left after exposure to 35% phosphoric etching gel for 30 seconds, followed by rinsing and drying (Fig. 3).¹⁶
___.The Velcro™ system is actually biomimetic. It is claimed that the system’s inventor, George de Mestral, was inspired by the cockleburs sticking to his clothes. Upon returning home, he discovered the small hooks that enabled the seed-bearing bur to be transported to new areas. It may be only a question of time before the system is duplicated using hard materials (crystals): mesothelioma is known to be generated by hook-shaped asbestos.
___.It is our opinion that the system can be used to foster better understanding of the effect of the various forces applied to the tooth/bracket system. Indeed, descriptions of enamel fracture under different debonding loads using such modern means as finite-element modeling (FEM), scanning electronic microscopy (SEM) and image software analysis fail to present a simple and intuitively easy method of viewing the phenomena involved.
Materials and method
___.The force needed to detach a model to which a standard-size “hook” fastener was glued was provided by a scale-type device, already described.^17-19 The only modification was the replacement of water with stainless steel balls as the weight. To enable the balls to flow evenly, a sand-timer-like arrangement was used: as soon as the model became detached, the direction of the ball flow was manually changed and the flow interrupted.
___.The model selected didn’t have as its goal the duplication of a direct-bonding bracket, but rather to evidence the forces involved in its detachment. Several prisms, 65 mm in length, were cut from a hardwood road having a square section of 1” x 1”. After adjusting the prisms’ bases to fit the size of the Velcro squares, we provided the model with detachable eyes or levers. The latter were used to delineate the different levels at which the force was applied (Fig. 4). The geometry of the typical edgewise bracket can be approximated by the prism formed by the model’s base and the level corresponding to position 1; higher levels were provided only to reveal detachment trends.
___.The “hook” part of a Velcro fastener was firmly attached to the model’s base with Krazy Glue™. Purchased as die-cut squares having a standard size of 22 mm x 22 mm (Fig. 5), the fastener had a weak adhesive as backing, which was removed. Interestingly, its interlocking ability has remained the same even after being subjected to denatured ethylic alcohol or butyl acetate, or napped with a fine steel brush. Tens of delaminating and debonding tests against its counterpart showed good resistance to fatigue. The “loop” counterpart, obtained from Home Depot as Industrial Velcro bands 2” wide, was bonded as is to a pad (actually, a wood board 18 x 13 x 2 cm). The latter, which we will call the “support,” was fastened to the lab bench vertically or horizontally, as shown in Figures 7 and 8, respectively.
___.The tests were performed by pressing the Velcro hook-covered base of the model into the desired position against its looped counterpart, rigidly attached to the support. A metal hook connected to the scale by a wire was made to be attached at any eye of or lever of the model. After making sure that all the desired angles were observed, we started to add weight to the other end of the wire. When the model became detached, weight was either added or subtracted about 20 times while we changed the model’s position on the support about every 5 minutes. When in each case the force exceeded the strength of the model’s attachment to the support, the corresponding weight was noted as the ultimate force and recorded on the charts.
Results
___.This simple test has been adapted to many more situations than those presented here. Fig. 8 shows the value of the ultimate forces parallel with the support at which the model remains still attached. When the angle of pull varied between ± 10^o, the force needed for debonding increased. Figure 9 shows the values of the ultimate forces perpendicular to the support at which the model remains attached.
___.In Figure 10 are represented values of the ultimate debonding forces applied at level 0, i.e., that of the support. With the exception of tension, the value of which remained constant, the higher the level of force application, the easier it was to debond. Figure 11 shows the variation in ultimate debonding forces at the model’s different levels.
Discussion
___.According to the American Society for Testing and Materials (ASTM) and the British Standards Institute,²⁰ shear is the mode of application of a force to a joint that acts in the plane of the bond. In other words, it is the state at which the stress is tangential to a face of the material, or applied parallel to the cross-sectional area tested. In contrast, peel is the mode of application of a force to a joint in which one or both of the adherents are flexible and in which the stress is concentrated at a boundary. In other words, stress consists in pulling away from the surface at a predetermined angle. Acting usually at right angles to the lap joint, it reaches a maximum at its ends. The interface is stressed in tension and shear, and the force is concentrated along the zone of contact of the substrate to the a dhesive. The contact zone can degenerate into a very small area, and the local tensile stress that develops becomes almost infinitely high, even when the peeling load is relatively small.
___.Figures 8 to 11 confirm that the two terms in the syntagm shear-peel are so different that they cannot be put together except for indicating an attempt to approximate pure shear, knowing that.a force-specimen misalignment is quite common,²¹

___.As can be observed, peeling is the most destructive (in our case, efficient) type of loading, followed by torsion and tension. Other information that can be easily translated to bracket debonding is, as far as tension is concerned, the fact that the higher the level at which force is applied, the more difficult to detach the sample. And the longer the lever (power arms, hooks), the easier the debonding.
Conclusions
___.The model used didn’t show significant fatigue of the rigid-converted Velcro™ fastening system. Although the elas-ticity modulus of the examined systems differs significantly, the information provided by the model is valuable.
___.Just as there is neither black nor white—just shades of grey—it is very likely that almost all tests and clinical debondings are variations of peeling. Pure shear is a fata morgana, while “shear-peel” is a pierced umbrella for both tests and office treatment, and therefore misnomers; the attempt to achieve it, however, helps the tester assess the bond’s resistance to the highest possible stress. the bond strength exhibited by a bracket under a variety of forces. The lowest effective debonding force is achieved at the adhesive/enamel interface either by peeling at 90^o or by cleavage. Tension and torsion are not efficient enough, unless substantially combined with peel. The more distant the point of force application from the bracket’s center, the easier the debonding.
References
1. Keim RG. The problems with bonding studies. J Clin Orthod. 2007; 41(4): 179-80.
2. Jena AK, Duggal R, Mehrotra AK. Physical properties and clinical characteristics of ceramic brackets: a comprehensive review. Trends Biomater Artif Organs 2007; 20 (2): 202-10.
3. Retief DH. Failure at the dental adhesive-etched enamel interface. J Oral Rehabil. 1974; 1: 265-84.
4. Swartz ML. Limitations of in vitro orthodontic bond strength testing. J Clin Orthod. 2007; 41(4): 207-10.
5. De Castro JC. Areas requiring further research in testing of orthodontic shear bond strengths. J Clin Orthod. 2007; 41(3): 135-37.
6. International Standardization Organization: Technical Report TR 11405, Dental materials-Guidance on testing of adhesion to tooth structure, 1994.
7. ASTM D 3330 (PSTC-101).
8. Leforestier E, Darque-Ceretti E, Costini JM, Muller M, Bola M. Adaptation of a standard adherence test to dentistry: the peeling test. Intern J Adhesion and Adhesives 2002; 22 (1): 23-30.
9. Littlewood SJ, Redhead A. Use of jigs to standardize orthodontic bond testing. J Dent. 1998; 26(5-6):539-45.
10. Akhoundi MSA, Mojtahedzadeh F. Problems in standardization of orthodontic shear bond. Journal of Dentistry, Tehran University of Medical Sciences, Tehran, Iran 2005; 2 (1): 36-40.
11. Katona TR. A comparison of the stresses developed in tension, shear peel and torsion strength testing of direct bonded ortho-dontic brackets. Am J Orthod Dent. Orthop. 1997; 112: 244-51.
12. Bishara SE, Fehr DE, Jakobsen JR. A comparative study of the debonding strengths of different ceramic brackets, enamel conditioners and adhesives. Idem, 1993; 104: 170-79.
13. Bishara SE, Fonseca JM, Fehr DE, Boyer DB. Debonding forces applied to ceramic brackets simulating clinical conditions. Angle Orthod. 1994; 64: 277-82.
14. Amditis C. Ceramic bracket debonding: the evaluation of two debonding techniques and their effect on enamel. Aust Orthod J. 1994; 13 (2): 80-85.
15. Letter of the President of the American Association of Orthodontists: ceramic bracket survey. The Bulletin, April 7, 1989.
16. Fjelda M, Ogaard B. Scanning electron microscopic evaluation of enamel surfaces exposed to 3 orthodontic bonding systems. Am J Orthod Dentofacial Orthop. 2006; 130: 575-81.
17. Matasa GC. A simple bond strength testing device and... the Trommsdorff effect. Orthod Mat Insider 2005; 17 (2): 6-8.
18. Matasa GC. Mending acrylic attachments. Idem, 2006; 18 (2): 4-8.
19. Matasa GC. Cyanoacrylate primers: a way to better bonds? Orthod Mat Insider 2007; 19(1): 6-8.
20. http://www.npl.co.uk/materials/programmes/mms11/design/glossary.html.. Accessed June 2007.
21. Katona TR, Chen J. Engineering and experimental analyses of the tensile loads applied during strength testing of orthodontic brackets. Am J Orthod Dentofacial Orthop 1994; 105: 543-51.