Orthodontic debonding: methods, risks and future developments

Cohesive bracket failure

Bracket failure is related to the force applied during bracket removal, along with the condition and the design of the bracket.^3,4 Although metal brackets may deform at debond, cohesive failure of the bracket is unlikely to occur. Occasionally, the bond pad peels off and remains on the tooth, but there are no reported studies in the literature describing this as a common event. By contrast, this type of bond failure is more of a problem when it comes to ceramic brackets. Ceramic brackets are brittle in nature, with a low fracture toughness (fracture toughness is a measure of a material's ability to resist fracture),⁵ some 20 to 40 times less than that of stainless steel in tension^5,6,7(3.0–5.3 mPa√m compared with 80–95 mPa√m.^5,8 This means that cohesive failure is much more likely to occur within a ceramic bracket when subjected to tensile stress, which might occur inadvertently at debond.

Whereas ceramic brackets will deform less than 1% before failing, metal brackets will deform by up to 20% before failing.⁵ The rigid and brittle nature of the ceramic bracket in combination with an underlying brittle substrate, namely the enamel, results in a poor environment for the absorption of stress during debonding and an increased risk of either bracket or enamel failure occuring at debond.^6,9

The two most important features that affect the strength of the bracket are the design of the archwire slot and the tie wing. High torsion in the slot, with large rectangular wires, can lead to fracture most commonly at the tie-wings. It has also been observed that applied stresses, for example during debond, tend to concentrate at the incisal portion of the base of the bracket, again radiating to the incisal tie-wings, which can then lead to bracket failure.^10,11 In addition, the presence of surface flaws, such as pits or scratches, can lead to crack propagation, which may ultimately result in the bracket fracturing. Scratches may arise from something as simple as a misplaced instrument and so care should be taken to avoid scratching ceramic brackets during use.

If cohesive bracket failure does occur at debond, particularly in the case of ceramic brackets, the following are of concern:

Adhesive failure at the bracket/bonding agent interface

The most common locus of bond failure is adhesive between the bracket base and the bonding agent, particularly with metal brackets where a tensile peel force is used to debond the brackets. Failure at this site minimizes the risk of enamel failure, but does increase the time required to remove the residual adhesive.¹³ However, depending upon the method chosen, the process of removal of the residual adhesive can still result in potential enamel damage.

Cohesive within the bonding agent

Similar to adhesive failure at the bracket/bonding agent interface, cohesive failure within the bonding agent also minimizes the risk of enamel failure at debond. Unlike adhesive failure at the bracket/bonding agent interface, there will be much less residual adhesive to remove following bracket removal and, hopefully, less risk to the enamel at final clean-up.

Adhesive failure at the bonding agent/enamel interface

Most orthodontic bonding is carried out using the acid-etch technique and composite resin-based bonding agents, with mechanical adhesion or a mechanical interlock between the two. As a result, purely adhesive failure at the bonding agent/enamel surface is very unlikely to occur. Instead, what is more likely is that, as the tags of adhesive tear out of the roughened enamel surface, some of the enamel will also fracture and be removed with the adhesive, even though macroscopically failure appears to be at the enamel surface.^14,15

Cohesive enamel failure

Unlike metal brackets, which can only bond to the adhesive by mechanical means, ceramic brackets can bond to composite resin-bonding agents either mechanically, chemically or a combination of the two. It is with ceramic brackets, particularly those that rely on chemical adhesion, that there is the greatest risk of cohesive enamel failure at debond^7,15 (Figure 1).

With the early ceramic brackets, the bonding base was smooth and adhesion to the bonding resin was purely chemical, via the use of an intermediate silane coupling agent. The fact that there were few stress-raising areas on such smooth bracket bases, and that the layer of bonding resin is likely to be thin and relatively homogeneous, means that stresses applied at debond are most likely to be concentrated at any areas of surface roughness or imperfection within the enamel. Although this increases the potential risk of enamel failure at debond, it is not a universal finding with the use of silane-coated ceramic brackets.¹⁶

Mixed mode of failure

As the name suggests, here the locus of bond failure is a mixture of adhesive and cohesive failure, for example adhesive at the bracket/bonding agent interface, cohesive within the bonding agent and adhesive at the bonding agent/enamel interface. This mixed mode of failure is very common and can be observed by the often varied adhesive remnant index (ARI) scores reported in clinical trials and laboratory investigations looking at bond failure.¹⁷ The locus of bond failure will not only depend on the bracket/bonding agent combination and the condition of the enamel surface, but also the technique of bracket removal.^3,4

Metal brackets

Metal brackets are usually removed using specially designed debonding pliers and can either be removed by squeezing under the tie-wings at the bracket stem, or by squeezing the bracket base close to the enamel surface. In both cases, a shear peel force is applied. Alternatively, lift-off pliers can be used. This is a plastic instrument with a wire loop that hooks under the tie-wings of the bracket. As the handles are squeezed so the loop applies a tensile peel force to the bracket with the plier head itself resting against the enamel surface. Indeed, work by Oliver,¹⁸ investigating the use of either debonding pliers or ligature cutters beneath the tie-wings or at the bracket base, found that there was an increased risk of enamel damage with the use of ligature cutters or other instruments at the bracket base. The enamel surface could end up being gouged.¹⁹ Instead, they recommended removal by applying a force just beneath the tie-wings using debonding pliers or by applying a tensile force using lift-off pliers. Using this method of bracket removal, the resultant mode of failure tends to leave more residual bonding agent on the enamel surface,¹⁷ but reduces the risk of enamel fracture. This would appear to be supported by the work of Bennett et al,¹⁹ who investigated the stress patterns created within the enamel during bracket removal. They found that forces applied to the outer wing of the bracket resulted in the least amount of stress to the enamel surface. Conversely, any force applied to the base of the bracket resulted in stress created and concentrated at the enamel surface which, as might be expected, could result in the locus on bond failure being at the adhesive-enamel interface or cohesive in the enamel.

Ceramic brackets

The composition of all currently available ceramic brackets is based upon aluminium oxide and this can be present in one of two forms, namely monocrystalline or polycrystalline alumina. The most significant difference between the two types of material is their optical clarity, with the monocrystalline brackets being more translucent due to the lack of grain boundaries.^20,21 However, it is the polycrystalline variety that tends to be more widely available owing to their relative ease of manufacture^6,7 (Figure 2). The chemically inert nature of ceramic brackets, although making them relatively stable within the oral environment, does also mean that they are unable to form direct chemical bonds with resin adhesives,^20,22,23 unlike some polymeric brackets, unless an intermediate silane coupling agent is used.

Bonding therefore occurs either by mechanical adhesion, chemical adhesion via a silane coupling agent, or a combination of the two. Examples of the types of mechanical interlock on the bracket base include dovetails, balls and/or dimples. To facilitate the use of a silane, glass is added to the aluminium oxide base so that the silane bonds to both the glass of the ceramic and the glass filler particles in the adhesive.²¹ The bond produced is very strong and the force required to remove the bracket has been found to be significantly greater than that required to remove stainless steel brackets.^{7,24,25,26,27,28} As a result, a number of methods of ceramic bracket removal have been suggested since their introduction.

Whichever method is used, it is important that any adhesive flash is removed from the periphery of the bracket with a debonding bur before attempting to debond. This is because the presence of such flash, known as a spew fillet, reduces stress concentration at the margins of the joint²⁹ and, instead, may promote unwanted bond failure elsewhere. For example, it may occur cohesively in the bracket or enamel, rather than adhesively at the interface of either with the bonding agent.

The idea of promoting a site of stress arising in order to make bracket removal easier and more predictable resulted in the introduction of the Clarity bracket by 3M Unitek. This bracket has a vertical notch between the tie-wings that promotes failure, initially within the ceramic. The crack then propagates through the centre of the bracket and then along the bracket/bonding agent interface, well away from the enamel.

Debonding pliers

These are routinely used for debonding metal brackets and come in a variety of forms, but principally comprise two blades that can be placed either beneath the bracket tie-wings, or at the adhesive close to the enamel surface (Figure 3). A study by Zarrinia et al,¹³ investigating the use of Howes pliers, debonding pliers and ligature cutters for the removal of ceramic brackets, reported that bracket-removing pliers provided the most consistent bond failure at the bracket/adhesive interface, leaving the resin intact on the enamel surface. A further study by Arici and Minors³⁰ investigated the in vitro force levels required to debond polycrystalline ceramic brackets using debonding pliers with different types of blades, including wide, narrow and pointed. The study was conducted using standardized techniques within the laboratory and found that the predominant locus of bond failure was at the bracket/adhesive interface, with no visible enamel damage. Their main conclusions were ‘that force required to initiate the debond of ceramic brackets is related to the contact area between the tips of the pliers and adhesive, and that this can be minimized, either by using pointed plier tips or by changing the direction of application of the force to diagonally opposite corners of the bracket’.

Specific ceramic bracket debonding instruments have been introduced by various manufacturers over the years, with early instruments applying either a tensile force or, in one instance, a rotational shearing force. However, not only can their use lead to failure of the ceramic bracket or, worse still, fracture of the enamel,^7,22 but there have been reports that the sudden and unpredictable bracket failure could also result in an inhalation injury.^3,4 The forces generated by these instruments were also sufficiently high to cause considerable patient discomfort during use.

Electrothermal debonding

Using this method of removal, the bracket is heated using an electrothermal debonder, whilst simultaneously applying a tensile force to remove the bracket from the tooth.^7,31,32 For this to work, sufficient heat needs to penetrate to the bracket/adhesive interface. Once the temperature exceeds the glass transition temperature of the adhesive, the bracket can be gently peeled away from the tooth¹³(the glass transition temperature (Tg) being the temperature at which the adhesive, a polymer, changes from a hard glassy state to a more viscous state).

Although the electrothermal debonder is expensive, a study by Bishara and Truelove,⁴ found it to be a quick and effective method of ceramic bracket removal, with a mimimal risk of bracket or enamel fracture. There is the potential to use this system when debonding stainless steel brackets, although debonding these brackets with conventional debonding pliers is fairly easy to accomplish and with minimal risk to the enamel. For metal brackets the cost of the electrothermal debonder does not outweigh the benefits of its use.

Even with ceramic brackets there are potential problems associated with this technique, principally the bulky nature of the handpiece, which can be difficult to use in the intra-oral environment, and the heat that is generated which can increase the intra-pulpal temperature, leading to pulpal inflammation.³³ If the operator is careless with the instrument, then there is also the potential to cause damage to the surrounding soft tissues.¹³ In the study by Bishara and Trulove,⁴ comparing conventional debonding techniques, ultrasonic and electrothermal debonding, they found that the conventional debonding technique led to a greater percentage of ceramic bracket failure when compared to electrothermal debonding or the use of an ultrasonic scaler.

Ultrasonic scaler

Using this technique, bond failure is reported to occur principally at the enamel-adhesive interface. The ultrasonic scaler has customized tips that are applied at the bracket/adhesive interface, which is said to reduce the risk of enamel damage since very low debonding forces are involved.^3,4 The same instrument can also be used to remove the remaining adhesive following bracket removal. However, there are also disadvantages with this technique. It takes significantly longer to remove the brackets than with conventional debonding,^3,4 as it must be preceded by composite flash removal, making the whole process more uncomfortable for the patient. It also leads to rapid wear of the customized tips, particularly when used with ceramic brackets.

Laser-aided debonding

Here the bracket is irradiated on its labial surface by a laser, generating heat that is transmitted to the resin-based adhesive, resulting in softening when the glass transition temperature is exceeded. Therefore, it works in a similar manner to the electrothermal debonder. There have been reports that this technique reduces the risk of enamel damage,^7,34,35 with no evidence of enamel tear-outs or bracket fracture.³⁵ However, early research into the use of dental lasers showed that laser irraditation generated too much heat, resulting in pulpal damage.^35,36,37,38 Improvements in understanding and advances in laser technology have resulted in a decrease in undesirable thermal effects,^35,39 but the cost of the equipment and the time taken to perform the debond has resulted in a limited uptake and use of this technique.

Debonding agents

Peppermint oil can plasticize the resin component of composite bonding agents and has been suggested as a method of simplifying ceramic bracket debond.⁷ Although it has been shown to promote bond failure and therefore make it easier to remove ceramic brackets, to have a significant effect the oil has to be left in place for a very long time, up to an hour. Leaving it in place for just 5 minutes has been found to have little effect on ease of bracket removal.⁴⁰ The most probable reason for the somewhat disappointing results with peppermint oil is the thin bond line, and therefore the very small surface area of adhesive exposed at the periphery of the bracket to the oil. Other disadvantages of this technique include the cost of the material and the fact that peppermint oil can be traumatic to the soft tissues.

Band removal

It is commonplace to use bands on posterior teeth rather than bonds. These are often cemented using a glass polyalkenoate cement (eg Ketac cement, 3M ESPE) or, more recently, a light-cured compomer cement, which is command set with the ability to release fluoride (eg Transbond^TM Plus light cure band adhesive, 3M Unitek) cement. Bands are then removed using specific debanding pliers followed by enamel clean-up.

Diamond bur

It has been suggested that a diamond bur in a high-speed handpiece, under water coolant, can be used for enamel clean-up at debond; the water coolant being necessary to avoid potential thermal damage to the pulp. In a study by Eliades et al,⁴¹ they include a description of the mechanism of action of a diamond bur. As the bur rotates, large tensile stresses are created in the near surface region, resulting in brittle fracture of the surface and creation of grooves.^41,42 The nature of this cutting mechanism generally suggests that the bur is designed to be used on more brittle surfaces, for example, enamel, ceramic or hard alloys. Orthodontists, because of the increased potential for enamel damage, do not generally use diamond burs in enamel clean-up. Other workers have also cautioned against the use of a diamond bur for adhesive removal owing to the increased risk of excessive enamel loss/damage.^13,43,44

Notwithstanding the adverse effects to the enamel surface, there is also the risk of particulate inhalation which will be discussed later (see ‘Particulate production’).

Tungsten carbide bur: plain cut or spiral fluted

This is the most common method of adhesive removal at debond and these burs can be used in both slow-and high-speed handpieces. In the case of the slow-speed handpiece, this can be with or without water coolant. Care must be taken because carbide burs are harder then enamel and, as such, if run at high speed, damage can be caused to the underlying enamel.^13,45,46 A study by Oliver and Griffiths⁴⁷ rated the tungsten carbide bur as the ‘gold standard’ for bonding agent removal. Its use creates high shearing forces within the bonding agent between the blades of the bur and the tooth surface, resulting in plastic ploughing of the surface, with flow driven and subsequent brittle fracture of the material. The eight-bladed tungsten carbide has been shown to be the superior method of removal of bonding adhesive when compared with manual removal using a scaler, various other fluted tungsten carbide burs, Soflex discs, special composite finishing systems and ultrasonic scalers.⁴¹

In a later study, Hosein et al⁴⁸ compared four methods of adhesive removal:

They found that the least enamel loss was seen with either the slow-speed tungsten carbide bur or the debanding pliers. Equally, and of concern, was the finding that the greatest enamel loss was observed when a tungsten carbide bur was run in a high-speed handpiece. This contradicts the findings of Zarrinia et al¹³ where enamel clean-up with a high-speed tungsten carbide bur was recommended. Although Hosein et al⁴⁸ found enamel loss to be low with the debanding pliers, on closer inspection the enamel surface showed distinct evidence of surface scratches and surface gouging and so this instrument was not recommended for adhesive removal.

Within orthodontic practice the spiral-fluted tungsten carbide bur remains the safest and hence most common method of enamel clean-up following debond. However, in addition to the least enamel loss, the use of this bur in a slow-speed handpiece is to be recommended over its use in a high-speed handpiece. This is because the use of the high-speed handpiece increases the level of potentially hazardous inhalable particulates (see ‘Particulate production’).

Ultrasonic scaler

As discussed earlier, an ultrasonic scaler can be used not only to debond orthodontic brackets, but can also be used to remove the bonding agent. However, there is some conflicting evidence as to the possible risks of enamel damage with its use. Whilst some studies suggest that using an ultrasonic scaler is less likely to result in damage to the enamel,^48,49 others have reported greater degrees of enamel loss when compared with the use of a tungsten carbide bur in a slow-speed handpiece.⁴⁸ Other potential disadvantages of the use of an ultrasonic scaler for enamel clean-up is the time it takes and the wear of the ultrasonic tips, as previously described.

Hand scaler or bond/band removers

The use of these instruments can lead to the use of excessive force, resulting in gouging of the enamel surface.^43,45

Polishing systems

There has been some suggestion that polishing systems alone can be used to remove the bonding agent following debond. These systems usually comprise rubber points and/or brushes with a prophylactic polishing paste. However, in reality what can happen is that any bonding agent remaining on the tooth surface is essentially polished and, although less noticeable, is still present.⁵⁰ The enamel surface index was used in a study by Zachrisson and Arthun,⁴³ to assess the condition of the enamel following removal of bonding adhesive. Polishing instruments scored well according to the index, but again this was thought to be due to large amounts of bonding adhesive still found present on the enamel surface.⁴³ This is supported by the work of Hong and Lew⁴⁴ who found them to be inefficient at removing bonding adhesive, leaving a polished bonding adhesive surface as opposed to a smooth enamel surface.

The most common place for polishing during enamel clean-up is following removal using a tungsten carbide bur. A study by Eliades et al⁴¹ found that the sequential use of multiple polishing tools (soflex discs) was not consistent in producing a roughness-reducing effect, whether the initial clean-up had been with a diamond bur or 8-fluted tungsten carbide bur.

Enamel loss at debond

The degree of enamel loss at debond will depend a great deal on the bracket material used and the method of debond and can vary from almost zero⁴⁸ to as high as 149.87 μm.⁴⁹ In addition to the overall loss, there is also concern that it is the uppermost layer of enamel, with its high mineral content and greatest concentration of fluoride, that is being lost, leaving the remaining enamel more prone to demineralization.^41,51

Surface roughness of the enamel

The surface roughness of intra-oral hard surfaces is of clinical importance in the process of bacterial retention;⁵² specifically marked scratches and grooves have been found to contribute to the following: plaque accumulation, staining, odour and consequently demineralization due to the activity of bacteria on the tooth surface.^43,50In vivo studies assessing plaque formation on polymer strips, one side being smooth (with a surface roughness (Ra) of 0.1 μm) and the other half rough (Ra >2 μm), have found that surface roughening resulted in a four-fold increase in plaque formation.^53,54 It has been suggested that an Ra of greater than 0.2 μm is the threshold surface roughness for bacterial accumulation. At values above this level, it is likely that an increase in plaque accumulation will be seen.⁵²