Aesthetic Brackets

A E S T H E T I C B R A C K E T S A N D A R C H W I R E S 2 2 3 E X C E L L E N C E I N O R T H O D O N T I C S 2 0 0 6

Aesthetic brackets and archwires David Birnie

CChhaapptteerr

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2 2 4 A E S T H E T I C B R A C K E T S A N D A R C H W I R E S E X C E L L E N C E I N O R T H O D O N T I C S 2 0 0 6

Introduction The appearance of fixed orthodontic appliances has always been of particular concern to many patients. The development of appliances, which would combine both acceptable aesthetics for the patient and adequate technical performance for the orthodontist, has remained an elusive goal. Three methods of achieving these criteria have been attempted:

• altering the appearance of or reducing the size of stainless steel brackets • repositioning the appliance on to the lingual surfaces of the teeth • changing the material from which brackets are made.

Early attempts to coat metal brackets with a tooth coloured coating were unsuccessful due to failure of the coating to adhere and to its poor translucence. There has recently been a firm trend towards the development of smaller stainless steel brackets but although these generally provide the technical performance required by the orthodontist, they offer a worthwhile but necessarily limited aesthetic advantage over conventionally sized appliances.

Aesthetic brackets have been a feature of Excellence in Orthodontic courses since 1988. Since that time the market for these brackets has matured considerably. Modern aesthetic brackets are made of:

• resin o polycarbonate o polyurethane

• hybrids of resin and o ceramic o glass o glass fibre o metal o any combination of the above

• ceramic o polycrystalline alumina o monocrystalline alumina

There is little doubt that at the time of their introduction in 1987, ceramic bracket manufacturers had over-estimated the bond strength required to retain brackets on teeth and that the strength of individual brackets varied considerably but that the idea of a less visible bracket was popular with the general public. In the 1990 Course Manual we cited 16 manufacturers/suppliers of ceramic brackets and while there have been some changes, there has been little significant alteration in this number. The products however have evolved continuously and are now very much more useful clinical tools.

Lingual orthodontics Lingual orthodontics satisfies aesthetic criteria by repositioning the fixed appliance on the lingual surfaces of the teeth but in doing so produces a significant decrease in the performance of the appliance and considerable additional technical difficulty and time requirement for the orthodontist. Lingual orthodontics has therefore gained only a limited following and this is likely to remain the case, although it is undergoing somewhat of a revival at the moment. Rafi Romano has written an excellent textbook entitled Lingual Orthodontics which comes with an accompanying CD-ROM for those who wish to pursue this further.

In addition, lingual orthodontics is not without its problems for the patient. Hohoff et al (2003) carried out a prospective study to evaluate comfort, function and oral hygiene over a one-month period in 22 adult patients with lingual appliances.


76% to 91% of patients reported restriction of tongue space, changes in tongue position or tongue lesions after three months and 10% to 21% of patients these were perceived as significant.

Speech articulation was the greatest problem as perceived by both the patients and by others. Mastication was a problem for many patients with 43% reporting severe problems in chewing and 44% in biting; masticatory problems were worse in patients with deep bites. Oral hygiene was a problem in approximately 50% of the patients.

Resin brackets Early attempts to produce brackets of different materials included the use of polycarbonate. These brackets, while aesthetically satisfactory in the early stages of treatment, deteriorated in appearance with time and were insufficiently strong to withstand long treatments or transmit torque.

Polyurethane has recently been introduced as a bracket material.

Ceramic, fibreglass and metal reinforced resin brackets More recently, ceramic, fibreglass, metal and ceramic and metal reinforced resin brackets have become available and while these are more durable than polycarbonate brackets, their ability to maintain their integrity over long treatments remains suspect. Their performance is however significantly better than polycarbonate brackets. The ceramic, fibreglass and metal reinforcement enhances the brackets ability to transmit torque by increasing its stiffness.

Fernandez and Caput (1999) showed that Ormco Spirit MB brackets had almost the same tensile bond strength (12.0 N) as metal brackets (13.2N) but a much higher bond strength than GAC Allure brackets.

Manufacturers include GAC (Elation) and Ormco (Spirit MB). The latter bracket was also available as a “glow in the dark” version (Wild Spirits) which was rather less translucent than its more conventional cousin. These brackets are useful where aesthetic brackets are essential on the lower teeth (such as for stage, film and television performers) but suffer from tiewing wear (in both upper and lower arches) and usually require at least one replacement per bracket during treatment.

The performance of these brackets continues to improve and they probably have the potential to challenge ceramic brackets with further development. Nevertheless, their performance currently is below that of metal or ceramic brackets because of the flexibility of the resin component of the bracket. Harzer et al (2004) compared the ability of the Forestadent mini-Mono bracket, the Forestadent Brilliant bracket (a homogeneous polyoxymethylene bracket) and the Dentaurum Elegance bracket (a polycarbonate bracket with a metal slot to improve torque transmission) in a laboratory experiment. This showed that the metal bracket achieved the highest torque moments and was considerably more effective than both polycarbonate brackets. The Forestadent Brilliant bracket was the least effective at torque transmission and the Dentaurum Elegance bracket performed between the two but closer to the Forestadent Brilliance bracket.

Eliades et al (2004) tested the hardness of resin brackets (Dentaurum Brilliant, Forestadent Align, Leone Leone and American Orthodontics Silicon) in vitro and in vivo. There was no difference between the as received state and the in vitro aging. However, in vivo, the polycarbonate brackets showed a significant reduction in hardness due to intra-oral aging factors such as fatigue, abrasion, temperature fluctuation, pH fluctuation and moisture ingress as well as polycarbonate’s intrinsic low modulus of elasticity

Semi-aesthetic brackets Where other materials such as glass fibre, ceramic or metal are used to reinforce resin-based brackets, the reinforcement is usually (almost) invisible when the bracket is placed on a tooth and an archwire tied in.


Ormco’s Damon 3 bracket represents a new class of bracket which is best described as a semi-aesthetic bracket. In this bracket, the bracket base and bracket body is made of reinforced resin and the bracket slot and self-ligating mechanism is made of MIMmed metal. D3 brackets are shown in Figures 12.1 and 12.2.

Aging Although resin-based materials have improved considerably and continue to improve, they are not suitable for all patients for an entire treatment. The resin tiewings may be subject to wear and this may require replacement of brackets during treatment. Brackets are subject to modification of their structural properties due to biofilm adsorption (Eliades and Bourael 2005). This adsorption may affect brackets differently depending on their:

• porosity • sorption • susceptibility to corrosion • biodegradability

Ceramics In late 1986, the first brackets made of ceramic materials became available and by the time that the 87th Annual Session of the American Association of Orthodontists was held in Montreal in May 1987, almost all major orthodontic manufacturers had either announced, or were about to announce ranges of ceramic brackets. This offered the possibility of a major advance in aesthetic orthodontics.

Ceramics are materials which are first shaped and then hardened by heat. This includes clays, glasses, some precious stones and metallic oxides. The ceramic material used in almost all orthodontic brackets is alumina, either in its polycrystalline or monocrystalline form. A few brackets are made from the chemically similar zirconia. The advantages of using alumina for orthodontic brackets is that its appearance is very good, its chemical resistance is excellent, and it is both hard and, in certain respects, very strong. The

disadvantages are that it lacks ductility, and is difficult and expensive to manufacture.

Ceramic brackets come in a variety of edgewise morphologies including true siamese, semi-siamese, solid and Lewis/Lang designs. Begg brackets are also now available. Many brackets are made by specialist ceramic manufacturers

Figures 12.1 and 12.2: Ormco Damon 3 semi-aesthetic brackets

Property MCA PCA Stainless Steel

Hardness (Rockwell) 97.5 82.5 5-35 Tensile strength (psi x 1000) 260 55 30-40 Fracture toughness (MPa√M) 2-4.5 3-5 80-95

Table 12.1: Comparison of hardness, tensile strength and fracture toughness of monocrystalline alumina, polycrystalline alumina and stainless steel


and sold under proprietary names by manufacturers of orthodontic products or orthodontic supply companies. Some brackets from different manufacturers may therefore be almost identical products. At the present time the only ceramic Begg brackets are made by TP.

Monocrystalline (MCA) versus polycrystalline alumina (PCA) brackets Since 1987, both monocrystalline and polycrystalline ceramic brackets have been available and varied arguments put forward in favour of one or other material. Monocrystalline brackets are machined from long extrusions of synthetic sapphire with subsequent treatment to 'anneal' the surface and therefore reduce sites of stress concentration. Polycrystalline alumina brackets on the other hand are made by injection moulding submicron sized particles of alumina suspended in a resin, sintering them to fuse the alumina and finally machining the bracket as necessary to produce the finished article.

The physical properties of the raw materials (as opposed to brackets) compared with stainless steel are given in Table 12.1.

The figures for hardness show that both monocrystalline and polycrystalline alumina are significantly harder than stainless steel and that for tensile strength monocrystalline alumina is much stronger than polycrystalline alumina, which in turn is significantly stronger than steel. This is reflected in the fact that the only true siamese brackets made from a ceramic material have been made from monocrystalline alumina.

Zirconia brackets Zirconia brackets were opaque and have been claimed to have lower frictional resistance than alumina brackets. This is not supported by the study by Keith et al (1994) in which zirconia brackets had similar or greater frictional resistance than alumina brackets and suffered surface damage after sliding tests.

Advances in design of ceramic brackets The principle changes in the design of brackets has related to three areas:

• bond strength • bracket strength, especially when subjected to torque forces and in relation to tie wing

strength • frictional resistance • debonding

Bond strength Harris, Joseph and Rossouw (1990) compared the bond strengths produced by Heliosit, Transbond and Ortho Concise using Transcend and Ormesh brackets. All combinations provided clinically acceptable bond strengths above the 6-8 N/mm2 recommended by Reynolds (1975). Ceramic brackets produced higher bond strengths than metal brackets. Metal brackets tended to fail at the bracket adhesive interface whilst ceramic brackets failed more commonly at the enamel resin interface.

Most companies have now moved to a mechanical method of bracket base retention in order to provide more predictable bond and debonding strengths. Wang et al (1997) revisited the bond strengths of a range brackets using a chemically coated base and a mechanical interlock. Brackets with a chemically coated base had higher bond strengths than metal and ceramic brackets with a mechanical interlock. Higher bond strengths showed that the debonding interface was at the enamel-resin interface and with lower bond strengths, at the resin-bracket interface. Higher bond strengths were found to produce some enamel fractures and detachments.


Bishara et al (1993) have shown that considerable variability occurs between chemically bonded, chemical/mechanically bonded and mechanically bonded brackets, different adhesives and different types of enamel conditioners. In addition, different etch times affect the bond strength with no etching and five second etches giving significantly reduced bond strengths (Olsen et al 1996).

An alternative approach has been tried with TP's Ceramaflex bracket which interposed a plastic pad between the ceramic bracket and the tooth to allow easier debonding. Fox and McCabe (1992) suggested that:

• the bond strength of Ceramaflex brackets was similar to that of metal brackets

• the Ceramaflex brackets debonded easily without fracture of the ceramic or damage to the tooth surface

• the Ceramaflex bond may be less reliable than of metal brackets

The probability of failure relative to an applied force is described by Weibull (1951). Figure 12.3 demonstrates the reliability of Ceramaflex versus metal bracket bonds.

It is interesting that Fox and McCabe's findings are very similar to those published in the TP literature and that the bracket base was subsequently modified to produce a window where the adhesive contacts the ceramic. More recently, the whole idea of a plastic interlayer was dropped and TP reverted to a mechanical base retention system (MXi - Maximum Integrity).

Arici and Regan (1997) have compared the bond strength and mode of debonding of metal, ceramic reinforced resin brackets and the two generations of TP Ceramaflex brackets. The debonding strengths were measured in tensile/peel mode; a large number of aesthetic brackets suffered structural failure – tie wing breakage for the ceramic reinforced American Orthodontics Silkon bracket and delamination between the ceramic body and the polycarbonate bracket base for both generations of the Ceramaflex bracket. The bond strengths of both aesthetic brackets were significantly lower than for the conventional metal bracket.

Bond strength is also modified by the choice of adhesive. Winchester (1991) showed that in a comparison of Heliosit and Prismafil, Heliosit produced consistently higher bond strengths than Prismafil and that high

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Figure 12.3: Ceramaflex and metal brackets - probable failure strengths

Figure 12.4: SEM picture of zirconia balls on the base of an Ice bracket; note the small ‘neck’ securing the ball to the base of the bracket thus providing undercuts for the retention of the adhesive

Figure 12.5: SEM of base of Ice bracket coated with zirconia balls. At the top of the picture (gingival edge) can be seen a clear zone which facilitates debonding


bond strengths were associated with an increased incidence of bracket and enamel fractures. This effect has been reported by other authors (Evans and Powers 1985). However, Ostertag et al (1991) found a slight trend in the other direction. With in vitro tests, more highly filled resins required marginally higher debonding forces.

Ormco’s Ice bracket has a bracket base coated with a monolayer of hollow zirconia balls – see Figure 12.4. These are attached to the bracket base by a combination of chemical adhesive and fusion generated by heat during the manufacturing process. The part of the bracket base closest to the gingival margin has a reduced coating of zirconia balls in order to facilitate debonding by making it easier to initiate a crack in the adhesive as shown in Figure 12.5. The gingival edge was chosen to reduce the possibility of spontaneous

debonding during treatment which might have occurred had the reduced concentration of zirconia balls been placed on the occlusal edge.

Bracket strength One of the early debates relating to ceramic brackets was that of monocrystalline alumina versus polycrystalline alumina. Patent restrictions meant that there would be only one brand of monocrystalline alumina bracket. The promised theoretical advantages of Starfire were slow to materialise; the technical design of the bracket which was almost identical to a metal bracket meant that certain aspects of treatment mechanics required no change when switching from metal to ceramic brackets. The Ormco/“A” Company Inspire bracket replaced Starfire in 1999.

Scott (1988) has pointed out that the tensile strength of ceramics is very dependent on the surface condition of the ceramic and this can make tests on bulk samples misleading and irrelevant. In addition, an important physical property related to the behaviour of ceramics is fracture toughness, the ability of a material to resist fracture. This is determined by stressing the material by impact and measuring the size of crack produced. The units of measurement are metres pascals per square root metre. It can be seen that in this mode of testing, both types of alumina perform poorly compared with stainless steel and this reflects their lack of ductility.

Kusy (1988) examined the morphology of polycrystalline brackets under a scanning electron microscope and demonstrated defects, predominantly intergranular fractures which might have a detrimental effect on bracket performance as would scratches. Research by Flores et al (1990) at Loma Linda University (see Figure 12.6) reveals the extent of this effect. The monocrystalline brackets (Starfire and Gem) are the strongest when unscratched,

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Transcend Allure Starfire Gem Metal

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Figure 12.6: Strengths of non-scratched and scratched ceramic brackets

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Figure 12.7: Torque values at bracket failure

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Figure 12.8: Degree of rotation at bracket failure


yet the most susceptible to the effects of surface scratches.

Figures 12.7 and 12.8 show the resistance of various types of ceramic bracket to torque and rotational forces when tested by Holt et al (1991) at Oklahoma

University. It can be seen that the monocrystalline Starfire brackets are the strongest, but there is a large variability in strength, reflecting the difficulty of consistent manufacture of all such brackets.

In 1992, the Starfire TMB (Totally Machined Bracket) bracket was introduced to counter the difficulty in use of the original Starfire bracket. These brackets were manufactured by producing “boules” of synthetic sapphire using the Czocharlski crystal growth technique and then machining the bracket into a twin bracket configuration. This technique makes possible the production of crystal blocks free of grain boundaries and other defects.

Finite element analysis (FEM) of ceramic bracket designs show that brackets with rounded corners and slots and an isthmus have lower stresses than those that do not (Ghosh et al 1995).

Figures 12.9 and 12.10 give “A” Company data on the torque strength of Starfire TMB brackets. The results for Starfire, Transcend and Allure in Figure 12.9 are very similar to those shown in Holt's work

(Figure 12.7) which supports the “A” Company data.

Matasa (1999) has investigated the impact resistance of ceramic brackets. Polycrystalline brackets with bulkier structures and glazed surfaces were more resistant to fracture than monocrystalline brackets. Protruding tiewings and bases were liabilities and domed structures seemed to deflect blows. The brackets tested produced the results in Table 12.2.

Johnson et al (2005) investigated the tiewing strength of several makes of ceramic bracket including 3M Unitek Clarity, American Orthodontics Virage, Dentaurum Fascination, GAC Mystique, Ormco Inspire, Rocky Mountain Luxi II and TP Orthodontics InVu. The results are shown in Figure 12.11 and show the progress made in the design and manufacture of ceramic brackets over the last decade. The tiewing of the Inspire bracket did not fracture at all under the test conditions and the authors consequently eliminated it from the study. The authors went on to conclude that semitwin designs (Virage, Fascination and Mystique) had stronger tiewings than true twins (Clarity, Luxi II, InVu and Inspire) although this may have not been the case had the Inspire bracket been included.

High resistance to impact Medium resistance to impact

Low resistance to impact

American Orthodontics 20/20 Dentaurum Fascination

Ormco Lumina GAC Allure GAC Allure 3 Unitek/3M Transcend Unitek/3M Transcend 2000

“A” Company Starfire TMB Ortho-Organisers Illusion Lancer Orthodontics Intrigue

Table 12.2: Relative resistance to impact of ceramic brackets (from Matasa 1999)

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Figure 12.9: Torque strength of ceramic brackets ("A" Company data)

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Figure 12.10: Tie wing strength of ceramic brackets ("A" Company data)


Clinical application The mechanical properties of ceramic brackets which give rise to potential clinical problems are low fracture toughness, lack of ductility and hardness. A useful list of clinical tips for the use of ceramic brackets is given by Ghafari (1992) and is worth reading. Many of these problems have been substantially reduced with the continued development of ceramic brackets; nevertheless they remain inferior technical performers to modern metal

brackets.

Reported problems with ceramic brackets • tooth abrasion • bracket breakage • loss of tooth control • increased archwire friction • debonding difficulty/damage

Tooth abrasion Ceramic brackets are much harder than enamel and rapidly cause wear if occlusal interferences are present. The American Association of Orthodontists carried out a survey in 1988 of members experience with ceramic brackets. As a result of the survey, the President of the American Association of Orthodontists, Dr John Lindquist, suggested that both health and safety concerns existed on the part of the orthodontic specialty regarding ceramic brackets and prudent practitioners might wish to discuss the potential risks at an informed consent meeting with the patient and/or parent (Lindquist 1989). The results were again summarised in the next AAO bulletin supplement (7:4 Winter 1989). Of the 21% who at that time reported seeing enamel damage, 59% of that damage was caused by abrasion. The first report of this cause of damage had only recently appeared (by Professor Scott in 1988) and this problem is now entirely avoidable by careful selection of cases, bracket location and treatment mechanics. Ceramic brackets should only be placed on lower teeth in cases where the overbite is at no stage increased. If such brackets are to be placed on the lower incisors in a deep bite case, then the initial use of an upper bite plane or selective sectional mechanics to fully reduce the overbite in the early stages of treatment are highly advisable. Sometimes, the simple expedient of placing lower bonds at a more gingival level suffices. Placement of ceramic brackets on lower posterior teeth would rarely seem sensible.

Methods of avoiding this problem included the use of elastomeric rings with covers for the occlusal part of the bracket on lower incisors (Unitek/3M Alastigards), but reliance on these elastomeric guards to overcome the effects of an occlusal interference would seem imprudent. A better solution is to use ceramic-metal reinforced resin where contact between enamel and a ceramic bracket is likely and aesthetic brackets are necessary.

Bracket breakage The low fracture toughness leads to a higher rate of bracket breakage than with stainless steel brackets. Anecdotally reported breakage rates vary widely. Odegaard (1989) reported three breakages in 500 brackets (Transcend) used throughout treatment. Interestingly, his in vitro tests showed that the force required to break a ceramic bracket was equal to that required to deform a metal bracket. One of the

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Clarity Virage Fascination Luxi II InVu Mystique Inspire

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Figure 12.11: The in vitro tiewing strength of ceramic brackets; note that the Ormco Inspire bracket did not fracture at all but that the value given is the failure value of the steel ligature (from Johnson et al 2005). True twin brackets are coloured blue but include Inspire which did not break and semitwin brackets are coloured plum.


course presenters (NH) has found a clinical breakage rate of 12 in 340 brackets (mainly Starfire brackets). Only four of these brackets required replacement since the fully siamese bracket morphology of Starfire brackets leads to one of the four wings fracturing and the remaining wings frequently provide adequate control if treatment is due to finish before too long. This contrasts with semi-siamese or single brackets which almost always require replacement. Fracture rates have declined as manufacturers improve their production processes. For example, the introduction of the second generation of Starfire brackets in 1989 was claimed by the manufacturers to more than halve the fracture rate in clinical use, (to less than 4%) principally as a result of the heat treatment reducing sites of surface stress concentration. Winchester (1991) found lower breakage rates in vitro with more recently developed brackets. Increasing production control will probably also result in less variability in reported breakage rates.

Ligation with ceramic brackets Meanwhile, several precautions should be taken to minimise bracket breakage. External trauma to and scratching of the brackets during archwire changes must be avoided. Ligatures and the ligating instruments are the main potential culprits. Careful ligation is necessary and elastomeric rings or Teflon coated ligatures (both conventional and Kobayashi) are recommended to prevent tie wing fracture. Monocrystalline ceramic brackets have a true siamese configuration which allows the use of figure of eight elastomeric ligation methods as used for metal brackets whereas most polycrystalline brackets have a semi-siamese tiewing design which is a significant drawback. Semi-siamese tiewing designs may also make it difficult to place both elastomeric chain and ligating modules on the same bracket because of the reduced depth of the tiewing. The loss of tooth control so noticeable with some of the narrower single ceramic brackets is attributable to this difficulty in obtaining effective ligation on such brackets.

Archwires and ceramic brackets The risk of excessive forces when placing or removing rectangular archwires which almost completely fill the slot can be reduced by using a more resilient wire (e.g.: nickel titanium or TMA) before proceeding to the stainless steel wire or as a full size finishing wire or as a substitute for a stainless steel wire.

Placement of additional torque in archwires may cause tiewing fracture on insertion with ceramic brackets. In addition to the use of large size lower modulus wires as recommended above, consideration should be given to increasing the amount of torque by inverting the bracket where appropriate (such as on a lateral incisor that was originally palatally positioned).

Contact sports and ceramic brackets Patients who participate in contact sports which involve a high risk of injury to the face and teeth are less suitable for ceramic brackets. All patients wearing ceramic brackets should be advised to wear a mouthguard when participating in contact sports.

Orthognathic surgery with ceramic brackets During orthognathic surgery, there is clearly a potential for instrumentation to cause bracket fracture. The transparency/translucency and radiolucency of ceramic brackets makes fragments of bracket hard to locate at operation and during recovery when the cough reflex is suppressed. The use of these brackets in such patients must therefore be considered very carefully.

However, given a careful surgeon and an anaesthetist aware of the problems we would not preclude the use of ceramic brackets in cases requiring orthognathic surgery.

Friction and ceramic brackets The problem of friction and ceramic brackets is well known. Angolkar et al (1990) demonstrated that:

• rectangular wires produce more friction than round wires • nickel titanium and beta titanium archwires produce more friction than stainless steel or

cobalt chromium wires


• ceramic brackets generate significantly more frictional resistance than stainless steel brackets.

These findings were broadly confirmed by Pratten et al (1990) who added the finding that artificial saliva increased the

frictional resistance.

Bazakidou et al (1997) evaluated friction in several types of aesthetic bracket and a metal bracket to serve as a baseline. The brackets used are shown in Table 12.3.

The lowest frictional resistance for both slot sizes was produced by the GAC bracket without a metal slot liner. There was no distinct trend between composite brackets with and without a metal slot liner and the frictional resistance with wire ligation was approximately three times more variable than that with elastomeric ligation.

Injection moulded polycrystalline ceramic brackets have been developed (Class One Orthodontics) which claim to have a smoother slot finish thus reducing friction. More importantly, 3M Unitek have introduced a ceramic bracket (Clarity) with a metal slot insert which is designed to solve the problem of increased frictional resistance between archwire and bracket slot.

The effectiveness of this innovation is supported by Nishio et al (2004) who found that the least friction was generated by all metal brackets followed by ceramic brackets with a metal slot and then ceramic brackets with a ceramic slot.

Saunders and Kusy (1994) have looked at the surface topography and frictional characteristics of ceramic brackets. It was concluded that:

• monocrystalline brackets (MCA) have a smoother slot finish than polycrystalline brackets (PCA)

• the frictional characteristics of MCA and PCA are comparable • archwire material has more effect on friction than bracket material • multiple tests demonstrate polishing and smearing of PCA slots

This is supported in a thesis supported by Class One Orthodontics (Omana 1991) which found that Class One Orthodontics ceramic brackets had a lower frictional resistance than other ceramic brackets and a metal bracket. Wire type effects in this study were insignificant.

The increased frictional resistance is also in part due to the hardness of the ceramic causing gouging of the relatively softer wire surface.

It is likely that further progress in this area will be made by attention to the engineering of the bracket slot.

Clinically, it is suggested that:

• ceramic brackets are not used on premolar teeth where sliding mechanics are used • if the patient has a wide smile and ceramic brackets are placed on the premolars, then

space closure with loops may be helpful • use stainless steel wire rather than a nickel-titanium wire for space closure

Bracket Type 0.018” slot 0.022” slot Unitek Miniature Twin metal RMO Signature polycrystalline ceramic ACO Starfire monocrystalline ceramic Ormco Spirit composite/metal slot GAC Elan composite/metal slot GAC composite/no metal slot AO Silkon composite/metal slot

Table 12.3: Brackets used in the study by Bazakidou 1997


• use round rather than rectangular wire in the labial segment if there is difficulty in closing space

• changing to a new archwire may also reduce friction if the previous wire has become gouged and space closure has halted

Interestingly, a paper by Ireland et al. (1991) supported the view that in the buccal segments, the choice of wire and the method of ligation had much more effect on friction than did the use of ceramic or metal brackets. Indeed, in tests on a single bracket, friction was higher in metal brackets. Frictional forces in sliding mechanics are clearly complex and hard to model comprehensively.

Debonding ceramic brackets Removal of ceramic brackets has been an area of significant concern. The lack of ductility means that there is no scope for breaking the bond by flexure of the bracket. This flexure has been shown to contribute to the lower bond strength that exists between metal bracket bases and composite than between composite and tooth when bonding is achieved without moisture contamination. With ceramic brackets, the relative bond strengths are easily reversed and bond failure at the enamel-composite interface with the necessarily higher level of debonding force has caused instances of enamel loss. It is sensible to avoid using ceramic brackets or to use a very careful debonding technique on structurally compromised teeth. In the AAO survey previously referred to, 36% of those who had noticed tooth damage at any stage had seen enamel flaking at debonding. This was in 1988 and it is probable that manufacturers initially overestimated the bond strength required to retain the bracket throughout treatment and did not take account of the differences necessary in debonding technique between ductile metal and brittle ceramic brackets. Odegaard and Segner (1988) showed that for one make of ceramic bracket, both paste/paste and 'no-mix' adhesives produced bond strengths that were slightly higher than for mesh-backed brackets although the differences were not statistically significant.

Winchester has shown lower tensile forces are needed to debond Transcend 2000 brackets than were required with the previous Transcend brackets. Jeiroudi (1991) has reported a single case of serious enamel damage when removing ceramic brackets. In 1989, Odegaard reported that out of 576 debonded brackets, only three had significant enamel loss, the largest defect being 2.5 mm across and very shallow. One of the course presenters (NH) has examined 340 teeth following removal of ceramic brackets at the end of treatment and has observed no enamel damage. Almost all of these brackets were removed using Starfire debonding lift-off pliers and the remainder using the Transcend debonding key(s) which were originally advocated. DJB has observed one very small lesion which ironically occurred when he was using an air-rotor to remove a bracket in the belief that this would provide no potentially damaging forces. Winchester (1991) reported 4 enamel fractures in 210 teeth tested in vitro where the bond strength is probably higher than in vivo. These findings are clearly encouraging because they indicate that since this problem was first identified, it has now become a potentially rare complication.

A further variable must surely be the variation in bond strengths of the various bracket types and the recent work by Winchester has shown that for one bracket type, the type of applied debonding force is important, lower tensile forces than shear forces being required to debond. Further work is currently in hand, but these factors may well explain the widely varying clinical impressions of the force required to remove ceramic bonds. Meanwhile, a clinically important point is that removal of all excess composite flash before debonding any ceramic bracket to reduce the force level required for debonding.

The following methods of debonding ceramic brackets have been described:

• conventional debonding pliers • ceramic bracket specific debonding pliers • air rotor • ultrasonic scaler • electrothermal debracketing


• laser-aided debonding • collapsible brackets with a designed ‘fault line’

Debonding has become much easier than in the late 1980s; however it still remains more of a challenge than debonding conventional Siamese metal brackets. Operators will learn about their particular combination of adhesive and bracket:

The specialised ceramic bracket debonding pliers produced by ceramic bracket manufacturers were originally a mixed success but are now much improved. All require practice to use well and it is worth persevering with a system that has merit. The Starfire debonding pliers have a guard to contain fragments of broken bracket. Bishara and Fehr (1993) have demonstrated that narrow debonding pliers generate a lower debonding force than wide pliers.

However, 3M/Unitek Clarity brackets are designed to collapse on debonding. Debonding forces are similar to other ceramic brackets and in 50% of tests on the 3M/Unitek Clarity bracket, one half of the bracket only debonded while the other half was left on the tooth; this was usually easy to remove however (Bishara et al 1997).

Theodorakopolou et al (2004) compared the debonding characteristics of Ormco Inspire and 3M Unitek Clarity brackets. They found that the shear bond strengths (using Transbond XT as the adhesive) when tested in an Instron machine were 20.3 ± 8 MPa and 21.7 ± 5.2 MPa. This is a relatively high figure for an in vivo shear bond strength. An additional sample of the same brackets were debonded using the manufacturer’s recommended technique and using the manufacturers recommended pliers. Over 90% of the brackets of both types debonded at the bracket-adhesive interface. No enamel fractures were observed using the manufacturers recommended technique. The clear recommendation is therefore to follow the manufacturer’s instructions!

In 2004, 3M Unitek introduced the APC-Plus Clarity bracket. This bracket has a colour sensitive adhesive which is sensitive to the degree of cure being pink when uncured and turning clear when completely cured. Staribratova-Reister and Jost-Brinkmann (2004) examined the effect of APC-Plus adhesives on bonding and debonding time. Two operators required substantially more time for the bonding of APC-Plus Clarity brackets compared with APC Clarity brackets and no difference was found in the time required to remove residual adhesive on debonding between the two brackets.

In Ormco’s Ice bracket, the area of the bracket base closest to the gingival margin has less retention to allow a failure zone in the adhesive to propagate as the bracket is debonded by rotating it occlusally.

Electrothermal debracketing Now largely of historical interest only!

Electrothermal debracketing (ETD) of ceramic brackets was available for “A” Company Starfire brackets only. It was based on work carried out by Zach and Cohen (1965) and by Sheridan et al (1986a and 1986b). The original ETD instrument was powered by a rechargeable battery and consists of a heated probe controlled by a finger switch. The later version, ETD Plus, was powered from a small transformer connected to the mains. The tip of the instrument fitted into the vertical slot between the tiewings. When fully seated the operator applied a twisting force to release the bracket. This happened in about 5 seconds. The tip of the instrument retained the bracket which was very hot!. Patients felt some thermal sensation and occasionally mild discomfort. It was essential to:

• trim any flash away from the bracket • dry the bracket • press the tip of the instrument firmly into the vertical slot between the tie wings

before applying the twisting force. If the bracket could not be debonded in ten seconds then an alternative method was to be used.


A paper by Jost-Brinkmann (1992) made several interesting points about electrothermal debonding.

• most teeth are likely to recover completely from the thermal stress involved • the bracket should be removed in the first heating cycle to ensure that the greatest reservoir

of heat capacity is removed from the tooth surface • enamel fractures and bracket fractures can still occur • adhesives soften at different temperatures - Transbond at 170° Heliosit at 100°. These

findings should be contrasted with those of Winchester (1991) in relation to the bond strength of Heliosit and Prismafil in mechanical debonding.

Brouns et al (1993) compared the use of the De-Bond 200 and the Dentaurum Ceramic Bracket Debonding Unit with a control group in which teeth were exposed for one second to water at 50°C and 70°C. The average pulpal temperature rise for both debonding units was below the 5.5°C suggested by Zach and Cohen as the threshold for pulpal damage in humans although the maximum temperature rise was 15.5°C. The temperature rises in the control group were an average of 6.3°C and 10.7°C respectively.

Kearns et al (1997) have shown intrapulpal temperature rises of 6.7°C to 7.1°C for different ceramic brackets using the Dentaurum Ceramic Bracket Debonding Unit. This is higher than the figure recommended by Zach and Cohen. The shear force required to remove the brackets was a mean of 4.6 MPa for the electrothermally debracketed group and 12.4 MPa for the mechanically debracketed group.

Takla and Shivapuja (1995) showed that there was significant hyperaemia 24 hours after debonding and that the pulpal response 30 days after debonding varied from complete recovery, through residual inflammation to pulpal fibrosis. These investigators also questioned the effects of electrothermal debracketing on pulps that were already compromised due to previous injury or large restorations. Studies should examine the pulp at least 56 days after debonding to assess pulpal health.

Lee-Knight et al (1997) concluded that the use of ceramic brackets on veneers and electrothermal debracketing results in veneer damage on debonding compared with 21% of veneers being damaged with metal brackets removed with Howe pliers and 35% of veneers when metal brackets are removed with a lift off debracketing instrument (LODI).

The use of ultrasonic scalers and lasers for debonding are mentioned for completeness and are noted practical methods of debracketing. A good review of different debracketing techniques is given by Bishara and Trulove (1990) while the use of CO2 and YAG lasers is described by Strobl et al (1992).

Key facts The temperature range of ingested food in the oral cavity may vary between 7°C and 75°C and the temperature range at the tooth surface 5°C to 48°C (Graf 1960, quoted in Crooks et al 1997)

Thermal pulpal injury appears to be reversible provided the pulpal temperature increase does not exceed 5.5°C; at 11.1°C, 60% of pulps show abscess formation and at 16.6°C, all pulps show necrosis (Zach and Cohen 1965).

Aesthetic wires Tooth coloured wires have been available for many years. The earliest were plastic coated and their mechanical properties suffered badly from the thick coating which reduced the effective diameter of the wire from .020" to .012" in one instance. The coating also discoloured and came off in use.

The next step was the introduction of Teflon coated wires. These coatings have the major advantage of being very thin and the effect on their clinical performance is undetectable. An additional potential advantage is the reduction in friction, especially on the nickel-titanium wires which were initially the only type on which a coating could be placed. However, the coatings still tend to come off in use. More


recently, coated stainless steel wires have become available, but the range of these wires is still limited and the retention of the coating is yet to be demonstrated as satisfactory.

In 1992, Talass described his work in developing the Optiflex 'wire'. This transparent fibre had a concentric, layered construction and a very low modulus of elasticity. It did cause detectable tooth movement and appears sufficiently robust if supported across extraction sites, but the performance was not nearly as good as a nickel-titanium wire and the scope for applying this technology to the later stages of treatment where more rigid wires are needed, seems remote. Ireland et al (1991) reported on a super-drawn Polyacetyl 'wire' (Asahi Chemical Industries Company Ltd) and found it to be unsatisfactory.

Work by Kusy (1997) on composite archwires is promising, and aesthetic archwires with significantly improved performance may yet become a reality. Work on composite archwires (unidirectional fibre-reinforced polymeric composites – UFRPs) with unidirectional ceramic fibres embedded in a linear or cross-linked matrix is promising. Prototypes are tooth coloured, extremely strong and have now reached the stiffness of beta-titanium wires as shown in Figure 12.12. They are formed by pultrusion – a process which produces continuous lengths of material with a constant cross-section by passing continuous fibres with a polymeric resin through a sizing die to preform the composite and establish the resin/fibre ratio, and then through a curing die to finalise the shape as curing takes place. Curing is undertaken by electromagnetic radiation. These wires have comparable resilience and springback to NiTi. Preformed archwires are possible through a technique known as beta staging which takes place between the sizing die and the curing die. Low coefficients of friction and enhanced biocompatibility should be possible by modifying the surface chemistry of the polymer.

Aesthetic ligatures and elastomerics Teflon coated ligature wires are available and work satisfactorily as ligatures whilst suffering the same tendency to loss of the coating as do the archwires. All wire ligatures involve an increased chance of scratching of the ceramic brackets compared to elastomerics and consequently a higher risk of bracket failure.

Elastomeric ligatures have two potential problems when used with ceramic brackets. Firstly, the majority of brackets are only semi-siamese and therefore do not permit figure of eight ligatures. This greatly diminishes the control possible with elastomeric ligatures in such brackets. Secondly, all elastomerics absorb stain and discolour, detracting significantly from the aesthetics. In general, opaque/white elastomerics are better in this respect than transparent elastomerics. Patients should be warned to avoid curry spices (turmeric!), mustard and oranges which can produce particularly marked discolouration. All elastomerics should be replaced at each visit. There is scope for significant improvement in this respect.

Summary Ceramic brackets have been understandably welcomed by patients and they are the best attempt so far at producing an orthodontic appliance which combines the aesthetic needs of the patient with the technical performance required by the orthodontist. Recent improvements in bracket manufacture and bracket removal have significantly lessened the potential for problems, as has the awareness of the possibility of enamel abrasion. Nevertheless, the only advantage that ceramic brackets have over stainless steel

0

50

100

150

200

250

NiTi UFRP Beta-Ti SS

Ela

stic

mod

ulus

GP

a

Figure 12.12: The elastic modulus of unidirectional fibre-reinforced composite archwires is similar to that of initial and intermediate stage conventional archwires such as NiTi and beta-titanium. (From Kusy 1997)


brackets is one of appearance and important questions about bracket fracture and tooth damage during bracket removal remain only partially answered at the present time. Further progress is required in the development of aesthetic archwires and ligatures.

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Useful related references not referred to in this chapter

Douglass JB (1989) Enamel wear caused by ceramic brackets American Journal of Orthodontics and Dentofacial Orthopaedics 95: 96-98

Rueggeberg FA and Lockwood P (1990) Thermal debracketing of orthodontic resins American Journal of Orthodontics and Dentofacial Orthopaedics 98: 56-65

Storm ER (1990) Debonding ceramic brackets. Journal of Clinical Orthodontics 24: 91-94

Swartz ML (1988) Ceramic Brackets Journal of Clinical Orthodontics 24: 91-94

Viazis AD, DeLong R, Bevis RR, Douglas WH and Spiedel TM (1989) Enamel surface abrasion from ceramic brackets: a special case report American Journal of Orthodontics and Dentofacial Orthopaedics 96: 514-518























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