An Overview of Orthodontic Bonding

Enamel

Direct bonding to enamel utilizes three principal agents: an enamel surface conditioner, a primer solution and an adhesive resin.

Surface conditioner

This creates micro-porosity and a high-energy enamel surface. Scanning electron micrographs are presented of normal enamel (Figure 1) and enamel that has been etched with 37% phosphoric acid for 15 seconds (Figure 2).

Bonding to defective enamel

It is not uncommon to encounter enamel surfaces that have developmental defects, such as those in amelogenesis imperfecta and molar incisor hypomineralization. Figures 3, 4 and 5 highlight the poor enamel formation in amelogenesis imperfecta, whilst Figures 6, 7 and 8 present a mild case of molar incisor hypomineralization. Clinical experience has shown that bond failure rates are higher in these cases; one reason for this may include the increased protein content of affected enamel. To address this, Venezie et al described the use of sodium hypochlorite to remove excess protein and improve the quality of etch in amelogenesis imperfecta cases.¹² The evidence for improved bracket retention with these methods, however, remains weak and would not routinely be recommended.¹³ Whilst conventional etching is discouraged in these cases, as phosphoric acid may result in more enamel loss, SEPs may be used as an alternative because they produce a milder etch pattern and remove less enamel.^14,15 Furthermore, the use of SEPs may help to reduce sensitivity that may be experienced by the patient during etching, rinsing and air-drying. Alternatively, the banding of molars may also be preferable.¹³

Bonding to composite labial veneers may result in increased bracket retention in cases affected by severe defects of enamel, as it is proposed that bonding to a larger area of composite resin increases bond strength when compared to bonding to the defective enamel alone.

Bonding to fluorosed enamel

For bonding to mildly fluorosed teeth, it has been reported that there is no significant difference in sheer bond strengths compared to that of normal enamel.¹⁶ An in vitro study by Isci et al, however, found that SEPs showed lower shear bond strength values for orthodontic brackets bonded to mildly fluorosed enamel.¹⁷

Composite resin

Bonding to composite resin requires superficial roughening, either through sandblasting with aluminium oxide or with diamond burs.¹⁸ Furthermore, an in vitro study concluded that a clinically acceptable bond strength can be achieved by surface conditioning of aged resin composite via the application of hydrofluoric acid, sandblasting with aluminium oxide, sodium bicarbonate particle abrasion, or a diamond bur.¹⁹ Subsequent bonding of brackets can be achieved by traditional orthodontic composites.

Porcelain

Bonding orthodontic brackets to porcelain/ceramic surfaces has a greater failure rate compared to enamel bonding.²⁰ Therefore, several techniques have been suggested, which include:

Zachrisson and Büyükyilmaz :²¹

Bourke and Rock:²²

Grewal Bach et al:²⁰

Hydrofluoric acid is highly corrosive, and should be used under rubber dam isolation and with high volume suction to prevent injury to the patient. Several primers are available for bonding to Zirconium crowns, for example, Assure®Plus (Reliance Orthodontic Products), All-Bond Universal® (Bisco, Schaumburg, Ill) and Scotchbond™ Universal (3M Unitek).²³

Gold

Conventional acid etching is ineffective in the preparation of gold surfaces for mechanical retention of orthodontic attachments. Büyükyilmaz et al suggested that intra-oral sandblasting is utilized.²⁴ This can be followed by bonding with a methacryloyloxyethyl trimellitate anhydride, (4-META) metal-bonding adhesive resin. Subsequent bonding of brackets can be achieved by traditional orthodontic composites. Research has shown that the bond strength achieved is comparable to that of acid-etched enamel.²⁴

Amalgam

Successful bonding of orthodontic attachments to an amalgam surface requires conditioning of the amalgam (for example sandblasting), and use of a 4-META resin. Subsequent bonding of brackets to sandblasted and alloy primer-coated amalgam surfaces can be achieved by traditional primers and orthodontic composites.²⁵

An alternative is to use a hydrophilic primer containing biphenyl dimethacrylate, such as Assure® (Reliance Orthodontic Products). This allows for composite bonding to amalgam following sandblasting without the use of a separate metal primer.²³ Subsequent bonding of brackets can be achieved by traditional orthodontic composites.

Acrylic

Acrylic teeth are often incorporated into orthodontic appliances as prosthetic teeth to mask spaces. Orthodontic brackets can be bonded to acrylic teeth using mechanical and chemical methods, or a combination of both. Mechanical retention includes sandblasting with aluminium oxide particles,²⁶ the creation of undercut holes to facilitate a micro-mechanical ‘lock’ or roughening the surface with diamond or tungsten carbide burs.²⁷ Chemical retention can be achieved using adhesive materials, such as cyanoacrylate.²⁸

Discussion

Advancements in dentistry over the last 50 years have meant that teeth previously considered of hopeless prognosis can now be restored and maintained. Orthodontic clinicians must therefore possess the knowledge and skills to modify conventional bonding techniques, which are summarized in Table 1. A summary table (Table 2) has been developed as an aide memoire for required alterations to the enamel bonding process that allow for orthodontic brackets to be bonded to composite, porcelain, zirconia, metal and acrylic.

This article has provided an overview of some of the evidence and techniques available for bonding to enamel and other surfaces (composite, porcelain, gold, amalgam and acrylic). Despite all efforts to improve bond strength in compromising situations, repeated bond failures may still occur. In these situations, it may be necessary to resort to banding teeth and accepting the associated disadvantages.

Conclusion