Sadowsky PL, Retief DH, Cox PR Effects of etchant concentration and duration on the retention of orthodontic brackets: an in vivo study. Am J Orthod Dentofacial Orthop. 1990; 98:417-421
Alzainal AH, Majud AS, Al-Ani AM, Mageet AO Orthodontic bonding: review of the literature. Int J Dent. 2020; 14 https://doi.org/10.1155/2020/8874909
Defective enamel can render conventional methods for bonding orthodontic brackets ineffective and compromise orthodontic tooth movement. There are various techniques proposed in the literature to optimize the orthodontic bond strength. A clinical case is presented here with some additional lesser-known techniques to aid bonding. These include etching the enamel surfaces twice, curing the resin adhesive (primer) after etching, and placing composite ‘pads’ to maximize the surface area for bonding. These techniques may be implemented when the primary objective is to improve the bond strength. However, the need to maximize bond strength must be balanced against the risk of causing iatrogenic damage to weaker enamel.
CPD/Clinical Relevance: Defective enamel can present an orthodontic challenge owing to compromised bond strength between the adhesive and enamel.
Article
Successful orthodontic tooth movement relies upon the predictable bonding of brackets to the enamel surface. Conventionally, this is achieved by first conditioning the tooth surface with 37% phosphoric acid etch for 15–30 seconds to create surface microporosities.1,2 The resultant uneven surface texture facilitates micromechanical bonding, usually via an uncured primer (an unfilled resin), to composite resin, which is frequently the adhesive material of choice for bonding orthodontic brackets. This is predominantly owing to its good shear and tensile bond strength, which helps it withstand orthodontic and masticatory forces.3 While the primer is usually cured to increase the strength of the bond during usual restorative composite bonding, in orthodontics, the primer is often left uncured to help facilitate de-bonding of the brackets without shearing off the enamel.
There are, however, additional complexities to consider when bonding brackets to atypical tooth surfaces, especially defective enamel, which can be seen in conditions such as dental fluorosis, molar–incisor hypomineralization (MIH), and amelogenesis imperfecta (AI). The literature highlights that the risk of bond failure is higher in these cases, potentially owing to the higher protein content of the enamel and, consequentially, resistance to acid etch.4
In such cases, various techniques may be used to optimize the bond strength of orthodontic brackets. A number of these techniques are referenced in the literature and are highlighted in this article. The authors also present additional techniques that proved beneficial when treating a particularly challenging clinical case.
The clinical case
A medically fit and well 9-year-old child was assessed within the orthodontic department after being referred by their dentist for unerupted maxillary permanent central incisors. The patient had previously sustained trauma to his maxillary primary central incisors when he was 1 year old. These teeth became non-vital and mobile, and were extracted 2 years later. This trauma may have contributed to changes in the form and eruption of the permanent incisors.
Clinical and radiographic investigations were undertaken for the unerupted UR1 and UL1 (Figures 1 and 2). The findings from these investigations included:
Severely dilacerated and unerupted UR1 and UL1;
Tuberculate unerupted UR2.
Figure 1. (a) CBCT scan showing dilaceration of the UR1 and UL1. (b) OPT radiograph demonstrating delayed dental development, including the unerupted UR2, UR1 and UL1.Figure 2. Clinical photographs were taken at the initial assessment. (a) Extra-oral smiling view. (b) Intra-oral central view demonstrates the absence of the UR2, UR1 and UL1 and a lack of vertical interocclusal space for their eruption. (c) Intra-oral maxillary view. (d) Intra-oral mandibular view.
The patient was subsequently assessed at the joint orthodontic and restorative clinic, where various treatment options were proposed. Owing to the severe dilaceration of the maxillary incisors, they were regarded to have a poor long-term prognosis. However, extraction of these teeth would leave little bone in the premaxillary area, which would render future restorative management very challenging. Thus, the agreed plan was to attempt to extrude the UR1 and UL1 orthodontically. The primary aim was to derive alveolar bone that could be maintained to facilitate prosthodontic replacement of these teeth in the future, including the provision of dental implants.
Open exposure of the UR2, UR1 and UL1 was then undertaken. This revealed that the UR1 and UL1 were not only dilacerated but had significant enamel hypomineralization (Figure 3). Initially, orthodontic brackets were bonded to the UR2, UR1, and UL1 after etching the enamel surfaces with 37% phosphoric acid for 20 seconds, followed by an uncured primer and then a composite resin. The brackets were to be used with a powerchain to hook on a fixed labial arch bar fitted through the headgear tubes of the UR6 and UL6. A lower removable appliance with poster bite planes was also used to help improve the vertical interocclusal space for erupting the incisors (Figure 4).
Figure 3. Open exposure of UR2, UR1 and UL1. (a) Initial incision to expose UL1. (b) Complete exposure of UL1 crown. (c) Complete open exposure of UR2, UR1 and UL1 with apical repositioning of the buccal flaps. (d) Clinical outcome after post-operative healing.Figure 4. (a) The appliance is fitted to the maxillary dentition with orthodontic brackets bonded to UR2, UR1, and UL1, and the lower removable appliance is fitted with posterior bite planes. (b) Labial bow with incorporated molar bands bonded to UR6 and UL6.
However, shortly after completing this treatment, the brackets de-bonded from the UR1 and UL1. Re-bonding the brackets was complicated by the hypomineralized enamel, which rendered usual bonding techniques ineffective. This necessitated the use of additional techniques to improve the bond strength.
Orthodontic bonding techniques
Various techniques have been suggested for orthodontic bonding to defective enamel, as outlined in Table 1.
Bonding to composite resin may provide increased bond strength compared to defective enamel4
Etching for 30–60 seconds with 37% phosphoric acid3
For severely affected enamel (where the whole enamel surface has marked opacity), a longer etch time is required to produce a similar etch pattern to normal enamel3
Grind away superficial enamel followed by 37% phosphoric acid etch for 15–30 seconds2
For more severely affected enamel (where the whole enamel surface has marked opacity with pitting of the enamel) increasing etch time poorly correlates with the quality of etch pattern, therefore, grinding affected enamel can be attempted2
This was found to increase the effectiveness of orthodontic bonding5
Amelogenesis imperfecta
Application of 5.25% NaOCl prior to phosphoric acid etch3,4
This aims to remove excess protein in the enamel and optimize the effectiveness of acid etch3,4
Provisional restoration with composite veneers/crowns3,4
Bonding to composite resin may provide increased bond strength compared to defective enamel3,4
Use of glass ionomer-based cement (GIC) adhesive3,5
May improve bracket retention and reduce the risk of enamel demineralization3,5
Impact on the clinical case
To optimize orthodontic bond strength to the hypomineralized and dilacerated UR1 and UL1, the following techniques were considered.
Using GIC as the adhesive cement;
Sandblasting the enamel with aluminium oxide for 10 seconds;
Etching twice with 37% phosphoric acid, rubbed into the enamel for 40 seconds each time;
Applying and curing the primer for 20 seconds after etching;
Placement of a composite ‘pad’ in a similar approach to the use of a composite veneer as outlined in Table 1.
A Cochrane systematic review found no clear evidence to determine which adhesive material provides better outcomes for bonding orthodontic brackets.6 GIC was attempted, but proved to be ineffective as bracket de-bonding occurred. A sandblaster was not available within the department.
The other listed techniques, however, were used in this case (etching the enamel twice with phosphoric acid, curing the primer after etching and placing a composite ‘pad’). The decision to cure the primer was based upon evidence that reported curing the primer prior to the application of composite resin increases the bond strength for surfaces contaminated by saliva or blood.7 The decision to ‘double-etch’ was informed by advice from restorative colleagues of the authors who suggested that this may optimize the bond strength by maximizing the creation of surface microporosities.
The combination of these techniques worked successfully, with no further bracket de-bonds, and the UR2, UR1, and UL1 were all subsequently extruded (Figure 5).
Figure 5. (a) Composite ‘pads’ are visible on the buccal surfaces of UR1 and UL1. (b) Completion of orthodontic extrusion of UR2, UR1 and UL1. No iatrogenic damage to the enamel was caused following de-bond.
The evidence highlights that for cases with defective enamel, increasing the strength of orthodontic bonding will increase the risk of adverse damage to the enamel during orthodontic treatment or de-bonding, such as enamel fracture.3,5
In this clinical case, the maxillary incisors had a poor long-term prognosis, and the primary objective was to orthodontically extrude them to induce alveolar bone growth and facilitate prosthetic tooth replacement. While no iatrogenic damage was caused to the enamel following the de-bond, the authors acknowledge that the techniques used focused primarily on optimizing bracket bond strength, accepting the risk to the enamel. The authors do not suggest that the techniques used should be routine in every case of defective enamel. Instead, they may be implemented when adhering orthodontic brackets to the tooth surface is the priority.
Conclusion
Bonding orthodontic brackets to defective enamel can pose significant challenges, but various techniques can be used to reduce the risk of treatment failure. These include increasing the duration and number of times of etching with 37% phosphoric acid, increasing the surface area of composite bonding prior to bracket placement, and, when absolutely necessary, curing the primer (resin adhesive) prior to composite placement. There will usually be a compromise between the need for optimal bracket bond strength and the need to preserve the health of defective and weaker enamel, and this must be assessed on a case-by-case basis.
