Millett DT, McCluskey LA, McAuley F, Creanor SL, Newell J, Love J. A comparative clinical trial of a compomer and a resin adhesive for orthodontic bonding. Angle Orthod. 2000; 70:233-240
Norevall LI, Marcusson A, Persson M. A clinical evaluation of a glass ionomer cement as an orthodontic bonding adhesive compared with an acrylic resin. Eur J Orthod. 1996; 18:373-384
O'Brien KD, Read MJ, Sandison RJ, Roberts CT. A visible light-activated direct-bonding material: an in vivo comparative study. Am J Orthod Dentofacial Orthop. 1989; 95:348-351
Keim RG, Gottlieb EL, Nelson AH, Vogels DS. 2008 JCO study of orthodontic diagnosis and treatment procedures. Part 1. Results and trends. J Clin Orthod. 2008; 42:625-640
Hu H, Li C, Li F, Chen J, Sun J, Zou S, Sandham A, Xu Q, Riley P, Ye Q. Enamel etching for bonding fixed orthodontic braces. Cochrane Database Syst Rev. 2013; 11
Fleming PS, Johal A, Pandis N. Self-etch primers and conventional acid-etch technique for orthodontic bonding: a systematic review and meta-analysis. Am J Orthod Dentofacial Orthop. 2012; 142:83-94
Lowder PD, Foley T, Banting DW. Bond strength of 4 orthodontic adhesives used with a caries-protective resin sealant. Am J Orthod Dentofacial Orthop. 2008; 134:291-295
Nandhra SS, Littlewood SJ, Houghton N, Luther F, Prabhu J, Munyombwe T, Wood SR. Do we need primer for orthodontic bonding? A randomized controlled trial. Eur J Orthod. 2015; 37:147-155
Banks PA, Richmond S. Enamel sealants: a clinical evaluation of their value during fixed appliance therapy. Eur J Orthod. 1994; 16:19-25
Bazargani F, Magnuson A, Löthgren H, Kowalczyk A. Orthodontic bonding with and without primer: a randomized controlled trial. Eur J Orthod. 2016; 38:503-507
Tang AT, Björkman L, Lindbäck KF, Andlin-Sobocki A, Ekstrand J. Retrospective study of orthodontic bonding without liquid resin. Am J Orthod Dentofacial Orthop. 2000; 118:300-306
Wang WN, Tarng TH. Evaluation of the sealant in orthodontic bonding. Am J Orthod Dentofacial Orthop. 1991; 100:209-211
O'Brien KD, Watts DC, Read MJ. Light cured direct bonding – is it necessary to use a primer?. Eur J Orthod. 1991; 13:22-26
Tang AT, Björkman L, Adamczak E, Andlin-Sobocki A, Ekstrand J. In vitro shear bond strength of orthodontic bondings without liquid resin. Acta Odontol Scand. 2000; 58:44-48
Imazato S. Antibacterial properties of resin composites and dentin bonding systems. Dent Mater. 2003; 19:449-457
Ahn S-J, Lee S-J, Kook J-K, Lim B-S. Experimental antimicrobial orthodontic adhesives using nanofillers and silver nanoparticles. Dent Mater. 2009; 25:206-213
Chung S-H, Cho S, Kim K, Lim B-S, Ahn S-J. Antimicrobial and physical characteristics of orthodontic primers containing antimicrobial agents. Angle Orthod. 2017; 87:307-312
Othman HF, Wu CD, Evans CA, Drummond JL, Matasa CG. Evaluation of antimicrobial properties of orthodontic composite resins combined with benzalkonium chloride. Am J Orthod Dentofacial Orthop. 2002; 122:288-294
Sehgal V, Shetty VS, Mogra S, Bhat G, Eipe M, Jacob S, Prabu L. Evaluation of antimicrobial and physical properties of orthodontic composite resin modified by addition of antimicrobial agents – an in-vitro study. Am J Orthod Dentofacial Orthop. 2007; 131:525-529
Takeuchi Y, Guggenheim B, Filieri A, Baehni P. Effect of chlorhexidine/thymol and fluoride varnishes on dental biofilm formation in vitro. Eur J Oral Sci. 2007; 115:468-472
Saito K, Hayakawa T, Kawabata R, Meguro D, Kasai K. In vitro antibacterial and cytotoxicity assessments of an orthodontic bonding agent containing benzalkonium chloride. Angle Orthod. 2009; 79:331-337
Rodrigues MC, Natale LC, Arana-Chaves VE, Braga RR. Calcium and phosphate release from resin-based materials containing different calcium orthophosphate nanoparticles. J Biomed Mater Res B Appl Biomater. 2015; 103:1670-1678
De Almeida CM, da Rosa WLO, Meereis CTW, de Almeida SM, Ribeiro JS, da Silva AF, Lund RG. Efficacy of antimicrobial agents incorporated in orthodontic bonding systems: a systematic review and meta-analysis. J Orthod. 2018; 45:79-93
Noble J, Karaiskos NE, Wiltshire WA. In vivo bonding of orthodontic brackets to fluorosed enamel using an adhesion promotor. Angle Orthod. 2008; 78:357-360
Lai SC, Tay FR, Cheung GS, Mak YF, Carvalho RM, Wei SH, Toledano M, Osorio R, Pashley DH. Reversal of compromised bonding in bleached enamel. J Dent Res. 2002; 81:477-481
Santin GC, Queiroz AM, Palma-Dibb RG, Oliveira HF, Nelson Filho P, Romano FL. Glass ionomer cements can be used for bonding orthodontic brackets after cancer radiation treatment. Braz Dent J. 2018; 29:128-132
Banks P, Thiruvenkatachari B. Long-term clinical evaluation of bracket failure with a self-etching primer: a randomized controlled trial. J Orthod. 2007; 34:243-251
Jung A, Egloff B, Schweitzer T, Aranda L, Rapin C, Albuisson E, Filleul MP. Comparison of adhesive seal morphology between APC™ PLUS and APC™ Flash-Free Adhesive Coated brackets. Orthod Fr. 2018; 89:191-197
Mohammadi A, Pourkhameneh S, Sadrhaghighi AH. The effect of different force magnitudes for placement of orthodontic brackets on shear bond strength, in three adhesive systems. J Clin Exp Dent. 2018; 10:e548-e554
Kindelan JD. In vitro measurement of enamel demineralization in the assessment of fluoride-leaching orthodontic bonding agents. Br J Orthod. 1996; 23:343-349
Shammaa I, Ngan P, Kim H, Kao E, Gladwin M, Gunel E, Brown C. Comparison of bracket debonding force between two conventional resin adhesives and a resin-reinforced glass ionomer cement: an in vitro and in vivo study. Angle Orthod. 1999; 69:463-469
Achieving high bond strength of orthodontic brackets to enamel, and hence a low failure rate, are the basic demands for orthodontic bracket-bonding systems. Given that continuous replacements of loose brackets are clinically inefficient, time-consuming, and costly, the search for an ‘ideal orthodontic adhesive’ has been a hot topic for many years. Since a substantive number of studies have been focusing on brackets, adhesive systems, and enamel surface conditioning methods in recent years, the authors feel that a review article of this kind can be helpful for the busy clinician to update his/her knowledge about the various adhesive systems available on the market, supported by relevant evidence.
CPD/Clinical Relevance: As there are no clinical guidelines currently in the literature for the selection of the ‘ideal orthodontic adhesive’ and the availability of a substantial number of materials, this review article might help clinicians in the selection process by presenting the available bonding materials, along with supporting evidence from the literature.
Article
Dental bonding was introduced back in 1956 by Bowen.1 In 1968, Buonocore pioneered the work on enamel preparation techniques.2 Since then, these bonding agents were subsequently applied to orthodontics, with multi-banded systems becoming obsolete and superseded by bonded appliances.3
New orthodontic glass ionomer cements, adhesive resins and hybrid cement-resin combinations offer much-improved physical properties and clinical benefits, but there are clear differences in the clinical indications and contra-indications for each type of adhesive. Moreover, an understanding of the features, benefits and limitations allows the clinician to select the material wisely in order to facilitate optimal clinical results.
Optimal material selection and application requires an understanding of the different chemical compositions and physical limitations of today's orthodontic adhesives. Therefore, a clear understanding of the features, benefits and limitations of various adhesives is required to select materials wisely and obtain optimal results. This review article studies the various adhesives used in orthodontics. Information about all other types of cements can be found elsewhere.
Search method
Review of the literature was carried out using the following search methods: MEDLINE, EMBASE, the Cochrane Central Register of Controlled Trials (CENTRAL) and the Cochrane Oral Health Group's Trials Register. The search was focused on various keywords including: ‘orthodontic adhesives’, ‘orthodontic bonding’, ‘orthdont$’, ‘systematic review’, ‘controlled trial’, as well as hand literature searches, which were conducted on studies published until October 2018.
What are the properties of an ‘ideal orthodontic adhesive’?
The terminology can be somewhat confusing when we talk about dental bonding. In this article, an adhesive refers to the cement itself, while a bonding agent refers to the unfilled resin primer. There are a large number of bonding agents available. These bonding agents can be categorized chronologically according to generation, a historical system of identification commonly used by adhesive manufacturers. The generation simply refers to when and in what order a specific bonding agent was developed by the dental industry, ranging from 1st generation in the 1960s to modern 7th generation adhesives and Universal (Figure 1).
Figure 1. Classification of bonding agents according to generation, year of development, steps involved during application and mechanism of action. Reproduced with permission from Ivoclar Vivadent.
There are many properties that an ideal adhesive should possess, including:
An adequate bond strength – strong enough to avoid debond during treatment and weak enough to permit easy removal of brackets at debond, without damage to the enamel surface and with the least discomfort to the patient. The clinically acceptable bond strength ranges from 5–8 MPa;
A long working time – to facilitate command set with no drifting of the bracket;
Easy to remove from the teeth/brackets – to aid easy handling;
Fluoride releasing – to reduce the risk of enamel demineralization;
A long shelf life – where they can be stored for extended periods of time;
Cost a reasonable amount – to be affordable for commercial use;
Compatible with life and non-toxic, ie biocompatible.
Therefore, manufacturers are continuously attempting to find an adhesive system that fulfils the requirements of the ‘ideal orthodontic adhesive’.4 Moreover, at present, there is a wide range of bonding materials on the market. Fortunately, in the last few years there has been an enormous amount of research investigating the properties of these bonding materials. In the next few sections we will explore in more depth each bonding material with reference to the relevant literature.
How can we classify orthodontic adhesives?
Dental adhesives can be broadly classified into two main categories, namely:
Diacrylates (composite resin); and
Glass ionomer cements (GICs).
These adhesives may be set by either chemical reaction (chemical curing) or by photopolymerization (light curing). More recently, composite and GIC adhesives have been modified to form compomers (polyacid-modified resin composites) and resin-modified glass ionomer cements (RMGIC) respectively.
Orthodontic adhesives can be classified into 6 different types, based on the composition of the adhesive used to bond the fixed orthodontic appliances.
1. Chemically-cured diacrylates (composites)
These are composed of methacrylate and dimethacrylate monomers such as Bis-GMA. The filler particles consist of glass beads or rods, aluminium silicate, barium, strontium and borosilicate glass. This filler content can vary greatly from 50–80% by weight of the material. Autopolymerization occurs when all the components are mixed with each other. The chemically cured systems are available as two pastes or as powder and liquid formation.
2. Light-cured diacrylates (composites)
These have similar constituents to chemically cured resin but have a light-activated initiator (photo-initiator) instead which utilizes an external blue light to initiate the setting reaction. The light-cured system is a single component material available in opaque syringes. It is convenient to use because it does not need to be mixed and allows for a longer working time. It is important that the light has a wavelength of 440–480 nm for photoinitiation to take place.
3. Dual-cured diacrylates (composites)
There are several systems that use both chemical cure and light cure and are referred to as dual-cure systems. The polymerization of the material is brought about by chemical cure in approximately four minutes or by conventional light cure in 5–30 seconds. The material can be ‘tacked’ with a 10 second light cure and then allowed to cure chemically.
4. Polyacid-modified resin composites (compomers)
These are similar in composition to resin composite but with the addition of a polyacid modified monomer and silicate glasses, which are fluoride releasing. Setting is initiated after light activation photopolymerization of the acidic monomers to change to rigid materials. The set material absorbs water from the saliva, allowing a delayed acid-base reaction. This reaction releases fluorides and other remineralizing ions from the aluminosilicate glass. Clinical bond failure rates of brackets bonded using compomer have been found to be comparable to those seen with diacrylate bonding agents and are indicated in cases when a conventional etching pattern cannot be achieved, such as in amelogenesis imperfecta or fluorosis.
5. Glass polyalkenoate cements (Conventional GIC)
These are based on the reaction of silicate glass powder (calcium-alumino-fluoro-silicate glass) and polyacrylic acid. Malaeic and tartaric acids can also be used but these affect the physical properties of the cement. The first-generation glass ionomer cement consists of aluminosilicate glass powder and alkenoic acid liquid, which undergoes an acid-base reaction. The second-generation glass ionomer cements have freeze-dried powder blended with glass and mixed with distilled water. The bond strength is a problem and some authors have reported higher bond failure rates with glass ionomer cements (20–50.9%) than with composites (5–7.8%). Also, glass ionomer cements do not reach their maximum strength for 24 hours.
6. Resin-Modified Glass Ionomer Cements (RMGICs)
These are hybrid materials of traditional glass ionomer cements with small additions of light-cured resin or self-curing resin and hence exhibit properties superior to conventional glass ionomer materials. RMGIC sets by photochemical polymerization independently of the GIC reaction setting, which is acid based and it increases the strength of the cement. These cements have the advantages of controlled setting reaction, early improved physical properties, further hardening on maturation, sustained fluoride release, caries inhibition and chemical bonding in the presence of moisture.
Is there a Cochrane review?
A recent Cochrane review by Mandall et al (2018) included 3 trials in the meta-analysis.5 A chemical-cured composite was compared with a light-cured composite (one trial),6 a conventional glass ionomer cement (one trial)7 and a polyacid-modified resin composite (one trial).8 The quality of the trial reports was generally poor. The authors concluded that: ‘It is not possible to draw anything other than tentative conclusions from this systematic review of orthodontic adhesives, primarily because of the weakness in the design and reporting of existing trials. Therefore, at present, there is no clear evidence on which to make a clinical decision of the type of orthodontic adhesive to use’.
What about self-etch primers?
Self-etch primers (SEPs) are gaining in popularity. It is estimated that SEPs are routinely used by almost 30% of practitioners in the United States.9 These systems incorporate methacrylated phosphoric acid esters; after application to the enamel, the phosphate group dissolves and removes calcium ions from hydroxyapatite, becoming incorporated in the network before the primer polymerizes, neutralizing the acid. The proposed advantages of SEPs include:
Reduced chairside time; and
Reduced sensitivity to moisture.
To date, there are only two systematic reviews with meta-analyses published about the difference between SEPs and conventional acid-etch techniques.10,11 A summary of these two meta-analyses is shown in Table 1. From the current evidence, it can be concluded that the choice of enamel preparation technique in orthodontics is dictated by individual preference in view of a lack of conclusive evidence.
5 RCTs included3 RCTs: split mouth design2 RCTs: parallel designA meta-analysis of these 5 studies, with follow-up ranging from 5 to 37 months, provided low-quality evidence that was insufficient to determine whether or not there is a difference in bond failure rate between SEPs and conventional etchants.
‘Low quality evidence that was insufficient to conclude whether or not there is a difference in bond failure rate between SEPs and conventional etching systems when bonding fixed orthodontic appliances over a 5–37 month follow-up’
5 RCTs included3 RCTs: split mouth design2 RCTs: parallel designA random effects meta-analysis demonstrated a tendency for a higher risk of failure with self-etch primers. A small but statistically significant time saving was also associated with the self-etch primer technique. There was insufficient evidence to assess the effect of bonding modality on demineralization rates.
‘Weak but statistically insignificant evidence suggests that the odds of attachment failures differ between SEP and acid-etch orthodontic bonding techniques over a minimum period of 12 months. Use of 1-step bonding techniques is likely to result in a modest time saving compared with 2-stage techniques’
Is the application of a primer before bonding necessary?
A bonding agent (unfilled resin primer and adhesive) may be used as part of the bonding process with light-cured composite. The primer improves enamel surface penetration and hence improves the effectiveness of the final bond.12 It is suggested that, if a primer could be avoided during bonding brackets, this would be more cost-effective and potentially saves time by omitting a step in the bonding process. There is a body of evidence in the literature to suggest that omitting the application of the primer does not have a statistically significant effect on the failure rates of brackets (Table 2). Therefore, it might not be necessary to use a primer during the bonding procedure. However, the use of a primer may offer extra protection to the enamel from possible damage during de-bonding.
Ninety-two patients requiring orthodontic treatment with fixed appliances were randomly allocated to the control or test groupsControl group: (n = 46) bonding with primerTest group: (n = 48) bonding without primer
Failure rate with primer was 11.1% and without primer was 15.8%Bonding without primer was shown statistically to be non-inferior to bonding with primer odds ratio 0.95–2.25 (P = 0.08)
Eighty patients undergoing fixed appliance therapy were includedControl group: (n = 40) were treated withviscous visible light-cured sealant and bonding agentTest group: (n = 40) were treated using bonding system without primer
For the non primer group, bond failures were under 4% and for the primer group, under 3%. The advantages of primer in improving bracket retention are not substantiated by this study
Fifty-two patients were randomized into two groups to assess failure rates of lingual bonded retainerControl group: (n = 26) the lingual retainers bonded using primer and compositeTest group: (n = 26) primer application was omitted
In the primer group, the incidence of retainer failure was 4%; in the non primer group, the incidence was 27%. The difference between the groups was statistically significant (P = 0.049)
Seventy-four patients assessed retrospectively from same specialty clinicControl group: (n = 37) composite bonded with the use of primerTest group: (n = 37) brackets bonded without liquid primer
The total percentages of bond failure, roughly 6%, were similar in both test and control groupsApproximately 73% of all patients in the test group and 57% of the patients in the control group experienced no bond failure at all throughout the entire course of treatment
The sample consisted of 50 permanent premolars previously extracted. A two-paste type of self-polymerizing resin used. The bond failure interfaces of the bracket bases and the enamel surfaces examined with SEM Control group: (n = 25) teeth bonded with using primerTest group: (n = 25) primer application omitted
Failure rates for the test group was 41% andfor the control group was 43%There were no significant differences in the tensile bond strength and the various debonding interfaces for the primer and the non primer groups in the bonding of brackets
Twenty human premolar teeth were taken, sectioned crown from root, and mounted in acrylic resin contained in a metal cube Brackets bonded. Shear bond strength was measuredControl group: (n = 10) primer usedTest group: (n = 10) no primer
The shear bond strength was almost similar in the 2 groups with no statistical difference. In the control group 12.1 N mm2 and in the test group 13.1 N mm2
Sixteen previously extracted premolar teeth included. Transbond XT® and Phase II composites were used with and without primersControl group: (n = 8) primer usedTest group: (n = 8)
Shear bond strengths for Transbond XT®Primer group, 18 MPa and 20.6 MPa for non primer. For Phase II primer group, 18.5 MPa and 15 MPa for non primer groupThese data suggest that Transbond XT® and Phase II might enable enough shear bond strength without the use of primer
What about incorporating antimicrobial agents?
In order to improve the antimicrobial activity of orthodontic bonding systems, some studies have suggested the incorporation of antimicrobial agents into orthodontic bonding systems to prevent demineralization through bactericidal and bacteriostatic action. Examples of some of the substances with potential for bacterial inhibition are listed in Table 3. Studies have suggested that these antimicrobial agents are selectively toxic to oral Streptococci, and their incorporation in orthodontic bonding systems helps to prevent demineralization of the enamel without compromising mechanical adhesion properties. A recent systematic review included 32 studies in the qualitative analysis; of these, 22 studies were included in the meta-analysis.28 The conclusion of this systematic review perhaps reflects the current understanding of the effect of antimicrobial agents in orthodontic adhesive systems: ‘Although there is evidence of antibacterial activity from in vitro studies, clinical and long-term studies are still necessary to confirm the effectiveness of antibacterial orthodontic bonding systems in preventing caries disease’.
Bonding to surfaces other than healthy enamel of permanent teeth?
Bonding to dental restorations or unhealthy enamel can be challenging. In this section, detailed surface conditioning strategies are discussed, before bonding orthodontic appliances on surfaces other than healthy enamel of permanent teeth (Figure 2).
Figure 2. Surface preparations during orthodontic bonding.
Most widely used manufacturers?
Dental manufacturers have come up with a vast array of adhesives in order to get as close as possible to the properties of the ‘ideal orthodontic adhesive’. Each of these can be classified into the categories of adhesives previously proposed. With the large number of orthodontic adhesives currently available on the market, it can be difficult for the clinician to decide which of these to use. For this reason, a summary of some of the most widely used orthodontic adhesives can be found in Table 4.
Manufacturers
Type of Adhesive
Supporting Research
Transbond XT® by 3M Unitek
Light-cured resin
‘Highest material retention on the enamel surface and mean adhesive shear bond strength compared to Fuji ORTHO® LC and Ketac™ Cem’31
Transbond™ PLUS self-etch primer (SEP) by 3M Unitek
Light-cured resin
‘There was no difference in the failure rates of brackets bonded with either Transbond™ PLUS SEP or conventional acid etch (AE) using Transbond XT® paste. Bonding with SEP was significantly faster than using conventional AE’32
APC™ PLUS Flash-Free by 3M Unitek
Light-cured resin
‘The APC™ Flash-Free adhesive system is able to reduce the time needed during orthodontic bracket bonding’33
eXact® by TP Orthodontics
Light-cured resin
See supporting evidence for other light-cured resins as no research specifically about eXact®
Turbo Bond® II by TP Orthodontics
Light-cured resin
See supporting evidence for other light-cured resins as no research specifically about Turbo Bond®
Alpha-Dent® by Dental Technologies
Light-cured resin
See supporting evidence for other light-cured resins as no research specifically about Alpha-Dent®
Right-On® by TP Orthodontics
Chemically-cured resin
‘Bonding time was shortest with Right-On® compared to adhesive pre-coated brackets and Transbond XT®’4
Unite® by 3M Unitek
Chemically-cured resin (no mix)
‘Greater mean bond strength compared to Concise® and Transbond XT® under a placement bonding force of 50, 100, 200 and 300 g’34
Concise® by 3M Unitek
Chemically-cured resin (two paste mix)
‘Greater mean bond strength compared to Unite® and Transbond XT® under a placement bonding force of 400, 600 and1000 g’34
Dyract Ortho® by Dentsply
Light-cured compomer
‘Median first failure time of brackets bonded with Dyract Ortho® was 546 days, compared to 542 days for Right-On® and 442 days for Transbond®, but this was not statistically significant’6
Ketac™ Cem by 3M ESPE
Conventional GIC
‘Ketac™ Cem performed statistically significantly better than Concise®, Bond-fast®, and Rely-a-bond®, in resisting enamel demineralization’35
Fuji ORTHO® LC by GC
RMGIC
‘There was no significant difference between survival rates among three bonding materials (Resilience L3®, Light Bond® and Fuji ORTHO® LC) with respect to the type of malocclusion, type of orthodontic treatment, or the location of the bracket. Resin-reinforced glass ionomer cement (Fuji ORTHO® LC) can withstand occlusal and orthodontic forces despite having a bond strength lower than that of conventional resin adhesives’36
Conclusion
With the large number of orthodontic adhesives currently available on the market, it can be difficult for the clinician to decide which of these to use. In this article, the development of orthodontic adhesives was reviewed, a classification system proposed and the current adhesives on the market outlined, including the relevant supporting evidence. Extensive research of the current products was carried out to help clinicians better understand the difference between the various adhesives that can be used. It is difficult to conclude which adhesive is most effective as this should be assessed on a case by case basis, however, it is hoped that this review clarifies some of the basic concepts of orthodontic adhesives.
