**4. Dentine Bonding Agents (DBAs)**

The concept of bonding a restorative material to the dentine surface is by no means a new idea. Even at the time of Buonocore using phosphoric acid to bond to enamel, the idea of bonding to dentine was considered. However, due to limitations of materials and knowledge of the structure and nature of dentine the dream remained just that until the late '70s. In fact Buonocore did try to introduce a dentine adhesive but was unsuccessful [15]. The earliest bonding agent which showed some success was introduced by Fusayama [27]. At the same time Bowen [12] in the USA started investigating new formulations of resins

Compomers have been widely used in Class V restorations. For example, the compomers Dyract AP, Compoglass F and F2000 were evaluated for use in this application over a 2-year period [37]. This study concluded that, after this time, all three materials showed an

Colour stability has been found to be somewhat of a problem with compomers in a few studies. This is not entirely surprising, given that they are designed to take up water, which is likely to alter appearance through a change in refractive index, and also to carry with it coloured chemical species (stains) from certain foodstuffs such as coffee and red wine. In a 3-year study of Class V restorations of Dyract, Demirci et al. [21] found that all Ryge criteria were good, except those relating to colour change, *i.e.* colour stability and marginal

Compomers have been used as fissure sealants [28], and a clinical study examined the teeth of children aged between 7 and 10 years sealed by the compomer Dyract Seal. Sealed teeth were examined post-operatively at 3, 6, 12 and 24 months, and were also evaluated by the Ryge criteria. In general Dyract Seal behaved as well as a conventional composite resin sealant, except on the criterion of marginal integrity, showing that this material was acceptable for its clinical application, at least of the 24 months period of the study [28].

Compomers have also been used for Class I [56] and Class II restorations. In the Class I study, they were used in composite laminate restorations, and were shown to perform as well as conventional composite resins [56]. In the Class II study, they were studied over 7 years in children aged between 3.6 and 14.9 years. Again performance was indistinguishable

Lastly, compomers have been employed as cements for orthodontic bands and there have been a number of full studies of compomers in this application [34]. Results have been generally extremely good for compomers, except in the realms of taste, as determined by the patient, and in which compomers scored less well than glass-ionomers. Thus, compomers have been shown to have acceptable performance as materials for use in orthodontic band retention, though the final choice of cementing agent could be left to patients. If they found the taste of the compomer particularly objectionable, a resin-modified glass-ionomer could

Overall, the major conclusion from these clinical results is that compomers perform well, and are suited to their suggested uses in dental restoration. The reduction in strength due to water uptake does not seem to be important clinically and these materials are suited to use

The concept of bonding a restorative material to the dentine surface is by no means a new idea. Even at the time of Buonocore using phosphoric acid to bond to enamel, the idea of bonding to dentine was considered. However, due to limitations of materials and knowledge of the structure and nature of dentine the dream remained just that until the late '70s. In fact Buonocore did try to introduce a dentine adhesive but was unsuccessful [15]. The earliest bonding agent which showed some success was introduced by Fusayama [27]. At the same time Bowen [12] in the USA started investigating new formulations of resins

discoloration. In both of these there were significant changes [21].

acceptable level of clinical performance.

from that of conventional composite resins.

be used equally effectively instead [50].

**4. Dentine Bonding Agents (DBAs)** 

in vivo.

that were more water tolerant as well as methods of treating the dentine with oxalates to gain adhesion. The concern of many clinicians at that time was the potential damage phosphoric acid was going to cause the dental pulp if dentine was etched [79]. The first work to investigate the mechanism of bonding to the dentine was by Nakabayashi [47]. His paper of 1982 has now become one of the classic papers to first identify a layer between the resin and dentine substrate referred to as 'hybrid' dentine, in that it was the organic components of the dentine that had been permeated by resin (Fig. 2). The term 'hybrid layer' has now become synonymous with bonding of resins to etched dentine. There has been a tremendous amount of research done on the hybrid layer, its structure, formation and how it can be improved. Without a hybrid layer a bond will not be formed to the dentine. Therefore, it is essential for some modification to be made to the dentine surface so a mechanical interlocking of resin around dentinal collagen can occur. This layer has also been referred to as the 'resin-dentine interdiffusion zone' [79].

Fig. 2. Bonded specimen in which the dentine (mineral and protein) has been removed. The infiltration of resin into the acid-etched dentine can be seen with an associated permeation of resin throughout the dentine tubular network and its lateral branches.

#### **4.1 Classification**

Dentine bonding agents have gone through many changes over the last 10 years. This has led some people to refer to the changes as 'generations' of material, implying that there has been some chronological development. This is a falsehood — for example, the first 'selfetching' type material was introduced by Coltène (Altstätatten, Switzerland) as 'ART Bond'. Therefore, it is more logical to classify materials by the number of steps needed to complete the bonding process.

#### **4.1.1 'Three-step' or 'Conventional' systems**

This group represents those materials that have separate etching, priming and adhesive steps. It just so happens that this group of materials is also the oldest. However, they are still widely used and have been shown to provide reliable bonding. The greatest problem with this group would seem to be that three distinct steps are needed, which gives rise to possible

Filling Materials for the Caries 347

water after washing. This allowed the priming solution to diffuse throughout the collagen fibre network more successfully. However, when it comes to clinical practice, it is very difficult to find the correct balance of residual moisture. Sano *et al*. [65] showed in their work on nanoleakage that most resin-based DBAs allowed the ingress of silver nitrate along the base of the hybrid layer. However, the clinical significance of this is unclear. It may be a pathway for fluid to affect collagen not coated by resin, and the outcome may be degradation of the bond over time. However, the degree of nanoleakage is very much material dependent rather than system dependent, meaning that there are conventional systems and self-etching priming systems that show small amounts of nanoleakage whereas others show more. For the self-etching systems, these are able to solubilize the smear layer and demineralize the underlying dentine, forming a quite thin hybrid layer [25, 35, 36, 79].

Dentine is quite a variable tissue. Within the tooth itself the dentine approaching the dentino-enamel junction is more highly mineralized and the area occupied by the tubules is less than that of dentine adjacent to the pulp. In addition to this, dentine should be considered as a dynamic tissue that changes due to ageing, in response to caries and restoration placement. Most changes relate to occlusion of tubules and also an increase in the mineralization of the dentine. The implication of this is that the dentine becomes slightly more difficult to etch and exposure of collagen fibrils can also be reduced, hence there is a potential for the bond to be somewhat tenuous. This is particularly the case for the highly sclerosed dentine of non-carious cervical lesions (NCCLs). Laboratory studies indicate that the hybrid layer of the dentine surface of NCCLs is thinner than that of normal dentine [29, 63]. In addition, it seems that some bonding systems do not adhere as well to this surface

A considerable amount of work has also been done looking at the variation of the bond to caries-affected dentine. Some of the early studies used artificial caries like lesions. However, this does not reproduce the situation that occurs in the oral cavity since caries is a process of demineralization and remineralization associated with the damage of the supporting collagen matrix [48, 53]. The increased thickness of the hybrid layer is mainly because the dentine is already partially demineralized from the caries and the action of the acid etch is therefore somewhat greater. This provides a clear basis for not etching for longer than that recommended by the manufacturer. In addition, the water content of caries-affected dentine is believed to be greater than normal dentine. This too will also have an effect on the ability of the resins to penetrate to the full depth of the demineralized dentine. In the case of cariesaffected dentine treated with chemo-mechanical caries removal solutions, there appear to be

However, the bond to radicular and pulp chamber dentine does seem to vary quite a lot depending on the DBA used. This perhaps provides a strong case for being careful with the selection of a DBA for these regions of the tooth. It is believed that it may be necessary to use different DBAs for different regions of the tooth, or a system needs to be selected where it has been shown to provide a reliable bond to all parts of the tooth. Another alternative is the use of GIC restorative materials when then is a deep cavity on the radicular surface of a tooth, as it is known that a reliable bond can be achieved and moisture control is not such a

**4.3 Bonding substrate** 

and show a slightly decreased bond strength [25, 35, 79].

no adverse effects on the bond with a DBA [79].

problem [25, 35, 79].

problems through contamination of the bonded surface prior to placement of the resin composite filling material; in other words, they are more technique sensitive [47, 79].

#### **4.1.2 'Two-step' systems**

This group has two subgroups; the first includes those systems that have a separate etch and have combined the priming and bonding steps. These systems are often referred to as 'Single-bottle' systems. In general, the problems experienced with the Conventional Systems still exist with the Single-bottle systems. Although one step has been eliminated, the great problem is ensuring good infiltration of the priming-bond into the demineralized dentine. The other subgroups combine the etching and priming steps together and are referred to as 'Self-etching primers'. These systems also have not been without their problems. The major concern has been their ability to etch the enamel to a great enough extent to ensure a good seal. This seems to be overcome now [42]. The problem of technique sensitivity also seems to have been significantly reduced with these systems compared with the Conventional and Single-bottle systems. This is attributed to the fact that the self-etching priming agent does not have to be washed off the dentine, therefore eliminating the need to maintain the dentine in a moist state. The method of demineralization of these materials is by the use of an acidic resin that etches and infiltrates the dentine simultaneously. The dentine is an excellent buffer, so the acidity of the self-etching primer is rapidly reduced and after polymerization is neutralized [32].

#### **4.1.3 'One-bottle' or 'All-in-one' systems**

This fourth group is the simplest of all the DBAs. They combine all steps into one process. Their mode of demineralization is identical to that of the self-etching priming materials, but the bonding resin is also incorporated. These systems also have the problem of not etching the enamel as effectively as phosphoric acid. In addition these systems are the newest and have no long-term clinical data to demonstrate their effectiveness, although early studies are showing some variability in the success of these materials [14].

#### **4.2 Bonding mechanism**

As already mentioned, the mechanism of bonding of resin-based DBAs is via a hybrid layer. This is a micromechanical interlocking of resin around dentinal collagen fibrils that have been exposed by demineralization. The interlocking occurs by the diffusion of the resins in the primer and bonding resin. The formation and structure of the hybrid layer has been extensively studied, and has also been referred to as the resin-impregnated layer, the resindentine interdiffusion zone. The thickness of the hybrid layer ranges from less than 1μm for the all-in-one systems to up to 5μm for the conventional systems. The strength of the bond is not dependent on the thickness of the hybrid layer, as the self-etching priming materials have shown bond strengths greater than many other systems but exhibit a thin hybrid layer. Sugizaki [72] showed that the etching, washing and drying process caused the dentine to collapse due to the loss of the supporting hydroxyapatite. Further work showed that this collapse of the collagen was an impediment to the successful diffusion of the resin to the base of the region of demineralization. To overcome this problem, Kanca [33] introduced the 'wet bonding technique' which left the demineralized collagen fibres supported by residual

problems through contamination of the bonded surface prior to placement of the resin

This group has two subgroups; the first includes those systems that have a separate etch and have combined the priming and bonding steps. These systems are often referred to as 'Single-bottle' systems. In general, the problems experienced with the Conventional Systems still exist with the Single-bottle systems. Although one step has been eliminated, the great problem is ensuring good infiltration of the priming-bond into the demineralized dentine. The other subgroups combine the etching and priming steps together and are referred to as 'Self-etching primers'. These systems also have not been without their problems. The major concern has been their ability to etch the enamel to a great enough extent to ensure a good seal. This seems to be overcome now [42]. The problem of technique sensitivity also seems to have been significantly reduced with these systems compared with the Conventional and Single-bottle systems. This is attributed to the fact that the self-etching priming agent does not have to be washed off the dentine, therefore eliminating the need to maintain the dentine in a moist state. The method of demineralization of these materials is by the use of an acidic resin that etches and infiltrates the dentine simultaneously. The dentine is an excellent buffer, so the acidity of the self-etching primer is rapidly reduced and after

This fourth group is the simplest of all the DBAs. They combine all steps into one process. Their mode of demineralization is identical to that of the self-etching priming materials, but the bonding resin is also incorporated. These systems also have the problem of not etching the enamel as effectively as phosphoric acid. In addition these systems are the newest and have no long-term clinical data to demonstrate their effectiveness, although early studies are

As already mentioned, the mechanism of bonding of resin-based DBAs is via a hybrid layer. This is a micromechanical interlocking of resin around dentinal collagen fibrils that have been exposed by demineralization. The interlocking occurs by the diffusion of the resins in the primer and bonding resin. The formation and structure of the hybrid layer has been extensively studied, and has also been referred to as the resin-impregnated layer, the resindentine interdiffusion zone. The thickness of the hybrid layer ranges from less than 1μm for the all-in-one systems to up to 5μm for the conventional systems. The strength of the bond is not dependent on the thickness of the hybrid layer, as the self-etching priming materials have shown bond strengths greater than many other systems but exhibit a thin hybrid layer. Sugizaki [72] showed that the etching, washing and drying process caused the dentine to collapse due to the loss of the supporting hydroxyapatite. Further work showed that this collapse of the collagen was an impediment to the successful diffusion of the resin to the base of the region of demineralization. To overcome this problem, Kanca [33] introduced the 'wet bonding technique' which left the demineralized collagen fibres supported by residual

composite filling material; in other words, they are more technique sensitive [47, 79].

**4.1.2 'Two-step' systems** 

polymerization is neutralized [32].

**4.2 Bonding mechanism** 

**4.1.3 'One-bottle' or 'All-in-one' systems** 

showing some variability in the success of these materials [14].

water after washing. This allowed the priming solution to diffuse throughout the collagen fibre network more successfully. However, when it comes to clinical practice, it is very difficult to find the correct balance of residual moisture. Sano *et al*. [65] showed in their work on nanoleakage that most resin-based DBAs allowed the ingress of silver nitrate along the base of the hybrid layer. However, the clinical significance of this is unclear. It may be a pathway for fluid to affect collagen not coated by resin, and the outcome may be degradation of the bond over time. However, the degree of nanoleakage is very much material dependent rather than system dependent, meaning that there are conventional systems and self-etching priming systems that show small amounts of nanoleakage whereas others show more. For the self-etching systems, these are able to solubilize the smear layer and demineralize the underlying dentine, forming a quite thin hybrid layer [25, 35, 36, 79].
