Chapter 11 Bonding systems
Probably one of the most significant aspects of dental materials advancement in the past 50 years is the development of adhesives for dental applications. This has greatly increased the options open to the restorative dentist. The advantages of an adhesive approach are:
• The bonding process may seal the margins of the restoration with the tooth so reducing or eliminating bacterial penetration into the dentinal tubules (microleakage). This in turn decreases postoperative pulpal sensitivity and the potential for recurrent caries.
• Polymerization shrinkage (which occurs when using resin composite) may be reduced. This shrinkage leaves a marginal gap which may be accentuated during thermal cycling and may allow bacterial ingress.
The obvious potential of dental bonding has led to the production of innumerable bonding agents and systems. This chapter explains the principles behind the bonding of restorative materials to dental hard tissue and attempts to simplify the confusion and complexities behind many of these systems.
Principles of Adhesion, Bonding and Sealing in Dentistry
What is meant by adhesion, bonding and sealing? In order to understand the processes, a clear understanding of what these terms mean is essential and a good knowledge of the structure of the substrate i.e. enamel and dentine, is also required. The reader is therefore advised to consult a dental anatomy textbook to review the structure of the dental hard tissues if required.
The difference between bonding and sealing
The terms bonding and sealing are commonly used synonymously but they have distinct meanings. Although a material may appear to be ‘stuck’ to another material, every part of the two surfaces may not be in intimate contact with each other. This may be illustrated by considering a piece of sticky tape stuck to a bench (Figure 11.1). Although the tape is stuck to the table surface, small air bubbles and voids are present between the tape and the bench. The tape has therefore not sealed.
Bonding and luting
• Luting: is the filling up of the potential gap between a cast restoration and tooth, which is essentially a grouting effect. An everyday example is that of kitchen or bathroom tiles, where the grout (cement) is used to fill in the gaps between the tiles but not necessarily to bond them together (Figure 11.2).
Fig. 11.2 Kitchen ceramic tiles with grout between them. The grout serves no adhesive function for the tiles, it merely fills the gap between them. This analogy represents the intimate contact between a cast restoration and the tooth with the gap between them being filled by a cement.
There is therefore an important difference between luting and bonding. Luting materials may be divided into conventional cements and luting composites. The latter are more wear resistant, aesthetic and insoluble in oral fluids. However, it is a paradox that although they fill up the microgap between the tooth and cast restoration, they are usually bonded to the underlying tooth surface by means of an intermediate agent.
When luting a cast restoration, the cement should be applied sparingly to the axial surfaces of the restoration. Filling the retainer with cement may prevent its full seating, however. The aim is to fill up the potential gaps between the cast and tooth but not have much excess (Figure 11.3).
Characteristics of dental bonding
In dentistry, two phenomena can occur in bonding. Firstly, it is a solid–liquid interface that is commonly encountered when bonding a dental material to tooth tissue. The intervening layer (adhesive) is generally applied as a liquid. The advantage of using a liquid is that the liquid will more readily wet the surfaces to be bonded to each other. This will achieve the intimate microscopic contact with the solid surfaces much more effectively than could be achieved with a solid on solid. Increased wetting results in better bonding. However, even with a liquid interface, there may be some limitations, as the viscosity of the liquid will limit the degree to which it wets the surface. This is the effect of surface tension (Figure 11.4).
Secondly, solid surfaces that need to be joined often have microscopic irregularities, giving the surface a rough texture. If both surfaces are uncontaminated, the irregularities on them may connect with one another. Depending on their respective roughness, the two surfaces will become intimately related. This means that any attempt to slide one against the other will be resisted by friction.
The aim with dental bonding is to use a combination of these two phenomena. The first surface, the tooth surface, is usually rough and an intervening layer of resin fills these micro- and macroscopic irregularities. The second surface is the restoration, which will be either a cast that may have a relatively rough fitting surface (perhaps achieved by sandblasting or etching) or a direct filling material. This material contains a filler that will provide microscopic irregularities on its surface (Figure 11.5).
Fig. 11.5A,B (A) Micrograph of the internal surface of a metal casting which has been sandblasted and (B) the macroscopic view of the fit surface of a full gold crown. Both show micro- and macroscopic irregularities into which the bonding agent will flow, providing mechanical retention.
Surface tension: is the property of the surface of a liquid that allows the liquid to resist an external force. The higher the surface tension, the lower is the ability of bonding to it. Oil has high surface tension although the oil is heavier than the water (Figure 11.4). Conversely from a bonding perspective, the low surface tension of the intervening layer of water between two sheets of clean glass will hold them together with the glasses only being easily separated by sliding one away from the other. If the water was absent, the glasses would not stay together unless supported.
Adhesion in dentistry
Mechanical adhesion involves the interlocking of the roughened surface of two substrates, which leads to mechanical bonding. This is clearly illustrated in the bonding of resin to etched enamel. A liquid resin (bonding agent) will flow into the irregularities produced by the surface modification of the enamel. On polymerization, the resin solidifies and the two materials become mechanically bound together.
There can be a physical attraction between two surfaces that need to be bonded. This is usually achieved using molecules with different charges at ends of the molecule. These molecules are referred to as dipolar. This means that if there is an opposing charge on the other substrate, the molecule will be attracted to it. The substrates will orientate themselves so that the oppositely charged ends of the molecules are adjacent to each other. This type of bond is relatively weak and readily breaks down.
In dentistry, the bonding agent normally infiltrates the substrate. In most cases it does not dissolve into the substrate but will infiltrate any irregularities in the surface. This is how the majority of the dentine adhesives function. It is, however, essential that the surface of the substrate is wetted effectively to achieve this.
It is possible that the structure of one of the substrates will result in its dissociation after application on to the surface of the other. This may result in different components bonding to the substrate. Glass ionomer cement attaches to enamel and dentine in this way. Failure of this type of adhesion occurs within one or the other of the substrates rather than at the interface.
Adhesion is a complex phenomenon and one which is difficult to achieve. The bonding of materials to one another is more successfully achieved when an intervening adhesive layer is used. This is possible mainly when there is a large surface area of tooth or restoration to which to bond. However, it is doubtful whether there is complete attachment over all the surfaces being bonded, which reinforces the difference between bonding and sealing. Whether dental adhesives can achieve a complete seal is a topic of great debate among dental clinicians.
Bonding to Enamel
The acid etch technique
Attempts to bond to tooth tissue date back to the 1920s but it was Buonocore in the 1950s who first reported the bonding of restorative materials to enamel using the acid etch technique. This technique has proved to be one of the most durable techniques in dentistry and defined one of the critical requirements of any bonding process: the need to prepare the substrate.
Any fluid with a low surface tension when applied will flow readily across the surface and adapt to its irregularities. In addition, when the fluid contains a chemical which will interact with the substrate surface then the bonding process will be enhanced.
Enamel as a substrate
Preparing enamel presents far fewer problems than preparing dentine due to its microscopic structure. Enamel is acellular, which means it is almost totally inorganic in nature (Figure 11.6). Enamel is generally prepared by using an acid to partly demineralize the crystalline structure. This part demineralization process results in preferential and differential removal of the crystallites so that the surface produced has micromechanical irregularities. These irregularities extend into the enamel structure, forming clefts and greatly increasing the surface area for contact by the bonding agent. The microscopic structure produced by etching enamel is shown in Figure 11.8 later in the chapter. The clefts usually penetrate between 20 and 30 μm and are found in the areas where the interprismatic material is present. The crystallites are partly removed. This effect is accentuated in freshly cut enamel and hence it is frequently recommended that enamel is roughened using a bur prior to bonding. The bond formed by acid etching is micromechanical in nature. Etching enamel with an acid will therefore:
The outer 5 μm of the enamel surface is amorphous and is less susceptible to etching, so the surface preparation of the enamel will ensure that the surface is clean. Pretreatment with a non-organic-based abrasive paste will remove organic smear. Furthermore, the effect of the acid will make the surface more receptive to the placement of a low-viscosity fluid. The nature of the structure of enamel means that it may also be dried sufficiently, so that its surface may be wetted by an intermediate resin without the risk of water forming a barrier between the adhesive fluid and substrate. In order to achieve a successful etch on unprepared enamel, the exposure time should be extended. This is also recommended with older enamel particularly if it has been exposed to fluoride for a significant period of time. The solubility of the enamel will be decreased due to the effect of the fluoride ion. Effective etching still forms a major part of any adhesive system available for dental use.
• The surface of the tooth should be thoroughly cleaning with a pumice slurry (prophylaxis) (Figure 19.49) prior to etching to remove pellicle and plaque.
Etching considerations for deciduous enamel
Primary enamel contains more prismless enamel at the surface but it is also less calcified than permanent enamel. It takes longer for the acid to penetrate the prismless layer to create the etch pattern on the underlying prismatic enamel. This means that the clinician should increase the etching time. Bond strengths to deciduous enamel are generally lower as a result.
Problems with etching
It is possible to over-etch enamel. Note the difference between a 30- and 60-second etch in Figure 11.8. There is a substantially greater removal of the enamel prism sheath after 60 seconds and the porosities produced are not so numerous. This will decalcify the substrate to too great a depth, that is, the etch pattern will be lost, thus decreasing ability of the resin to form tags which may penetrate the etched pattern. The result will be lower bond strength. It is impossible to determine clinically when the enamel has been over-etched, so attention must be paid to the length of time of application of the acid.
Etching can only be done once on the same surface. Repeating the etching process will result in over-etching (see Figure 11.8B). If the tooth surface becomes contaminated by blood or saliva during the bonding process then etching may require to be repeated. This is a common clinical problem when the dentist is working on teeth not isolated by rubber dam. If contamination does occur, then the dentist should re-prepare the tooth and re-etch, removing approximately 50 μm of enamel.
The quality of the etching pattern may also be improved by bevelling the enamel. Bevelling removes the outer amorphous enamel, so exposing fresh enamel for bonding and roughening its surface. In addition, this takes account of the angulations of the enamel prisms and ensures that no unsupported prisms remain. Hence bevelling aids in the production of a better etch pattern so that higher bond strengths may be achieved.
A number of acids have been proposed over the years to etch enamel. The most commonly used chemical is phosphoric acid (ortho-phosphoric acid), and the optimum concentration is between 30% and 50%. The acid must be strong enough to effect an etch pattern but not too concentrated so that the small amount of water present does not get saturated with reaction by-products quickly, as this would slow the dissolution rate. The most commonly used concentration is 35–37% and this is applied for a period of 15–30 seconds.
When the technique was first used clinically the recommended etching times were longer, but this increased the risk of over-etching the surface. Other acids have been introduced to market in an attempt to decrease the etch time, for example maleic acid was introduced by 3M some years ago. Unfortunately few dentists read the directions-for-use (DFU) accompanying the material, which recommended a shorter application time. The result was that the enamel surface was over-etched so bond failures resulted. As a result this product was withdrawn.
Presentation of etching agents
Modern etching materials are available as liquids or gels (Figure 11.9). Liquids are difficult to control as they may run off the tooth surface, causing undesirable etching of enamel that will not be bonded. There is also the potential for causing chemical burns to the gingival tissues. The viscosity of the acid liquid may be increased with the addition of fine particles of colloidal or amorphous silica, which is used in many industries as a thickening agent. This aids localization of the acid solution, which then may be applied precisely to the areas to be etched (Figure 11.10).
Fig. 11.10A,B The difference in behaviour of an etching liquid and gel. (A) The liquid has run off the area being etched. It is now in contact with enamel that does not need to be etched and, worse still, the soft tissues, which may result in a chemical burn if not removed. (B) The etching gel retaining its position on the periphery of a cavity.
When a thickening agent is added to the aqueous 37% phosphoric acid solution in quantities less than 2%, a transparent paste is produced, which may be extruded from a syringe. A colouring agent is frequently added to make gel identification easier against the white of the tooth surface. When using an etching gel, great care must be taken to ensure that the gel is washed away completely by thoroughly washing with air and water from the three-in-one syringe before the application of the bonding system. Otherwise the fine particles of colloidal silica will remain within the retentive features of the clefts within the enamel. This will prevent the bonding material adapting to the surface of the tooth tissue and will reduce the performance of the bond.
The surface of the enamel should not be scrubbed during etching as the newly etched surface and exposed crystallites are friable and may break down. However, the acid should be gently agitated during its application as this will remove etch solution at the surface of the tooth which has been contaminated with products of dissolution. The movement of the fluid will introduce fresh acid to the surface so enhancing the efficacy and effectiveness of the etch process.
• It is essential to wash the tooth thoroughly after etching to remove all the acid, the products of etching and the gel thickening agent. Failure to do this may mean that the silica carrier is retained in the enamel clefts, so reducing the bond strength.
• When etching (high acidic) products are being used, the patient and dental team should wear protective equipment such as protective clothing and eye wear. The application of rubber dam is also advisable.
Normally the bonding material is a dilute dimethacrylate resin system with a low viscosity. The most commonly used material is bis-GMA diluted with TEGDMA. Urethane dimethacrylates are rarely used. These materials are applied to the enamel surface after etching and flow into the crevices formed during the etching process. The resin monomer is then polymerized to form a solid polymer. The resin tags impregnate the enamel surface to a depth of about 30 μm (Figure 11.11).
Fig. 11.11 (A) Cross-section of enamel showing the orientation of the enamel rod structure. (B) After acid etching there is preferential etching of the enamel rods to a depth of between 10 and 30 μm. (C) After resin application and polymerization, formation of tag within the enamel. P, prism sheath; T, resin tag.
During the polymerization process, oxygen inhibition of the curing process means that the surface of the resin layer is only partly polymerized. This part-polymerized resin should be wiped away with a cotton pellet before the patient is discharged. Alternatively, the addition of a restorative or another methacrylate-based material to the resin followed by polymerization results in union of the overlying material with the bonding resin and indirectly with the enamel.
One of the difficulties with this type of procedure is that during the polymerization process the resin tags shrink and have a tendency to neck. This means that just beneath the enamel surface the resin tag is narrower than the aperture it is occluding. During thermocycling, this thin neck of resin is likely to be stressed and fail, resulting in separation of the tag from the overlying surface resin.
In addition the resin may not penetrate to the full depth of the fissure that has been created by the etching process as air becomes entrapped during the resin application. This may lead to microleakage. The newer self-etch bonding systems have attempted to overcome this.
Bonding to Dentine
The smear layer has to be removed but when this has been done fluid starts to flow out from the dentinal tubules. This will continually contaminate the surface. Any material that is designed to bond onto dentine must therefore be miscible with water. Unfortunately most of the materials used as bonding agents are hydrophobic, which presents a problem as these are not compatible with the bonding agent.
The above processes occur whatever bonding system is used. Each of these stages should be completed without any antagonistic processes intervening to ensure that the resulting bond is as good as possible. Ideally, each stage should be carried out alone, but to be more time efficient, most dental adhesives are designed to do at least two of these stages together. This means that frequently there are two chemical reactions taking place at the same time. For example, the demineralization of the dentine occurs at the same time as the impregnation of the dentine.
Since smear layer removal opens the dentinal tubules, the dentinal fluid outflow that follows will always cover the surface of dentine with a thin fluid film. The clinician, therefore, may need to make a choice between (Figure 11.12):
Fig. 11.12 Cross-section of dentine. (A) The smear layer is intact and the dentinal tubule openings are plugged with debris. When the smear layer is removed, fluid may flow out from the now opened dentinal tubules (B).
Removal of the smear layer and dentine etching
Dentine conditioning agents are generally acids which are designed to remove the smear layer produced by cavity preparation and modify the surface of the underlying dentine. Depending on both their concentration and the time period of application, these materials modify or remove the dentine smear layer and preferentially partly demineralize the intertubular dentine and the periphery of the dentinal tubules. This normally extends to a depth of approximately 10 μm, leaving the collagen matrix intact and uncollapsed. The partly demineralized collagen matrix acts as a scaffolding which may be impregnated with the primer (Figure 11.13). Sclerosed dentine requires a longer exposure time for the same effect to be produced. Some conditioning agents also incorporate glutaraldehyde, which acts on the collagen fibres by fixing them by a process of cross-linking. This is supposed to strengthen the fibres and prevent their collapse.
Fig. 11.13 The conditioning process: application of the conditioner demineralizes the smear layer and removes the inorganic phase of the dentine; after drying of the surface the excess conditioner infiltrates the collagen and interlocks when the resin is polymerized. (NRC: no rinse conditioner)
The conditioning process is fraught with problems as the smear layer is of variable thickness at different points on the surface. The time available to treat the underlying dentine is dependent on how quickly the smear layer is removed, making the process less predictable. Figure 11.14 shows the variable results achieved with application of a conditioning agent on a smear layer. The varying thickness of the smear layer has led to differential opening of the dentinal tubules. Figure 11.15 shows a much more aggressive etching process. Here the demineralization has been much more extensive and extends more deeply into the dentine. This may cause an inflammatory response in the pulp if the cavity is deep.
Fig. 11.14 Dentine surface after treatment with conditioner: as a result of the very thick smear present the conditioner has only been partly effective. Note the open dentinal tubule (A), a dentinal tubule still completely occluded with debris (B) and a dentinal tubule partially cleared (C).
If the dentine is over-treated and the dentine is excessively demineralized, the residual collagen will not act as a scaffold but will collapse. Excessive drying may also have this effect, making infiltration of the primer very difficult as no inter-penetration of the dentine structure is achieved. The selection of the conditioner and its concentration is therefore critical.
Use of too strong an acid will lead to the various stages of the process being completed too quickly in some areas of the preparation, leading to over-etching. If the acid is too weak then the dentine preparation will only be partially complete and less inter-penetration of conditioner with tooth will occur. In either case the bonding process will be sub-optimal.
Priming the dentine surface
The next stage in the bonding process is priming of the prepared dentine surface with a material that can bond the hydrophobic material (such as resin composite or compomer) to the hydrophilic dentine. A primer is a solution which is applied to the conditioned surface of the dentine. It infiltrates the collagen network to stabilize it and provides a link between the dentine and the sealer, i.e. between the hydrophilic dentine and the hydrophobic sealer. The composition of the primer is generally a bifunctional monomer (coupling agent) in a solvent (carrier). The bifunctional monomer has the role of ionically linking to the (hydrophobic) methacrylate groups in the sealer to the collagen and hydroxyapatite in the (hydrophilic) dentine. These molecules are referred to as amphiphilic and the linking is achieved by having a molecule with a methacrylate group at one end. This is connected to an inert backbone and on the opposing end is a reactive group that carries a charge which will be attracted to the hydroxyapatite in the tooth. Whether there is any chemical interaction with this reactive group varies with each manufacturer’s adhesive. The molecule should not be too rigid, however, as strains may be set up in the bond or sites for bonding may be reduced due to the decreased ability of the reactive groups to line up. All dentine bonding agents therefore have similar basic structure:
• Unfortunately there are a number of terms which are frequently defined in different ways by different authors and manufacturers. This can be very confusing to the student and qualified clinician alike! For simplicity the following guide may be helpful:
• Sealers flow into the dentinal tubules and seal the dentine with a surface layer rich in methacrylates and bond to the resin composite. They are sometimes referred to as the bond, resin or adhesive.