11: Bonding systems

Chapter 11 Bonding systems

Introduction

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 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.

Bonding and luting

Further confusion arises when dentists discuss bonding and luting.

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.

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).

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

Three types of adhesion are possible at the interface:

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.

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:

When etching enamel, the dentist needs to look out for a loss of sheen on the etched area, which takes on a frosted appearance (Figure 11.7).

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.

Problems with etching

Over-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.

Re-etching

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.

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).

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.

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).

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

Dentine unlike enamel is a ‘living’ tissue. This causes problems for bonding, as:

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 process for bonding to dentine consists of three chemical processes:

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):

and

Removal of the smear layer and dentine etching

This is the initial stage of any dentine bonding process and a range of chemicals have been developed for it. These are normally known as dentine conditioners and include:

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.

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.

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.

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:

image

where M is the methacrylate molecule bonding to composite, R is the linking molecule on the backbone and X is the molecule interacting with dentine or smear.

The coupling agent within the primer varies from manufacturer to manufacturer since no one molecule appears to be have universal acceptance. The most commonly used ones are:

Jan 31, 2015 | Posted by in Dental Materials | Comments Off on 11: Bonding systems
Premium Wordpress Themes by UFO Themes