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.

The structure of the substrate plays a considerable part in the success of any adhesive process. Any adhesive system demands that:

• The surface should be clean, that is:

 Free from debris and organic material

 Dry

• The surface must have low surface energy

• It must have good wetting properties.

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.

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:

Fig. 11.6 Surface of human enamel showing its amorphous structure, which provides very limited means of retention for a restoration.

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).

Fig. 11.7 The well-recognized frosted appearance of enamel after acid etching.

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 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.

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. 11.9 Advantages and disadvantages of a liquid versus gel presentation of acid for etching enamel.

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.

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.