F08. Describe the proper method of manipulating and applying a dentin bonding agent and identify the mechanism that makes it possible to bond to a dentin surface.

Dentin Bonding  

Modern dentin bonding agents consist of a conditioner (often an acid), primer and a bonding resin. Several different types of conditioners and concentrations have been explored through the years, and today it seems, as one of the most reliable conditioners is phosphoric acid with a concentration ranging from-27 - 37%.

In addition to the conditioner (Etchant), a typical dentin-bonding agent also comes with a Primer. The Primer can be described as the "premier resin" being placed on the conditioned dentin surface. The characteristic property of a primer is that it should be hydrophilic.

After the conditioned dentin surface has been primed, the Bonding Agent is placed on top of the primed surface. The bonding agent is a mixture of the priming monomer and the key monomer present in the composite. The Bonding Agent must be cured before the composite is placed.

Bonding Procedure

When dentin is cut, the inorganic hydroxyapatite crystals are broken up and the collagen is stretched, torn, and smeared over the cut surface. 

The above picture shows how a cut dentin surface is covered with such a "smear layer." The picture below shows how the smear layer covers the dentin and form so called "smear plugs." 

Before any serious bonding attempt can be made, this "smear" layer must be removed. Such a removal is achieved with a so-called "conditioner." In most cases, the conditioner consists of an acid. However, in one product, the conditioner consists of EDTA (pH=7.4). Because of the concern dentists had a few years ago regarding etching dentin, manufacturer preferred to use the more general term "conditioner" rather than "acid" to describe their acids. As mentioned under enamel etching, dentin should be etched for 15 - 30 s. During this time, the smear layer is removed and the outer surface of the dentin is demineralized leaving a demineralized collagen mesh, attached to the mineralized dentin located a few microns under the collagen surfaces.  

The above figure shows how the dentin surface looks after it has been etched for 15 s.

As long as this collagen mesh remains moist, it is fluffy. However, if it is desiccated, the mesh structure densifies. 

Fortunately, it is possible to re-swell the mesh by rubbing the surface with a water-saturated pellet.

In order to infiltrate the moist collagen mesh with a primer, one must use a primer that consists of a molecule that has both a hydrophilic and a hydrophobic end. Regarding the previously mentioned generic molecule MrX, the hydrophilic component is X, while the hydrophobic component is the methacrylate group. The X group consists of -OH, -COOH and -H₂PO₄.  

HEMA is one of the most efficient primers. It is used as a primer in 3M's Scotchbond MP

By polymerizing methacrylic acid, polyacrylic acid (PAA) is formed. PAA is often used as a conditioner when polycarboxylate cements and glass ionomers are used. PAA is also used as the liquid in these two materials.

PENTA-P is used in bonding agents such as Prime-and-Bond (Dentsply) and in Optibond (Kerr). This monomer is to some extent self-etching. It is also capable of forming a heavily cross-linked structure.  

When the primers are lightly rubbed into the surface of the moist collagen, the M groups cluster in the center of droplets, while the X groups orient toward the surrounding water matrix. 

If one would try to polymerize the primer while there is still a lot of water or solvent present in the blue regions, polymer spheres would form within the collagen mesh, and the bond strength would become rather low. It is therefore important at this point to remove as much as possible of the water matrix. Blowing air on the primed surface does such a removal. The air pressure forces the primer deeper into the tubules and facilitates its infiltration into branches from the tubules.  

At the same time, as the water volume shrinks due to water evaporation, the primer droplets start interfering with each others, and at a certain point the primer forms a matrix that contain small water droplets. If at this point the primer is cured, a polymer film will form and the bond strength will increase. At the same time, the dentin surface will become organophilic and hydrophobic. The rein infiltrated tooth surface is called the hybrid layer.  

The primed surface is now coated with a bonding resin (often a mixture of the primer and the monomer used in the composite).

This bonding resin will wet the primed dentin surface and mix to some extent with the already placed primer. The bonding resin is also the resin that is curable (contains the light activating chemicals). Depending on the composition of the bonding resin, different procedures should be used. If for example the bonding resin is unfilled, it is recommended to thin the resin by a short air blast. The reason is simply that a thick film of unfilled resin can result in misinterpretation of x-rays. For example, if another dentist checks a x-ray taken of a patient with such a thick unfilled resin layer, the dentist may interpret the resin as being a gap between the restoration and the tooth. At the same time, air thinning will remove some of the "ligament" effect of the bonding agent and also increase the oxygen inhibition. Whether the two latter effects are of clinical significance is not yet clearly proven. However, if a filled bonding resin is used instead of an unfilled resin, one does not need to air-thin the bonding resin because the x-rays will not show a gap as long as the material is bonded. Whether or not one uses a filled or unfilled bonding resin, this resin should be cured before the composite is placed. The manufacturer of the different products gives the light curing time.  

After the dentin bonding procedure has been completed, the composite restoration is built.

From the above we can conclude that the dentin bonding agents that are used today are mechanically retained and that the primers work as "surfactants." During the last few years, some manufacturers have combined the primer and the bonding agent into one single bottle. These systems are not better than the two bottle systems (are not as efficient primers as the two bottle systems), but have the advantage of being fool-proof in the hands of individuals who do not understand how important it is to use the two bottles in the right order. The newest development trends is to combine everything into one single bottle (including the acid) in order to produce so called "self-etching" bonding systems.

Comparing the principles behind enamel and dentin bonding, it is obvious that there is a conflict between the old instructions (dry enamel) and the more modern treatment concepts (moist dentin). How can a dentist desiccate enamel and keep dentin moist? The answer relates to whether or not a hydrophilic primer is placed on the enamel. If the enamel is not absolutely dry, a hydrophilic primer should be placed on the enamel before the hydrophobic resin is placed. If on the other hand the dentin is desiccated, the dentin can be rewet before the hydrophilic primer is placed. The latter procedure, though, requires some additional time, because the collagen structure does not swell instantaneously when the water is placed. Because of the latter, the most efficient bonding technique is to leave the dentin surface slightly moist even though this means that the enamel surface may not be completely dried, and then place the hydrophilic primer on both enamel and dentin before the bonding resin is placed.