F07. Until recently, the majority of the dental profession believed that dentin bonding could be achieved by chemical bond formation to the tooth tissues. By considering factors such as surface energy, variations in chemical compositions, and problems associated with keeping the surface free from moisture, such a bond mechanism may not be achieved clinically. Explain why chemical bonding may be difficult to achieve under clinical conditions.

The idea behind chemical bond formation to dentin is based on the assumption that resins could contain an active group that could bond to either calcium or collagen. The other end of the resin molecule would contain a methacrylate group, capable of reacting with the resin during polymerization. To simplify, such a molecule can be described as MrX, where M is the methacrylate group, r is the backbone structure (the spacer) of the molecule, and X is the group capable of bonding to tooth components (Ca, PO₄ or collagen).

Through the years, many attempts have been made to achieve such a bonding, but up until now there are very few evidence that such bonds form. The reason may simply be that the tooth surface is too poorly standardized regarding chemical composition and that the bonds are not stable when other ions, including H-ions, diffuse along the interface and compete with the different bond sites. In addition, because of the biological environment, the chemical components that can be used are limited which limits our ability to bond to dentin.  

Bonding to Hydroxyapatite

The easiest way to understand chemical bonding in dentistry is to look at the formula M-R-X (or just MrX) shown to the right. The meaning of MrX is shown to the left. M stands for the methylmethacrylate group seen at the leftmost position (including the -CO-O- bond). R stands for a spacer consisting of a hydrocarbon chain, while X is the group capable of bonding to calcium present on the tooth surface.  

One of the compounds that have been explored as a dentin-bonding agent is the amino-carboxylate compound shown above. The potential bonding effect can be found at the nitrogen atom, which, as you might remember, has a sp3 orbital configuration of which one is filled with two electrons. The electron filled orbit could thus interact with the positive charge from the calcium ion present at the surface of the tooth. In addition, the end group of this molecule also has a carboxylate group (-COOH) originally, which donates its hydrogen and become negatively charged. That negative charge could also interact with a positive charge such a calcium ion.  

One of the most popular compounds being used in dental bonding agents has been resins with a phosphate group as the end group. The reason for its popularity relates to its ability to become negatively charged, but also because the phosphate group will "mimic" the original phosphate group present in the hydroxyapatite. 

Another chemical that is known to promote dentin bonding is polyacrylic acid (PAA) present in materials such as carboxylate cements and glass ionomers. The bonding ability has been related to the negative charges present along the PAA after the hydrogen ions have reacted with the surface. A mechanism that has not been considered, but should not be excluded, is that the PAA as well may form micro mechanical interlocking and wet the surface because of its hydrophilic nature.  

Bonding to Collagen

One attempt to bond a resin to collagen has been to first treat exposed collagen fibers with an aldehyde (e.g., glutaraldehyde). During that treatment, the C=O double bond opens up and withdraw the hydrogen from the collagen surface. 

When that happens, the oxygen has a free electron available to interact with the free electron that available at the previous hydrogen location at the nitrogen atom of the collagen. When that reaction has happened, one can say that a hydroxyl group has been grafted to the collagen protein.

The second step of the reaction is to allow the hydroxyl group of hydroxyethylmethacrylate (HEMA) to react with the grafted -OH group present on the protein surface. Thus, through a condensation reaction, the HEMA molecule with its methylmethacrylate group is now bonded to the collagen surface.

The dentin-bonding agent that introduced the above concept was GLUMA (glutaraldehyde and HEMA). Whether the above reactions occur clinically has never been proven.  

Conclusion

As the different dentin bonding agents discussed above were developed, the acidity of the systems increased. This acidity resulted in some etching and bond improvement. The use of GLUMA was combined with EDTA conditioning. In the later case, collagen fibers were exposed. After some time it was discovered that HEMA without glutaraldehyde bonded as well to dentin as the GLUMA composition did, suggesting that the bond be mainly of a micromechanical nature.