Dentin Adhesives

It took a considerably long time to achieve first success in dentin bonding. This was due to two major problems: 

  • Dentin provides a moist structure with tubules filled with liquid, resulting in considerable hydrophilicity. Therefore, it is difficult to bond hydrophobic resins to hydrophilic dentin surfaces. 

  • After rotary treatment of dentin, a smear layer prohibits direct contact with the underlying dentin.

The transfer of an etch‐and‐rinse technique from enamel to dentin was initially unsuccessful. Therefore, in order to utilize the whole cavity surface, successful dentin bonding was desirable. The development of different classes and stages of adhesive systems can be recognized in so‐called generations, which have recently been displaced by functional classifications and the number of steps.

A functional adhesive monomer was first introduced by Hagger in 1951 -glycerophosphate dimethacrylate (GPDM)- which was able to penetrate the dentin surface and form an intermediate layer between the dentin and the restorative material which was later known as a hybrid layer by Kramer and McLean in 1952. Kramer and McLean made the first remarks concerning the presence of the hybrid layer when they noticed that the product developed by Hagger had a tendency to penetrate the dentinal surface. 

The modern era of adhesive dentistry was initiated in 1955 by Buonocore. Buonocore reported the use of phosphoric acid at an initial concentration of 85% to improve the retention of an acrylic resin on enamel and suggested that tooth enamel should be treated in order to improve its bonding with the restorative resins. Buonocore and his team also tried to bond to etched dentin in 1956. Unfortunately, this procedure was met with very limited success due to the poor wetting of the dentin by the former hydrophobic resins. This first generation of dentin adhesives resulted in poor performance mainly from the weak adhesion of the smear layer to the underlying dentin. Researchers and manufacturers tried to develop chemical dentin bonding agents to be placed on the top of the smear layer because etching of dentin was suggested of potentially harmful effects on the pulp. 

The development of different treatments of the smear layer resulted in an improved but still insufficient bond strength. The smear layer was maintained but with poor results. This second generation of adhesives was quickly followed by the third and fourth generations which relied on the complete removal of the smear layer and partial decalcification of the dentin by phosphoric acid.

The first- and second‐generation adhesives never reached clinically relevant efficacies because they only bonded to smear layers;  however, the adhesion of smear layers to underlying dentin was too low to counteract polymerization shrinkage of resinous materials. 

Owing to its high efficiency, the procedure of acid etching of tooth enamel to increase the bond strength with the composite resin has continued to be used, although it has been subject to some changes in principles or techniques, such as decreasing the phosphoric acid concentration from 85% to 30-40% and the etching time from 40-60 to 20-30 seconds, as well as the use of etching products in the form of gel, etc.

The almost unanimously acknowledged attribute of the smear layer to protect the pulp-dentin system by decreasing dentin permeability led the specialists to initially ignore Fusayama’s suggestion in 1980 to acid-etch not only the enamel but also the exposed dentinal surfaces. This very controversial procedure was finally accepted, which resulted in “total-etch” (or total acid etch). The first documented clinical success was accomplished with so‐called third‐generation adhesives. In this scenario, prepared enamel is selectively conditioned with 30–40% phosphoric acid followed by the application of primers providing acidic monomer mixtures for smear layer dissolution on dentin. 

Under clinical circumstances, selective enamel etching is nearly impossible. Therefore, simultaneous etching of enamel and dentin was desirable and was realized by the fourth generation including a simultaneous etching of enamel and dentin with phosphoric acid, which is removed by rinsing (etch-and-rinse). After conditioning of enamel and dentin with phosphoric acid, the application of first a hydrophilic primer followed by a more hydrophobic adhesive is a characteristic of these fourth‐generation adhesives. 

In 1982, Nakabayashi studied the characteristics of the hybrid layer and laid the foundation of the dentin hybridization theory. The formation of the hybrid layer eliminates the risk of marginal micro-infiltration and leads to an adhesive attachment between the substrate and the restorative material. This historical turning point in dentin bonding resulted in the gold standard dentin bonding steps. 

The golden standard dentin bonding protocol

To be continued...