The latest development in dental adhesion is based on simplification and reduced application time. The 1990s and early 2000s were marked by a strong desire from both clinicians and manufacturers to develop simplified adhesive systems. The wishes for more simplification led to the development of the fifth generation as “one bottle bonds”. Its easier handling guaranteed marketing success, however, studies repeatedly confirmed that conventional three-step etch-and-rinse adhesives still perform more favorably and are most reliable in the long term. When the primer is mixed with the adhesive (etch-and-rinse two-step), the adhesive becomes more permeable and absorbs water over time. This permeability can lead to accelerated degradation of resin-dentin bonds. 

Phosphoric acid etching the dentin always has one central problem: the collagen network exposed after phosphoric acid etching must be penetrated by hydrophilic monomers and therefore the fibrils must not collapse; otherwise, penetration is insufficient. The main drawback of this technique is the significant delay between the moment when the microporosities are formed by acid etching and the moment when the monomeric resin infiltrates these gaps. In this time period, the conditions for filling the etch pits by diffusion must be created and maintained.

To avoid this problem in general, sixth‐generation adhesives were developed, without phosphoric acid etching, called self-etch adhesives, containing primer having acidic pH values. They allowed demineralization of enamel and dentin while simultaneously penetrating the demineralized structures, providing a depth of demineralized dentin identical to that of the penetrated area. The applied acidic components are intentionally not rinsed off to guarantee the appropriate function of the priming parts. After the self‐etching primer, a conventional adhesive is applied. Due to problems regarding the chemical stability of monomers in acidic environments, earlier products had to be mixed before application. The primers of the more recent products are premixed, containing short‐chain self‐etch monomer mixtures that stay hydrolytically stable even in acidic pH environments. 

Even more, simplification was introduced with the seventh generation of adhesives, with self‐etch systems being available with only one liquid. This consists of mixtures of hydrophilic and hydrophobic monomers, which are so acidic that they are able to act as etchant, primer, and adhesive in one. Earlier products were mixed; recent adhesives are non‐mix versions. One-bottle systems (sixth and seventh generations) also show a more pronounced fatigue behavior compared to multistep systems. 

The newest eighth generation of adhesives has been defined. The latest developments are so‐called “universal adhesives” and they can be applied in three different modes. They include the functional monomer 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP), which is more adapted to the self-etch approach compared to GPDM (thicker molecule). The idea behind these adhesives is to combine universal primers (e.g. Clearfil Ceramic Primer, Monobond Plus) with primers for conditioned enamel and dentin surfaces.

Universal primers are applied to ceramic surfaces like hydrofluoric acid-etched glass‐ceramic, airborne pretreated oxide ceramics, as well as tribochemically pretreated ceramic or metal surfaces. The mixture of different monomers is able to enhance bonding to all mentioned surfaces after surface roughening by different approaches.

This is a tremendous advantage in repairing insufficient restorations when it is not exactly known what kind of ceramic was used or if different material surfaces are affected at one site.

Another -and the most important- approach of universal adhesive is to use the adhesive as an etch‐and‐rinse system, even on dried or on moist dentin surfaces, as well as a self‐etching primer at the same cavity in order to facilitate handling in daily situations when adhesives are used. As mentioned before, it has been shown that the performance of self‐etch adhesives in enamel may be increased by selective phosphoric acid etching of enamel margins. However, it cannot always be totally avoided that phosphoric acid is also applied to dentin, which can cause some adhesives to show a decrease in efficiency. Universal adhesives are developed to promote bonding to dentin as etch‐and‐rinse adhesive, independently of whether the phosphoric acid‐etched surface is kept moist or dry, and as a self‐etch adhesive. Today, MDP is incorporated into most universal adhesives, which was shown to be effective in reducing aging effects, primarily in self‐etch mode, which was attributed to nanolayering as well as chemical bonding to calcium in dentin. This eight-generation adhesive system remains vulnerable to a number of problems inherent to simplified adhesives.

Despite the increased popularity of self-etch adhesives, etching with phosphoric acid is still considered the golden standard against which new materials are tested. In a brief, the etch-and-rinse three-step and self-etch two-step systems demonstrate not only improved performance but also better bond stability which can be related to the fact that both systems use a separate adhesive resin coating. They proved to increase the survival of ceramic inlay/onlay restorations when compared to the simplified systems. 

To be continued...
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