16 July 2019

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What to expect from the adhesive world of tomorrow.

Lorenzo Breschi

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Nowadays, adhesive dentistry has reached a very high standard of quality and is able to offer clinicians a great variety of treatment options to obtain optimal adhesion to the different dental substrates. Three-step etch-and-rinse adhesives, together with 2-step self-etch systems used with a selective enamel-etching protocol are considered the gold standard. Recently, universal adhesives has been introduced on the market to simplify the restorative procedures and to reduce the variables dependent on the operator experience. They are claimed to equally work both with a etch-and-rinse or a self-etch protocol and to be efficient on direct and indirect restorations.
Despite the presence of highly performing adhesive and composites, the effort to develop the ideal material, with better mechanical properties and clinical outcomes, continues at a rapid pace.

Current research is focused in the development of adhesive materials able to do more than just bond to tooth tissue. The aim is to produce materials with additional therapeutic action to allow the preparation of even less invasive cavities and to prevent early restoration replacement due to bond degradation or caries recurrence. The above mentioned therapeutic effect is generally defined as the “bioactivity” potential of the resin and may involve anti-bacterial, anti-enzymatic, and/or re-mineralization effects. A key strategy to counteract damaged teeth is to restore the carious structure and, at the same time, to promote the remineralization process by means of a biomimetic approach.
Therefore, resin-based composites with bioactive remineralizing agents are in huge demand. The chemical composition of these materials is usually based on the presence of synthetic calcium phosphate (Amorphous Calcium Phosphate, Hydroxyapatite, Tetracalcium Phosphate, etc.) or Bioactive glass that consists mainly of silicon, calcium, sodium, phosphorous, and oxygen. These materials are biocompatible, harmless, non-irritating, non-inflammatory, and non-toxic, and have the benefits of minimal bioresorption. Bioactive fillers incorporated into the resin matrix of the composite have proved to release calcium, phosphate and silica ions and, consequently, to activate the remineralization process.
Another important issue for a restorative resin is the ability to interact with oral biofilms; therefore, appealing features for the development of new materials should consider:
1) inhibiting biofilm adhesion,
2) discouraging biofilm growth,
3) affecting biofilm metabolism,
4) adjusting microbial ecosystem.

Nowadays, efforts have been made to improve the chemical compositions of bonding agents to increase the bond strength. In addition, antibacterial bonding agents have been synthesized to eradicate the residual bacteria in the tooth cavity demonstrating antibacterial activity without compromising the bond strength. An example is the adhesive containing dimethyl-aminododecyl methacrylate (DMADDM), which inhibites the growth of S. mutans with no adverse effect on dentin bond strength. Furthermore, recently, a quaternary ammonium methacryloxy siliane molecule was also incorporated into experimental adhesives to evaluate the possible antimicrobial and anti-proteolytic potential to prevent secondary caries and hybrid layer degradation. The suppression of biofilm growth and acid production represents only one of the different strategies to contrast caries formation and progression. An interesting alternative should be to modulate the biofilm species toward a healthy composition, instead of eliminating all bacteria. Indeed, several studies investigated how bioactive materials could influence the compositions of biofilm species. However, for clinical longevity and success, not only the bioactive nature of the material, but also the morphology of the particles included in the matrix is of fundamental importance and determines the resultant properties of the composite. Therefore, in future, more researches should focus on the improvement of the physical and mechanical properties of the new bioactive composites, without affecting theirs biological potential.

To conclude, in this new Era of dentistry, where the evolution of caries management is shifting to a “non-invasive” approach, restorative materials are invested with highly demanding expectations. However, most of the studies based on the development of a new generation of adhesive materials are in vitro, thus in vivo study are still necessary to complete the process. Nonetheless, multidisciplinary research, is expected to offer tremendous benefits to oral health.