Please find Part 1 by clicking here and learn more about the basics of bonding as used in cosmetic dentistry and explained in the 'Procedures' section.
The visible part of your tooth consists of two different principal hard tissue components. The outer shell is called dental enamel, which covers the entire tooth above the gum line. The deeper layer is called dentin, which forms the bulk portion of your tooth. It surrounds the dental pulp, which contains a network of nerve fibers, blood vessels, and connective tissue. I am not going too much into dental biology, because this may be too boring for you.
Just a few years ago, enamel was a requirement for dental bonding. Due to its crystalline composition and distribution, we are able to create retentive patterns by etching the dental surface. This allows dental materials to be micro-locked to the dental surface. Enamel has a stable crystalline hydroxyapatite structure that does not collapse due to its high mineral content when exposed to acidic dental-conditioning materials.
Compared to enamel, dentin has a very high water content and lots of organic matrix (collagen etc.). The first dental resins used for white fillings were only usable on etched enamel, because they were hydrophobic (like oil on water). Enamel has a very low water content and is densely packed with calcium crystals that are variously orientated. These stacked calcium crystals dissolve differently once exposed to the acid of the dental etchant. This creates a surface that is highly irregular on a microscopic level and provides a mechanical micro-retention for dental resins, even the hydrophobic ones.
The trick of the new bonding agents is to overcome the high water content and hence the high amount of hydrophilic matrix of the dentin. The dental bonding agents form a so-called 'hybrid layer' at the surface/interface of the dentin. This layer seals exposed dentin tubules, mixes an organic matrix with synthetic molecules, and hence creates high bonding strengths for anything that is linked to it via a regular resin.
The hybridized dentin layer is a mixture of synthetic polymers and natural dental hard tissues, a true hybrid of body tissue and synthetic material. Diffused resin monomers intricately lock into the acid-etched dentinal surface.
The mineralized aspect of the dentin is purposely dissolved with acid to expose the superficial network of the collagen matrix. This layer of a loose collagen fibril network is vulnerable to dehydration. Only water keeps them afloat, and one has to be careful to maintain the surface wet to prevent the collapse of the dental collagen fibers (see picture below). The placement of rubberdam around the treated tooth is important during this procedure (see the blue rubberdam on the animated version below) to prevent contamination with saliva that contains numerous biological molecules that could interfere with the bond strength. Upon application of a dental primer, the collagen network is diffused with hydrophilic resins, leading to the fixation of a hybrid layer of resin-embedded collagen fibers. The hybrid layer is chemically and physically very different from the original tooth structure. It is a true hybrid that even resists the effects of acid in the future.
The development of the chemiccal elements of dentin bonding has led to a dental revolution. The spectrum of dental treatment modalities, such as porcelain veneers, has significantly widened and become more predictable and safer today.
Recently, there has been a discussion about "touch-cure polymerization" at the interface between the dentin surface of the prepared tooth and the dental resin/composite. The idea is that the polymerization starts at the tooth surface, hence pulling the dental composite towards the dentin and rendering a more intimate and stronger bond of the porcelain veneer. Research is ongoing and has been controversial:
Examples of dental adhesive systems:
Do you want a "bonding" experience?
Several new bonding agents (adhesives) were tested for their shear bond strength recently. The two best systems observed were Adhese Universal used in combination with Variolink Esthetic and Scotchbond Universal used in combination with RelyX Ultimate. The continuous advances in developing adhesive/cement systems allows the cosmetic dentist to utilize cosmetic dental materials more broadly and more predictably. This includes silica-based ceramics, zirconia, hybrid ceramics, as well as new indirect composites.
References:
DiMatteo AM. Esthetic Bonding: How to avoid catastrophe before it starts. Inside Dentistry. Jan. 2017; 34-38.
Sorensen JA et al. Shear bond strength of adhesives to dentin with varying moisture conditions. J Dental Res. 2016; Spec Iss. Ab#1342.