Subir Banerji and Shamir B. Mehta
The term adhesion may be defined as the ‘force that binds two dissimilar molecules together when they are brought into intimate contact’.1 Contemporary advances in the field of adhesive dentistry have now made it possible to predictably bond a variety of different materials (namely resin-based and glass ionomer cements) to the dental hard tissues (notably where there is copious enamel of a desirable quality available). This can take place without the need to prepare a significant mechanical retention or resistance form to the involved/affected tooth surface, with concomitant biological conservation.
Glass ionomer cements do not offer the aesthetic and mechanical merits of resin composite materials. They do, however, offer the potential for direct, inherent dynamic chemical adhesion to enamel and dentine without the need for an adhesive bonding agent.
This chapter will focus on the use of adhesive systems in conjunction with resin composite restoration placement and also discuss the difficulty of bonding to ‘living’ dentine tissue, which appears to be less predictable than enamel bonding, primarily on account of its higher water content, the structural heterogeneity of dentinal tissues (with the presence of a tubular arrangement and a higher organic content as opposed to the heavily mineralised, prismatic, hydrophobic nature of enamel), as well as the long-term stability of the bond developed.2 Histological variations in dentine, such as those that occur with the process of ageing (culminating in a hypermineralised substrate, by the process of sclerosis of dentine as well as the formation of reparative and reactionary dentine) or those that occur as dentine approaches the pulp (resulting in a ‘wetter’ bonding surface with a concomitant increase in the number of tubules), also compound the predictability of the adhesion of resin composite to dentine.
There are a large number of adhesive bonding systems in the marketplace. For the purposes of simplicity, it is perhaps easier to consider any commercial dentine bonding agent by virtue of its chemical content, thus having a component that acts as a:
- Provider of management of the smear layer.2
The etchant most commonly used is 37% orthophosphoric acid, which not only demineralises the hydroxyapatite structure in enamel (as well as in dentine, although with less uniformity), creating pores in the microstructure and thus allowing the permeation of resin of flowable consistency to enter these pores and form a micro-mechanical lock of resin tags, but also leading the modification of the smear layer. The latter is a thin layer comprised of debris formed by the process of cavity preparation. Typical shear bond strengths of resin composite to phosphoric acid–etched enamel are in the range of 20 MPa, which is considered to be sufficient to resist the stresses produced by polymerisation shrinkage of resin composites.3
Primers are adhesion-promoting agents with a bi-functional molecular structure comprising a monomer component with hydrophobic properties that will have an affinity to any exposed collagen present in the dentinal tubules and a hydrophobic monomer, which can form a chemical link with the resin present in the bond. This results in the formation of a hybrid layer