Abstract
The phenomena of adhesion and cohesion are reviewed and discussed with particular reference to dentistry. This review considers the forces involved in cohesion and adhesion together with the mechanisms of adhesion and the underlying molecular processes involved in bonding of dissimilar materials. The forces involved in surface tension, surface wetting, chemical adhesion, dispersive adhesion, diffusive adhesion, and mechanical adhesion are reviewed in detail and examples relevant to adhesive dentistry and bonding are given. Substrate surface chemistry and its influence on adhesion, together with the properties of adhesive materials, are evaluated. The underlying mechanisms involved in adhesion failure are covered. The relevance of the adhesion zone and its importance with regard to adhesive dentistry and bonding to enamel and dentin is discussed.
Highlights
Every clinician has experienced the failure of a restoration, be it loosening of a crown, loss of an anterior Class V restoration, or leakage of a composite restoration
Where there are thick transition zones and/or narrow adhesion zones, the behavior of the entire bonding interface may be dependent on the properties of the transition zone because the properties, notably strength, of the adhesive may be impaired because of inadequate cohesion within the adhesive
Laboratory bond strength test values and the resistance of luted restorations to clinical loads will be maximized when the propagating crack that causes bond failure has to travel through the adhesion zone rather than the bulk adhesive
Summary
Every clinician has experienced the failure of a restoration, be it loosening of a crown, loss of an anterior Class V restoration, or leakage of a composite restoration. Cohesion may be defined as the internal strength of an adhesive due to various interactions within that adhesive that binds the mass together, whereas adhesion is the bonding of one material to another, namely, an adhesive to a substrate, due to a number of different possible interactions at the adhesive-substrate surface interface Zinc phosphate cement has good cohesive strength but exhibits poor adhesion to smooth surfaces It does not bond, chemically to surfaces and its bonding or adhesion, that is, its application as a luting agent, is possible only through mechanical interlocking at the interface with the restoration and that with the tooth. The adhesive bond will fail if the adhesive separates from the substrate or there is internal breakdown of the adhesive (i.e., cohesive failure), Figure 1
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