The Evolution of Adhesives
Understanding today’s universal capabilities
Stephen Poss, DDS
Based on an understanding of the structures, composition, and interaction of dentin, enamel, and resin materials, successful adhesive dentistry is predicated typically on first preparing the teeth with instrumentation, then conditioning enamel and etching the dentin surfaces, after which bonding resins can be placed.1 The etching process in most cases removes or modifies the smear layer and/or imparts irregularities in prepared enamel to enhance micromechanical and chemical retention.1 This protocol is intended to enhance adhesion in the wet oral environment by improving the attachment of restorative materials to tooth structures.
Over the years, demands for less technique sensitive and more predictable bonding has led to the ongoing development of new adhesive materials with different compositions, mechanisms of action, and techniques for their application. In fact, eight generations of adhesive bonding agents have evolved, along with three different etching protocols—total etch, self-etch, and selective etch.2,3
With each generation, manufacturers have sought to enhance adhesive predictability while simultaneously reducing the number of components/bottles required (separate etchant, primer, adhesive), simplifying application protocol, and improving material chemistry for stronger and more durable adhesion.
Although newer generations of adhesives have demonstrated effectiveness in clinical and laboratory studies, total-etch adhesives containing 30% to 40% phosphoric acid have long remained the most effective for etching. Total-etch adhesives comprise a two-bottle system requiring application of a separate acid and primer. The phosphoric acid is applied to enamel for up to 30 seconds and to dentin for 15 seconds, after which it is rinsed from the preparation. Typically this is accomplished by tracing the cavosurface of the preparation with acid first, and then filling in exposed dentin, taking care to avoid over-etching the dentin. The primer is then applied to promote the ideal bond between the resin, preparation, and restorative material.4 Despite their many benefits (superior enamel etching, greatest efficacy for etching compromised dentin), total-etch adhesives are more technique sensitive, which can result in over-etching/over-exposing dentin and gap formation between the adhesive and dentin. These challenges could lead to postoperative sensitivity and decreased bond strengths.
Self-etch systems typically contain a 10% maleic acid that is milder than phosphoric acid and a formula combining the primer and adhesive resin. This one bottle combination effectively modifies the smear layer, facilitating penetration of the resin into the dentin tubules and forming a consistent adhesive layer with decreased technique sensitivity. In addition, self-etch adhesives eliminate the need for rinsing and drying after application.5
In most cases self-etch adhesives can be used for selective etching techniques where only the enamel edges of preparations are carefully etched with phosphoric acid, after which dentin is either conditioned with a primer or the self-etching adhesive.
Today, there are 8th generation universal adhesive bonding systems, such as Prime & Bond Elect (Dentsply Sirona, www.dentsplysirona.com), Adhese Universal (Ivoclar Vivadent, www.ivoclarvivadent.com), All Bond Universal (Bisco, www.bisco.com) and Scotchbond Universal (3M ESPE, www.3m.com), that are indicated by manufacturers for use as either a self-etching bonding agent or an etch-and-rinse bonding agent with phosphoric acid. They have also been reported to demonstrate enhanced material characteristics for adhesively bonding both enamel and dentin to a variety of restorations (Figure 1).3
However, regardless of the advanced material composition of universal adhesive bonding systems or the conditioning/placement protocol employed, certain characteristics ensure successful and predictable adhesive bonding,6,7 including high bond strength values regardless of etching modes,8,9 resistance to degradation,10,11 and low film thickness for passively fitting indirect restorations.12 They should also be compatible and/or function similarly with other restorative materials, such as light-cure and dual-cure cements, ceramics, composites, etc., in a variety of clinical situations to ensure durable predictability.5-7
Today’s universal adhesives can be applied across varied moisture levels. In particular, dipentaerythritol pentaacrylate monophosphate (PENTA) technology, found in Prime&Bond Elect, can be used in virtually all restorative bonding procedures, contains acetone to repel water, inhibits water uptake, and enables the material’s resins and fillers to penetrate deep into the dental tubules. It also reduces microleakage and postoperative sensitivity.