Principles of Adhesion and Bonding
Considerations for making best clinical choices
The way a restoration is planned and then attached to a tooth has changed dramatically since Buonocore first introduced the idea of bonded restorations in 1955.1 Practitioners have become so confident that the bond to enamel will not fail, few actually check for the telltale “chalky white appearance” after they place the acid on enamel to confirm whether there has actually been a proper etch. There is little concern once the acid is placed and rinsed off that the enamel bond is strong. This is now taken for granted because, morphologically, enamel is usually consistent, and the results are predictable if the instructions are followed. The bond to enamel is micromechanical due to the changes in its surface caused by the action of the acid.
When dealing with dentin, however, the results are not as predictable. Dentin is not consistent from patient to patient, from tooth to tooth, or from one area within the tooth to another. Dentin is 13% water by weight,2 which can adversely affect its ability to allow a hydrophobic resin to attach. Another factor affecting the bond to dentin is the formation of the smear layer during the tooth preparation. This smear layer consists of small particles from the mineralized collagen matrix and bacterial contamination.3 Properly handling the smear layer has been the subject of much discussion over the years. At first, it was felt that it could not be removed without damaging the pulp. Then, it became imperative to remove it with some acid to increase the bond strength using the available materials. Presently, the move is towards leaving it in place and infiltrating it to reduce the potential for postoperative sensitivity.
Available Bonding Systems
There are now numerous different bonding systems, named first through seventh generation so far, that are distinguished by how the system works, particularly by the way they treat the smear layer. The systems can be classified in three basic categories: etch-and-rinse (two or three step), self-etch (one or two steps), and most recently, universal (the choice of etching with acid or not).
As these systems evolve, manufacturers attempt to make the bonding process simpler and less technique sensitive by eliminating steps or by making existing steps less complicated. All systems consist of an acid, a dentin primer, and an unfilled resin to seal the dentin and enamel.
Etch and Rinse
In etch-and-rinse systems, the acid is usually phosphoric acid (30% to 40%) and is applied to the enamel and dentin for 15 seconds (or 20 seconds on enamel and 10 seconds on dentin). The potential for over-etching the dentin or under-etching the enamel exists during this process.
The next step is to remove the acid by rinsing. All acid must be removed, so the tooth must be rinsed completely. The problem then becomes the removal of the remaining water. Too much remaining water can be detrimental to the bonding with some systems, and too little water (over-drying) can be a problem with other systems.
The etching and rinsing steps may contribute to problems with postoperative sensitivity that arise when using these systems. Over time, an idea developed that eliminating these steps might make bonding systems more user-friendly, with more predictable results. This led to the development of self-etch and, eventually, universal bonding systems.
Self-Etch and Universal
Self-etching systems can be classified by the strength of the acid in the systems. Self-etch adhesives can be classified as strong (pH ≤1), intermediate (pH approximately 1.5), and mild (pH ≥2).4 Water is necessary to initiate the demineralization in these systems. However, excess residual water during polymerization may be a reason for poor bond strength.5 Because these have water as part of the system, rubbing during application is required to aid diffusion of the monomer and obtain good clinical performance,6 and manufacturers recommend applying multiple layers to ensure a sufficiently thick resin film on top of the hybrid layer.7
Since some of these systems contain weak acids, a separate enamel etch is recommended to achieve a better bond to enamel.8 However, with some of the earlier systems, there was a weaker bond to dentin if the etch got on the dentin. The newer systems (universal) can be used with or without an enamel etch to achieve an acceptable bond strength. This gives the clinician more flexibility when using a bonding system.
Some simplified adhesives (eg, one-step systems) have proven to be less durable over time, whereas three-step etch-and-rinse, and two-step self-etch adhesives continue to show the highest performance.9 The simplification of the systems has not always led to a better system. Insufficient resin impregnation of dentin, high permeability of the bonded interface, suboptimal polymerization, phase separation, and activation of endogenous collagenolytic enzymes are some of the recently reported factors that reduce the longevity of the bonded interface.9 MMPs (matrix metalloproteinase) have been identified as a factor that can cause deterioration of the hybrid layer,10 and the addition of 2% chlorhexidine (CHX) to the tooth surface before bonding has been reported to greatly reduce the degradation of the smear layer.11
Conclusion
Bonding successfully to enamel and dentin is an extremely important step in the placement of a long-lasting restoration. This success is not only dependent on the material that is used, but also on the skills of the person using the material. Reported bond strengths are usually based on results achieved in the laboratory on a flat surface and under ideal conditions. The geometry of the cavity is much different than the flat surface of the laboratory, leading to different bond strengths in different parts of the preparation. At chairside, the conditions are not always ideal, as patient-related factors can adversely affect the conditions. A recent study showed that the results are more dependent on the operator and interactions between the operator and the materials than the choice of material.12
References
1. Buonocore MG. A simple method of increasing the adhesion of acrylic filling materials to enamel surfaces. J Dent Res. 1955;34(6):849-853.
2. Mjör IA. Human coronal dentine: structure and reactions. Oral Surg Oral Med Oral Pathol. 1972;33(5):810-823.
3. Pashley DH. Smear layer: physiological considerations. Oper Dent Suppl. 1984;3:13-29.
4. Manuja N, Nagpal R, Pandit IK. Dental adhesion: mechanism, techniques and durability. J Clin Pediatr Dent. 2012;36(3):223-234.
5. Shiratsuchi K, Tsujimoto A, Takamizawa T, et al. Influence of warm air-drying on enamel bond strength and surface free-energy of self-etch adhesives. Eur J Oral Sci. 2013;121(4):370-376.
6. Zander-Grande C, Ferreira SQ, da Costa TR, et al. Application of etch-and-rinse adhesives on dry and rewet dentin under rubbing action: a 24-month clinical evaluation. J Am Dent Assoc. 2011;142(7):828-835.
7. Van Meerbeek B, Vargas M, Inoue S, et al. Adhesives and cements to promote preservation dentistry. Oper Dent. 2001;Suppl 6:119-144.
8. Brackett MG, Brackett WW, Haisch LD. Microleakage of Class V resin composites placed using self-etching resins: effect of prior enamel etching. Quintessence Int. 2006;37(2):109-113.
9. Breschi L, Mazzoni A, Ruggeri A, et al. Dental adhesion review: aging and stability of the bonded interface. Dent Mater. 2008;24(1):90-101.
10. Pashley DH, Tay FR, Yiu C, et al. Collagen degradation by host-derived enzymes during aging. J Dent Res. 2004;83(3):216-221.
11. Brackett MG, Tay FR, Brackett WW, et al. In vivo chlorhexidine stabilization of hybrid layers of an acetone-based dentin adhesive. Oper Dent. 2009;34(4):379-383.
12. Karaman E, Yazici AR, Aksoy B, et al. Effect of operator variability on microleakage with different adhesive systems. Eur J Dent. 2013;7(Suppl 1):S60-S65.
About the Author
James F. Simon, DDS, MEd
Director, Division of Esthetic Dentistry
Department of Restorative Dentistry
University of Tennessee College of Dentistry
Memphis, Tennessee
