Dental adhesion review: aging and stability of the bonded interface.
Commentary by Howard E. Strassler, DMD
Breschi L, Mazzoni A, Ruggeri A, et al. Dent Mater. 2008;24:90-101.
OBJECTIVE: Most of current dental adhesive systems show favorable immediate results in terms of retention and sealing of bonded interface, thereby counteracting polymerization shrinkage that affects resin-based restorative materials. Despite immediate efficacy, there are major concerns when dentin bonded interfaces are tested after aging even for short time period, ie, 6 months.
METHODS: This study critically discusses the latest peer-reviewed reports related to formation, aging, and stability of resin bonding, focusing on the micro and nano-phenomena related to adhesive interface degradation.
RESULTS: Most simplified one-step adhesives were shown to be the least durable, while three-step etch-and-rinse and two-step self-etch adhesives continue to show the highest performances, as reported in the overwhelming majority of studies. In other words, a simplification of clinical application procedures is done to the detriment of bonding efficacy. Among the different aging phenomena occurring at the dentin bonded interfaces, some are considered pivotal in degrading the hybrid layer, particularly if simplified adhesives are used. Insufficient resin impregnation of dentin, high permeability of the bonded interface, sub-optimal polymerization, phase separation, and activation of endogenous collagenolytic enzymes are some of the recently reported factors that reduce the longevity of the bonded interface.
SIGNIFICANCE: In order to overcome these problems, recent studies indicated that (1) resin impregnation techniques should be improved, particularly for two-step etch-and-rinse adhesives; (2) the use of conventional multi-step adhesives is recommended, since they involve the use of a hydrophobic coating of nonsolvated resin; (3) extended curing time should be considered to reduce permeability and allow a better polymerization of the adhesive film; (4) proteases inhibitors as additional primer should be used to increase the stability of the collagens fibrils within the hybrid layer inhibiting the intrinsic collagenolytic activity of human dentin.
Fortunately, when bonding to enamel etch-and-rinse (fourth and fifth generation) adhesives have proven to be durable and strong with very predictable clinical results. Bonding to dentin has proven to be more elusive. While bond strength numbers to dentin reported at intervals of 1 day, 1 week, 3 months, and even 1 to 2 years have been clinically adequate, the ability of adhesives, both etch-and-rinse and self-etch, to form a stable bond is very much in question.
This review article describes the problems with long-term adhesion. The interface between the dentin and the adhesive demonstrate aging and lack of stability over time. At the 2-year point the bond can drop off almost 50%. The evidence presented demonstrates that one-step self-etch adhesives are the least durable, while both three-step and two-step etch-and-rinse adhesives have the best clinical performance.
About the Author
Howard E. Strassler, DMD
Professor, Division of operative Dentistry
Department of endodontics, Prosthodontics and operative Dentistry
University of Maryland Dental School