Objectives : The aim of this study was to explore the therapeutic opportunities of each step of 3-step etch-and-rinse adhesives. Methods : Etch-and-rinse adhesive systems are the oldest of the multi-generation evolution of resin bonding systems. In the 3-step version, they involve acid-etching, priming and application of a separate adhesive. Each step can accomplish multiple goals. Acid-etching, using 32–37% phosphoric acid (pH 0.1–0.4) not only simultaneously etches enamel and dentin, but the low pH kills many residual bacteria. Results : Some etchants include anti-microbial compounds such as benzalkonium chloride that also inhibits matrix metalloproteinases (MMPs) in dentin. Primers are usually water and HEMA-rich solutions that ensure complete expansion of the collagen fibril meshwork and wet the collagen with hydrophilic monomers. However, water alone can re-expand dried dentin and can also serve as a vehicle for protease inhibitors or protein cross-linking agents that may increase the durability of resin–dentin bonds. In the future, ethanol or other water-free solvents may serve as dehydrating primers that may also contain antibacterial quaternary ammonium methacrylates to inhibit dentin MMPs and increase the durability of resin–dentin bonds. The complete evaporation of solvents is nearly impossible. Significance : Manufacturers may need to optimize solvent concentrations. Solvent-free adhesives can seal resin–dentin interfaces with hydrophobic resins that may also contain fluoride and antimicrobial compounds. Etch-and-rinse adhesives produce higher resin–dentin bonds that are more durable than most 1 and 2-step adhesives. Incorporation of protease inhibitors in etchants and/or cross-linking agents in primers may increase the durability of resin–dentin bonds. The therapeutic potential of etch-and-rinse adhesives has yet to be fully exploited.
Introduction to state of the art etch-and-rinse adhesives
Buonocore was the first to demonstrate that acid-etching enamel with phosphoric acid increased resin–enamel bond strengths. He believed that acid-etching simply increased the microscopic surface area available for resin retention. However, one of his students, John Gwinnett, who was a trained electron microscopist, looked at the interface more closely. He reported that adhesive resins could penetrate into acid-etched enamel prisms where they could actually envelop apatite crystallites rendering them acid-resistant. This was the first true hybrid layer, although that term had not yet been introduced. Resin-treatment of acid-etched enamel created a new structure that was neither enamel nor resin but a hybridization of the two materials. It was the first example of in situ dental tissue engineering.
Nakabayashi et al. were the first to demonstrate true hybrid layer formation in acid-etched dentin. This was best observed by transmission electron microscopy but was later demonstrated by scanning electron microscopy following argon ion beam etching . Nakabayashi’s group was the first to demonstrate that resins could infiltrate into acid-etched dentin to form a new structure composed of a resin-matrix reinforced by collagen fibrils. He named this new biocomposite the hybrid layer ( Fig. 1 ).
Evolution of etch-and-rinse adhesives When Fusayama introduced the revolutionary concept of total-etching of cavities (i.e., simultaneous etching of enamel and dentin), the technique was resisted by U.S. and European dentists. They thought that 40% phosphoric acid would induce adverse pulpal reactions when allowed to etch dentin. Later work revealed that acid-etching dentin more than 0.5 mm thick produced no adverse pulpal reactions if the etched dentin could be sealed from oral bacteria. The adverse pulpal reactions seen in the U.S. and Europe were due to bacterial leakage, not acids per se . The lack of pulpal reactions to total-etching in Japan was due to the fact that they only excavated carious dentin. As part of their minimal invasive restorative philosophy, Japanese dentists did not extend outline forms into normal dentin as was the practice in the U.S. and Europe. Excavated caries-affected dentin, unlike normal dentin, is almost impermeable to all solutes and solvents , thereby protecting the pulp from irritants.
The introduction of dry bonding The first marketed etch-and-rinse adhesive was Clearfil Bond System-F (Kuraray Co., Ltd., Tokyo, Japan) in 1978. It utilized 40% phosphoric acid used in the total-etch manner. Adverse pulpal reactions continued to be reported in the U.S. following acid-etching of dentin with phosphoric acid because clinicians were performing “dry bonding”. That is, after total-etching, they would dry the cavity walls to confirm that the enamel margins were “frosty” or had a chalk-like color. This meant that the enamel was properly etched. What was not realized at that time was that drying the cavity caused the acid-etched dentin to collapse. Such collapsed demineralized dentin had lost the interfibrillar spaces between exposed collagen fibrils that serve as inward diffusion channels for monomer infiltration. Consequently, resin–enamel bond strengths were high (ca. 20 MPa) but resin–dentin bond strengths were very low (ca. 5 MPa). Such low resin–dentin bond strengths were not sufficient to resist the forces of polymerization shrinkage (about 24 MPa in class I cavities) . Thus, during polymerization of resin composites, one or more of the bonded walls would debond, creating bacterial leakage through normal permeable dentin that could irritate the pulp.
The introduction of wet-bonding The low resin–dentin bond strengths associated with “dry” bonding created dentin sensitivity, microleakage, secondary caries and loss of bonded restorations. Kanca found that water was an excellent rewetting agent and this led to him to introduce the concept of “wet-bonding”. This technique increased the strength of resin–dentin bonds, allowing good sealing of dentin and much less post-operative pain. At this point resin–dentin bonds equalled or exceeded resin–enamel bonds and the era of safe, reproducible resin–dentin bonding began.
Dentin bonding as a form of tissue engineering In most tissue engineering applications, one uses a 3-D scaffold (often made of collagen) that is designed to be resorbed over several weeks to months, to provide replacement by regenerating tissues of the host . Unlike classical tissue engineered constructs, where the scaffold is designed to be resorbed and replaced by normal tissue, in erupted teeth there are no tissues available for regeneration of occlusal hard tissue surfaces while teeth are in function. Instead, biocomposites must be engineered within minutes, in situ , with the expectation that they will last for decades! Because progress in adhesive dentistry has been incremental, we fail to recognize how far we have come in 55 years since Buonocore first acid-etched enamel. Each day, practitioners bond relatively hydrophobic resins to enamel and dentin within a few minutes and in doing so have completely transformed the surface chemistry of these hard tissues from wet, crystalline, hydrophilic surfaces that are acid-labile, to softer but tougher, hydrophobic, drier dentin surfaces that are chemically compatible with resin composites. These tooth colored biocomposites are also acid-resistant. They can be made to be antibacterial .
Adhesive bonding begins by acid-etching to increase the permeability of resins to enamel and dentin . In dentin, this is a unique form of tissue engineering. Acid-etching with 37 wt.% phosphoric acid completely demineralizes the surface 5–8 μm of the intertubular dentin matrix to create nanometer-sized porosities ( Fig. 2 ) within the underlying collagen fibrillar matrix. This permits infiltration of solvated comonomers into and around collagen fibrils to gain retention for tooth colored resin-composite fillings . Even more amazing is the contrast between the porosities of most bioengineered scaffolds (5–20 μm) compared to the porosity of interfibrillar spaces between resin-infiltrated collagen fibrils in hybrid layers that are only 10–30 nm wide. Thus, the dental biocomposites that are made by dentists in situ are created at a nanometer scale over a distance of 5–8 μm!