Enamel adhesives dentin

E.J. Swift Jr., Dentin/enamel adhesives review of the literature, Pediatr. Dent 24 (2002) 456-461. 

The potential benefits of the use of adhesive monomers both in free-standing dentin-enamel adhesives and in self-adhesive formulations are still somewhat areas of debate. Current restorative resin/dentin adhesive systems have demonstrated outstanding long-term clinical performance in prospective clinical studies [49-53]. 

Because of its complexity bonding to dentin has presented more of a challenge than bonding to enamel. Adhesion of restorative resins to dental tissues has been attributed to chemical and/or mechanical factors (2). Theoretically, chemical bonding can occur with either the inorganic (apatite) or organic (collagen)... 

Acrylic dental adhesives have been modified with a phosphate ester linkage by reaction of a glycidyl methacrylate or hydroxyethyl methacrylate with phosphorus oxychloride (158). One such product, a 3M Scotchbond, adheres to dentine as well as to tooth enamel. Adhesion to enamel can be improved in dental bonding cements by including an acryloxyalkyl- or acryloxyaryl acid phosphate component (159). Japanese products utilize an imsaturated phosphinic acid monomer to improve adhesion (160). 

Phosphoric acid has traditionally been used as a conditioner of enamel and dentin for adhesive restorative treatments, as desalbed in section Polymers in Dental Adhesion . 37 % phosphoric acid has been shown to decalcify enamel and dentin in a desirable pattern, thus facilitating impregnation of adhesive monomers required for placement of composite resin restorations in the tissue matrix. Despite the known efficiency of 37 % phosphoric add gels in acid-etching enamel and dentin, it has been shown that its effects in increasing the surface porosity of the pseudo-intact surface layer of enamel lesions was not sufficient [117]. To overcome this limitation, 15 % hydrochloric acid (generally for 2 min) has been tested and shown to remove about 30 pm of surface enamel, thus allowing for much improved penetration of viscous resins in white spot lesions [116]. 

The precise nature of the adhesion of the polyelectrolyte cements to untreated dental enamel and dentine has yet to be established. The earliest theory was due to Smith (1968) who speculated that the polyacrylate chains of the cement formed a chelate with calcium ions contained in the hydroxyapatite-like mineral in enamel and dentine. Beech (1973) considered this unhkely since it involved the formation of an eight-membered ring. Beech studied the interaction between PAA and hydroxyapatite, identified the formation of polyacrylate and so considered that adsorption was due to ionic attraction. 

The most important modification of these materials was the discovery of the effect of adding stannous fluoride (Foster Dovey, 1974, 1976). Originally added to provide fluoride release, it was found to improve the mixing qualities of the cement and to increase strength by about 50 %. This is reflected also in improved adhesion to enamel and dentine (Section 5.7.4). 

The bond strength to enamel (2-6 to 9-9 MPa) is greater than that to dentine (1-5 to 4-5 MPa) (Wilson McLean, 1988). Bond strength develops rapidly and is complete within 15 minutes according to van Zeghbroeck (1989). The cement must penetrate the acquired pellicle (a thin mucous deposit adherent to all surfaces of the tooth) and also bond to debris of calciferous tooth and the smear layer present after drilling. Whatever the exact mode of bonding to tooth stmcture, the adhesion is permanent. The principles and mechanism of adhesion have already been discussed in Section 5.2. 

Powis, D. R., Folleras, T., Merson, S. A. Wilson, A. D. (1982). Improved adhesion of a glass ionomer cement to dentin and enamel. Journal of Dental Research, 61, 1416-22. 

Cements based on phytic add set more quickly than their glass polyalkenoate or dental silicate cement cormterparts, but have similar mechanical properties (Table 8.2). They are unique among add-base cements in being impervious to acid attack at pH = 2-7. Unfortunately, they share with the dental silicate cement the disadvantage of not adhering to dentine. They do bond to enamel but this is by micromechanical attachment - the cement etches enamel - and not by molecular bonding. Lack of adhesive property is a grave weakness in a modern dental or bone... 

Table 8.5. Adhesive bond strength of the magnesium phosphonate cement to dentine and enamel (Ellis, 1989)...

Good bonding was obtained to several substrates under aqueous conditions. Values obtained were 41 to 10-3 MPa to composite resins, and 9-8 to 15-6 MPa to stainless steel (Table 9.6). They were also reported as adhering to porcelain. No adhesion was obtained to untreated dentine or enamel. The cements could be bonded to enamel etched with add (3-5 MPa) and to dentine conditioned with poly(acrylic acid) (10 MPa). 

The mechanism of adhesion to various substrates has not been fully explained. Brauer Stansbury (1984b) consider that bonding to composite resins occurs by the diffusion of methacrylate polymer chains into the resin. Bonding to base metals is, perhaps, by salt or chelate bridges. Here it is significant that ZOE cements do not bond, so perhaps bonding is due to the action of free EBA on the substrate. The adhesion to porcelain is surprising. Porcelain is inert so that the attachment can hardly be chemical. Also, it would be expected that if a cement adheres to porcelain then it should adhere to untreated enamel and dentine, but this is not so. 

Acid etching pretreatment of dentin prior to application of a resin restorative leaves the collagenous component of this substrate substantially untouched. The collagen at the dentin surface interferes with the mechanical adhesion of the hydroxyapatite, (the inorganic constituent of dentin) to the adhesive. Thus, in contrast to its successful use with enamel, etching pretreatment produces a bond between dentin and restorative resin that is too weak to withstand the forces encountered at the interface under clinical conditions. 

In the realm of dentistry, restoration and protection of tooth enamel are of great importance in operative and conservative dentistry. Yamamoto et al. (2013) used PLD to create a freestanding flexible double-layered sheet composed of a 4 pm thin hydroxyapatite (HAp) layer coated with a 0.5 pm thin TCP layer. The adhesive strength between the HAp/TCP sheet and enamel was 5.7 MPa, decidedly higher than that between the monolayered HAp sheet and enamel (1.9 MPa). Electron microscopical observation revealed that the HAp/TCP sheet was largely fused with the enamel. Therefore, the double-layered HAp/TCP sheet can be used as a material to promote the repair of tooth eruption and to maintain healthy dentine. 

A combination of modified features of the last-named two bonding systems is realized in an adhesive applieation known as the Kanca technique, in which dentin and enamel pre-treatment by phosphorie acid etching is followed by the consecutive layering of NTG-GMA, PMDM, and HEMA-bis-GMA adhesive resins, onto which the restorative is placed by conventional manipulation. Low microleakage, and composite shear bond strengths to enamel/dentin at the 18-MPa level, have been reported [54]. 

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