Tooth adhesive bonding

To achieve such compatibility the primary requisite is that the restorative adheres to tooth material. This concept of adhesion is hardly to be foimd in the literature of the 1920s and 1930s. For that reason we find no attempt at developing tooth adhesives in that period. Adhesion was, apparently, only recognized as a desirable property in the 1950s. It seems for some reason to be associated with the introduction of simple resins as dental restorative materials. Although they were not a great success, attempts were made to bond them to tooth material. 

There are many obstacles to permanent adhesion under oral conditions. The substrate is a biological tissue and subject to change, and the presence of moisture represents the worst kind of situation for adhesion. Water is the great barrier to adhesion. It competes for the polar surface of tooth material against any potential polymer adhesive. It also tends to hydrolyse any adhesive bond formed. These twin obstacles gave rise to considerable doubt as to whether materials adhesive to tooth material could be developed at all (Cornell, 1961). 

Misra, D. N., Bowen, R. L. Wallace, B. M. 1975. Adhesive bonding of various materials to hard tooth tissues. VII nickel and copper ions on hydroxyapatite role of ion exchange and surface nuclea-tion. Journal of Colloid and Interface Science, 51, 36-43. 

Composite resins consist of blends of large monomer molecules, filled with unre-active reinforcing filler. As such, they are hydrophobic, which means that they are unable to bond to the hydrophilic prepared tooth surface [1]. Glass-ionomer cements, by contrast, consist of aqueous solutions of polymeric acid, typically poly(acrylic add) and powdered reactive glass. These two components react together in an acid-base reaction, and thus cause the cement to set. These materials are hydrophilic, and therefore capable of wetting the prepared tooth surface and forming tme adhesive bonds. 

To understand some of these challenges, it is necessary to consider the anatomy of the tooth. In particular, the composition and stracture of the two main tissues, enamel and dentine, need to be examined in order to understand how they influence adhesive bonds. Details of the composition of these tissues are shown in Table 5.1, from which it can be seen that the enamel comprises a much greater amount of mineral phase than the dentine. Consequently it is harder and stronger, and is also more brittle [4]. 

The ability of glass-ionomers to form a natural adhesive bond to the surface of the tooth is one of these material s most important clinical advantages. They were originally prepared from poly(acrylic acid), a substance chosen because of its use in the zinc polycarboxylate cement, a material known to adhere to the tooth surface [123]. The advantages of adhesion by these materials were apparent right from the start, when they were used for the repair of cervical erosion lesions and as pit and fissure sealants [124,125]. 

These findings represent the initial reports on the clinical use of these materials. Further work is needed to confirm (or otherwise) the positive findings from these first 6 months of clinical observations. Additional work is also necessary to determine the ideal method of bonding them to the tooth and also the extent to which they can form natural adhesive bonds to the surface of teeth. 

Unlike adhesive bonding in most areas of technology, in dentistry adhesion involves a living substrate, which means it is more variable than most surfaces to which adhesive materials are applied. To understand the significance this, the nature of the substrates that are encountered in dentistry must be considered, which means beginning with a consideration of the anatomy of the tooth. 

Glass-ionomers of all types are able to form adhesive bonds to the surface of the tooth. In order to do so under clinical conditions, the newly cut enamel and/or dentine surfaces are generally treated with a dilute solution of polyacrylic acid, typically at 10% concentration. This is a much milder treatment than that used with current bonding agents for composite resins and is more akin to pretreatments used with third generation bonding agents. 

When freshly mixed, the carboxyHc acid groups convert to carboxjiates, which seems to signify chemical adhesion mainly via the calcium of the hydroxyapatite phase of tooth stmcture (32,34—39). The adhesion to dentin is reduced because there is less mineral available in this substrate, but bonding can be enhanced by the use of minerali2ing solutions (35—38). Polycarboxylate cement also adheres to stainless steel and clean alloys based on multivalent metals, but not to dental porcelain, resin-based materials, or gold alloys (28,40). It has been shown that basic calcium phosphate powders, eg, tetracalcium phosphate [1306-01-0], Ca4(P0 20, can be substituted for 2inc oxide to form strong, hydrolytically stable cements from aqueous solution of polyacids (41,42). 

The glass polyalkenoate cement uniquely combines translucency with the ability to bond to untreated tooth material and bone. Indeed, the only other cement to possess translucency is the dental silicate cement, while the zinc polycarboxylate cement is the only other adhesive cement. It is also an agent for the sustained release of fluoride. For these reasons the glass polyalkenoate cement has many applications in dentistry as well as being a candidate bone cement. Its translucency makes it a favoured material both for the restoration of front teeth and to cement translucent porcelain teeth and veneers. Its adhesive quality reduces and sometimes eliminates the need for the use of the dental drill. The release of fluoride from this cement protects neighbouring tooth material from the ravages of dental decay. New clinical techniques have been devised to exploit the unique characteristics of the material (McLean Wilson, 1977a,b,c Wilson McLean, 1988 Mount, 1990). 

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. 

A variety of organic adhesives which are capable of forming strong bonds between a polymeric (acrylate) restoration and the hydrophilic tooth material have recently been developed. A number of these monomers, which possess a pendent ionizable group, are polymerized in the mouth to form an adhesive layer. Alginates, which are used as impression materials, are formed by the reaction of the sodium salt of anhydro-beta-d-mannuronic acid with calcium sulfate. Calcium ions crosslink the linear polymer to form a gel. This reaction is carried out inside the mouth, and the gel formed retains the shape of the oral cavity. 

Orthodontics is concerned with tooth movement to optimal positions, using metallic archwires ligated to brackets bonded to enamel or dental restorations by adhesive resins, as well as using suitable other metallic appliances, to provide appropriate forces and bending moments in vivo. The force generated by a bent orthodontic wire is proportional to its elastic modulus, and relatively light and continuous forces are considered to be optimum. There is considerable interest in nickel-titanium orthodontic wires, which have the lowest elastic modulus of the major wire alloys [7]. 

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