Adhesion, glass-ionomers

M.J. Tyas, Milestones in adhesion Glass-ionomer cements, J. Adhes. Dent. 5 (2003) 259-266. 

J.W. McLean, A.D. Wilson, Fissure sealing and filling with an adhesive glass-ionomer cement, Br. Dent. J. 136 (1974) 269-276. 

The glass-ionomer cement is the most versatile of all the dental cements and has been developed for a variety of applications (McLean Wilson, 1974, 1977a,b,c Swift, 1988b van de Voorde, 1988 Wilson McLean, 1988 Mount, 1990). Many of its applications depend on its adhesive quality which means that, unlike the non-adhesive traditional filling materials, it does not require the preparation of mechanical undercuts for retention and the consequent loss of sound tooth material. 

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. 

Laswell et al., 1971 Arato, 1974). All were prone to excessive dissolution and only one had adequate strength and film thickness. Their working characteristics were found to be unduly sensitive to changes in temperature and humidity (Simmons, D Anton Hudson, 1968). All were inferior to conventional zinc phosphate cements. No further development of these cements has taken place, nor is it likely that interest in them will be revived. The modem water-activated glass-ionomer cement has filled this niche and has vastly superior properties including adhesion to tooth material. 

The main line of development now lies with its successor, the glass-ionomer cement, which uses a similar glass, but in which phosphoric acid is replaced by poly(acrylic acid) this cement is more resistant to acid erosion and staining and has the great advantage of adhesion to tooth material. 

Polyelectrolyte-based dental cements or restorative materials include zinc polycarboxylates, glass ionomers, a variety of organic polyelectrolyte adhesives as well as alginate-based impression materials. Dental cements are primarily used as luting (cementing) agents for restorations or orthodontic bands, as thermal insulators under metallic restorations, and as sealents for root canals, pits and fissures. They are also sometimes used as temporary or permanent (anterior) restorations. For further introduction to dental materials the reader is referred to standard texts [122,123]. 

Lack of adhesion of a dental restoration to tooth structure results in microleakage at tooth-restoration interface. This occurrence can result in discoloration at the margin of the restoration, or in the formation of caries. Occlusal forces on the restoration and differences between the coeffidents of thermal expansion of the cement and tooth material can lead to leakage. In addition, oral fluids and moisture may affect the adhesion. Microleakage of composite resin restorations has been reviewed by Ben-Amar [233]. Microleakage is not as serious a problem with glass-ionomer cements as it is with resin-based restorative materials, due to reduced polymerization shrinkage [234]. 

The results of Figure 8.18 show that most commercially available dental restorative materials have wear rates that are lower (better) than human enamel. All of the materials listed in Table 8.15 have nominal colors equivalent to that of human teeth and are of acceptable biocompatibility. In particular, glass ionomer ceramics have become increasingly popular due to their favorable adhesion to dental tissues, fluoride release, and biocompatibility. 

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. 

Use of resin-modified glass-ionomers has grown considerably since their introduction in 1991, and versions are available that are suitable for use as full restorations [34]. However, because of limited penetration by light, deep cavities may need to be filled using the incremental build-up technique usually associated with composite resins. Resin-modified glass-ionomers show good adhesion to dentine [31] and also release useful amounts of fluoride [31,35]. 

The term polyacid-modifled composite resin was originally proposed for these materials by McLean et al. in 1994 [6], and was felt to be a more accurate description than the term compomer under which they had been first marketed. The latter word was coined as a hybrid of the terms composite and glass-ionomer , but lacked any indication that the materials in question more closely resembled conventional composite resins than glass-ionomer cements. In particular, they are formulated without any water present, and are substantially hydrophobic, albeit less so than conventional composite resins. Also, despite early claims, they show no inherent adhesion to the tooth surface, and have to be used in association with bonding agents of the type used with conventional composites [1,6]. 

This means that polyacid-modified composites are essentially composite resins. As such, they must be bonded to the tooth with appropriate bonding agents, applied in increments, and show no ion-exchange properties, though they will release fluoride [38]. Similarly, resin-modified glass-ionomers are very similar to conventional glass-ionomers. They show inherent adhesion to the tooth [30], long-term fluoride release [31] and ion-release under neutral and acidic conditions [59]. 

S.B. Mitra, Adhesion to dentin and physical properties of a light-cured glass-ionomer liner/base, J. Dent. Res. 70 (1991) 72-74. 

One of the key features of polyacid-modified composite resins is their lack of adhesion to tooth tissnes [5]. This is a feature that they share with conventional dental composite resins, and the contrasts with the behaviour of the glass-ionomer cement. It is further evidence that these materials are essentially composite resins, and have very little of the anticipated hybrid character of composites and glass-ionomers. Bonding therefore reqnires the type of bespoke bonding agents used for conventional composite resins, together with the appropriate preparation of the freshly cut tooth surface [6]. 

Despite the presence of glass-ionomer components, compomers are not adhesive to tooth tissue and must be used with bonding agents. 

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]. 

Adhesion of glass-ionomer cements appears to be the result of two inter-related phenomena These are ... 

Glass-ionomers have other advantages over composites in this appUcadon, namely that they are hydrophilic and dimensionally stable. Their hydrophilic character enables them to absorb fluid that can be left at the bottom of the fissure without jeopardizing the adhesion to enamel. The dimensional stability is important because it allows the cement to retain its marginal adaption and seal, so that there is no risk of caries developing under the fissure sealing material. Fluoride release is also potentially advantageous. 

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