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Limitations of Dentin and Enamel Adhesion

Numerous studies have suggested that the resin-dentin interdiffusion zone or hybrid layer is a complex anatomic and histologic entity. Variation in the extent and reliability of adhesive infiltration is frequently reported with adhesive systems.1-4 Bond strengths decrease with time, and the ultrastructure of the resin-dentin interface undergoes changes that may weaken the bonding.5-8 A reduction in bond strengths up to 48% has been reported at 1 year with an all-in-one self-etching adhesive.7 Degradation of the bonds may be a result of hydrolysis that occurs either in the adhesive resin or in the collagen fibers that are not fully enveloped by the adhesive in the hybrid layer, especially when margins are in dentin.9,6,7 When all margins of the restoration are in enamel, the quality and integrity of the bonds remain unchanged with time.9 

Both total-etch and self-etch adhesive systems may have difficulties to infiltrate and to embed the collagen fibrils. In total-etch systems the infiltration of the exposed collagen fibrils by the hydrophilic resin may not fill all porosities created by etching procedures.1 On the other hand the self-etch adhesive leaves the solublized mineral phase in the resin as a suspension (Figures 1 and 2) supporting possible nano-porosities of the hybrid layer. In all cases the lack of sealing ability of adhesives is related to the presence of exposed or uninfiltrated collagen fibrils which are susceptible to proteolytic enzymes and may represent a pathway to secondary bacteria colonization.1  

In a recent study the quality of the hybrid layer was evaluated using an immunocytochemical technique and high resolution SEM.10 This study evaluated the morphologic labeling pattern of the collagen fibrils within the hybrid layer using both etching systems. These immunohistochemical data confirm that in all cases collagen fibrils remain uncovered and not enveloped by resin.

 

LIMITATIONS OF ENAMEL ADHESION 

More recent developments in dental adhesion include self-etch (SE) adhesive materials;11 they are easy to apply, and no rinsing is required (Figures 3-4-5-6-7). The same solution serves as conditioner, primer, and adhesive (Table 11-1). The rationale behind the action of self-etching agents is the formation of a continuum between tooth surfaces and adhesive material, which is accomplished by the simultaneous demineralization and penetration of resin in enamel and dentin surfaces.11 However, omitting the conventional etching step with phosphoric acid may result in the absence of the characteristic demineralization of enamel and dentin (Figure 8).

The enamel etching capability of SE adhesives has been studied profusely.12-21 One of the shortfalls of SE adhesives is that they may not etch enamel to the same depth achieved with phosphoric acid.18,19 Other studies, however, demonstrated that enamel bonding with SE adhesives is of the same magnitude as enamel bonding after phosphoric acid etching.20,21 The low bond strengths of SE adhesives to intact enamel, which is present in the cervical aspect of some restorations, raise clinical concerns.22 Optimal bonding to intact enamel is important to accomplish good clinical performance and to prevent microleakage.23 Long-term clinical studies with SE adhesives have demonstrated that these materials do not etch enamel adequately.24,25 

The lower adhesion strength of SE adhesives to intact enamel may be a result of the less pronounced enamel etching pattern obtained with SE adhesives on intact enamel than the pattern obtained with phosphoric acid.18,20 Intact enamel is less accessible to SE adhesives because it is hypermineralized and may contain more fluoride than instrumented enamel.14 Changes occur in the outermost enamel layer after eruption.26 A prismless enamel layer may be present and prevent further penetration of SE adhesives.27 Longer application times may actually compensate for the higher pH of SE adhesives compared to that of phosphoric acid etchants.28 In fact, when a self-etching primer was applied for 60 instead of the recommended 30 seconds, Ferrari et al demonstrated that an adequate seal is provided in Class V cavities, in vitro and in vivo.13 

When the acid-etch technique2 was relatively new, the formation of tag-like resin extensions into the enamel microporosities was considered the mechanism of bonding of resin to phosphoric acid-etched enamel,27,31 comparable to that of phosphoric acid etched enamel.32 Other studies reported that the etching pattern of SE adhesives was not as well defined as that of total-etch (TE) adhesives on roughened enamel, but resulted in similar enamel bond strengths.18,20 This similarity in bond strengths for SE adhesives, in spite of their shallower etching pattern in comparison to TE adhesives, may seem conflicting. The length of the tags created on phosphoric acid-etched enamel, however, may contribute little to the bond strength. The adhesive strength may be a result of the ability of the resin to penetrate between the enamel crystallites and rods.33 An ideal etch pattern is not essential to produce a strong bond.34 Nevertheless, etching enamel with non-rinsing conditioners of pH higher than that of phosphoric acid remains controversial. The unpredictable behavior of SE adhesives around enamel margins is the reason why etching with 30% to 40% phosphoric acid is still recommended clinically.18 Total-etch adhesives bond better to intact enamel than the corresponding self-etch adhesive from the same manufacturer.

The stability and durability of bonded interfaces formed with self-etching adhesives has been the subject of some controversy.6-9 The hydrolysis of the hybrid layer formed with self-etch adhesives has now been established. An NMR study found that the ester portion of the HEMA molecule undergoes hydrolysis, as a result of the dissociation of the phosphoric acid component in the MDP molecule.35 Upon hydrolysis, HEMA breaks down into methacrylic acid and ethylene glycol.

In summary, bonding to dental hard tissues has transformed the conceptualization and practice of operative dentistry. Developments in adhesive materials have not necessarily corresponded to improvements in their clinical success, as manufacturers and clinicians have prioritized the simplification of the clinical technique over the clinical efficacy of the newest materials. Therefore, clinical trials should be mandatory prior to the introduction of new adhesive materials.

References: 

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