Smear layer

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. 

The SEM micrographs of dentin surfaces treated with an aqueous acetone solution (mass ratio 1 1) containing 0.3 mol/L 3-methoxy PIDAA indicated significant removal of the smear layer by 3-methoxy PIDAA (3 MeOPID), similar to that achieved with PIDAA (Figure 8). Similar observations were made with the other PIDAA derivatives. 

Figure 8 SEM Photomicrographs of dentin with (a) smear layer untreated, (b) treated with PIDAA, and (c) treated with 3-methoxy-PIDAA (3 MeOPID). 

This is especially true for enamel where details of the prepared surface are masked by debris. A smear layer often covers the dentin surface. Complete drying of the surface cannot be accomplished under in vivo conditions, since fluid seeps rapidly from the dentinal tubules. Thus the tooth surface is highly heterogeneous and bonding is only accomplished at localized active sites. 

Another important feature of the pre-treatment of the dentine surface is that the smear layer is removed and so also are the smear plugs. The latter is especially important as it opens to dentinal tubules, which means that liquid bonding agent can flow into the tubules (see Fig. 5.2). When polymerization occurs, this results in the formation of... 

Self-etching primers are blends that can both etch and prime the surface in one step. They contain at least one acidic monomer, in order to allow them to undertake the etching process. They also eliminate the need to rinse away the solubilized products to the etching step. Instead, these substances become incorporated into the primer layer. This has the advantage of reducing the technique sensitivity associated with the process of bonding [4]. However, there is some evidence that the resulting adhesive bonds are weaker than those with substances that do not incorporate any mineral components of the smear layer [24]. 

The acidity of self-etching primers allows them not only to penetrate the smear layer and incorporate its component, but also to interact with the underlying intact dentine. In doing so, it forms a hybrid layer of the type previously identified with three-layer bonding systems [20,21], and which is essential in promoting adhesion. This acidity varies between formulations, and can be classified as either strong or weak, depending on the acid involved in the blend and also its concentration [25]. 

On the other hand, strong self-etch primers have a pH of around 1, and hence contain strong acids based on phosphoric acid groups only. They interact more aggressively with the smear layer of the freshly cut tooth, removing more of it from the surface. Though these substances nonetheless include the components of the mineral phase, the resulting bonded surface closely resembles that created by the etch-and-rinse technique. 

The essential mechanism of action of self-etch adhesives is that they simultaneously etch and prime the freshly cut tooth surface, and in doing so, incorporate the components of the smear layer [58,60]. Because of the presence of the smear layer, and also because of smear plugs blocking the dentinal tubules, self-etch adhesives have been claimed to provoke less post-operative sensitivity in patients than the etch-and-rinse approach [61,62]. However, this has not been confirmed by scientific studies [63,64], but rather any reduction in patient discomfort has been attributed to operator technique [15]. 

H. Koibuchi, N. Yashuda, N. Nakabayashi, Bonding to dentin with a self-etching primer the effect of smear layers. Dent. Mater. 17 (2001) 122-126. 

F.R. Tay, H. Sano, R. Carvalho, E.L. Pashley, D.H. Pashley, An ultrastructural study of the influence of acidity of self-etching primers and smear layer thickness on bonding to intact dentin, J. Adhes. Dent. 2 (2000) 83-98. 

Tensile bond strength (TBS) were determined using a testing protocol and assembly previously described (6). To assess the efficacy of smear layer removal by aqueous AA the surface of 1 mm thick dentin cross sections were pretreated with one drop (0.05 mL) of AA (17.6 wt. % in distilled HjO pH = 2.0). The durations of AA treatment were 15, 30, 45, 60, and 120 s. l ch AA treated dentin surface was rinsed with distilled water for 10 s and then was air dried. The dentin specimens were then sputter coated with gold for evaluation by scanning electron microscopy (SEM). 

Compared to the SEM photomicrographs of an untreated dentin surface, photomicrographs of the AA treated dentin cross sections for times of 15, 30, 45, 60, and 120 s all showed significant surface changes(Figures 3-8). Optimal smear layer removal and opening of dentinal tubules occurred after 30-60 s of AA etching. 

SEM results clearly demonstrate removal of the smear layer from dentin using AA as a dentin etchant/conditioner. Various water soluble salts are probably formed, i.e., calcium hydrogen phosphates or calcium ascorbates, during interaction of AA... 

Figure 3. SEM photomicrograph of dentin surface with smear layer untreated (3000 X).

The smear layer is formed during the cavity cutting process and can consist of cutting debris, smeared odontoblasts (one of a layer of connective tissue cells lining the pulp cavity, which takes place in the formation of dentine) and micro-organisms. It cannot be formed in a reproducible manner. There are three main approaches to dealing with the smear layer. 

The smear layer is often removed with phosphoric acid, which again deminerahzes the superficial dentine and exposes collagen. The amphiphilic monomer creates the hybrid layer and diffuses into the dentinal tubules where it forms tags and provides additional micro-mechanical retention. If the collagen layer becomes desiccated, then it collapses, monomer infiltration is inhibited and the hybrid zone is not successfully formed. 

DBAs are now classified according to the number of clinical steps involved in treatment of the smear layer ... 

Type 1 Remove smear layer 3 clinical steps etch, prime, bond. 

Type 2 Remove smear layer 2 clinical steps etch, prime and bond combined. 

In a typical Type 1 system, the smear layer is removed using 10% maleic or 35% phosphoric acid. The dentine is then primed with an aqueous solution of HEMA, and the adhesive is a combination of bis-GMA and HEMA together with an amine that can be used to initiate a light-cured reaction. In the type 4 systems, all steps are combined to ensure no desiccation and the successful formation of the hybrid zone. A typical type... 

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