Discussion - References.
The ability of root-canal sealers to adhere to dentine and gutta-percha is expected to result in superior sealing ability, which in turn should reduce leakage in clinical situations. Adhesion should also improve the stability of the root filling, e.g. during preparation for post space. Adhesion depends on a multitude of interacting factors including the surface energy of the adherend (dentine or gutta-percha), the surface tension of the adhesive (sealer), the adhesive’s ability to wet the surfaces and the cleanliness of the adherend surface (Bayne 2001).Moreover, stresses caused by differences in thermal expansion coefficients and dimensional changes during setting of the adhesive may affect adhesive bonds. Two mechanisms of adhesion may be distinguished: chemical and mechanical. In case of chemical bonding a smooth surface generally results in better adhesion. Micromechanical bonding, on the other hand, requires the presence of irregularities on the surface of the adherend into which the adhesive can penetrate.
The endodontic smear layer forms over the surface of dentinal walls when the root canals are instrumented (McComb & Smith 1975). The significance of the smear layer in endodontics has been the subject of extensive debate since it was first described. Certain factors would indicate removal of the smear layer prior to root-canal filling. Not only can the smear layer act as a reservoir or substrate for microorganisms (Pashley 1984), but it will also obstruct the extension of sealer tags into the dentinal tubules and thereby decrease adhesion by micromechanical forces (Kouvas et al.1998). On the other hand, it has been argued that removal of the smear layer increases dentine permeability with possible diffusion of noxious substances from the root canal to the external root surface (Galvan et al. 1994). Presence of the smear layer may also discourage bacterial penetration and colonization of the dentinal tubules (Michelich et al. 1980, Drake et al. 1994). Furthermore, the constituents of the smear layer may improve adhesion by altered wet ability and chemical bonding. For such reasons, retaining the smear layer on the root-canal walls has been considered to be beneficial.
No adhesion test has yet been generally accepted. Most previous studies have measured the bond strength of endodontic sealers in bulk and to dentine only. In the present study, tensile bond strength measurement was performed by testing the sealers in a thin layer between dentine and gutta-percha rather than in bulk, as they would be used in a clinical context (Wennberg&Irstavik 1990). Since tensile bond strengths of low magnitude are highly susceptible to transverse forces (Irstavik et al. 1983), special care was used when handling the specimens during mounting in the testing machine to avoid influences from such forces.
All the sealers tested showed measurable adhesive properties. The resin-based sealer (AH) had the highest bond strength amongst the sealers investigated, whilst the calcium hydroxide-based sealer (AP) demonstrated the lowest values. These findings are in general agreement with most other studies (Wennberg & Irstavik 1990, Gettleman et al.1991, Webster et al. 2001).
Phosphoric acid, citric acid and EDTA were used as dentine pretreatments in the present study. The pretreatment of dentine with17% EDTA is known to remove the smear layer, whereas distilled water leaves it intact (Goldman et al.1981, Yamada et al.1983). Both phosphoric and citric acids are similarly known to be effective in removing the smear layer (Tidmarsh1978,Baumgartner et al.1984).Pretreatment of dentinewith35%phosphoric acid or 6% citric acid was found to be more effective than EDTA in removing the smear layer (Timpawat et al. 2001).
In the present study, the effect of smear layer removal on adhesion varied amongst the different sealers investigated. For the resin-based sealer, AH, dentine pretreatment resulted in no improvement in adhesion. A stronger bond might have been expected, as it has been suggested that the exposure of the dentinal tubules creates a much more irregular surface allowing for penetration of resinous tags (Gettleman et al. 1991). However, we found that pretreatment of dentine, with EDTA in particular, significantly decreased the bond strength as reflected in the change of the failure pattern towards debonding at the sealer-dentine interface. Similar findings have previously been reported (Wennberg & Irstavik1990, Webster et al.2001). A possible explanation may be that the weak demineralization by EDTA leaves a relatively smooth surface of the organic dentine structure, which does not offer an increased area for adhesion. By contrast, dentine treated with EDTA showed greater adhesion to the resin-based sealers than dentine that received no treatment in other studies (Gettleman et al. 1991, Pecora et al. 2001). With all pretreatments other than EDTA, inspection with the stereomicroscope revealed the bond failure to be predominantly cohesive within the sealer (Fig. 3A). It has been suggested that the very slow setting of resin-based sealers might allow sufficient time for the development of adhesion to dentine, but that shrinkage stresses might fracture the still weak unset sealer cohesively (DeGee et al.1994).The present findings suggest that improving the cohesive strength of AH may further promote its sealing ability.
Similar to AH, the adhesion of the calcium hydroxide based sealer, AP, was significantly decreased by EDTA treatment. These findings are in disagreement with previous studies (Wennberg & Irstavik 1990, Fidel et al. 1994) where a significant increase in bond strength was reported for calcium hydroxide-based sealers when the dentine surface was pretreated with EDTA. Other studies, however, indicated that the tensile bond strength of calcium hydroxide-based sealers did not significantly increase when the smear layer was removed (Gettleman et al. 1991). AP failed cohesively within the sealer, with some areas debonding in the sealer-dentine interface. When the dentine was pretreated with EDTA, failure became predominantly adhesive to dentine (Fig. 3B), reflecting that smear layer removal reduced adhesion. Debonding at the sealer-dentine interface (Gettleman et al.1991), as well as cohesive failure within the sealer (Wennberg & Irstavik 1990), has been observed for calcium hydroxide-based sealers.
In contrast to adhesion of the majority of sealers, which mostly relyona mechanical type of bonding, glass ionomer cements also bind by chemical interaction (Wilson et al.1983). It is generally accepted that in case of chemical bonding, a smooth surface is desirable for adhesion (McComb&Smith1976).The present investigation supports this suggestion, as the highest bond strength( although not statistically significant) resulted when the smear layer was kept. The mean tensile bond strengths recorded for the glass ionomer-based sealer (KE) ranged from 0.35 to 0.41MPa. A wider range (0.1- 1.4 MPa) has been previously reported (Timpawat et al. 2001). They found pretreatment of dentine with 35% phosphoric acid or6%citric acid to result insignificantly higher bond strength for KE than treatment with EDTA. In the present study, the effect of etching dentine with phosphoric and citric acids could not be evaluated as KE debonded at the gutta-percha interface. The effect of pretreatment with EDTA, however, could be shown by the mode of failure being partly adhesive to dentine. The shear bond strength of glass ionomer-based sealers has been found to be higher with the smear layer present (Lalh et al. 1999a, Webster et al. 2001). The finding that the failure of KE was mainly adhesive to gutta-percha may be attributed to the high setting contraction of the fast setting glass ionomers leading to premature adhesive failure (De Gee et al. 1994). Improving the bond between glass ionomer sealers and gutta-percha may be a step towards achieving a better seal.
The bond strength of the recently introduced silicone (polydimethylsiloxane)-based sealer (RS) has not been previously reported. In the present study, no significant difference in bond strength was found, whether dentine was pretreated or not. However, the application of an experimental primer prior to RS significantly increased its bond strength. This occurred only when the smear layer was not removed. The ability of this primer to enhance the adhesion of RS to untreated dentine was reflected in the change of the pattern of failure of the specimens. The failure changed from a predominantly adhesive failure to dentine to a predominantly adhesive failure to gutta-percha, following application of the primer (Fig. 3C).These results suggest a possible interaction with the smear layer.
For the zinc oxide-eugenol-based sealer, GS, the decrease in bond strength when dentine was pretreated with EDTA was of no statistical significance. The sealer debonded at the dentine interface, whether dentine was pretreated with EDTA or left untreated, as reported in another study (Gettleman et al. 1991). In contrast to the present findings, pretreatment with EDTA was found to result in significant increase intensile bond strength for zinc oxide-eugenol-based sealers (Wennberg & Irstavik1990). In another study (Gettleman et al.1991), the increase in bond strength was not statistically significant. The use of phosphoric or citric acid resulted in a cohesive failure in the sealer. This was reflected in the significant increase in bond strength recorded under these conditions and may be related to chemical effects of the acids, causing precipitation of zincphosphate aggregates at the interface.
The present investigation confirms that the smear layer may play an important role in the bonding of endodontic sealers to dentine (Lalh et al. 1999b).Whether it is beneficial to recreate the smear layer under aseptic conditions prior to root-canal obturation or not remains to be answered. In further studies, scanning electron microscopy and energy dispersive X-ray spectroscopy will be used for analyses of effects of various dentine pretreatments on the smear layer.
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