Is the bonding of self-adhesive cement sensitive to root region and curing mode?

Abstract Objectives To evaluate the influence of two curing techniques on the degree of conversion (DC) of resin cements and on bond strength (BS) of fiber posts in different regions of root dentin. Material and Methods Twenty single-rooted premolars were endodontically treated, and the post spaces were prepared. The roots were randomly divided into two groups (n=10), according to the activation mode of the resin cement RelyX™ U200 (3M ESPE Saint Paul, MN, USA): conventional (continuous activation mode) and soft-start activation mode (Ramp). The posts (WhitePost DC/FGM) were cemented according to the manufacturer’s recommendations and, after one week, the roots were cross-sectioned into six discs each of 1-mm thickness, and the cervical, medium, and apical thirds of the root canals were identified. The DC was evaluated under micro-Raman spectroscopy and the BS was evaluated by the push-out test. The data were analyzed by two-way ANOVA and Tukey’s test (α=0.05). Results Neither the activation mode nor the root regions affected the DC of the resin cement. Higher BS was achieved in the soft-start group (p=0.036); lower BS was observed in the apical third compared to the other root regions (p<0.001). Irrespective of the activation mode and root region, the mixed failure mode was the most prevalent. Conclusion The BS of fiber posts to root canals can be improved by soft-started polymerization. The DC was not affected by the curing mode.


Introduction
Endodontically treated teeth usually demonstrate an extensive loss of dental structure and require the 29 . In this context, the cementation restorations that resemble the natural dental structure because the modulus of elasticity of the adhesive 14 .
Unfortunately, several factors can affect the histological and anatomical characteristics of the root canal, density and orientation of the dentinal tubules in the different root canal regions 11 , as well as accessibility to the different root canal regions 12 .
Different areas of the same root canal also do not respond to acid etching, and thus the ability of adhesion to root dentin may be different at different depths in the same root canal 11 . Higher bond strength values at the cervical third are generally expected due to the ease of conditioning, and polymerization of the cements in this region 9 . However, this is still   The root canals were irrigated with 10 mL of distilled water and dried with paper points (Maillefer, Dentsply Ind. e Com. Ltda., Petrópolis, RJ, Brazil).

Experimental groups
At this point, the teeth were randomly divided into  (Table   1).

Bond strength
The average values of bond strength in MPa (mean and standard deviation) can be seen in Table   ( Table 2and soft-start) for the different root regions (cervical, medium and apical) The self-adhesive resin cement used (RelyX U200) is a new product, released as a substitute for the RelyX U100 resin cement. This resin cement has the same bonding mechanism of its predecessor, RelyX U100.
Both products have micromechanical retention, but it seems that their bonding relies mainly on the chemical adhesion to hydroxyapatite 25 . This may be the rationale behind the product's good performance in the apical third of the root canal in the present and earlier study 3 , different from what occurs with conventional resin cements.
diminishes toward the apical region of the intraradicular dentin 11 . In the apical third of the root, there are fewer dentinal tubules 11 , the dentin is irregular, and it may be devoid of dentin tubules 9 . When present, these resemble those from peritubular dentin 26 . Altogether these factors increase the availability of calcium for chemical adhesion with the self-adhesive RelyX U200 in the apical region, which yields a higher bond strength at this third, as observed in this study. Conventional resin cements, on the other hand, rely mostly on micromechanical retention. Therefore, better bonding is expected to occur in areas with a high density of dentinal tubules, such as the cervical region 9 .
Another factor that favors self-adhesive cements, as claimed by manufacturers, is that that this type Is the bonding of self-adhesive cement sensitive to root region and curing mode? In the continuous activation mode, the light intensity remained constant from the beginning to the end of the polymerization process, decreasing in the pre-gel phase. This in turn produced a polymeric material with potential for stress relief. In interface, which may have debonded in some areas, producing low bond strength for this group 8,23 .
Another technique used in this study was micro-Raman spectroscopy, which has been proven to be well suited for the characterization of the chemical structure and characterization of adhesive resins, collagen and minerals at a resolution of up to 1 mm.
It is also very useful in determining the degree of conversion of dental adhesives by providing a direct measurement of the percentage of converted double bonds 16,19 .
For light cured and dual cured resin cements, an adequate curing of the resin material by light is essential. Light intensity is higher at the cervical third 27 , yielding a higher degree of conversion than other regions 9,24 . This does not seem to be essential for self-adhesive resin cements, as the degree of conversion of Rely X U200 was neither affected by the curing technique, nor the root region. This was also observed in another published study 29 that employed similar resin cement. These cited studies employed conventional, dual-cure resin cements and not selfadhesive cements, as were used in the present study.
Little has been published on the light-curing potential of conventional dual-cure cements. Earlier to achieve maximum cement hardening 7 and this was also seen to be true for more recent resin-luting cements 21 . Perhaps, the dual-cure self-adhesive resin cements are capable of reaching maximum mechanical properties under light or chemical cure modes, explaining the similar degree of conversion observed in the different root regions. Additionally, both groups employed very similar energy densities, which may also be the reason for the similar degree of conversion between the two groups. However, further Regarding failure modes, there was no statistically difference between the two activation modes and root regions. The most frequent failure mode was the mixed type, which agrees with the results of some authors 1,13 who used self-adhesive resin cement. The present study evaluated only one type of resin cement (dual self-adhesive), which does not necessarily future studies should investigate more resin cements in order to investigate the differences between the The present study has some limitations, for instance, no thermal cycling or mechanical stress was applied. These factors may limit the direct application