Ultrastructural Evaluation of the Hybrid Layer After Cementation of Fiber Posts Using Adhesive Systems with Different Curing Modes

Metallic cores or prefabricated posts are often needed as an auxiliary method of retention for the filling or reconstruction of endodontically treated teeth. Since their introduction by Duret et al. in the 1990’s (1), the use of fiberglass posts has increased rapidly due to their advantages (2) when used as recommended. An adhesive technique with resin cement is recommended for this type of post. However, this operative step can be complex due to various factors, such as the heterogeneity of the dental root substrate (3); instrument access at increased depths (4); adjustment of the post (5); adhesive systems; and resin cements from different presentations, application techniques, and reaction types (6-13). Due to these various aspects, the performance and interaction among these materials Ultrastructural Evaluation of the Hybrid Layer After Cementation of Fiber Posts Using Adhesive Systems with Different Curing Modes


INTRODUCTION
Metallic cores or prefabricated posts are often needed as an auxiliary method of retention for the filling or reconstruction of endodontically treated teeth.Since their introduction by Duret et al. in the 1990's (1), the use of fiberglass posts has increased rapidly due to their advantages (2) when used as recommended.
An adhesive technique with resin cement is recommended for this type of post.However, this operative step can be complex due to various factors, such as the heterogeneity of the dental root substrate (3); instrument access at increased depths (4); adjustment of the post (5); adhesive systems; and resin cements from different presentations, application techniques, and reaction types (6)(7)(8)(9)(10)(11)(12)(13).Due to these various aspects, the performance and interaction among these materials teeth were removed with an Isomet 1000 cutter (Buehler, Lake Bluff, IL, USA), standardizing the root length to 14 mm.The root canal was opened with #2, 3, 4 and 5 (Maillefer, Tulsa, OK, USA) to obtain a 9.0 mm long canal space.
Canal walls were etched with 37% phosphoric acid (Super Etch; SDI, Victoria, Australia) using a syringe for 15 s, followed by washing for 30 s. Excess water was removed by light air-drying and absorbent paper points.The smooth fiber glass posts, 1.5 mm in diameter (Angelus, Londrina, PR, Brazil), specially made for this study, were treated with 37% phosphoric acid for 1 min, rinsed, and silanized (Monobond S; Ivoclar Vivadent, Schaan, Liechtenstein).Next, the adhesive system was applied.The interactions between the adhesive systems and resin cements of different types of chemical reaction were analyzed according to the following groups: G1: self-cure adhesive Lok (SDI) + self cure Post-Cement Hi-X (Bisco Inc., Schaumburg, IL, USA).The lok (SDI) primer was applied to the root dentin using an ultrathin microbrush.Next, the Lok base and catalyst mixture was applied.After having applied the mixed base and catalyst pastes, the resin cement was introduced into the root canal with a Centrix injector.The post was then introduced, and the excess cement was removed with a brush.
G3: light-cure adhesive One Step (Bisco Inc.) + self cure Post-Cement Hi-X (Bisco).The adhesive was applied and light cured for 20 s.The application of resin cement followed the same procedure as in G1.
G4: self cure adhesive Lok (SDI) + dual cure resin cement Variolink II (Ivoclar Vivadent).The adhesive was applied in the same manner as in G1.The dual-cure resin cement was mixed and inserted into the root canal with a Centrix injector.The excess was removed with a microbrush and light cured for 40 s.
G5: dual-cure adhesive Excite DSC (Ivoclar Vivadent) + dual-cure resin cement Variolink II (Ivoclar Vivadent).The application and curing of the adhesive system was carried out in the same manner as in G2.Mixing, insertion and polymerization of the cement followed the same procedure as in G4.
G6: light-cure adhesive One Step (Bisco) + dual-cure resin cement Variolink II (Ivoclar Vivadent).The adhesive and resin cement were mixed, inserted, and photoactivated in the same manner as in G3 and G5, respectively.Light curing of the adhesive or cement was necessary in this group.Then, except for G1, in which self cure materials were used, the root surfaces were covered with aluminum foil to prevent the introduction of any external light that was not intentionally introduced through the upper end of the post in an axial direction.
All specimens were stored for 7 days and then the roots were bisected longitudinally.The resulting 96 root halves was processed according to the each type of analysis: hybrid layer or resin tags.The specimens were subjected critical point drying ( 14) and examined with a scanning electron microscope (JSM 840A; Jeol, Tokyo, Japan).
For analysis of the adhesive interface by continuity of the hybrid layer, 48 root halves were treated with 37% phosphoric acid, rinsed and deproteinized with sodium hypochlorite at 2.5% for 2 min.The specimens were fixed on aluminum stubs and sputter-coated (Edwards Pirani 501; Scancoat Six, UK) for 2 min.SEM analysis was performed at magnifications of ×400 and ×1500, in three bands of observation: cervical third (1st, 2nd and 3rd mm); middle third (4th, 5th and 6th mm); and apical third (7th, 8th and 9th mm), using the cervical border as a reference.A descriptive measure of 0 to 3 was used to evaluate the continuity of the hybrid layer.Thus, "0'' represented the continuity of the hybrid layer in all thirds of the root, i.e., an absence of fissures in all thirds; "1" represented a continuity in two thirds; "2" represented a continuity in at least one third; and "3'' indicated that all thirds presented a discontinuity in the hybrid layer or fissures.
The non-parametric Kruskal-Wallis test (α=0.05)was used for the statistical analysis of the 3 levels studied and comparison among the 6 treatment groups as regards the presence of gaps between post and cement (GPC), gaps within the cement layer (GC), gaps between the adhesive layer and cement (GCA), gaps within the adhesive layer (GA), and gaps between the adhesive layer and dentine (GAD) for the continuity of the hybrid layer in thirds.
For analysis of resin tags, the remaining 48 root halves were completely demineralized with 37% hydrochloric acid for 48 h and 2% sodium hypochlorite for 10 min, dissolving the dental substrate completely, in order to permit visualization of the resin, extensions and adhesive side branches.The specimens were stored, fixed, sputter-coated and evaluated as described for the hybrid layer analysis.A magnification of ×2500 was employed.The density and morphology of resin tags were ranked with scores from "0" to "4" where: "0" represented the absence of tags, "1" few tags from 2 to 7 μm, "2" few tags from 7.1 to 15 μm, "3" many tags greater than 15 μm, and "4" many tags of more than 15 μm and with side branches.
Comparisons among the 3 levels, in terms of density score and morphology of resin tags by group, were analyzed statistically by the Friedman test (p<0.05).

Analysis of the Adhesive Interface by Continuity of Hybrid Layer
Statistical analysis of the 6 groups in relation to the examined thirds showed significant differences.The cervical third presented the smallest number of gaps, followed by the middle and apical thirds, respectively (Table 1).
Type GA gaps were the most common, followed by GAD, GCA and GPC.The adhesive Lok showed fewer GPC gaps, followed by Excite DSC and One Step, mainly in the apical third (Fig. 1).As regards the cements used, Hi-X showed no GPC gaps, whereas Variolink II showed GPC gaps in the middle third and even more strikingly in the apical third.
In relation to the continuity of the hybrid layer, groups using self cure adhesives, G1 and G4, presented the best results among the three root thirds.Groups using dual cure adhesives (G2 and G5) presented intermediate results, while groups using light cure adhesives (G3 and G6) presented the worst results (Table 1).

Analysis of Resin Tags
Comparative analysis among the groups regarding the scores of the density and morphology of resin tags per groups and thirds revealed that the apical third was Table 1.Comparative analysis of the 6 experimental groups regarding the number of gaps and the continuity of the hybrid layer in the three root canal thirds.Descriptive measure scores of 0 to 3.    the most critical region for all groups (Table 2).In the cervical third, G1, G2, G4, and G5 showed the best results, while G3 and G6 had the worst.In the middle third, G1 presented the best results, followed by G2, G4, and G5 (Fig. 2), with intermediate results, and G3 and G6 with the worst results.In the apical third, the groups with the best results were G1 and G4.The worst result was obtained in G3.The highest mean scores among all root thirds was obtained in G1 and G4, while the lowest mean score was obtained in G6.

DISCUSSION
The results of the present study suggest that the density and morphology of the hybrid layer is directly related to the type of polymerization of adhesive materials and to the root depth, thus rejecting the null hypothesis.
The best performance in all thirds, with respect to the continuity of the hybrid layer and the resin tags, was observed for the self-cure adhesive and resin cement, followed by the dual cure and light cure systems, respectively.The hypothesis that seems to explain these results is related to the luminous intensity in the deepest regions of the post space, the morphology of the root substrate, the stress generated by the cavity configuration (C-factor), and the interaction between adhesive and resin cement systems.
The reduction of luminosity due to the distance of the light source from the apical third, results in incomplete polymerization of resin materials, whose chemical reaction is dependent on light energy, especially at greater depths (15,16).The relevance of this fact was demonstrated by Kanehira et al. (17), who found a directly proportional relationship between the degree of adhesive conversion and bond strength, and Giachetti et al. (18), who showed the importance of light in the polymerization of light cure or dual cure adhesives.
Another aspect to be analyzed is the heterogeneous morphology of root dentin.According to Ferrari et al. (19), a reduction of tubular density can be observed in the deeper regions of root dentin.This seems to justify, in part, the results obtained in the comparisons among the three thirds within the post space, regardless of the group evaluated.In this manner, the apical third of the post space was the most critical in producing continuous hybrid layers and producing resin tags, as reported elsewhere (20).Difficulty of access of adhesive applicators to the deepest regions may also influence the quality of the adhesive interface.The brushes used in this experiment were of the ultra-fine type for G1, G3, G4, and G6.In G2 and G5, the small/endo that comes with the Excite DSC kit was used its own applicator.The performance of the ultra-fine type of brush present better results in the study by Vichi et al. (20), and for this reason does not appear to have influenced the results.
The intraradicular region has a comparatively unfavorable cavity configuration.According to Bouillaguet et al. (21), the C-factor is considerably greater than in intracoronal restorations.Therefore, it is acceptable that dual cure resin cements and adhesives or light cure resins develop higher tension at the adhesive interface.As a result of this tension caused by polymerization, gaps may form in the two adhesive interfaces: the post/adhesive and the adhesive/root interfaces.These gaps were mainly found in G6, which combined light cure adhesive and resin cement.
The incompatibility between adhesive and resin cement systems should be considered in the analysis of the types of gaps found in the present study.When the chemical characteristics of the adhesive system are antagonistic, cement gaps can be observed (22,23).In this study, in G5 and G6, the GCA-type gaps occurred in a larger number for the combination of dual cement with simplified, two-step adhesives.Thus, the classic, threestep adhesives do not present a risk of incompatibility.This is because the acid monomers that are contained in the first step and the final application of the adhesive prevent these monomers from coming into contact with the cement.
Generally, the simplification of steps in the adhesive system and the polymerization reaction of adhesives and resin cements have a direct effect on the adhesive post/dentin substrate interface.When a lightcure component was added to the experimental cementing protocol, inferior results were always observed.When light cure components were incorporated either in the adhesive or the cement the results worsened.
Clinically, it appears to be more reliable to use adhesive systems that use polymerization reactions that are strictly linked to self curing approaches.
The results of this study led to the following conclusions: 1) the best results regarding the uniformity of the hybrid layer, absence of gaps, and presence of resin tags were obtained when using self cure adhesives and resin cement systems; 2) dual cure adhesive in combination with dual cure resin cement showed fair results for the analyzed variables; 3) light cure adhesive system in combination with dual cement showed the worst results; 4) the apical third was the most critical region for all combinations of adhesives and cements analyzed in this work.

Figure 1 .
Figure 1.Presence of gaps in the hybrid layer in the middle third in G3 (A) (Original magnification ×500) and in the apical third in G6 (B) (Original magnification ×1500).A: adhesive; D: dentin; GAD: gaps between the adhesive layer and dentin; GA: gaps within the adhesive layer; GC: gaps within the cement layer; P: post area.

Table 2 .
Comparative analysis among the experimental groups regarding the scores of the density and morphology of resin tags per groups and thirds.Scores from 0 to 4.Uppercase letters compare values per column (thirds) (p<0.001;Kruskal-Wallis test) and lowercase letters compare values per rows (groups) (p<0.001;Friedman test).Mean of thirds: cervical third + middle third + apical third/3.Different notations indicate statistically significant differences.S.D.: standard deviations.