Adhesive post-endodontic restorations with fiber posts: push-out tests and SEM observations

Abstract

Objectives. Nowadays, the restoration of endodontically treated teeth is based on the use of materials with a modulus of elasticity similar to that of dentin (18.6 GPa). Fiber posts, resin cements and some composite resins all have this characteristic. This study evaluated the bond strength between luting materials, root dentin and fiber posts through push-out tests and examined the integration among these three components through scanning electron microscopy.

Methods. Endodontically treated extracted teeth and plastic plates were used to test the interface between luting agent and dentin and luting agent and post.

Results. Chemical affinity between different components (luting materials and fiber posts) is extremely important in achieving high bond strength. The bond strength tests and SEM observations showed that in vitro, composite resins perform better than resin cements.

Significance. The in vivo use of these materials may significantly reinforce residual tooth structure therefore reducing the risk for fracture and debonding.

Introduction

In recent years, the choice of materials used in the pre-prosthetic restoration of endodontically treated teeth has changed from the exclusive use of very rigid materials (stainless steel, gold and zircon dioxide) to materials which have mechanical characteristics that more closely resemble dentin (composite resins and fiber posts). In this way, a mechanically homogeneous unit can be created.

These new materials are easy to use and have the advantage of reducing fracture risk. In a previous study, we used finite element analysis to demonstrate that less rigid core materials distribute stress better than rigid ones. These materials do not generate forces in the interface area but in the dentin around the central third of the canal. Thus the critical interface between dentin and restorative material is preserved. 

All of the materials which compose this kind of ‘mono-block’ should ideally have similar modulus of elasticity:

• resin cements (6.8–10.8 GPa);
• composite resins (5.7–25 GPa);
• fiber posts (16–40 GPa).

The presence of fibers in some materials is a further advantage because fibers distribute stress on a wider surface area, remarkably increasing the load threshold at which the material begins to show micro-fractures. The properties of fiber-reinforced materials are well known:

• high impact resistance;
• attenuation and softening of vibrations;
• shock absorption;
• increased fatigue resistance.

In vivo longitudinal study results support the use of techniques that conserve crown and root dentin, and show that though posts are important for retention, they should no longer be perceived as an implement for tooth reinforcement. However, if residual crown dentin is scarce, manufacturers’ instructions as well as findings in the literature suggest cementing a pre-fabricated fiber post in the canal with resin cement before reconstructing the crown with light-curable composite. This technique is indicated when there is a close fit between canal and post. When the post’s section greatly differs from that of the canal, the classic luting technique creates a thick layer of cement between dentin and post. The cement has a lower modulus of elasticity than the two materials it joins and thus a zone of highly concentrated loads and stresses is created as we previously described in a study carried out with finite element analysis. These considerations suggest that composite resin which has mechanical characteristics similar to dentin would be a better material to use in the critical interface between post and dentin, especially when cement thickness exceeds 500 µm. In this way, the thickness of the material no longer presents a problem.

A choice must also be made between different types of posts based on their light-transmitting capacities:

1. Non-translucent posts considerably block light passage; therefore, light-curable materials must be substituted with self-curable composite resins. These materials must be very fluid and have a long setting time. They should be applied with a thin disposable metal tip to minimize the formation of air bubbles.
2. Translucent posts can be easily luted with light-curable composite resins.

The purpose of our study was to compare the performance of traditional resin cements with composite resins (both self- and light-curable) through bond strength tests and SEM observation.

Materials and methods

Push-out test sample preparation

All materials used and their Young’s modulus of elasticity are summarized in Tables 1 and 2. Bond strength of the various luting materials was tested at the interface with dentin and at the interface with different posts through two separate push-out tests.

The push-out test that evaluated bond strength between luting material and root dentin, was carried out on 50 extracted single rooted teeth. Radiographs were taken to eliminate elements with irregularly shaped canals. The crown was removed with a section at the cemento-enamel junction using a low speed diamond saw (Isomet, Buehler Ltd, Lake Bluff, NY, USA). The teeth were endodontically treated according to the Ruddle technique which combines chemical action (5% NaOCl and 17% EDTA) and mechanical instrumentation. The canal was filled with gutta percha and Pulp Canal Sealer ET (Kerr, USA) and the gutta percha was compacted using the continuous wave technique (SystemB, Analytic Technology, Redwood, USA). The first 8 mm of the canal was shaped with a cylindrical diamond bur (Komet 837/016, Brasseler, Lemgo, D) so that a consistent thickness of luting material could be obtained from the coronal to the apical portion of the root canal. The luting materials were inserted with a disposable metal tip. The root was then sectioned transversally and four 2 mm thick sections were obtained (Fig. 1).

Samples were also prepared to evaluate the bond strength between luting material and post. Sixty plastic 3 mm thick plates were prepared by placing a post in the center and surrounding it with luting material using a disposable metal tip (Fig. 2). All specimens were stored in distilled water for 24 h before testing.

Push-out tests were performed (Fig. 3) at a cross-head speed of 0.5 mm/min using a universal testing machine (Acquati, Varese, Italy). The maximum failure load was recorded in daN and then converted into MPa.

Statistical analysis

Statistical analysis was performed applying one-way analysis of variance (ANOVA) followed by Scheffe’s test as post-hoc comparison at a significance level set at p < 0.05.

SEM sample preparation

The different combinations of fiber posts and luting materials analyzed through SEM observation (JSM9-840A, JEOL Ltd, Tokyo, Japan) are summarized in Table 3.

Thirty extracted elements with straight and regular single canals were used. The crown was sectioned off at the cemento-enamel junction using a low speed diamond saw. After endodontic treatment, application of the adhesive system and post luting were performed following manufacturers’ instructions. Disposable metal tips were used to inject composite resin into the canal while traditional resin cements were  introduced using  the  post as  a  carrier. The samples were then longitudinally sectioned with a low speed diamond saw and each half smoothed under running water with 600 grit silicon carbide paper. One half of each sample was gold sputtered (Polaron E5100, Polaron Equipment Ltd, Watford, UK) and the other half was pre-treated with 6N HCl for 20 s and 1% NaOCl for 10 min to remove the organic and mineral components of the dentin so that the hybrid layer and resin tag formation could be better analyzed.

Results

Push-out tests

Data from the push-out tests are presented in Tables 4 and 5 and Graphs I and II. All the samples gave high bond strength values (range: 26–30 MPa) although the values obtained for the combination of Tech 2000 or Tech 21 with Panavia F or Liner Bond 2V were significantly higher (p < 0.05). There is a statistically significant difference between the resin cement group and the adhesive/composite resin group.

SEM observations

Figs. 4–9 are a representative selection of the SEM observations. The interface between adhesive and root dentin was observed first. Dentin pre-treatment with ED Primer created a layer of hybridized dentin. This layer was not homogeneously distributed along the canal walls and the resin tags, when present, were irregular and showed different lengths.

With the use of another self-etching adhesive system (CLB2V), different results ranging from adhesive layers with no resin tags at all to hybridized dentin layers with many resin tags were obtained.

The use of a 4 generation adhesive system (All Bond 2) created a completely different situation. Very long and numerous resin tags were observed. This structure was consistently present from the coronal to the apical portion of the space prepared for the post.

The distribution of the luting material in the canal and the interface between luting material and post were also evaluated. Samples in which resin cement was used showed inconsistent results. At times optimal integration between post and cement was obtained with no air bubbles or voids in the cement layer. In other samples the cement was full of micro-bubbles and in one case, a single bubble occupied the entire cement thickness.

When self-curable composite resin was used, the situation varied from the total absence to the presence of a small number of bubbles. Most of the bubbles occurred at the tip of the post and never occurred at the interface between different materials.

The best results were obtained with the combination of a translucent post and a light-curable composite resin. In all the samples examined no bubbles or voids were found and this material, like the self-curable composite, adapted perfectly to the post surface.

Discussion

With the use of a 4 generation adhesive system(All Bond 2, Bisco Inc., Schaumburg, IL), SEM analysis demonstrated the presence of numerous and very long resin tags all over the root surface due to acid etching pre-treatment with 37% phosphoric acid. However, the corresponding push-out test showed only a slight superiority in the bond strength of this material. This may suggest that clinical performance is analogous. These results are in accordance with previous studies conducted by Mannocci et al.

Chemical affinity between post and luting material plays an important role in bond strength. The manufacturer asserts that the resin matrix of Tech 2000 and Tech 21 X-op fiber posts contains resin obtained through the polymerization of diphenilpropane and metiloxirane (dppMor), a resin monomer that should be compatible with 10 methacriloxy decyldihydrogenphoshate (MDP) found in some cements. The push-out tests carried out for Panavia F and Clearfil LB2V (Kuraray Co., Osaka, Japan) (MDP based) combined with Tech 2000 or Tech 21 X-op (dppMor based) gave the highest bond strength values.

The injection technique used for application of the self- and light-curable composite resins guaranteed less air bubble and void formation in all the samples examined. The use of disposable metal tips made this technique more predictable. The air bubbles present in the self-curable resin sample do not derive from a fault in the luting procedure but from the mixing phase of the substrate paste with the catalyst.

Conclusion

Adhesive luting of posts is an alternative technique that is comparable and in some ways superior to the traditional technique that uses resin cements. Composite resins are easy to use and ergonomically advantageous because the same material may be used to lute the post and restore the core.

Particular attention should be paid to the association between translucent posts and light-cured composite resin. This technique has the advantage of a prolonged working time.

Further investigation is needed to demonstrate the complete conversion of light-cured composite at different depths.

Authors: Luca Boschian Pesta, Giovanni Cavallib, Pio Bertanic, Massimo Gagliania

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