Evaluation Of Effect Of Activation Mode And Bonding Agent On The Push-out Bond Strength Of Post To Root Canal Dentin.

INTRODUCTION

Pulpless teeth pose several challenges due to the loss of tooth structure as a result of caries, defective restoration and endodontic access preparations^1,2. Several endodontically treated teeth require the use of a post to retain its final restoration.³

Certain disadvantages were observed with the conventional cast posts, such as rigidity due to its high modulus of elasticity when compared to dentin, in addition they also require multiple 4 sittings for fabrication. Other problems such as loss of retention of post or crown, root fracture and risk of corrosion when different metals were used have also been reported^5,6. In the past decades, the increasing demand for aesthetics has led to the development of metal-free post-and-core systems, especially fiber- reinforced posts^7,8. Many studies on fracture strength of fiber reinforced posts show more favorable mode of failure. The modulus of elasticity of fiber posts is closer to that of dentin and distributes stress evenly over a broad surface area.^9,10

With newer adhesive systems and resin-based luting agents it is possible to create a genuine adhesive continuum between the tooth and the post core complex. The use of these bondable materials allows the practitioner to unify the structure and morphology of root systems₃. These post systems also improve the optical effect of esthetic restorations¹¹.

The most common failure in fiber-reinforced composite type of posts is due to debonding¹². Failure due to debonding can be minimized by controlling the factors essential in achieving a good bond between post-cement-dentine interface either by variation in the technique or by use of different materials for bonding of the post in the root canal¹³.

The objective of this study was to compare the effect of use of different dentin bonding agents and different activation modes of resin cement on the push-out bond strength of fiber posts.

MATERIALS AND METHOD

Forty single rooted, human mandibular premolar teeth (extracted for orthodontic reasons) were used in this study.

The teeth were decoronated one mm above the CEJ with diamond disk. The root canals were cleared of pulpal remnants using barbed broaches. The patency of the apical foramen was determined by passing no.10 k-file into the root canal until the tip of the file was visible at the apical foramen, and then 1mm was subtracted from that length. The teeth were instrumented to a no. 30 size master apical file upto the apical foramen. Then the canal was prepared by step back technique and flaring of the canal was done followed by circumferential filing using H-files. 10 ml of sodium hypochlorite was used as an irrigant in between successive files.Recapitulation with no.15 size files was done during biomechanical preparation.The smear layer was removed by irrigation with 10 ml sodium hypochlorite and 10 ml of 17% EDTA solution, each for 3 min. Final rinse was done with 10 ml of sterile water.

All canals were dried with paper points. The teeth were obturated with lateral condensation using no.30 size gutta-percha and AH Plus sealer. o All the roots were stored at 37 C in distilled water for 1 week.Theroot canals were prepared with peeso reamers. After the post space preparation 17% EDTA solution was used to remove the smear layer. Then the teeth were randomly assigned to four groups containing ten teeth each, as shown in Table 1

For group G-1 and G-2, Xeno-V adhesive was applied inside the root canal with a disposable micro brush, excess was removed with paper points. The adhesive was light polymerized for 20 seconds.

In group G-1 The dual cure resin Rely X ARC was applied and Tenax aesthetic fiber post was inserted into the canal. The resin cement was polymerized with Coltolux LED curing unit. However, for group G-2, the adhesive and post were similarly placed but it was not light cured. 

For group G-3 and G-4, three step adhesive Scotch bond multipurpose plus was used. Etchant applied for 20 seconds rinsed with water and dried. Primer was applied and cured for 20 secondsthe adhesive was light polymerized for 20 seconds. In group G-3 the dual cure resin Rely X ARC was applied and Tenax aesthetic fiber post was inserted into the canal. The resin cement was polymerized with Coltolux LED curing unit. For group G-4, the adhesive and post were similarly placed but it was not light cured.

All the roots were stored at 37⁰ C in distilled water for 1 week.

Push out bond strength testing

The coronal portion of each root was sectioned perpendicular to the long axis with diamond disc in low speed with water spray to get 2.2 to 2.5 mm thick slices. For the push-out test, the specimens were mounted in a customized device (a stainless steel base with a 2.5 mm diameter central orifice) fixed to the lower part of the universal testing machine (Lloyd ,LR 50K United Kingdom.) The metallic test point (1.0 mm diameter) was attached to the upper part of the universal testing machine and centralized in relation to the fiberreinforced resin post. The load was applied to the apical side of the root slice to avoid resistance to movement of the post due to post space taper. The push-out strength was performed at a speed of 0.5mm/min until dislodgement of the post. This was further confirmed by the appearance of a sharp drop along the load/time curve recorded by the testing machine. The push out test values was recorded in Newtons. The results were statistically analyzed by using one way analysis of variance (ANOVA). 

RESULTS

The mean push out bond strength (Table 2) was higher for groups G-3 and G-4 where Scotch bond Multipurpose plus bonding agent was used.

Test of significance by one way ANOVA showed significant differences among the groups (p<0.05). Scheffe multiple comparison showed that there were no significant differences in mean values between Group 1 and 2 and between Group 3 and 4 (p>0.05). However statistically significant difference (p<0.05) was found between Groups 1 and 3, Groups 1 and 4, Groups 2 and 3 and Groups 2 and 4.

DISCUSSION

Intra-radicular posts have been used to provide anchorage for dental restorations for over 250 years¹⁴. The use of posts in cases where the tooth structure has been destroyed due to caries, trauma, or over aggressive endodontic procedures is gaining widespread acceptance among dental clinicians^15,16.

The most common disadvantage of post cores, is poor retention of the posts. The cement, post material, surface structure, post length and the amount of remaining tooth structure all affect the retention of the post¹⁷. Establishing good bonding of the post to the root canal is essential.

A variety of experimental tests were described for the evaluation of the bond strength between root dentin and posts, such as the pull-out test, the micro tensile test and the push-out test. The pulling out of the whole post from restored roots requires high loads and results in testing large adhesive interfaces, where the stress distribution is highly non-uniform¹⁸.

The trimming and non trimming modalities of the microtensile technique have been used to obtain specimens from root dentin. The most remarkable findings in the data collected with the trimming variant of the micro tensile technique were the extremely high number of premature failures and the large spread of the data distribution. In nontrimming technique most of the beams failed while being cut, suggesting that the root–post bond strength was too low to resist the stresses transmitted to the interfaces, particularly when sectioning at the post periphery.

When compared to pull-out and micro tensile bond strength testing data variability is low and less force is needed for push-out bond strength tests. Taking into account the relative weakness of the post–root bond, the push-out test seems to be the most accurate and reliable¹⁹.

In this study posts cemented with total etch bonding agent showed higher bond strength than those using self-etch adhesives. Self-etching primers, which have a weaker etching capability, do not result in complete elimination of the smear layer, and only a relative penetration through the smear layer may be seen. Self-etch adhesives often leave a residual smear layer in between adhesive material and dentin.²⁰

SEM micrographs of the studies demonstrated more numerous and longer resin tags by the etch and rinse system in all the three regions of the root canal compared to the corresponding root regions in self-etch and self-adhesive luting agent groups. Therefore, the bond strength in the adhesive systems using a separate etching step is significantly greater than that in the systems without a separate etching step²¹.

CONCLUSION

Within the limits of this study it may be concluded that Scotch bond multipurpose plus adhesive exhibited the higher bond strength values than Xeno-Vadhesive, the mode of activation light curing or autopolymerizaion for curing dual cured resin cement had no effect on the push out bond strength.