|Year : 2012 | Volume
| Issue : 2 | Page : 111-115
Evaluation of push-out bond strengths of two different adhesive root canal obturation systems: An in vitro study
Lakshman V Uppalapati, Jyothi Mandava
Department of Conservative Dentistry and Endodontics, GITAM Dental College and Hospital, Visakhapatnam, Andhra Pradesh, India
|Date of Web Publication||11-Jul-2012|
Department of Conservative Dentistry and Endodontics, GITAM Dental College and Hospital, Gandhi Nagar, Rushikonda, Visakhapatnam - 530045, Andhra Pradesh
Background: Bonded obturations of the root canal are important to eliminate any space that allows the percolation of fluids, to resist disruptions in the filling due to mechanical stresses, and to possibly reinforce roots.
Aim: To evaluate the regional bond strength of two adhesive endodontic obturating systems - EndoREZ and RoekoSeal.
Methods and Materials: Thirty extracted human maxillary central incisors were decoronated at the cemento-enamel junction. After cleaning and shaping of the root canals, the samples were randomly divided into two groups of 15 specimens each. In group A, samples were obturated using EndoREZ sealer and size #35, 0.06 taper EndoREZ points. In group B, samples were obturated with RoekoSeal sealer and size #35, 0.06 taper gutta-percha points. Each root was then sectioned into one mm thick slices with the help of a diamond saw under water coolant. The root slices were then subjected to push-out bond strength test in universal testing machine at a crosshead speed of 0.5 mm/min. After calculating the bond strength for each slice, data was subjected to student paired t-test.
Results: The results showed statistically significant (P value < 0.05) higher mean bond strength for EndoREZ, when compared to RoekoSeal. Bond strength values showed a slightly increasing trend from coronal to apical regions, though the values are statistically insignificant.
Conclusion: Under the conditions and scope of this study it can be concluded that EndoREZ performed better with higher mean push-out bond strength than RoekoSeal.
Keywords: Methacrylate resin sealers, Push-out bond strength, Regional interfacial bond strength, Self-adhesive sealers
|How to cite this article:|
Uppalapati LV, Mandava J. Evaluation of push-out bond strengths of two different adhesive root canal obturation systems: An in vitro study. J Dr NTR Univ Health Sci 2012;1:111-5
|How to cite this URL:|
Uppalapati LV, Mandava J. Evaluation of push-out bond strengths of two different adhesive root canal obturation systems: An in vitro study. J Dr NTR Univ Health Sci [serial online] 2012 [cited 2013 May 22];1:111-5. Available from: http://www.jdrntruhs.org/text.asp?2012/1/2/111/98356
| Introduction|| |
The main goals of endodontic treatment are to eliminate infection within the root canal system and to avoid its recontamination, which are sought by cleaning, shaping and filling procedures.  The use of a root canal sealer with a thermoplastic core filling material like gutta-percha is considered as a standard procedure in endodontic obturation. Recent concepts in obturation suggest that sealers should demonstrate adhesive properties to dentin and core material, thus decreasing the chances of endodontic treatment failures. Though resin bonding in dentistry was introduced in the mid-1950s by Buonocore, initial attempts at using low viscosity resins in endodontics were reported only in 1978 by Tidmarsh. 
The role of an endodontic sealer in maintaining the integrity of the apical seal is of paramount importance when the placement of an intra-canal retainer is required. In this regard, during mechanical preparation of the post space, root canal filling material might be dislodged, creating voids in the obturation that will affect the quality of the seal.  As noted by Adams et al. in 1997 "Strengthening of the roots with an adhesive joint should be reflected by improvements in interfacial strength and dislocation resistance between the root filled material and intra-radicular dentin, which may be evaluated by thin-slice push-out tests."  The push-out test intends to assess the extent to which the sealer and core material are bonded together into a solid mass, as well as the strength of the bond to the canal wall. Thus, its results can be interpreted as interfering with the stress suffered during post space preparation. 
Although predictable clinical results have been reported with the use of gutta-percha and epoxy resin based adhesive obturations, there is an increasing interest in the use of methacrylate resin based sealers in endodontics. These sealers may be used with dentin adhesives for bonding to intra-radicular dentin and hydrophilic resin monomers may be incorporated into the root canal sealers to facilitate better resin penetration into the dentinal tubules.  Simultaneously, development in the sealers aimed at countering the polymerization shrinkage of the resins led to the development of silicon based sealers which do not shrink but expand slightly (0.2%). The ability of these materials in improving the resistance to displacement of endodontic fillings has not been addressed much so far. The present study was done to evaluate the bond strengths of two adhesive endodontic obturating systems incorporating; EndoREZ - a hydrophilic, urethane dimethacrylate sealer with resin coated gutta-percha i.e. EndoREZ points and RoekoSeal - a polydimethylsiloxane or silicon based sealer with gutta-percha.
| Materials and Methods|| |
Thirty freshly extracted, periodontally involved, caries free human maxillary central incisors, having a single patent root canal with closed apices were selected. All the collected human teeth were immersed in 5% sodium hypochlorite (NaOCl) immediately after extraction and left for two hours for surface disinfection and dissolution of the superficial soft tissue. Calculus was removed using an ultrasonic scaler and the teeth were then stored in distilled water.
Each tooth was decoronated at or below the cemento-enamel junction using a flexible diamond disk in a slow speed hand piece under copious amount of water coolant, so that all the roots were 12 mm in length. The working length for each root was established by inserting the #10 K file into the canal until the tip of the instrument could be visualized at the apical foramen and then subtracting 0.5 mm from this length. The cervical portion of the root canals were prepared by using #2 and #3 Gates glidden drills, then the root canals were instrumented by means of endosequence 0.06 taper NiTi instruments to apical size #35. During instrumentation canals were irrigated with two ml of 3% sodium hypochlorite using hypodermic syringe with a 27 gauge needle. Prior to obturation, each canal was irrigated with three ml of 17% EDTA solution for one minute and 3% NaOCl to remove the smear layer. Canals were then irrigated with five ml of 10% ascorbic acid solution, followed by five ml of normal saline as a final rinse.
The 30 root samples were then randomly divided into two groups (15 samples in each) according to the obturation systems used. First 15 root samples in group A were obturated using EndoREZ sealer and size #35, 0.06 taper EndoREZ points according to the manufacturers' instructions. Obturation was completed by introducing accessory points using the lateral compaction method. Excess gutta-percha was removed with heated plugger and the coronal surface of the root filling was light cured for 40 seconds to achieve an immediate coronal seal. In group B, 15 samples were obturated with RoekoSeal sealer and size #35, 0.06 taper gutta-percha points using lateral condensation technique. Coronal seal was obtained by light curing the coronal part. On completion of the obturation procedure, the specimens were radiographed to check the compactness of root filling and the presence of voids. All the obturated samples were then stored at 37°C in 100% humidity to allow complete polymerization of the sealer.
Obtaining Root Sections
After one week, all the root samples were embedded in methacrylate resin blocks. Each root was sectioned into six one mm thick slices i.e. two slices each for coronal, middle and apical thirds by using the markings made on the roots as a guide. Uniform root slices were obtained with the help of a slow speed diamond saw (Leica Saw microtome, Lieca SP 1600 Nussioch, Germany) under water coolant. This resulted in 83 slices for group A (EndoREZ) and 82 slices for group B (RoekoSeal), as seven samples from group A and eight samples from group B were lost during the slicing process. The thickness of each root slice was measured and verified to an accuracy of one mm ± 0.05 mm by means of a digital caliper. The coronal surface of each slice was marked and positioned on a base with a central hole, in a universal testing machine (Dak system Inc UTB 9103-ACHiTech, Mumbai) for the push-out test.
Bond Strength Evaluation
The punch was lowered at a crosshead speed of 0.5 mm per minute making sure that it contacted only the obturation material, generating shear stresses on the obturated areas to be deboned. The load was applied in an apico-coronal direction so as to displace the root filling towards the larger coronal part of the root slice. The peak load at dislodgement was noted for all the samples using real-time computer program. To determine the mode of failure, the specimens were observed under 100X magnification with an inverted microscope (Olympus Corporation, Japan). Push-out bond strength in megapascals (MPa) was calculated using the following formula:
The data was transferred to Microsoft Excel software and statistical analysis for group wise comparison was done by student t-test. The independent variables for the regional bond strengths at coronal, middle and apical thirds of the root samples were analyzed.
| Results|| |
There was statistically significant difference between the two test groups for the displacement resistance values which is in favour of group A (P < 0.05) [Table 1]. The mean bond strength of group A (EndoREZ) was significantly higher (1.39126 MPa), when compared to group B (RoekoSeal) (1.19550 MPa). Comparisons in terms of root segments revealed an increase in bond strength values from coronal to middle and middle to apical root specimens in both the groups, though the variations were statistically insignificant (P > 0.05). According to the results, in terms of the sealers used, RoekoSeal had lower bond strength to root dentin [Figure 1].
|Figure 1: Representing regional bond strength analysis of groups: A (EndoREZ) and B (RoekoSeal)|
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|Table 1: Bond strength comparison between groups A (EndoREZ) and group B (RoekoSeal)|
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| Discussion|| |
Adhesion of root canal filling material to dentinal walls is important in both static and dynamic situations. In a static situation, it should eliminate any space that allows the percolation of fluids between the filling and the dentinal wall. In dynamic situation, it is needed to resist disruption of the filling because of mechanical stresses caused by tooth flexure, restorative procedures or post space preparation.  By establishing monoblocks between the intra-radicular dentin and adhesive root fillings, potential improvements in sealing ability and strengthening of endodontically treated teeth may be anticipated. 
The hydrophilic polymer based resin material as root canal sealer may obtain better dentinal tubular penetration, bond to the collagen matrix and consequently increase adhesive strength to dentin. However, the root canal system has an unfavorable geometry for resin bonding.  Sodium hypochlorite has been widely used as an irrigant in endodontic therapy along with EDTA for smear layer removal. Nikaido et al.  and several other studies demonstrated that the bond strength to root dentin significantly decreases with NaOCl irrigation. Studies by Vongphan et al. and Weston et al. revealed that usage of reducing agent like ascorbate significantly increased the bond strength of sealers to root dentin contacted by NaOCl. , Thus in the present study optimum bonding was achieved by using 10% ascorbic acid.
It has been suggested that push-out test provides a better evaluation of bond strength than the conventional shear test because using the push-out test, fracture occurs parallel to the dentine-bonding interface, making it a true shear test for parallel-sided samples (Drummand et al. 1996, Ureyan Kaya et al. 2008). Interfacial strength and dislocation resistance between the root filling material and the intra-radicular dentin have been successfully evaluated using thin-slice push-out tests (Gesi et al. 2005, Skidmore et al. 2006). 
Few studies have investigated the bond strength of EndoREZ to root dentin. The mean bond strength of 1.39126 MPa achieved for EndoREZ group in this study is in corroboration with a study by Ayce Unverdi Eldeniz et al., (2005), who demonstrated mean shear bond strength of 1.17 MPa (±0.36) to smear free dentin surface.  Higher bond strengths of around 2.5 MPa with EndoREZ were reported in a thin slice push-out bond strength analysis by Bouillaguet et al.  The results of Fisher et al.,  who reported a bond strength for EndoREZ of 0.09 MPa 0.07 were significantly low in comparison with the present study. Even though the methodology was similar to the present study, usage of smaller diameter plunger (0.45 mm) in their study for push-out bond strength analysis in all the coronal, middle and apical segments would have led to non-uniform stress distribution and premature failures. The single cone obturation used in their study might also have led to low bond strength values. Further, the advantage of 10% ascorbic acid in reversing the oxidative substrate might have led to better bond strengths in the present study. Superior performance of EndoREZ system may be due to better infiltration of hydrophilic monomers into the radicular dentin and simultaneously attaining better bonding to resin coated gutta-percha.
Polydimethylsiloxane or silicon based sealer-RoekoSeal, has been developed for obturation of the root canals with promising initial clinical data. The expanding (0.2%) nature is claimed by the manufacturers to help in filling lateral canals and dentinal tubules leading to high sealing ability. An 18-month longitudinal study on leakage of RoekoSeal showed minimal leakage.  Both the sealers tested in the present study showed measurable adhesive properties, though RoekoSeal with mean bond strength of 1.19550 MPa was inferior to EndoREZ.
There is no consensus among the authors regarding regional variations in bond strength along the root dentin. Very few studies evaluating bond strength of obturation to root dentin, detailed the regional variations. In the present study location of the section in the root did not have a significant (P > 0.05) influence on bond strength, which correlates well with other studies by Fisher et al.  and Bouillaguet et al.  Slight increase in bond strengths from coronal to apical regions observed in the present study correlate with the study by Sly et al.  Following the manufacturers' instruction for immediately light-curing the coronal part of the root filling to create a coronal seal might cause rapid polymerization and prevent stress relief by resin flow.  Light curing the coronal part, absence of the effect of oxygen inhibited area in the coronal regions and the presence of larger volume of sealer could be the reasons for lower bond strengths coronally.
| Conclusion|| |
Within the limitations of the present study, methacrylate resin based sealer EndoREZ has better bonding when compared to the silicone based sealer RoekoSeal. Though slight increase in the bond strengths from coronal to apical region could be noticed, significant regional variations between the coronal, middle and apical thirds of the root were not demonstrated.
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