Push-out Bond Strength Evaluation for Different Endodontic Sealers (A Comparative Study)

Aim s : The aim of this study was to compare the push-out bond strength values between four different types of sealers. Materials and method: Forty extracted straight, single-rooted, sound human mandibular first premolar teeth were selected for this study. The crown portion of each tooth was decoronated to the level of the cement-enamel junction (CEJ) to standardize the root length to 16 mm. The working length was measured and the canals were instrumented using Nickel-Titanium ProTaper Universal Rotary System up to size F3. The canals were rinsed with 2.5% sodium hypochlorite followed by 17% EDTA, then the samples were divided into four groups (n=10) according to the sealer to be used with F3 gutta-percha. These are (AH Plus, GuttaFlow 2, GuttaFlow Bioseal and MTA Fillapex) sealers. The pushout bond strength test was performed and the data were analyzed using one way ANOVA, and post hoc Duncan’s multiple range tests at (p ≤ 0.05). Results: There was a statistically significant difference in the push-out bond strength among the four sealers’ groups at (p ≤ 0.05). The highest mean value of bond strength (3.605) MPa was shown in the AH Plus group. Bioseal group produced (1.833) MPa followed by (0.822) MPa for GuttaFlow2 group. The lowest value (0.645)

This bioactive glass contains silica, sodium oxide, calcium oxide, and phosphorus oxide. GuttaFlow Bioseal has a nanosilver component also instead of a microsilver.
These bioactive components are able to form hydroxyapatite crystals when they contact tissue fluids so that they could result in stimulation of tissue regeneration and healing (13)(14) .  (15) .
Push-out test has been commonly used to assess the dislodgement resistance of the filling materials and the sealer-dentin bonding strength (16) .
The study aimed to compare the push-out bond strength of four different sealers (AH Plus, Gutta Flow 2, GuttaFlow Bioseal and MTA Fillapex) to root dentin. The surface area was measured by the following formula:

DISCUSSION
Sealers play a crucial role in the obturation process and they can influence the quality of the root canal treatment (17) . The sealer bond strength to root canal dentin wall is a very desirable property because it helps keep the integrity of this sealer-dentin interface without disruption in long term (18) . In the present study, throughout canal preparation, irrigation is done using (3ml) of (2.5%) sodium hypochlorite (NaOCl) since it is the simplest available endodontic irrigant having an organic tissue dissolving property. Inorganic component of smear layer was removed using (5ml) of (17%) EDTA remained in the canal for 1 minute since it may cause peritubular and intertubular erosion of the dentin if it is applied for more than this time (19) . The combined use of these two irrigants represents the most commonly used protocol in clinical practice (20) .
After that, sodium hypochlorite (NaOCl) was used to ensure the complete removal of the remnants of EDTA from the canal. This irrigation protocol yields more exposure of dentinal tubules and dentinal collagen network, thus enhances the bond strength for most sealers to dentin via increasing sealers' penetration into the dentinal tubules and their mechanical interlocking to the root canal wall (21)(22) . The canals were finally washed with distilled water in order to eliminate the negative effect of sodium hypochlorite (NaOCl) as it is a potent agent that leaves behind on the dentinal surface an oxygen rich layer leading to the reduction in the bond strength value (23) .
In the current study, four different types of sealers were examined, therefore the use of gutta-percha as the main core material was to be considered as a constant, also to firmly simulate the clinical conditions (20) (24) . In the present study, three sizes of plungers were used for each root third (apical, middle and coronal) in order to completely cover the core material in each third of the sample. The plunger was centralized to avoid contact with dentin and the force was applied on the obturating material in an apical to the coronal direction in order to avoid any constriction interference caused by root canal taper (25) . The dentin thickness used for push-out test has also been considered a variable in several studies. Thin slices, about 1mm, are in risk of sealer detachment during slicing as mentioned by Gesi, et al., 2005 (26) . A 2mm thickness slices were used so as to avoid premature debonding (25) .
According to the findings of this study, AH Plus has shown the greatest bond strength value in comparison to the other sealers used in the study. This performance of AH Plus sealer is well documented in many studies (2)(13) (27)(28) .
This can be related to the inherent volumetric expansion property of AH Plus sealer that can form a covalent bond between open epoxide ring of epoxy resin sealer and the exposed amino-groups of radicular dentin (29) . In addition, it could be discussed that AH Plus displays a significant cohesion between its molecules which could be translated to a great adhesion property. Also, AH Plus deeply penetrates the micro irregularities as a result of its creep capability, excellent flowability with a long polymerization time. This can increase its mechanical interlock with the radicular dentin and its dislodgment resistance (30) .
The lower bond strength of GuttaFlow 2 can be related to the hydrophobic nature of root canal dentin following the use of chelating agent (EDTA). The use of EDTA alone or combined with NaOCl decreases the surface free energy of radicular dentin because it removes the inorganic part of dentin and demineralizes the intertubular and peritubular dentin leading to collagen fibers exposure and patent dentinal tubules with increased surface roughness, hence making the dentinal surface hydrophobic and decreases its wettability that interferes with the wettability and thus the bonding of GuttaFlow 2 (31)(32) . Also, the presence of silicone resin in the composition of Gutta Flow 2 can increase their surface tension, resulting in more difficult flow and spreading of these materials with poorer wetting effects (21) . Subsequently, it can be supposed that there is no chemical but only slight mechanical interaction between silicone-based sealers and radicular dentin (2) .
Regarding GuttaFlow Bioseal sealer, which is a novel polydimethylsiloxane sealer similar to GuttaFlow 2 but it contains calcium silicate, it was found under the conditions of this study that it has a bond strength lower than AH Plus but higher than both GuttaFlow 2 and MTA Fillapex. Once GuttaFlow Bioseal contacts with the tissue fluids, the bioactive material yields calcium silicate that will form a physical bond with the dentinal surface through the formation of hydroxyapatite interface deposits (34)(35) . After the release of such ions, tag-like structures will extend into the dentin. This will significantly improve the sealer's adhesion and hence the push out bond strength (36)

Conflicts of interest
There are no Conflicts of interest.