The Effect of Smear Layer on the Push-out Bond Strength of Silicone-based Root Canal Sealers

Aims: The study aimed to evaluate the effect of smear layer on the push-out bond strength of silicone-based root canal sealers. Materials and methods : Sixty extracted, 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 cement-enamel junction to standardize the root length to 16 mm. The working length was measured by subtracting 1mm from the visually determined canal length and the canals were instrumented using ProTaper universal rotary system up to size F3. The canals were divided into two groups according to the irrigating solutions used. Group.1 was rinsed with 2.5% sodium hypochlorite followed by 17% EDTA to ensure complete smear layer removal. Group.2 was rinsed using 0.9% normal saline to keep the smear layer. Then the samples were subdivided into three subgroups according to the sealer used with gutta-percha. These include AH Plus, GuttaFlow 2, and GuttaFlow Bioseal groups. The push-out bond strength test was performed using the universal testing machine and the data were analyzed using independent sample T-test at (p ≤ 0.05) to compare the results of each sealer group in the presence and absence of the smear layer. Results: For all sealers’ groups the results has shown a statistically significant difference in the bond strength at (p ≤ 0.05) between group 1 (without smear layer) and


INTRUDUCTION
The main goal of root canal treatment is to eliminate all vital and necrotic tissues from the root canal (1) . During root canal therapy a layer of material is formed with all kinds of cutting instruments. This layer consisted of organic and inorganic substances that cover the prepared root canal walls. It appears as an amorphous granular and irregular which was named as Smear layer (2)(3) . It may have some adverse effects regarding preventing irrigants and sealants from penetrating the dentinal tubules (4) . The smear layer was shown to be composed of particles ranging from less than 0.5-15 µm in size (5) . Removal of the smear layer may permit for more cleaning of the walls of the root canals and allow for better adaptation of the root canal sealers that enter the dentinal tubules.
Nevertheless, some studies decline the effect of smear layer removal on sealer bond strength (3) .
The invention of silicone-based root canal sealers was in an attempt to improve the properties of root canal sealers involving physical, chemical and biological leading to a series of material generations (6) . GuttaFlow 2 (Coltene/Whaldent, Switzerland) was available in 2012 as an advancement of the previous GuttaFlow material, having the same excellent properties but with a stiffer consistency and changes in the form of the silver particles used. GuttaFlow 2 is composed of a mixture of gutta-percha powder with particle size smaller than 30 µm, polydimethyl-siloxane, zirconium dioxide, platinum catalyst and a preservative of microsilver particles. This cold flowable system combines both gutta-percha and sealer, having a solubility close to zero (7)(8)(9)(10) . GuttaFlow2 also does not shrink, instead, it expands slightly by about 0.2%.
Also it adheres considerably to guttapercha points and to dentin walls (11) .

GuttaFlow
Bioseal (Coltene/ Whaldent, Switzerland) has been produced in late 2015. The main difference of this sealer than the other silicone-based sealers is in its composition as it has a bioactive ceramic glass in addition to gutta-percha, polydimethylsiloxane, zirconium oxide and platinum (12) . GuttaFlow Bioseal has a nanosilver component instead of microsilver. These bioactive components are capable to form hydroxyapatite crystals when they contact tissue fluids, so that they could result in stimulation of tissue regeneration and healing (13) (14) . Regarding working and setting times, they are shorter than that of GuttaFlow 2 (15) . It has a minimal solubility and a good alkalinizing ability with pH (8-9) that prevents the activity of osteoclasts and stimulates alkaline phosphatase enzyme, which in turn helps in the reformation of periapical bone.
The flow of this material is slightly lesser than that of GuttaFlow 2 (16) .
The sealer bond strength to the root canal dentin wall is a very desirable property because it helps keep the integrity of the sealer-dentin interface without disruption in the long term (17) . Recent theories of dentin bonding mechanisms include either smear layer modification and direct bonding to it, or smear layer removal and bonding to the tooth structure (18)(2) .
This study aimed to assess the effect of smear removal on the bond strength of silicone-based sealers.  The surface area was measured by the following formula:

+ h²]
Where is the constant 3.14, r₁ is the coronal radius, r₂ is the apical radius, h is the thickness of the section in mm. .

RESULTS
For AH Plus sealer group, there was a statistically significant difference at (p≤0.05) between groups A1 and A2 as shown in (Table.1

DISCUSSION
Treating the surface of root canal dentin with different endodontic irrigants during chemomechanical preparation can induce some alterations in its structure and chemical composition and changing its solubility and permeability. This affects the bond strength of endodontic sealers to root canal dentin (19) .
Many authors have suggested the successive use of organic and inorganic solutions as endodontic irrigants because no single solvent has proved to be able to remove the smear layer alone (2) . In the present study, irrigation of group 1 was 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 (20) . The combined use of these two irrigants represents the most commonly used protocol in clinical practice. In this study, three different sealers were tested, therefore the use of gutta-percha as a main core material was to be considered as a constant, besides to firmly simulate the clinical conditions (21) . A single master cone of guttapercha was placed with a sealer to provide a final compact mass with no spaces (22) . The push-out bond strength test was conducted in the current in-vitro study as it is one of the most reliable and reproducible techniques (23) . It can evaluate the root canal sealer materials even with low values of bond strength (24) .  (11) . The chelating solutions not only eliminates the smear layer, but it can demineralize the intertubular and peritubular dentin leading to collagen fibers exposure and patent dentinal tubules with increased surface roughness thus it decreases the surface free energy of radicular dentin (26) (27) .
The resultant coarse hydrophobic surface of the dentin makes the wetting of silicone-based sealers to the radicular dentin poorer. In addition, the silicone presents in GuttaFlow sealers can produce greater forces of surface tension which makes the material flow more difficult into the rough surface (28)  . After release of such ions, tag like structures will extend into the dentin. This will significantly improve sealer's adhesion and hence the push out bond strength (31) . So that, the maintenance of the smear layer leads to increase of dentinal wall moisture condition which could have a positive influence on the adhesion of GuttaFlow Bioseal material.

CONCLUSION
According to the types of the sealer used in this study and smear layer treatment condition, the push out bond strengths was influenced by sealer types and by existence or absence of the smear layer.

Conflicts of interest
There are no Conflicts of interest.