The flow rate of endodontic sealers in various consistencies

The experimental group prepared samples of sealers by No. 57 A) Endomethasone N in liquid:powder ratio of 1:5, 1:6, 1:7 (standard), 1:8 and 1:9 according to the manufac-turer brochure depending on the clinical situation; B) Roth 801 as 1:7 (standard) and 1:8 mixtures C) Tubliseal EWT as standard preparation (base-catalyst 1:1). A volume of 0.05 ml sealer was spread on the glass plate and upon applying the load of 2 kg the diameter of sealer was measured. In the control group the sealer samples were loaded only by the weight of glass plate (0.1kg). for the (d>20mm) (En-domethasone The thin of


INTRODUCTION
One of the main roles of endodontic sealer (ES) is lubricating canal instruments and gutta-percha cones during its insertion and compaction. This feature is directly dependent on its velocity over the surface (dentin). The appropriate flow of ES is important to be able to reach the most distant labyrinth spaces (apical ramification, recurrent canals, pulpoperiodontal communications, cul de sac, irregular ampular spaces and fracture gaps etc.) and hermetically seal endodontic space [1]. One study has confirmed that presence of residual irrigant could accelerate sealer's setting time and slow down the sealer flow. Temperature rise during gutta-percha compaction has similar effect on setting time of ES resulting in slowing down the flow rate (warm compaction technique) [2]. The flow rate of ES is also dependent on material nature. In general, thinner consistency creates the higher flow rate and vice versa [3].
One study confirmed that denser ES had lower flow, where higher viscosity increased possibility of creating pools (reservoirs) within sealer itself and prevented sealer to line all canal walls affecting homogeneity of root canal filling. This study also showed that thinner ES created pores on dentinal wall-sealer junction as well as between canal wall and gutta-percha cones [4]. Sealer's flow is also correlated to the canal shape, width and convergence where for very narrow accessory canals and dentin tubules of diameter < 500 µm capillarity rule may be applied. The flow rate is also dependent on dentin structure. It is known that dentinal tubules become narrower with age but ES flow is also affected in vital tooth (filled by tubular liquor) [5].
Some authors reported different ES flow rate of AH Plus along dentinal walls after the use of different irrigants where Chlorhexidine improved while Cetrimide decreased sealer flow. The explanation for higher flow is the presence of liquid vehicular-carrier incorporated in Chlorhexidine while powder component of Cetrimide likely caused higher friction [6]. Some authors examined different compaction forces on gutta-percha cones (3.0-24.3 kg, 8N, 10-25 N, 20 N) where higher values were attributed to the compaction techniques and lower ones to mono-cone method. Higher values of ES flow rates were to be expected with higher compaction forces [3,[7][8][9][10].
A variety of experimental models have been used to measure the ES flow rates. One of them is microscope examination of dentinal tubules and pulp-periodontal communications filled by ES of different fluidity [2,11,12]. Kontakiotis et al. investigated the values of contact angle for Roth 801, AH26, RSA, Gutta Flow dropped onto the dentin surface indicating inverse correlation of their flow and contact angle (higher the flow, lower the contact angle) [13]. Japanese authors compared two study models for the same sealer: flow of sealer down the vertical glass slab (time and length) and spreading the sealer drop between two horizontally pressed plates (diameter of circle). A discrepancy in obtained results with these two methods for tested sealers (Sealapex, AH26, Canals and CH61) has been found [14]. Some authors used passive leaking through the rotational viscometer (round in shape) and examined rheological parameters: pseudoplasticity, i.e. increase of viscosity stress due to the increase of fluidity shear rate [15,16].
The aim of this research was to evaluate the flow of three zinc oxide-eugenol endodontic sealers of different consistency (density) under the load of 2 kg. The null hypothesis was that there was no difference between the flow rates among three standardly prepared sealers and no differences in flow rate between standard mixture and mixtures with different powder to liquid ratios.

Materials
The study included the following endodontic sealers: Endomethasone N (Septodont, St. Maur, France), Roth 801 (Roth Inter. Limit.) and Tublisel EWT (Kerr Romulus, Michigan, USA), all being zinc-oxide-eugenol based materials. The materials were mixed according to the manufacturer recommendation; eight samples of each ES were prepared. The powder to liquid ratio for standard mixture of Endomethasone N samples was 1.5 gr: 3.0 gr (standard mixture assumed one scoop powder and two oil drops that was the ratio of w/w = 1:7). The next Endomethasone N mixtures were made in very thin (1:5), thin (1:6), thick (1:8) and very thick consistency (1:9). Roth 801 sealer was prepared as standard mixture (1:7) and thicker mixture (1:8) with weight ratio powder/liquid (w/w) 0.13 gr: 0.03 gr. Tubliseal EWT was prepared by mixing equal parts of base and catalyst from original tubes. This variant was chosen due to the prolonged setting time that experiment required. The accurate weights of sealers' components were measured using the digital scale (with error of ±0.0005 gr) -Mettler PE 360, Germany.

Groups
The experiment was done using a pair of glass plates according to ADA specification No.57. and ISO standard number 6876/2001 [17]. The sealers were aspirated by insulin-graduated syringe with barrel of 2 mL. The amount of 0.05 ± 0.025 mL sealer was immediately injected after mixing on the middle portion of glass plate and spread in circle using dental probe to the size of 10 mm in diameter. Three minutes later another glass plate (120 gr) was gently placed over sealer ( Figure 1). The weight of 2 kg was then placed over second plate for each sample. Two-plates system was fixed by metal rings for the next 7 minutes.

Measurments
The biggest and the smallest diameter of each sample of spread sealer was measured using orthodontic ruler during 11th minute after mixing the sealer and immediately after removing the 2 kg weight. ADA standard required 10 minutes to display real flow rate. The sample that exhibited discrepancy of maximal and minimal diameter more than 1 mm was discarded. An orthodontic ruler was used to measure sealer diameters with accuracy of 0.5 mm ± 0.025 mm (error).

Statistical calculation
Student t-test, Boniferroni and Post-hoc test were used for statistical comparison of results within the control and experimental groups at the confidence level of 0.05.

RESULTS
The mean values of sealers diameters are presented in Table 1 and 2.
There was statistically significant difference in sealer diameter between Endomethasone N and other two sealers as well between Roth 801 and Tubliseal EWT (p < 0.05).
The obtained results of spread sealers diameter loaded by the glass plate weight only (control group) are shown in the

DISCUSSION
Our results showed the difference in the flow rate among tested sealers with the load of 2 kg, therefore, the null hypothesis can be rejected. In addition, the null hypothesis stating there was no difference in the flow rate between standard mixture and mixtures with different powder to liquid ratios was rejected due to found statistically significant difference between flow rate of 1:5 and 1:8 mixtures.
The study used protocol similar to the Grossman model [18] that is simple and allows easy comparison of results [3,[19][20][21]. The calculated SD values were less than 30% for experimental (13.0-29.8%) and control group (15.5-19.8%). This indicates homogeneity of the results and satisfactory measuring precision.
Literature data for root canal pressure during obturation was within the span of 8 to 35 N (0.8-3.5 kg) therefore we used 2 kg weight to be comparable to the similar literature results [3,[7][8][9][10]. In our study ISO protocol for the endodontic sealer flow was respected hence minimum 20 mm of diameter of spread sealer was noted [17]. Gambarini et al. results for the sealers: Roeko Seal Automix (Polyvynilsiloxan) 32.7 mm, Bioseal (ZOE sealer) 38.5 mm and Real Seal (composite resin) 37.9mm, are also in accordance to ISO requirements and similar to results in our study [19]. The differences in final results of the flow in different studies are the consequence of the experimental condition and can be correlated to the weight of cover glass plate (30-120-500 gr), additional load (1.0-3.5 kg), the amount of placed sealer on the plate (0.05 ± 0.005 -0.5 ± 0.05ml), pressure exposure time (30 sec to 10 min) and humidity values [3,[7][8][9][10].
Considering the depth of tubular penetration, Balaguerie et al. using SEM analysis found deeper flow of AH Plus sealer than Acroseal, RSA, Endobtur and Ketac-Endo. The authors explained lower flow of these sealers due to warm gutta-percha that created coagulant particles in hot sealers resulting in harder penetration along the dentinal tubules [20]. Our investigation satisfied ADA Number 57 standard (d > 20 mm) for each sealer in all experimental groups (22.1-39.9 mm) with loads of 2 kg.
In vitro study of Candeiro et al. revealed better flow of calcium-silicate sealer Endosequence BC Sealer (27 mm) than AH Plus (21 mm). The authors explained superior flow of first sealer due to its smaller particles although both satisfied ISO 6876/2001 standard [21].
The different ratio of powder in tested sealers in the current study resulted in corresponding diameter values for spread sealers (flow). Endomethasone N showed significantly lower diameter (around 30%) than Roth 801 that is in accordance to the results of Camps et al. [22]. They also compared the flow of ZOE-based sealer Esthesone and Pulp Canal Sealer (thicker and thinner mixtures) and obtained higher flow rate in thinner samples. Furthermore, they found significantly lower fluidity of tested sealers (31 mm and 40 mm) than manufacturers' referent values. Their study also showed that very fluid mixture of sealer (of low density) placed in root canal during application of mono-cone technique required around 80% lower pressure than in cases of the same sealer of higher density. This fact indicates the choice of obturation technique (that exerts different stress values to dentin canal walls) in various canal systems (wide opened apical foramen and thin root canal walls) [22].
Balaguer et. al. explained high flow rate of sealer in cases of thin prepared mixture by more present liquid (oil) constituent that lowers the friction i.e. viscosity (shear stress) over the surface [20]. Analyzing the mixtures with component ratio from 1:5 to 1:9 samples where the powder percentage is different for about 50%, statistically significant difference was found in sealer's diameters. Mendonca et al. reported similar findings in their study using comparable variations of powder and liquid components in ZOE-based sealers (Endomethasone N, Grossman sealer and Tubliseal). In low-density sealer samples they found smaller sealer penetration into dentinal tubules due to less resistance related to higher liquid percentage [23] and that was in accordance with other investigations [3,24].
Manufacturers instructions suggest using different density of sealers (thicker or thinner) (Endomethasone N, Roth 801 cement), what was included in experimental, mostly in in vitro studies [3,22,24]. To achieve more efficient and accurate dosing in standard manual mixing of ZOE-based sealers, it is mostly used as two-component paste (Tubliseal, TublisealEWT).  The SEM study of Mamootil et al. found slightly lower penetration of ZOE-based Pulp Canal Sealer into dentinal tubules in comparison to resin-based endodontic sealers [25] that is not in accordance to Gambarini et al. results [19]. The discrepancy might be due to different methodologies of these two studies. Namely, SEM examination of dentinal histological sections in the study of Mamotil et al. was analyzed used high magnification while Gambarini et al. used loupes only. Mamotil et al. in their methodology protocol did not mention patient's age (teeth donors) or teeth age (deciduous or permanent dentition) as well as angle of dentin section in coronal area that may have significant influence on tubule diameter and consequently flow rate of ES.
Mutal et al. stressed out the importance of gas bubbles that were created more in sealers with lower flow rate during obturation. They noted pores and vacuoles in diameter of around 500 µm, more present in thicker sealers (resin-based and glassionomer-based) than in ZOE-based sealers. They concluded that good obturation requires denser sealers because bubbles become entrapped within sealer and do not reach the canal wall. Therefore, the chance of microbial percolation between oral cavity and dentin tubules is then lower [2]. One study reported expansion of gutta-percha cones after using different eugenol concentration in ZOE-based sealer (Pulp Canal Sealer EWT). The increase of eugenol oil resulted in volumetric change of gutta-percha cones and higher sealer flow, i.e. better tubular penetration confirmed by spiral computerized tomography [26]. Tiwari found that AH Plus samples had optimal values for sealer penetration into dentin tubules as well as improved hermetic and antimicrobial effect compared to Perma Evolution paste (mixture of calcium silicate and calcium phosphate). This study analyzed the size of particles and concludes that flow rate is inversely proportional to the granule size [27].
Passing the sealer through the round opening is interesting model for flow analysis and it did not show significant difference between Roth 801 and TubliSeal EWT [16] while experimental model with two glass slabs (ISO 6876/2001) displayed different "flow" values in sealers regardless of 2 kg weight.
The differences in flow values are only to be explained by different experimental protocol. The differences in viscosity of sealers resulted in corresponding diameter of spread sealer (flow). In addition, Ono et al. involved gravity force as an important factor in fluidity of sealer placed to roll down the vertical glass slab. Our study included only friction factor without gravity due to its horizontal position as ADA standard required [17]. In our study humidity and heat factor (cold/warm gutta-percha) were not analyzed but they are known to affect flow and penetration of sealer (warm compacted gutta-percha vs. mono-cone technique or Thermafil) [27,28]. Relatively new calcium phosphate and calcium-silicate mixture, Capseal, showed lower flow values than AH Plus and Sealapex sealers using ISO 6876 standard. This result is caused by material structure itself. Namely, AH Plus and Sealapex have significantly smaller granulation of particles than Capseal resulting in better flow and spreading power [29].
One of the promising calcium-silicate sealers, MTA Obtura, did not show statistically significant difference in flow rate compared to Sealer 26 although both satisfied ADA No. 57 requirements [30]. Kyung et al. found that only bioceramic-based sealer Endosequence BC created smaller diameter of around 18 mm, while others (AH-Plus, AD Seal, Radic-Sealer, EndoSeal MTA, MTA Fillapex, AD Seal i Radic-Sealer) showed satisfying diameters (more than 20 mm) [31]. Jeanneau C et al. investigated anti-inflammatory effect of Endomethasone N and Pulp Canal Sealer and found that only standard or thick consistency (adequate saturation of powder with eugenol) had effect on lessening the secretion of interleukine-6 [32].
Some authors analyzed the sealer flow using ISO standard and viscosity of sealers using rheometer device and found the latter one to be more accurate (ρ = -0.8618). The greatest flow was found for Pulp Canal Sealer EWT and then for AH Plus, Sealapex and Capseal sealer [33].
It is to note that heterogeneity of results about flow rate of various sealers indicates to follow the proposed standard protocols in order to get comparable results. As for clinical aspect, the density of ES should be in function of every single case and chosen obturation technique.

CONCLUSION
All tested sealers showed satisfactory flow level. The greatest flow was observed in Tubliseal EWT paste, then Roth 801 cement and the lowest in Endomethasone N.