Introduction.
Microleakage can occur from a coronal direction in root-filled teeth (Madison et al. 1987, Swanson & Madison 1987, Magura et al. 1991). During root-canal treatment, it is important to create a seal in the access cavity in order to prevent the entrance of saliva and microorganisms into the root-canal system. However, several studies have reported that coronal microleakage can occur around temporary restorations (Saunders & Saunders 1990, Beckham et al. 1993, Carman & Wallace 1994, Beach et al.1996, Uranga et al.1999). If the coronal restoration becomes defective or is lost, the eventual contamination of periapical tissues could result in failure of endodontic treatment (Malone & Donelly1997).
To reduce leakage along temporary fillings and beyond, a variety of alternative methods have been suggested. These include an additional material placed into the canal orifices after removal of a portion of guttapercha and sealer (Roghanizad & Jones1996), or sealing the pulp chamber floor with a restorative material (Carman & Wallace 1994). Either technique should prevent the entrance of saliva and microorganisms into the root-canal system, and the escape of medicaments placed in the pulp chamber into the oral cavity.
Several investigations have assessed the sealing ability of restorative materials to prevent coronal microleakage; however, they have shown contradictory results (Noguera & McDonald 1990, Saunders & Saunders 1990, Beckham et al. 1993, Carman & Wallace 1994, Beach et al. 1996, Uranga et al. 1999). These differences are probably due to the different techniques and materials used for bacterial and radioisotope penetration over different periods of time (Dow & Ingle 1955, Noguera & McDonald1990,Torabinejad et al.1990).
Although it is impossible during laboratory studies to reproduce artificially the occlusal forces and variety of microorganisms observed in the oral cavity, it is important to simulate in vivo conditions as close as possible. Significant changes in temperature have been shown to adversely affect the marginal seal of restorative material owing to their different linear coefficient of thermal expansion. This may be the main factor in the aetiology of microleakage, and thermocycling should be included in any microleakage study (Oppenheimer & Rosenberg 1979).
The aim of this in vitro study was to evaluate the sealing ability of four materials placed on the pulp chamber floor as barriers to coronal microleakage.

Materials and methods.
One hundred extracted human mandibular molars with mature apices were stored in 10% formalin. After removal of calculus and organic debris, the teeth were individually stored in distilled water. Standard occlusal access cavities were prepared, and working lengths were determined visually by subtracting1mm from the point at which a size 10 ￠le just exited the apical foramen.
Cleaning and shaping procedures were executed using the hybrid technique (Valdrighi et al.1991).The coronal two-thirds of the canals were prepared sequentially with size 15-40 K-Flexofiles (Dentsply Maillefer Instruments, Ballaigues, Switzerland) and Gates Glidden burs sizes 2 and 3 (Dentsply Maillefer). The apical third was instrumented to a size 35 for mesial canals and up to a size 45 for distal canals. Root canals were further instrumented with step- back enlargement in1-mmincrements to three sizes larger than the master apical file. The irrigant solution was 5.25% sodium hypochlorite. The teeth were flushed with 17% EDTA  for 3 min to remove the smear layer. A final irrigationwith5 mL of5.25%sodium hypochlorite was performed.
The canals were dried with paper points and obturated with cold laterally condensed gutta-percha and Endomethasone sealer (Septodont, Saint-Maur, France). Sealer was mixed according to the manufacturer’s instructions. The master cone was coated with a thin layer of the sealer prior to placement into the root canal with a gentle pumping action at the working length. An endodontic spreader size B (Dentsply Maillefer) was placed into the root canal within 1mm of the working length, and lateral condensation was completed with accessory cones (Tanari, Tanamarim Industrial Ltda., Manacapuru, Brazil). Bucco-lingual and mesio-distal radiographs were taken to assess the quality of the root-filling. Excess gutta-percha was removed with a hot instrument and vertically condensed at the orifice of the canals.
The teeth were randomly divided into four experimental groups, one for each material, used for the coronal seal and a positive control group, not treated with restorative material. Each group had 20 specimens. All experimental groups had an approximately 2-mm thick layer of restorative material placed onto the chamber floor (Beckhamet al. 1993). A probe was used to measure the distance between the pulp chamber floor and the occlusal surface, 2 mm was then subtracted from this measurement and the restorative material was placed onto the pulp chamber floor to that depth. The probe was used to check the thickness of the materials.

• Group I: Glass ionomer chemically cured (Vidrion R, S. S. White Artigos Dentarios Ltda., Rio de Janeiro, Brazil).
• Group II: Reinforced zinc oxide temporary cement (IRM, L. D. Caulk Division, Milford, DE, USA).
• Group III: Zinc oxide and zinc sulphate hydrated temporary cement (Coltosol, Colte' ne, Altstatten, Switzerland).
• Group IV: Dentinal adhesive (Scotch Bond, 3M do Brazil Ltda., Rio de Janeiro, Brazil).
• Group V: No material (positive control group).

A2-mmthickness of the materials was used to seal the coronal access to the root canals. In groups I and II, the materials were prepared following the manufacturers’ instructions. The glass ionomer was placed into the pulp chamber by a syringe (Centrix Incorporated, Shelton, USA). IRM and Coltosol were placed with a number1 spatula.
In the Scotch Bond group, the specimens were etched with 37% phosphoric acid for 15 s, washed for 10 s and gently dried with cotton pellets. A thin layer of Primer (3M do Brazil Ltda., Rio de Janeiro, Brazil) and Bond (3M do Brazil Ltda., Rio de Janeiro, Brazil) was applied. In a pilot study, it was observed that, approximately two drops of Bond was equivalent to a 2-mm thickness of material. The dentine bonding agent was light cured for 20 s. If the two drops of Bond were insufficient, one more drop was placed onto the pulp chamber floor and light cured for 20 s. Even though it is not usual to apply 2 mm of dentine bonding agent in clinical procedures, it was considered acceptable in this study in an attempt to standardize the barrier materials.
All teeth were radiographed after the placement of the restorative materials to verify their uniformity and density, and the sealers were allowed to set for 7 days at 378C and 100% humidity (Beckhamet al.1993).The specimens were then thermocycled 1000 times using two baths of water maintained at 5 and 558C with1-min of immersion in each bath.
The apical foramen of the teeth was sealed with epoxy resin and the roots with two layers of red nail varnish, except over the coronal access. The teeth were then immersed in India ink for 5 days. Following exposure to the dye, the roots were rinsed in tap water and the nail varnish was completely removed with a scalpel.
The teeth were placed in 5% hydrochloric acid for 72 h, and then the specimens were rinsed, dehydrated in ascending concentrations of ethanol and cleared with methyl salycilate. Six specimens were lost during thermocycling and clearing procedures. The IRM, Coltosol and glass ionomer groups had 19 useful specimens, whilst Scotch Bond had17 and the control group 20.
The total length of the filling within the root-canal obturation (Y), and the greatest depth of dye penetration (X) along each canal were recorded in millimetres. The coronal entrance for dye penetration is shown in Fig.1. These measurements were converted to percentages of microleakage related to the total length of the root filling for each root canal. The dye penetration was not measured along each barrier material because it was partially, or in some cases totally lost during immersion in 5% hydrochloric acid. Data were statistically analyzed using the Kruskal-Wallis test, differences of P < 0.05 were considered to be significant.

Figure 1. The start point of dye penetration measurement was the coronal portion of the root-canal obturation up to the maximum point of dye displayed.

The mean percentage extent of leakage for each group are presented in Table 1. All positive control teeth showed dye penetration. Statistically significant differences between the means of each group were identified. Groups II (IRM) and III (Coltosol) had a significantly lower percentage of dye penetration than the other groups (P < 0.05). The prevalence of teeth presenting microleakage is recorded in Table 2. Scotch Bond had the greatest mean leakage; it did not differ significantly from the positive control group (P > 0.05).
For the Vidrion group, the mean dye penetration approached 32.2% of the root-canal length. Although this group demonstrated significantly worse results when compared to IRM and Coltosol, it was significantly better than the positive control group and Scotch Bond.

Table 1. Mean percentages of microleakage for each group.



Table 2. Prevalence of microleakage.

Discussion.
Previous investigations have pointed out the importance of coronal microleakage in the failure of root-canal treatment. The purpose of this study was to compare four materials used as barriers to coronal microleakage.
There are many factors that can affect coronal leakage. These include the thickness of the sealer cement (Magura et al. 1991), presence of voids within the rootcanal filling (Magura et al. 1991), solubility of sealer (Saunders & Saunders1995), smear layer removal, mastication forces, penetration of bacteria and the effect of saliva (Uranga et al. 1999). In addition, temperature changes in the oral cavity have been shown to adversely affect the marginal seal of a dental material (Uranga et al. 1999) because the linear coefficients of thermal expansion of materials and dentine are different, perhaps representing the leading a etiology of leakage (Noguera & McDonald1990). Thermocycling was therefore incorporated into the protocol of the present study.
Using cleared teeth to assess dye penetration has been employed in other leakage studies (Saunders & Saunders 1990, Torabinejad et al. 1990) and allows specimens to be examined in three dimensions, so that the extent and adaptation of the filling material can be observed and evaluated. Although the limitations of dye studies have been highlighted (Chailertvanitkul et al.1997), they continue to be used.
Taylor et al. (1997) and Saunders & Saunders (1994) have shown that removal of the smear layer decreases coronal leakage, regardless of the obturation technique used. This may occur owing to the increased tubule penetration of filling materials and better sealing ability. Therefore, in the present study we incorporated the use of17% EDTA to remove the smear layer.
Methylene blue dye was not used because it tends to be washed out during the clearing process. India ink, a histological stain, remained stable during this process and was used in this study. Chong et al. (1995) noted that bacterial ingress and India ink penetration provided a similar rank order for the sealing ability of the materials tested.
The results showed that IRM (Fig. 2) material and Coltosol (Fig. 3) allowed significantly less coronal microleakage than the other groups. The mean leakage for IRM was 6% with four of 19 samples displaying dye penetration. Beach et al. (1996) found that one of 18 IRM samples gave positive bacterial growth with a 4-mm layer of material. The present study demonstrated that there was no significant difference between IRM and Coltosol, which showed a mean leakage of 4.85%. Costas & Wong (1991) evaluated the sealing ability of two restorative materials used to retain bleaching agents inside the pulp chamber and reported that IRM provided a satisfactory seal. However, IRM is package das a liquid and powder that must be mixed before placement. This is a disadvantage because it takes considerably longer to place and adjust than Coltosol. However, Coltosol may be affected by occlusal loading and the manufacturer recommends the material not to be left in place for longer than 2 weeks.

Figure 2. IRM group showed mean leakage of 6.38%.



Figure 3. Mean leakage for Coltosol group was 4.85%.



Figure 4. Vidrion R group showed mean leakage of 32.2%, which was significantly different from the other groups.



Figure 5. Scotch Bond showed the poor sealing ability. The mean leakage was 54.35%, which did not differ significantly from the positive control group.



Figure 6. Control group showed mean leakage of 62.07%.

The glass ionomer group (Fig. 4), whose setting is an acid-base reaction, showed poor sealing ability, as reported by Beckham et al. (1993). This is probably due to shrinkage of the material upon setting, resulting in a potential avenue for microleakage and its use may be more technique sensitive.
Scotch Bond (Fig. 5) showed the greatest leakage, which did not differ significantly from the positive control group (Fig. 6).The bonding agent has a low viscosity and does not seem to provide an effective seal when used alone. Leonard et al. (1996) have reported that the latest generation of dentine bonding agents has shown increasingly greater bonding strengths, the ability to penetrate dentinal tubules through hydrophilic wetting and a greater tendency to resist microleakage, thereby justifying research efforts to incorporate dentine bonding agents and resin as sealers. Further long-term evaluations are indicated.

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