Influence of MTA powder handled with distilled water or phosphate buffered saline in push-out performance

Objective: to investigated the bond strength of MTA powder handled with distilled water (DW) or phosphate buffered saline (PBS). Material and Methods: two horizontal cross-section were prepared in the root of fifteen incisors, followed by drilling of two canal-like holes on its axial surface. Holes were filled with MTA with DW or PBS. Load was applied until MTA displacement. Mann-Whitney and Friedman tests were selected to establish the influence of independent variables on push-out strength. Results: MTA handled with DW and PBS showed similar values (p>0.05). Conclusion: DW or PBS do not interfere in push-out bond strength of MTA.


S
ince the introduction of mineral trioxide aggregate (MTA), it has been widely used for root-end filling, perforation repair, pulpotomy, pulp capping, and formation of an apical barrier in necrotic open apices of the teeth because of the good biocompatibility, sealing ability, and its ability to promote hard-tissue formation. 1MTA is composed of tricalcium silicate, dicalcium silicate, tricalcium aluminate and tetracalcium aluminoferrite, bismuth oxide, and other mineral oxides. 1 MTA is a water-based cement usually provided in pre-dosed powder and distilled water (DW) that are jointly mixed (in a powder/water ratio of about 0.33) to obtain a homogeneous paste.The powder consists of fine hydrophilic particles, which form a colloidal gel in the presence of water or moisture that solidifies to hard cement. 2 When immersed in diverse storage media such as DW and phosphate-containing solutions [e.g., phosphate buffered saline solution (PBS)], precipitates of feasible bioactivity are formed on MTA's surface. 3The precipitation occurrence was granted to a presumable interaction of the released calcium (Ca2+) with the surrounding fluid environment. 3Previous investigations showed the formation of apatite-like crystalline structures resulting from the interaction between MTA and PBS, 4,5 which might positively influence the sealing ability of these biomaterials. 5In such studies MTA was inserted in contact with such synthetic tissue fluids.The present study investigated the push-out bond strength of MTA powder handled with distilled water (DW) or phosphate buffered saline (PBS) solution.We hypothesized that MTA powder handled with PBS will take advantage regarding the push-out performance when compared to MTA handled with DW.

• Estimation of Sample Size
According to a previous study with similar methodology, an effect size (0.74) was added to a power β = 95% and a α = 5% inputs into an F test family between two independent means (G*power 3.1 for Macintosh). 6As a result, a minimum size of 16 disc samples was then requested.

• Selection and Initial Preparation of Specimens
The Local Ethics Committee approved this research.Fifteen human permanent upper incisors were selected and cleaned with periodontal curettes and 5.25% sodium hypochlorite (NaOCl).Coronal and apical segments were removed from all teeth, resulting solely in a segment from the middle third.From this portion, which presented 10 mm in length, two horizontal cross-section (1±0.1 mm thick) were prepared by the use of a low-speed saw (ISOMET, Buehler Ltd., Lake Buff, IL, USA) and a diamond disc (Buehler Ltd.Lake Bluff, IL, USA).All this process was performed under uninterrupted watering.The final thickness of specimens was proven with a digital caliper that presented an accuracy of 0.001 mm (Avenger Products, North Plains, OR, USA).

• Production of Canal-Like Holes in Root Slices
After preparation of the total of thirty root slices, a 0.8 mm cylindrical carbide bur (Kommet, Santo André, São Paulo, Brazil) was selected to drill two canal-like holes in such a way as to be parallel to the main root canal.Under permanent watering, holes were perpendicularly drilled to the surface of all slices using a vertical drill stand (Dremel Workstation 220, Mount Prospect, IL, USA).A minimum 1 mm space between holes, outer walls of slices and main root canal was respected.
6. Bidistilled water -1 min. 7anal-like holes were dried by the use of paper points.Then, the two holes of the same root slice were filled with gentle vibration by White MTA (Angelus, Londrina, PR, Brazil) handled with DW or PBS.For that, one sachet of MTA were mixed during 30 seconds with 1 drop of DW or PBS until obtaining an homogeneous mixture with a consistency similar to wet sand, according to the manufacturers' instructions.Afterwards, the two slices extracted from the same root were randomly distributed in order to be stored in one of the time periods: 7 days or 30 days (http://www.random.org).

• Push-Out Bond Strength Test
After storage of specimens during both time points, in which slices were maintained at 37°C, with gauzes moistened in DW, push-out test was initiated.For that, a device that presented a 0.6 mm plunger tip of was placed above the tested MTA.Load was stably applied in a coronal-apical direction, by an universal testing machine (Instron, Canton, MA, USA) at a crosshead speed of 0.5 mm/min.During loading, a graphic of load (N) × dislodgement (mm) was produced, until MTA union with dentine fail.The push-out bond strength was estimated to reach values in MPa, according to the following formula: 6,7 Push-out median value (MPa) = Maximum load (N) / Adhesion area of root canal sealers (mm²).

• Statistical Analysis
As data did not adhere to a Gaussian Curve (Shapiro-Wilk test, p<0.05), rank-based procedures were selected.Mann-Whitney and Friedman tests were respectively selected to establish the influence of the independent variables (MTAs tested and time-periods) on the push-out values (dependent variable).

Results
MTA handled with PBS showed similar bond strength values to MTA handled with DW, in both time-points (p>0.05).However, the periods of storage (7 and 30 days) significantly influenced the individual results of each MTA group (p<0.05)(Table 1).

Discussion
The preparation of MTA powder in association with PBS did not showed advantages in results regarding push-out values of MTA handled with distilled water group in both periods evaluated in the present study.Therefore, our hypothesis was rejected.Previous studies have demonstrated that the immersion or exposure of MTA in PBS can significantly increase the bond strength to dentine as a result of the formation of an apatite layer when in contact; 4,5 however, to the best of the authors knowledge, this is the first time that push-out of MTA handled with PBS is evaluated.The small amount of PBS used during MTA´s handling may be insufficient to generate any difference in the push-out outcomes when compared to MTA handled with distilled water.
Physical properties and hydration behavior of MTA are influenced by the effects of the storage medium -environmental pH and presence of ions -on MTA composition. 2herefore, in order to eliminate possible interferences that the storage medium may cause in the analysis of this material, specimens were preserved with gauzes moistened in distilled water before the push-out assay.Several studies employed distilled water as storage medium during experiments of MTA's properties. 1,2,5or both materials, push-out bond strength was higher at 30 days (p<0.05).This is consistent with previous reports, which demonstrated that bond strength values are prone to rise with time. 5,8This may be justified by the hydration and expansion processes of MTA. 1,9,10o increase the internal validity of the push-out model, two artificial standard holes were created in dentine slices, providing the same dentine source to both materials. 6Therefore, confounding factors associated with dentine heterogeneity present in different teeth and in different regions of the same root were eliminated. 7In addition to that, an equal cylindrical size for all canal-like holes eliminated relevant biological bias, since it standardized the internal root canal anatomy of the MTA's groups.Artificial canal-like holes undertaken allowed the creation of even cleaning and shaping circumstances, which is virtually impossible to reach when considering randomized small-sized test groups of root filled teeth.

Conclusion
In conclusion, MTA powder handled with distilled water or PBS solution presented similar performance and, therefore, the use of both of these vehicles do not interfere in push-out bond strength results.Future studies should evaluate the influence of handling MTA with PBS in other outcomes, such as biomineralization.

Table 1 .
Mean and standard deviation (±SD) of the push-out bond strength of MTA handled with PBS or DW in the different tested time points (7 days or 30 days) * DW = distilled water, MTA = mineral trioxide aggregate, PBS = phosphate buffered saline solution.Different letters indicate significant differences at P=0.05