Solubility, porosity and fluid uptake of calcium silicate-based cements

Abstract Objective: To evaluate the absorption/fluid uptake, solubility and porosity of White mineral trioxide aggregate (MTA) Angelus, Biodentine (BIO), and zinc oxide-eugenol (ZOE). Material and Methods: Solubility was evaluated after immersion in distilled water for 7 and 30 days. Porosity was evaluated using digital inverted microscope, scanning electron microscope (SEM) and micro-computed tomography (micro-CT). For the fluid uptake test, specimens were immersed in Hank's balanced salt solution (HBSS) for 1, 7, 14 and 28 days. Fluid absorption, solubility and porosity of the materials were measured after each period. Statistical evaluation was performed using one-way analysis of variance (ANOVA) and Tukey tests, with a significance level at 5%. Results: After 7 and 30 days, BIO showed the highest solubility (p<0.05). All methods demonstrated that MTA had total porosity higher than BIO and ZOE (p<0.05). Micro-CT analysis showed that MTA had the highest porosity at the initial period, after its setting time (p<0.05). After 7 and 30 days, ZOE had porosity lower than MTA and BIO (p<0.05). Absorption was similar among the materials (p>0.05), and higher fluid uptake and solubility were observed for MTA in the fluid uptake test (p<0.05). Conclusions: BIO had the highest solubility in the conventional test and MTA had higher porosity and fluid uptake. ZOE had lower values of solubility, porosity and fluid uptake. Solubility, porosity and fluid uptake are related, and the tests used provided complementary data.


Introduction
Root repair cements must have appropriate physicochemical properties. It should have low solubility in tissue fluids 7,24 , since the dissolution of materials may allow leakage, leading to treatment failure 7,10 . Solubility is evaluated using standardized samples of material, which are weighed before and after immersion in distilled water as defined by the ISO 6876:2002 16 or ANSI/ADA specification No. 57 1 .
Another important property of root repair cements is the porosity, which can affect the physical properties and, consequently, its behavior within its environment 4 . Standardized tests are used to assess this material property and alternative methods have been suggested. Therefore, to evaluate physical properties using different methods is relevant. The porosity of endodontic materials may be evaluated using microscopy 5 . However, this technique produces two-dimensional data and may be inexact to show the internal porosity of the material 26 . Therefore, microcomputed tomography (micro-CT) may be used as an alternative method for evaluating porosity and distribution of the size of pores within the material 23 .
The porosity may be associated with solubility 3,8,27 , suggesting that a porous material has higher solubility.  White Art. Dent. Ltda., Rio de Janeiro, RJ, Brazil) were used in the proportions described in Figure 1. These materials were manipulated and subjected to solubility, porosity and absorption (i.e., fluid uptake) analyses.

Solubility
The solubility test was performed based on a previous study 6  placed in plastic receptacles with lids containing 7.5 mL of distilled and deionized water, suspended by nylon threads attached to the containers. The receptacles remained in the oven at 37°C for 7 days. After this period, the test specimens were removed from the distilled water, dried and placed in a dehumidifying chamber. The mass was measured before and after immersion of the samples in distilled water, and every 24 hours thereafter, until attaining mass stabilization.
New samples were prepared and kept immersed in distilled water for 30 days. The mass loss was expressed as a percentage of the original mass.

Porosity analysis using inverted optical microscope
The methodology to evaluate porosity was based on a previous study 3 . The microstructure of the materials was observed using a digital inverted microscope (MIC-D Olympus, Philippines) on rectangular specimens (n=6) measuring 8x10 mm and 5 mm high. The specimens were prepared and then stored for 24 hours at 37°C and 95% humidity. The specimens were then unmolded and stored in distilled water for 7 days, after which they were sectioned in half along their cross section with a microtome cutter (IsoMet 1000, Buehler Ltd, Lake Bluff, IL, USA). They were subsequently polished using fine grit of silicon carbide abrasive paper numbers 320 and 600, and rinsed with distilled water after the polishing procedure.

Porosity analysis using digital inverted microscope
The number of pores was higher for MTA (p<0.05), and there was no statistically significant difference between BIO and ZOE (p>0.05) ( Table 1).
Representative images captured at 50x magnification of all materials evaluated are shown in Figure 2.

Porosity analysis using micro-CT
The porosity observed in micro-CT is represented in  Porosity analysis using SEM Qualitative analysis by scanning electron microscope showed a higher porosity for MTA (Figure 4)   Porosity occurs because of spaces in the nonhydrated cement 18 , and the porosity and solubility of materials may affect their stability, integrity, and durability 21    Portland Cement and MTA, the increase in weight after 7 days in distilled water, probably occurs due the empty pores and capillaries, which are rechargeable, and the newly captured water will then participate in the hydration process of these cements, increasing their weight 12 . ZOE showed solubility higher than MTA at 7 days and comparable values at 30 days, in agreement with previous studies 11, 25 . This solubility can be related to the setting reaction of zinc oxide-eugenol. Zinc is a metal likely to form a chelate (zinc eugenolate) and eugenol has a replaceable hydrogen and a nearby donor in the oxygen 9 . When ZOE is immersed in water, eugenol is eluted by leaching and a progressive decomposition of the zinc eugenolate matrix occurs, causing a disintegration of the material 28