The effect of pH on solubility of nano-modified endodontic cements.

AIMS: To evaluate the effect of storage pH on solubility of white mineral trioxide aggregate (WMTA), bioaggregate (BA), and nano WMTA cements.
MATERIALS AND METHODS: Forty-eight moulds randomly allocated into three groups of pH 4.4 (group A), 7.4 (group B), and 10.4 (group C); and one empty as control in each group. Each group was further divided into three subgroups according to the material studied; WMTA, BA, and nano WMTA. The specimens in subgroup A were soaked in butyric acid buffered with synthetic tissue fluid (STF) (pH 4.4), while the samples in subgroups B (pH 7.4) and C (pH 10.4) buffered in potassium hydroxide for 24 h and then the loss of cement was determined. A two-way analysis of variance (ANOVA) and Tukey post-hoc statistical tests were used to detect any statistically significant differences among the groups/subgroups.
RESULTS: Statistical analysis has showed the highest solubility in acidic pH for all tested materials. Nano WMTA samples in pH = 10.4 had the lowest and BA samples in pH = 4.4 showed the highest cement loss.
CONCLUSION: The solubility of all tested cements can be jeopardized in acidic environment which might affect on their sealing characteristic in clinical scenario. However, nano WMTA cement due to its small size particles and different additives was capable of producing lower porosity in set material, which resulted in showing more resistance in acidic environment.

RCT Entities:   Population Interventions Outcomes

INTRODUCTION

Mineral trioxide aggregate (MTA) fulfills many of the ideal properties of a root-end filling and repair material for different purposes such as pulp capping, pulpotomy, management of teeth with immature root formation, management of root perforations, and surgical root-end fillings.[1] MTA cement, however, has some clinical limitations. These drawbacks taken into consideration by many investigators and include its granular consistency,[2] slow setting time,[3] and porosity.[4] In addition, the sealing ability of MTA in low pH environment has been questioned. In an acidic environment, the sealing properties of MTA might be compromised due to a decrease in surface hardness[5] which can cause lower push-out bond strength.[6]

Many studies have reported on attempts to modify this Portland cement to improve on its shortcomings. Bioaggregate (BA) is one calcium silicate–based material that has been reported as a modified version of MTA.[7] It is composed of tricalcium silicate (C3S), dicalcium silicate (C2S), calcium phosphate monobasic, and amorphous silicon dioxide with the addition of tantalum pentoxide, instead of bismuth oxide in MTA for radiopacity.[7] This newly introduced cement is aluminium free and displays biocompatibility similar to WMTA.[16] However, dislodgment resistance of BA in low pH compared with WMTA has been discussed by others.[8]

A new version of MTA was prepared by reducing the size of particles and adding small amounts of zeolite and strontium.[3910] This cement was claimed to overcome some limitations of the original MTA, such as long setting time and decreased resistance to solubility in low pH environments.[11] The main ingredients of WMTA, nano WMTA, and BA are described in Table 1. Ideal cements used for purposes within the scope of endodontics should provide an impervious apical seal[12] especially in different clinical conditions, such as acidic environment, which can result from periapical diseases.[13] Therefore, the purpose of the present study was to evaluate the solubility of BA and nano WMTA in different storage pH in comparison with WMTA in order to determine the feasibility of applying these two modifications of WMTA in low pH conditions without significant changes in their acidic resistance compared with WMTA.

Table 1

The ingredients of Nano white mineral trioxide aggregate, WMTA, and bioaggregate[3711]

MATERIALS AND METHODS

The solubility of WMTA (ProRoot MTA, Dentsply, Tulsa, Okland, US), BA (BA; Innovative BioCeramix, Vancouver, Canada) and nano WMTA (Nano WMTA; Kamal Asgar Research Center, US Pat #13/211880) were determined according to the method recommended by the ISO 6876 method and with the American Dental Association (ADA) specification #30.[1415] This international standard specifies requirements for “materials used for permanent obturation of root canal with or without the aid of obturating points.”

Stainless steel moulds with an internal diameter of 20 ± 0.1 mm and a height of 1.5 ± 0.1 mm were used for sample preparation. Forty-eight moulds were cleaned with acetone in an ultrasound bath for 15 min. All moulds were weighed three times before use (accuracy, 0.0001 g) on a Mettler AE-163 (Mettler Toledo) balance, which was used throughout the experiment. The moulds were divided into three groups of A, B, and C (n = 5). Each group contained 15 moulds and an empty mould was used as control. Every experimental group was subdivided into three subgroups: 1, 2, and 3; each with 5 moulds. The moulds were placed on a glass plate in order to be filled to slight excess with the mixed materials. The first subgroups of each group (A1, B1, and C1) were filled with WMTA, while the second subgroups (A2, B2, and C2) were filled with BA, and the third subgroups (A3, B3, and C3) were filled with nano WMTA. Each material was mixed and placed according to the manufacturers’ instructions.

In group A, samples in each subgroup were wrapped with gauze soaked in butyric acid buffered with synthetic tissue fluid (STF) at pH 4.4. In groups B and C specimens were wrapped with gauze soaked in STF buffered in potassium hydroxide at pH of 7.4 and 10.4, respectively, for 24 h. Following exposure to the acidic STF, the weight of specimens was recorded to determine the amount of solubility. Loss of cement was identified by the decline in mass of specimens stored in deionized water. The weight of particles detached from the cement during the experiment was also recorded according to the ISO 6876. The plates with filled mould were weighed and 60 mL of distilled water (pH = 7.1, 37°C) was spread with a Luer-Lock syringe over the plate so that the lower and upper surfaces of the cement were completely covered with water (100% relative humidity). The sets were placed in an oven at 37°C from which they were retrieved at the indicated time intervals. Fifteen moulds were used for each experimental group and studied for up to 28 days. The moulds were hung over a Petri dish and both sides were gently rinsed with distilled water to collect the residues.

Samples were dried in desiccators and the moulds were weighed after 28 days. An industrial microwave vacuum drying system (Yantai Care Microwave System, China) was used to eliminate all free water trapped in the cement tablets to establish a consistent net weight. The initial amounts of cement inside the moulds were determined by subtracting the weight of the mould-Petri dish from the weight of the filled mould-Petri dish. A two-way analysis of variance (ANOVA) and Tukey post-hoc statistical tests were used to detect any statistically significant differences among the groups/subgroups.

RESULTS

The means ± standard deviations (SDs) of the cement loss of the experimental groups were as follows:

Group A: A1 WMTA (4.4) 2.68 ± 0.52, A2 WMTA (7.4) 0.87 ± 0.077, A3 WMTA (10.4) 0.38 ± 0.039

Group B: B1 BA (4.4) 4.16 ± 0.65, B2 BA (7.4) 2.15 ± 0.25, B3 BA (10.4) 1.20 ± 0.20

Group C: C1 nano WMTA (4.4) 0.98 ± 0.16, C2 nano WMTA (7.4) 0.53 ± 0.09, C3 nano WMTA (10.4) 0.35 ± 0.11.

Overall means of solubility in all examined pH for WMTA, BA, and nano WMTA were 1.31, 2.507, and 0.62, respectively [Figure 1]. The samples of WMTA and BA groups in pH = 4.4 showed significantly higher solubility in comparison with specimens in pH = 7.4 and pH = 10.4 (P < 0.05), however it was not significant between nano WMTA samples (P > 0.05). There was also a significant interaction between the effects of material and pH on solubility. Nano WMTA samples in pH value of 4.4 showed significantly lower cement loss in comparison with WMTA (P = 0.011) and BA samples (P = 0.0001). This difference was also significant between WMTA and BA specimens (P = 0.011). In pH value of 7.4, significant difference was only noticed between nano WMTA and BA groups (P = 0.019); however in pH value of 10.4, there were no significant difference between tested materials (P > 0.05) [Figure 1].

Figure 1

Box plot of the solubility of white mineral trioxide aggregate WMTA, bioaggregate, and nano WMTA in contact with different pH values (a) and overall box plot of solubility in experimental groups (b)

DISCUSSION

MTA has been used since 1993[16] due to its numerous advantages making it a recommended root-end filling material.[17] The sealing property of an ideal root-end filling material is vitally important.[17] This issue is becoming more important as Malamed reported that in most clinical cases MTA is applied in an inflamed area, which is considered to be an acidic environment.[18] This acidic environment can cause acid corrosion in which calcium hydroxide, C-S-H, and the calcium sulfoaluminate phases decompose and produce porosities.[19] Moreover, MTA becomes considerably porous after its setting time at the MTA-dental interfaces, which results in or causes microleakage.[20] Namazikhah et al., examined specimens with scanning electron microscope (SEM) and indicated that in low pH condition MTA shows great degree of porosity.[21] According to the results of the present study, samples of the WMTA group exposed to acidic pH showed higher amount of solubility compared to the other two pH values. It is obvious that WMTA might undergo structural changes in the presence of low pH resulting in altered sealing ability.[5]

BA is another modification of MTA, which is composed of C3S, C2S, calcium phosphate monobasic, amorphous silicon dioxide, and tantalum pentoxide for radiopacity.[7] The manufacturers indicate that C-S-H and CH are formed during the hydration phase, while in the presence of amorphous silicon dioxide the CH byproduct further reacts to form C-S-H. This reaction results in lower amount of CH in BA after setting.[22] Grattan-Bellew et al.,[23] reported that CH can be responsible for the weakness of the mixed cement and others indicated that more resistance to an acidic environment may be enhanced by reducing the amount of CH in BA.

In the current study, moulds filled with BA after exposure to different pH showed more solubility especially at pH 4.4 compared to WMTA and nano WMTA, which is found on literature as the same results achieved by evaluating the displacement property.[24] Previous studies indicated that BA initially is not affected by acidic environment and its dislodgement resistance is higher than WMTA. However after 34 days, WMTA showed significantly higher bond strength than BA.[24] This issue was also observed in the present study, where after 28 days the cement loss in WMTA was lower than BA. To explain this, we should note that MTA undergoes a hydration phase which can promote the strength and retention characteristics of this cement. Thus, this nanomodification of MTA has superior property to form hydroxyapatite crystals.[3]

A new version of MTA, nano WMTA was introduced by Saghiri et al.,[3] and they claimed that by modifying the size of the constituents of WMTA, the surface area of the powder might be increased. In addition, reducing the size of particles and their uniform distribution within the product is reported to play an important role in shortening the setting time and increasing the microhardness even at low pH [Figure 2]. The present study evaluated the solubility of this new root-end filling material exposed to different pH compared to WMTA and BA. The results of this investigation showed that nano WMTA had less solubility than WMTA in low pH. This finding may be explained by the characteristics of nano WMTA particle form, size, and different composition of this new cement. As indicated above, the uniform distribution of particles, and the increased surface area observed in nano WMTA, results in less porosity and better interlocking of particles in the set nano WMTA and reduced solubility of this cement in long-term.[391011]

Figure 2

Schematic diagram of particles and reaction of cements before and after hydration

Nano WMTA contains additive ingredients except from basic materials found in other two cements.[311] These substances are as follows: Tricalcium aluminate, calcium sulfate, zeolite, and strontium carbonate. Tricalcium aluminate in Portland cement causes an “interstitial phase” which is beneficial due to facilitating the formation of the desired silicate phases. It reacts most strongly with water of all the calcium aluminates, and it is also the most reactive component of the Portland clinker phases. Its hydration to phases of the form Ca2AlO3(OH)·nH2O leads to the phenomenon of “flash set” (instantaneous set). Due to this advantage, it was reported that tricalcium aluminate is associated with adverse effects that can reduce the durability of concrete by following effects:

Heat release: A large amount of heat is generated during the phenomenon of “flash set” which can cause spontaneous overheating in large masses of concrete. In order to overcome this negative effect first, tricalcium aluminate levels are reduced and secondly small amount of calcium sulfate (typically 4-8%) is added to the cement. Sulfate ions in solution form an insoluble layer of ettringite (3CaO·Al2O3·3CaSO4·32H2O) on the surface of the aluminate crystals, passivize them and slightly contribute to the strength of the cement[1122]

Sulfate attack: The elimination of the sulfate corrosion may be ensured by using low tricalcium sulfate content cement or by adding zeolite to the cement. Zeolite is composed of crystalline hydrated aluminosilicate of alkaline metals and metals of alkaline soils (Ca, K, Na, Mg).[24] Adding zeolite could decrease the volume of tricalcium aluminate in the cement. Authors have indicated that zeolite, beside anticorrosive action against sulfate, can be regarded as a stabilizing agent for cements. Previously done study[25] also confirmed that zeolitic cement can replace Portland cement in many applications with the advantage of higher resistance to acidic and sulfate attack, which could reduce the solubility of the applied cement. These findings confirmed the results of the present study where it was shown that Nano WMTA containing zeolite had better acidic resistance in comparison with BA or WMTA.

According to the recent patent, the amount of zeolite in nano WMTA is 2% in order to prevent its adverse effect on the compressive strength of cement.[11] Furthermore, others acclaimed that the benefits of adding zeolite in durability of the cement far outweigh the slow rate of strength development at early age, or a modest strength reduction of 20-30%.[25]

CONCLUSION

According to this comparative investigation, the authors have concluded that:

Acidic environments can significantly increase the cement loss of all three types of tested materials. However, these cements showed the minimal solubility in alkaline pH values

Nano WMTA showed the lowest cement loss in comparison with WMTA and BA, especially in low pH value. This issue can suggest nano WMTA to be applied in acidic environments such as preapical inflammation.