Influence of Powder to Liquid Ratio On Calcium Ion Release Of Zinc Oxide Modified Mineral Trioxide Aggregate – An In-vitro Study

Introduction

Mineral trioxide aggregate is one of the most commonly used material in reparative dentistry and endodontics,¹ which requires the material to be placed in direct contact with the pulpal or periapical tissues. One of the ideal requirement of restorative material is that it should not cause any discoloration to the tooth structure.

Grey MTA which was introduced initially was not suitable for use in anterior teeth^2,3. To overcome this problem white MTA was introduced. However, studies have shown that white MTA also causes dentin discoloration both in-vitro and clinically.^4,5 This discoloration may be due to the presence of radio opacifier bismuth oxide with in the formulation of white MTA.

This bismuth oxide when it comes in contact with the strong oxidizing agent like sodium hypochlorite or amino acidspresent in the collagen, will be destabilized i.e, the oxygen will be given off and sodium chloride will be formed on the surface as black precipitate which is responsible for dentin discoloration.^6,7 Several alternatives were suggested to overcome this problem like substitution of the bismuth oxide as radio opacifier with other radio opacifiers^8,9 like zirconium oxide and calcium tungstate, but these radio opacifiers need to be added in a larger quantity to produce the radio opacity similar to bismuth oxide which in turn deteriorated both the physical and chemical properties of MTA. To overcome this problem, in a recent study done by Angelica et.al,ZnO was added to the MTA in ratio of 5%, 15% and 45% and found that all the formulations were found to prevent dentin discolorationwithout significantly altering other properties of the tooth.¹⁰

For a restorative material to be ideal in its use as a pulp capping material or root end filling material it should have adequate physical and chemical properties, among the chemical properties calcium ion release and high pH are important because these conditions help in mineralization process.¹¹

MTA fulfill most of these requirements, when mixed with the liquid (distilled water), MTA forms a colloidal gel that later solidifies as hard mass.However, the characteristics of the mass can be influenced by various factors, among them the most important factor is P/L ratio.¹² The manufacturers recommendation for its use is by mixing three parts of powder to one part of distilled water, but several studies have shown that altering the P/L ratio can also influence the properties of the set cement.¹³

Hence, in the present study 5% ZnO was added to MTA to form a modified MTA which was used in different powder liquid ratios to evaluate the calcium ion release after 24hrs of mixing.

Materials and Methods

The MTA (Angelus MTA ) was divided into three groups according to the P/L ratio. The cement was mixed using Group A- 2:1, Group B- 3:1, Group C- 4:1 proportions.

Thirty acrylic teeth (n=10) with a cavity of 3mm depth were prepared and standardised and were filled with different materials which were placed individually in a 10ml of ultrapure water and stored in water at 37°C.13 After 24 hrs the sample solutions were collected, and calcium ion concentration was measured using EDTA titration method.Briefly, 5ml of distilled water and approximately 1ml of 8molL-1 Potassium Hydroxide solution were added to 1ml of sample solution and a small amount of Eriochrome Black T indicator. The solution was then titrated immediately with 0.01moll-1Disodium Ethylene Diamine Tetra Acetic Acid until the wine-red colour of the solution completely disappeared and a skyblue colour developed. The amount of Calcium in the sample was calculated by the following equation: 1ml of 0.01moll -1 Disodium Ethylene Diamine Tetra Acetic Acid= 0.4008 mg Calcium¹⁴.

Results:

The data obtained was analysed by One Way ANOVA test followed by Tukey’s post hoc analysis to evaluate calcium ion release. Statistical Package for Social Sciences [SPSS] for Windows, Version 22.0. Released 2013. Armonk, NY: IBM Corp., was used to perform statistical analyses.The level of significance [P-Value] was set at P<0.05.

The results were found to be statistically significant. The results show that greatest calcium ion release is shown by group A followed by group B and C.(Table 1) and (Graph 1).

Tukey HSDPost hoc analysis done (Table 2) showed that there is significance difference between all the groups when intergroup comparison was done

Discussion:

MTA is one of the most commonly used calcium silicate based restorative material and is currently used for a variety of purposes such as root end filling material, direct pulp capping, for apexification and for perforation repair.¹² The success of Ca(OH)2 containing products depend on two factors, its Ca+2ion concentration and its alkaline pH.

Ca ions play a major role in the success of pulp capping, apexification, and perforation repair and also bring about proliferation, differentiation of pulp cells in a dose dependent manner.^15,16

Earlier studies have shown that substances added to calcium hydroxide containing products may influence the amount of calcium ions released from it.¹⁷

In the present study 5% Zinc oxide was added to MTA based on a study done by Angelica et al¹⁸ which showed that addition of Zinc oxide to MTA prevented dentin discolouration. Three different P:L ratio of 2:1, 3:1, 4:1 was used and the calcium ion released after 24 hrs was evaluated.

The release of calcium and hydroxyl ions from the set restorative material depends on the size of the mineral particles and also on the network structure of the cement responsible for water sorption, solubility and porosity of the set cement.¹⁹

Results of the present study using ZnO modified MTA showed that when mixed using the P/L ratio of 2:1 it showed higher amount of calcium ion release than 3:1 and 4:1. Thus, the higher amount of Ca ion release at a lower P:L ratio could be due to higher water content during mixing which may have resulted in higher calcium ion release.

In a study by Fridland and Rosado²⁰ they have observed that when volume of water in the mixture was increased they found that there is a significant increase in solubility and porosity of the set cement.²⁰ According to Vivian et al this higher solubility and porosity is responsible for higher pH level and calcium ion release,²¹ suggesting that samples with greater amount of water content will have higher bioactivity than those with less water content thus confirming the results of the present study.

According to ANSI/ADI specification no.57/2000 and ISO 6876:20012 a set cement should have a solubility less than 3%, results of earlier studies have shown that an increase in the water content of the set cement showed an increase in solubility.¹³ In the present study we found that there is higher calcium ion release when p/l ratio of 2:1 this could be attributed to the higher water content which may have been responsible for increased solubility of the set cement.

From the present study it can be concluded that, P:L ratio of 2:1 can be considered ideal for the manipulation of ZnO containing MTA as it showed the maximum amount of Calcium ion release.