Comparison of Mechanical and Physical Properties of White and Gray Mineral Trioxide Aggregate Useable in Dentistry

Introduction Mineral trioxide aggregate (MTA) is widely used as a biomaterial and is an ideal bioactive agent for the obturation of the root canal system in dentistry. It is a mixture of Portland cement and bismuth oxide (1). Microanalysis of MTA has shown that its chief ion is calcium. MTA yields hydroxyapatite when it is mixed with interstitial fluid, which is the main component of bone and has proper compatibility with the body tissues. MTA has various applications in the repair of the root and bone in endodontics. Two commercial types of MTA are available, referred to as white and gray ProRoot MTA (2). Asgary et al showed that the main differences in the ingredients between gray and white MTA were differences in aluminum oxide (Al2O3), magnesium oxide (MgO) and in particular ferrous oxide (FeO). The amounts of Al2O3, MgO and FeO in gray MTA were reported to be approximately 122%, 130% and 100% higher than those in white MTA, respectively (3). One of the physical properties of root filling materials is their solubility; a material with very low solubility is an ideal material (4-6). Destruction and disintegration of these materials result in gap formation between the material and the root canal wall, giving rise to an increase in bacterial microleakage over time. Another important property of such materials is their microhardness which indicates strength and resistance of the material to deformation. This parameter is affected by different factors such as yield strength, tensile strength, elastic modulus, the moisture of the environment and crystalline structure stability of the materials (7,8). When the MTA powder is mixed with water, a special structure of micro-canals is formed. The majority of these structures are eliminated during the setting reaction and finally, the set cement will consist of a series of pores and micro-canals. Penetration of water results in better hydration of the powder and optimal formation of the Abstract Objectives: Currently, different materials are used to obdurate tooth root canals. An ideal root canal filling material should have some specific properties, including biocompatibility, dimensional stability and insolubility. Materials and Methods: One of the methods to evaluate these materials is to carry out solubility and microhardness tests. To this end, 20 mineral trioxide aggregate (MTA) samples were prepared for solubility test and 60 samples for microhardness test. MTA (Angelus, Brazil) was prepared based on manufacturer’s instructions by mixing the powder and catalyst at a ratio of 1:1 and was placed in Teflon molds. The samples were stored in a room with 95% relative humidity at 37°C for 390 minutes (3 times longer than the setting time). The solubility of the materials was tested by immersing the samples in distilled water and measuring weight loss at 24-hour and 7-day intervals. The microhardness was determined using Vickers test before and after immersion in Ringer’s solution at different intervals and the results were compared. Results: The results showed that solubility of white MTA after 1 day was similar to that of gray MTA; however, after 7 days, its solubility increased. There were no significant differences in microhardness on day 1 and on the subsequent days between white and gray MTA samples (P < 0.05). Conclusions: There was no significant difference in the solubility of white and gray MTA samples at 24-hour interval; however, at 7-day interval, the solubility of white MTA (WMTA) was significantly higher than that of grey MTA (GMTA) (with the solubility of both materials being <3%) In addition, there was no significant difference in microhardness of white and gray MTA between the first day and other intervals (P < 0.05).