Several new calcium silicate-based materials have been introduced to overcome the limitation of MTA. They are largely used in endodontic treatment, and in restorative dentistry, including indirect and direct pulp capping. These materials have similar basic components and biological properties; however, they differ from each other (setting time, physicochemical properties). MTA-like cements are known to have a high pH, resulting from the hydration process . In water, calcium silicates undergo hydrolysis producing calcium hydroxide and calcium silicate hydrate, which reacts in the presence of physiological fluids producing hydroxyapatite mostly at the surface of the tricalcium silicate paste . High pH (12.5) the hydrated cements contribute to presence of calcium hydroxide . The bioactivity of calcium silicate-based materials is associated with their ability to release hydroxyl and calcium ions . The release of alkaline phosphatase and bone morphogenetic protein 2 (BMP–2), which are necessary in the mineralization process, is stimulated by release hydroxide ions [2, 32]. Several studies confirmed that calcium silicate-based materials activate hard tissue to repair [2, 3, 39]. The released hydroxyl ions during the hydration reaction turn the pH environment into alkaline, which inhibits proliferation of bacteria [4, 19, 38]. The nature of the mineral particles and cement network structure determinate ion release [8, 20]. In the present study, an alkalinizing properties of TheraCal LC (Bisco Inc. Schamburg, IL, USA), MTA Plus (Avalon Biomed Inc, Bradenton, FL), Biodentine (Septodont, Saint-Maur-des-Fossés, France), RetroMTA (BioMTA, Seoul, Korea), MTA Flow (Ultradent Products, Inc., South Jordan UT, USA), and OrthoMTA (BioMTA, Seoul, Korea) were evaluated. These materials are commonly used in endodontic treatment as a pulp capping, pulpotomy, apexogenesis, apexification, perforation repair, and retrograde filling [1–10].
In the recent study, time intervals selected for testing were based on the setting times of the materials and previous studies [19, 36, 40, 41]. The pH was analysed immediately after immersion (baseline) and after 1 hour, 3 hours, 1 day, 2 days, 3 days, 1 week, 2 weeks, 3 weeks and 1 year. Comparison the results of our study with those obtained by other authors may be difficult due to differences in methodology. In this in vitro study, a deionized water at pH lower than 7 (6.5–6.9) was used as testing medium, in agreement with other authors [20, 29, 40, 41], to standardize the test conditions and allow a comparison of the data with other studies. Although many authors did not provide the initial pH value of solutions in which samples were immersed [19, 42–44]. In present study, samples were incubated in the same solution for the entire test period, without changing it for the new one after the measurement, although many authors after recording the pH placed the samples in fresh solution [20, 36, 43]. Those authors claimed that a regular exchange of the water where the tested samples were immersed in the experiment was performed to avoid saturation of the medium, since it would not present ion exchange, as it occurs in the clinical situation. The differences in methodology also apply to the size and the volume of medium in which the samples were stored. In our study, the materials were placed in a plastic material with a height 5 mm and inner diameter 4 mm. Thus, the samples were of the same volume and had an identical contact surface with the medium. Other authors placed materials in polyethylene, teflon or plastic tubes of length 3 mm, 2 mm, 10 mm, 1.6 mm, 5 mm and diameter respectively 1mm, 10 mm, 1 mm, 8 mm, 5 mm [36, 41–43]. In present study, all sample were immersed in 10 ml; in other studies, the volume varied from 10 ml to 20 ml [36, 40–42].
Several methods are available for measuring pH, but there is no universal standards. The use of a pH meter increases the accuracy of the results, as well as provides numeric data which can be analyzed. Although other method can be used, such as the pH curves (titration), this has lower precision and can hinder the accurate interpretation of the results . We used the first one method in the present study.
TheraCal LC (Bisco Inc. Schamburg, IL, USA), as a light-curable resin-modified silicate material that sets by hydration, does not include water. A moisture is taken up from the environment and its diffusion within the material. According to manufacturer instructions this material should be placed on moist dentin. In our study, TheraCal LC proved to be a light-cure material able to increase pH more than 1 year. Gandolfi et al.  reported that TheraCal LC was able to alkalize the surrounding medium initially to pH 10.96–9.28 (3h–3 days) and subsequently to pH 8.32–8.04 (1 week- 4 weeks). The results of another study by the same author  showed that TheraCal LC, Biodentine and MTA Plus induced the alkalization of the soaking water that decreased with time but was still present at 28 days. After 3 hours, the pH of soaking water was decreased by TheraCal LC at level 9.53. For Biodentine, MTA Plus and MTA Plus, the pH value of the soaking water after 3 days was 10–11, and after 14 days the pH was decreased to level 8–9. Only Biodentine was able to keep the pH higher than 9 after 28 days of soaking. TheraCal LC showed the most constant pH among all of the materials.
MTA Plus (Avalon Biomed Inc, Bradenton, FL) has a composition similar to the ProRoot MTA but having a finer mineral powder that can be mixed with 2 different liquids provided (water or a gel) to obtain materials with different setting times [21–23]. In our study, MTA Plus showed that the pH initially increased up to 2 weeks from 10.28 to 11.59, and in third week it decreased to the value 11.41, then it increased and one year it reached the value 11.91. In contrast, Gandolfi et al.  compared alkalizing properties of Dycal, MTA Plus and ProRoot MTA. They showed that all 3 materials created alkaline pH after 3 hours of soaking, MTA Plus mixed with gel achieved the pH level at 12. Over 28 days, the pH of all materials gradually decreased. After 28 days, the pH was the highest for Dycal (9.8), MTA Plus (8.29), MTA Plus gel (7.99) and the lowest for ProRoot MTA (7.1).
New calcium silicate-based materials have been developed to overcome the drawbacks presented by MTA. Biodentine (Septodont, Saint Maur-des-Fosses, France) is among these materials that has captivate attention in the last recent years and has been advocated to be used as a dentine restorative material in addition to endodontic indications similar to those of MTA [24–26]. In our study, Biodentine showed a gradual increase in pH throughout the observation period, to finally reached a pH level 12.19.The pH values observed in the present study were higher than those obtained by other authors. Gandolfi et al.  showed that Biodentine induced alkalization of the medium that decreased with time but was still present at 4 weeks. It alkalized the medium to 11.65 at short time (3h–1day) then decreased, and after 28 days the pH was 9.48 . Aksoy et al.  demonstrated similar results reported in other investigation [2, 45]. They showed that Biodentine had the highest hydroxyl ion rates (pH 9.6) compared to TheraCal LC (pH 8.2) in 24 hour examination while the pH gradually decreased during the following measurement periods (7 days- 28 days). In both materials, after 7 days, pH started to slowly decrease unlit the last measurement day (Biodentine pH 8.37 and TheraCal CL pH 8.06).
RetroMTA (BioMTA, Seoul, Korea) is a new hydraulic bioceramic material proposed for use in similar indications as MTA (pulp capping, perforations and root resorption repair, apexyfication and apical surgery)[27–29]. According to the manufacturer’s product specification, the pH of RetroMTA is initially 12.5, and decreased to 7.8–8 in 4 weeks . In the present study, the pH of RetroMTA was alkaline, varying from 10.10 to 11.86 after one year. The lower pH values observed in our study compared to the manufacturer may be explained by the different methodology of measurement. Sousa et al.  examined the release rate of OH- of ProRoot MTA and RetroMTA in 3, 24, 48, 72 hours and 7 days. As a result of this study, there were no significant differences in the pH levels of ProRoot MTA and RetroMTA throughout the periods of experiment. The pH of RetroMTA varied from 9.93 to 7.9; the pH of both materials tended to decrease over time.
The poor working properties of MTA-like cements results in a paste hard to manipulate. Considering the importance of the ideal flow ability that endodontic materials should present reducing the difficulty of handling and facilitate insertion . MTA Flow (Ultradent Products, Inc., South Jordan UT, USA), high plasticity cement was developed with the aim of improving these characteristic. Guimarães et al.  compared alkalizing activity of MTA Flow and MTA Angelus. Both cements showed alkalizing activity. The pH level of the soaking water was reduced with time but was still present until the end of the study (MTA Flow 8.5, MTA Angelus 8.7). At 186 hours, the pH level of the MTA Flow was significantly reduced in comparison with the initial time intervals. This result is not accordance with results of our study; where the pH of MTA Flow in 3rd hour was 10.72, in 24th hour 11.2, and after 168 hours 11.63.The pH of MTA Flow has reached one of the highest values (12.20) after one year.
The OrthoMTA (BioMTA, Seoul, Korea), a new calcium silicate-based material, developed mainly for orthograde root canal obturations as well as retrograde fillings and perforation repairs [34, 35]. This material also has a bioactive properties; it stimulates the apical foramen to release Ca2+ , which leads to formation of hydroxyapatite layer on its surface. The only study evaluating the alkalizing properties of OrthoMTA is the Kim et al.. They demonstrated that ProRoot MTA, OrthoMTA, and Endocem MTA inducted the pH values of the storage water respectively 11.90, 11.42, and 11.33 on day 7. This result did not differ significantly. In our study, the pH level of OrthoMTA increased gradually during all periods, and it was the highest value (12.29) among all recorded values after the one year observation.
Our results should be considered within the experimental conditions used, facing natural limitations of comparing in vitro and in vivo studies. In vivo the alkalizing abilities of calcium silicate-based materials can be reduced by buffering effect of dentin. However, dentin seems to be a stronger buffer for acids than for alkalis . It should also be remembered that there is a permanent exchange of tissue fluids at the material interface that will reduce pH level in clinical situation . Despite of these facts, the use calcium silicate-based materials with prolonged alkaline properties may be more beneficial to anti-inflammatory and mineralization activity.