The interaction between the calcium silicate and water leads to the setting reaction and final hardening of Biodentine. This reaction is based on hydration of the tricalcium silicate (3CaO. SiO2 = C3S), which results in a hydrated calcium silicate gel (CSH gel) and calcium hydroxide (Ca(OH)2) (Camilleri 2008, Camilleri et al. 2012)
Tay et al. (2007), Han et al. (2010) and Grech et al. (2013)
provided evidence that tricalcium silicate-based preparations are a source for production of hydroxyapatite after they have come in contact with synthetic tissue fluids.
|2(3CaO.SiO2) + 6H2O
||3CaO.2SiO2. 3H2O + 3Ca(OH)2
This solution process takes place on the surface of each individual calcium silicate particle. In the presence of the excess calcium hydroxide, the hydrated calcium silicate gel tends to precipitate on the surface of the Biodentine particle. This precipitation process is seen more in systems with an overall low water content.
Unreacted tricalcium silicate molecules are surrounded by a layer of the hydrated calcium silicate gel, which is relatively impervious to fluids. The water impermeability of this gel layer slows or prevents further reactions and their effects.
The CSH gel results from the permanent hydration of the tricalcium silicate gel. It successively fills the gaps between the individual tricalcium silicate molecules. The setting and hardening process represents the continuing gel crystallization process (formation of crystal nuclei), which takes place in supersaturated solutions (Camilleri et al. 2013)
For exact measurement, the initial setting time was defined in these studies by the manufacturer Septodont as the time at which the material's elastic modulus has decreased and reached a level of 10 MPa. The final setting time has ended when the material has an elastic modulus of 100 MPa. The interval between mixing and initial setting corresponds to the working time.
||Setting time (in min.)
|Fuji IX (GIZ)
These study results permit the conclusion that the processing time of Biodentine is about 6 minutes and the final setting time is 10-12 minutes (Nonat and Franquin 2006). Biodentine therefore falls into a similar time frame as that of conventional amalgams. It is apparent from the data that classic glass ionomer cement needs a much lower setting time of just under 4 minutes. By contrast, MTA with its setting time of several hours is in last place by far.
Another method of tracing the setting process is measurement of electrical resistance. This measures the mobility of the individual ions, which depends on the pore volume and the number of ions in the material during the setting reaction. Using this technique, it was shown that Biodentine's material properties improve further even after the actual setting time of c. 12 min. The structural density of the Biodentine increases while the porosity diminishes (Goldberg et al. 2009)
. This means that Biodentine is able to optimize its mechanical properties over time.
These results contrast with the series of studies by Grech et al. (2013)
. They determined the setting time of Biodentine using the indentation technique, while the sample was stored in Hanks solution, a buffered, Ca- and Mg-free salt solution used in the production of vital cell suspensions. In this way, they found a setting time of 45 minutes and concluded that the addition of calcium chloride to the liquid component of Biodentine is responsible for the comparatively rapid setting process. Bortoluzzi et al. (2009)
had already demonstrated this accelerating effect of calcium chloride with MTA. The results of Grech et al. initially contradict the manufacturer's product information, which gives a setting time of 9–12 minutes for Biodentine. It must be noted that the term setting time is defined differently in the articles in question and the experiment designs differ somewhat. The data published by the manufacturer Septodont correspond more to the duration of initial setting, while Grech et al.
have focused on the final setting time. In general, when comparing studies on this subject, the greatest care should be paid to comparability (study design, methodology, definition of terms) to avoid false or misleading interpretations of the study results.
Villat et al. (2010)
, for example, chose impedance spectroscopy, which measures the impedance of a substance, to evaluate the setting time of Biodentine. Interestingly, they came to the conclusion that the impedance levels of glass ionomer cements stabilized after 5 days, whereas calcium silicate-based products needed 14 days for this. The authors assume that the greater porosity of Biodentine allows greater scope for ion exchanges between the material and its surroundings.
Since Biodentine can be used as a lining material or dentin substitute under definitive restorations, several studies have dealt with its bonding ability to different adhesive systems. Odabas et al. (2013) investigated the shear strength of a conventional etch & rinse system, a 2-step self-etch adhesive and a 1-step self-etch adhesive system in relation to Biodentine at different times (after 12 minutes and 24 hours). The study showed that there was no significant difference in bond strength between the tested adhesive systems at the respective measurement times.
The study by Aggarwal et al. (2013) looked at the bond strength of Biodentine, ProRoot MTA and MTA Plus under the aspect of perforation cover in the furcation region of teeth. They found that the bond strength of all products generally increased with time but that the adhesive strength of the MTA after 24 hours was inferior to that of Biodentine. The authors noted the fact that contact with blood did not interfere with the bond strength or setting duration of Biodentine as a further plus point of Biodentine.
El-Ma’aita et al. (2013) concentrated in their study on the effect of leaving or removing the smear layer on the bond strength of root filling materials. The authors compared Biodentine, ProRoot MTA and Harvard MTA. It should be noted that it is not current practice to use these materials for complete root canal fillings but the authors demonstrated the very good adhesive characteristics of these products in their study so that their use as sole root filling material in future appears entirely possible.
The studies showed that removal of the smear layer reduces the bond strength of calcium silicate-based cements considerably. The authors concluded from this result that the smear layer plays an important part in the development and structure of the surface-active layer between dentin and Biodentine. In addition, the authors suspected that the smear layer can possibly interact at a mineral level both with the C-S-C and with the root dentin.
Guneser et al. (2013) found that Biodentine acts very well as a material for covering a perforation even when it has come in contact with endodontic irrigants such as NaOCl, CHX or other salt-containing solutions. By contrast, MTA showed the lowest adhesive values in this study.