# Narrative Review on Methods of Activating Irrigation Liquids for Root Canal Treatment
## 4 Irrigants Activation Techniques
### Thermal Techniques  
The boiling point of sodium hypochlorite is between 96 and 120 \({}^{\circ}\)C [43]. The potential of a heated NaOCl solution to dissolve pulp and cleanse a root canal is greater than that of a solution at ambient temperature [44]. Additional heating of sodium hypochlorite enhances its ability to dissolve necrotic pulp tissue as well as improves its efficiency against _E. faecalis_. However, heated solutions remove the organic part of dentine shavings more effectively than non-heated equivalents. High temperature raises the rate of the NaOCl reaction positively influencing its antibacterial properties as well as capability of dissolving organic residues. There is evidence that heating to 50-60 \({}^{\circ}\)C is the optimal temperature range [45] and frequent replacement of the irrigant in the root canal system is required due to the solution's rapid temperature decrease. It has been demonstrated that irrigants injected into the root canal at 66 \({}^{\circ}\)C reach 41 \({}^{\circ}\)C within 15 s and 36 \({}^{\circ}\)C within 4 min [46]. Low-concentration NaCl at 45 \({}^{\circ}\)C has the same tissue-dissolving ability as a 5.25 percent solution at 20 \({}^{\circ}\)C [7].  
Increasing the temperature of NaOCl improves its tissue-dissolution capacity, but on the other hand, enhances toxicity and tissue irritation [47]. From the electrochemical point of view, elevated temperature increases the oxidation-reduction potential of NaOCl making it a more potent agent [48]. Furthermore, electrical activation of NaOCl prior to application resulted in superior bovine muscle dissolution in comparison to inactivated one [49].  
There are two ways to increase the temperature of irrigating solution: extracanal (heating of the solution before injection to the root canal) and intracanal (heating directly inside the root canal with heat carriers). According to research, intracanal heating of NaOCl is more efficacious than extracanal heating [50]. The former technique caused much better cleansing of the walls of the root canal from debris in comparison to the latter [51]. It occurs most probably due to the rapid heat exchange between the tissue and environment in in vivo conditions. It results in less effective extracanal heating of the solution than was demonstrated in experiments [52].  
Heat carriers (System-B--Endodontic Heat Source, Kerr Endodontics, Gilbert, AZ, USA, or similar) are utilized for intracanal heating. The maximum temperature of the device is between 150 and 180 degrees Celsius, and the measurement of the tip is 30/04. Before introducing the heat carrier, processing of the root canal is necessary. The root canal is filled with sodium hypochlorite with an endodontic needle. The heat carrier is inserted at room temperature up to 3 mm from the end of the working length and subsequently activated. Each activation cycle of the heat carrier lasts 5 s with another 5 s of pause. During the process, carriers make short upward and downward movements with a few mm of amplitude in order to mix the NaOCl solution. After each of the aforementioned cycles, the irrigation solution is replaced with a fresh one [43; 53]. Clinicians recommend intracanal UA of heated NaOCl because it results in superior penetration into dentin tubules and canal purity compared to syringe activation or UA alone [52]. While combining thermal activation and UA, better pulp dissolution was obtained in the side dentin tubules that remained contaminated after using either UA or thermal activation alone [53].