Endodontics has the ultimate goal in cleaning the

Endodontics has reached a level where major developments and innovations are happening routinely. Needle ultra sonics, biocompatibility of filling materials, rotary devices, and imaging of the techniques have been undergoing major changes over the decades. But, the developmental changes in irrigating the root canal system have shown a slow progress. However, nowadays we are seeing developmental surge in advanced irrigation technology 1. The Endodontics has the ultimate goal in cleaning the root canal system effectively. The apical third of the root canal is the most critical area. This is due to the fact that the presence of deltas, fins, and canal spaces in the root canal makes it the highly complex anatomical network, making the instruments inaccessible. The apical third contains over 98% of the canal ramifications2.The mechanical instruments which we used normally have less cleaning capability in the apical third of the root canal. Researchers are being indulged in various studies to invent a superior quality mechanical device which would be implemented in irrigation techniques to achieve an effective cleaning. In a research, it was reported that only 35% of the canal space walls have actually been contacted by the the instruments 3. These instruments are helping in creating a pathway in the canal system and are also helpful in allowing the chemical irrigants solutions to flow through the created path. So we are now mostly dependent on the actions of these chemical agents to clean the complex canal space4. The technique of positive pressure needle irrigation does not allow the solutions to reach the full length of the canal system effectively. The most important aspect in order to clean the apical area is to have a irrigant solution to create a force and with that force to reach the apical region and take the particles away from the apical third of root canal. But these qualities are not present in classic technique5.The apical negative pressure irrigation technique has changed the common grounds in apical irrigation. This technology could allow the chemical irrigants to reach the full working length of the root canal space. This irrigation technique creates negative pressure by suction method. The negative pressure created is used to pull the chemical solutions injected into canal space from the reservoir to the working length using high speed suction. The high speed suction at the tip od he cannulae drives the negative pressure current force. When the blockage of the cannulae occurred or disturbance to the suction force occurred, solutions would not be present in the apex. This is due to the dependent nature of the technology on the suction force to deliver the chemical solution to the full working length of the canal space.This review of the negative pressure apical  irrigation will show a descriptive difference with the positive pressure irrigation, technology used, how it is one of the most advanced canal cleansing, minimisation of risks and maximisation of effectiveness and the clinical impact.OBJECTIVE OF IRRIGATIONThe endodontic treatment has always kept its priority to eliminate or to prevent apical infection. Instruments are made to shape the canals to their convenience and irrigants are injected into the roots to clean the canals. The irrigation of root canal system involves the canal filing and delivering chemical solutions to the full working length of the canal space safely. Irrigation of root canal has been aiming to eradicate the organic and inorganic contaminants. Chemical irrigants and physical devices are being applied in order to achieve the effective irrigation. But, the ability of the root canal irrigation system is very much dependent on the effectiveness of irrigants and the mechanical function of the irrigation technique. Hence, effective cleaning of roots could only be achieved by well crafted mechanical device and the ability of the chemical agents. Conventional positive apical needle irrigation has shown difficulties to improve the delivery of chemical solution into the apical third of the root canal. This had been proved by Nair et al.,6 who showed biofilms were found in mandibular molars even after the procedure of positive pressure irrigation. Mechanical instrumentation of the apical third becomes challenging due to the presence of fins, lateral canals, apical deltas7,8. The irrigants injected into the root canal usually destroys the biofilm in laboratory studies. Hence, it appears that debris formed during instrumentation gets compacted against the apical anatomy, fins and isthmus and made the irrigants hard to reach the apical most part of the root canal, especially when positive irrigation is carried out. In order to avoid such consequences, the need for the irrigation technique, that could avoid the accumulation of debris or remove the debris accumulated, developed. Hence the negative pressure irrigation techniques which sucks the debris in root canal is created. Fig.2 shows the difference in technique in both negative and positive pressure irrigation.APICAL NEGATIVE PRESSURE IRRIGATION SYSTEMThe EndoVac system (Fig.1.) uses suction technique to clear out the debris and promote the flow of the irrigant solution to reach the apical third of the root canal9. This suction pressure creates a movement of the chemical solutions placed in the chamber of the device. The chamber is located in between the master delivery tip and the end of the canal where the tip of the micro cannulae is placed.The EndoVac system uses two steps to remove the debris. The first step involves a macro cannulae of 0.55 mm diameter placed in the middle third to eliminate gross debris and pulp remains. The second step includes a micro cannulae of 0.32 mm diameter used for the full working length to remove the minute debris10. Therefore, appropriate apical enlargement to a minimum of a size 0.35 file must be created to ensure that the micro cannulae tip (0.32 mm) reaches the apical tip. Fig.3A and Fig.3B shows the macro cannulae and micro cannulae respectively.EndoVac system has the advantage of eliminating the debris formed from the instrumentation of shaping the root canals and also the necrotic remains by using a safe delivery of the chemical irrigants to the full working length in the root canal without the risk of apical extrusion11,12. Many researchers have reported the advantages in EndoVac system when compared to the other irrigation techniques. The advantages in EndoVac includes free from risks such as the extrusion of apex and improved cleaning quality13,14,15,16. The researches conducted by various authors in their report, showed that the histological studies showed the advantages of the EndoVac system which improved the quality of cleansing in the apical one third12,16,17. In a study by Baumgartner and Nielsen17, using teeth extracted with vital pulps, it was shown that improved cleaning efficacy of EndoVac system at 1 mm from the working length in comparison to conventional irrigation. The authors also showed that the volume of irrigant delivered by the EndoVac system was 3 times higher, compared to conventional needle irrigation. These results were confirmed in an in-vivo study where the EndoVac system showed a higher ability to clean the apical one third of vital pulp root canals.APICAL NEGATIVE PRESSURE IN PREVENTING EXTRUSION OF THE IRRIGANTSIrrigants are helpful in achieving the goal of cleanliness21,22. But certain irrigants could be extruded into the periapical tissues, which would lead to unwanted complications such as inflammation, hematoma and sometimes necrosis could also occur23,24,25. Hence, it is necessary to reduce the risks associated in root canal irrigation that would benefit the patient more26. In a study, Himel and Desai26, Brown27, Montgomery and Myers28, and Roy and Laurence29, reported that the positive pressure irrigation could result in extrusion of apical root canal. But, Desai and Himel26, Fukumoto30, and Mitchell31 demonstrated that negative pressure irrigation could be reducing the periapical extrusion. Desai and Himel researched the EndoActivator, Ultrasonic needle irrigation and RinsEndo. In EndoActivator, volume of extruded irrigants was very less comparing ultrasonic needle irrigation and RinsEndo. Hence, concerning extrusion negative pressure irrigation is comparatively safe. Apical negative pressure irrigation irrigated the root canal system effectively to the fullest of the working length32. This is due to the design of the micro cannulae, which avoids the vapour lock effect. However, the negative pressure irrigation could not completely activate the irrigants in the non-instrumented areas.IRRIGATION OF CURVED ROOT CANALSRoot canals are curved structures33, which makes them more difficult in accessing and cleaning. This difficulty is due to the contact between the device needles and canal walls. So, bending the instrument to follow the canal structure becomes a necessary, however, it could compromise the irrigation greatly34. Rodig et al.35, and Amato et al.36 , reported that the curvature reduced the cleaning efficiency of several irrigation techniques. But CUI and PUI reaches the working length in curved canals. This is due to the sufficient force exerted at the tip of the instrument to overcome the vapour lock effect. As a result, both exhibit an increase in penetration of the irrigants into the lateral canal and apical third of the canal37-40. Fig. 4. Shows the mode of operation of CANUI – A new concept of irrigation.In a study, Abarajithan et al,19 reported that apical negative pressure irrigation and positive pressure irrigation were equally effective in eliminating the smear layer from the coronal amped middle third of the root canals, while in the apical third the EndoVac system performed significantly better than the needle irrigation. Their results are ordinary considering their usage of large sized apical areas to improve the irrigant flow to the root canals. The difference in smear layer removal at all levels was significant and better than the positive irrigation. The reason behind this finding is due to failure of positive irrigation to avoid vapour lock effect in closed canal system20.Several studies have compared the effectiveness of both positive and negative apical pressure irrigation techniques16,18. Currently no differences between the treatments were found. This could be explained on the basis of the usage of single rooted teeth on these studies which were lacking the complex anatomy of the apical root canal. Also, single rooted teeth could be cleaned easily than the posteriorly positioned teeth. This would certainly explain the absence of difference in efficacy between the positive and negative pressure techniques in clinical settings. The challenge in endodontics treatment is the cleaning of complex regions in root canals. Considering this, the negative apical pressure has a slight advantage over the conventional methods in in-vivo researches.Though negative pressure irrigation would improve the irrigant volumes, contact with the root canal walls, mainly in the uninstrumented areas, which would increase the disinfection and debridement, it is still the questionable debate about the exposure and volume of irrigants in improving the effectiveness of cleansing in the root canal.CONCLUSIONThis review has shone some lights on the EndoVac system which has a slight upper hand over the conventional positive method. Hence, the negative apical pressure system’s improved cleaning ability would result in remarkable improvements in the treatment of complex regions in root canal. However, many studies are needed to be done in order to achieve this in real clinical settings. Since the introduction of the negative pressure technology, many studies have been carried out to modify the technique to attain the effective irrigation. But the majority of the studies have been carried out in in-vivo conditions which causes a great decline in the effectiveness of those successful studies. However, safety and effectiveness should be the primary task in inventing a new technique.