Optimizing Endodontic Irrigation: Advantages of Negative Apical Pressure Technology

INTRODUCTION
The objective of endodontic treatment remains and has always been to eliminate and/or prevent apical periodontitis. Instruments shape canals and irrigants clean canals. Root canal irrigation aims to eliminate organic and inorganic debris, such as biofilms, necrotic pulp tissue, and dentin debris from vital and/or contaminated root canal systems. To achieve these goals, this procedure should effectively remove the potential irritants from the orifice of the root canal to the apical limit. This is done by chemical and physical means.
In this article, the authors will review several advantages of negative apical pressure to improve the efficacy of endodontic irrigation and its ability to avoid sodium hypochlorite (NaOCl) extrusion in the periapical tissues.

SODIUM HYPOCHLORITE
Bacteria in the form of biofilms and their products in necrotic root canal systems are the main etiological factor of apical periodontitis.¹ Biofilms are the common “ecosystem” of bacteria in root canals, being found even in shaped root canals of teeth with complex anatomies or in retreatment cases.^2,3 The results of clinical and in vitro studies have shown that NaOCl is currently the more effective irrigant solution available for endodontic treatment.

Figures 1a to 1c. Confocal laser scanning microscopy of biofilms attached to dentin structure. Live bacteria present green fluorescence and dead bacteria present red fluorescence. (a) The biofilm was treated with 5 minutes of EDTA; no dissolution effect. (b) The biofilm was treated with a 2% chlorhexidine irrigant solution for 5 minutes. No dissolution effect was visible, and the dentin was covered by a large layer of bacteria. The effect of 5 minutes of 2.5% sodium hypochlorite (NaOCl) can be seen in (c). Residual biofilm layers are visible in red, also an evident dissolution effect can be observed in comparison to EDTA and chlorhexidine. Dentin structure is partially visible due to the limited time contact used (5 minutes).

Figure 2. Microcomputed tomography (MicroCT) sections of mesial roots of mandibular molars at the one- mm apical level. Multiple presence of isthmuses, fins, and irregular anatomy of the root canal system are evident. The goal of endodontic irrigation is to deliver the NaOCl irrigant solution to this level in a safe way to improve dissolution of necrotic debris and bacteria without the risk of NaOCl extrusion. It is noted the common presence of inaccessible areas that probably cannot be addressed by mechanical instruments.

Sodium hypochlorite dissolution ability assists not only in clearing remnants tissue but also it has proved efficacy to dissolve biofilms attached to dentin independently of the concentration used.4 Research in the biofilm area has shown that the more effective strategy to eliminate biofilm is to dissolve its physical structure.⁵ This property is lacking in chlorhexidine, EDTA, and other commonly used endodontic antimicrobial agents.⁶ Even if combinations of chelating agents with antimicrobial have the ability to diffuse into biofilms, residual biofilm layers (even if nonviable) can affect directly the interfacial adaptation of root canal sealers to root canal walls. This is because they will act as an intermediate organic layer between dentin and sealer, thereby increasing the potential for leakage (Figures 1a to 1c). The dissolution ability of NaOCl not only assists the killing of both planktonic and biofilm bacteria, but also can improve the cleaning of the dentin wall in nonshaped root canal walls.⁷ For this reason, NaOCl has been recommended as the primary endodontic irrigant for several decades.
Two limitations of NaOCl have been identified. It cannot remove the smear layer and it does not prevent the accumulation of hard-tissue debris.^7,8 During instrumentation, dentin debris may be compacted against fins or isthmuses.8 This is a common phenomenon when rotary instrumentation is used.⁹ The cleaning of these hard to reach areas, such as isthmuses and fins, is a real challenge for endodontic irrigation because dentin debris may physically limit the diffusion of NaOCl, thereby inactivating its antimicrobial activity and consequently decreasing its effectiveness.¹⁰

Limitations of Conventional Endodontic Irrigation
Despite several advantages of NaOCl, its ability to decontaminate the root canal system in a predictable way has not been consistent.¹¹ The effectiveness of root canal irrigants depends not only on their chemical characteristics but also is directly related to the mechanical effectiveness of the irrigation technique. Conventional positive apical needle irrigation has shown difficulties to improve delivery of NaOCl to the apical one third of the root canal system. This fact has been confirmed microscopically by Nair et al,² who showed that biofilms were found in mandibular molars’ accessory anatomy such as isthmuses, even after rotary or manual instrumentation and 5.25% NaOCl needle irrigation was delivered the conventional way.² Mechanical instrumentation of the apical one third can be challenging considering that the presence of fins, lateral canals, or apical deltas are very common^12,13 (Figures 2, 3a, and 3b). Due to the consistent abilities of NaOCl to kill biofilm in laboratory studies, it appears that apical anatomy and debris developed during instrumentation and compacted against the isthmuses and fins can prevent the perfusion of NaOCl into these hard to reach areas, especially when positive apical pressure is used. Thus, the search for irrigation methods that avoid the accumulation of debris during instrumentation will improve NaOCl delivery and consequently the antimicrobial and dissolution effect within the whole root canal system.

Figures 3a and 3b. MicroCT 3-dimensional (3-D) reconstruction of the mesial root canal of mandibular molar; the presence of an isthmus between the root canals and multiple foramina are evident. These areas are difficult to be cleaned mechanically.

APICAL NEGATIVE PRESSURE SYSTEM
Typically, conventional irrigation systems used positive apical pressure to deliver NaOCl into the apical third. Dynamic irrigation (using sonic or ultrasonic devices) is also used. The EndoVac (Axis|SybronEndo) system uses suction to promote the flow of the irrigant solution to the apical area of the root canal.¹⁴ This suction creates a movement of the irrigant that has been passively placed into the pulp chamber by the master delivery tip all the way to the end of the root canal where the tip of the microcannulae is placed.
Two steps occur in the use of the EndoVac in order to remove the organic and inorganic debris. In the first step, a macrocanulae is placed but not limited to the middle third to eliminate gross debris and pulp remnants. In the second step, a microcannulae is used at the full working length to eliminate microdebris. The macrocannulae tip size is 0.55 mm, whereas the microcannula has a diameter of 0.32 mm (Figures 4a and 4b). Therefore, appropriate apical enlargement to a minimum of a size .35 file must be created to ensure that the microcannulae tip (0.32 mm) reaches the apical terminus.¹⁵
Advantages of EndoVac system include the ability to eliminate debris created from rotary instrumentation and necrotic pulp remnants by using a safe flow of NaOCl to the full working length without the risk of apical extrusion.^16,17 Several studies have shown the advantages in using the EndoVac system, in comparison to other irrigations systems; mainly zero risk of NaOCl extrusion (when used according to the manufacturer’s recommendations) and improved cleaning ability of the root canal system.^19-27 (A representative case using negative apical pressure is shown in Figures 5a to 5f.)

Figures 4a and 4b. Microcanula of the EndoVac system (a). MicroCT tomography 3-D reconstruction of the microcanula. The flow of the irrigant solution is indicated by the arrows (b).

Figures 5a to 5f. Clinical case performed by using negative apical pressure. A mandibular molar with failed endodontic treatment and presence of fistula (a to c). After removal of post and carrier based obturation filling material (d), the cleaning of the apical area shows complex apical anatomy properly filled (e), similar to the 3-D microCT model that shows a similar pattern of complexity in the apical third (f).

REDUCED RISK OF SODIUM HYPOCHLORITE EXTRUSION
The efficacy of conventional positive pressure needle irrigation of NaOCl can be improved by placing the irrigation needle one mm from working length and injecting. This method creates a force that not only can be mechanically effective to remove debris from the root canal,18 but may also cause the extrusion of NaOCl into the periapical tissues.²¹ However, despite the NaOCl ability to kill bacteria, its effect can be also extrapolated to human cells; thus, it is considered toxic if the solution reaches the periapical tissues. In this context, placing the irrigation needle one mm from the working length can increase the chance of NaOCl extrusion or postoperative pain.
The inability of different devices to avoid extrusion of the irrigant solutions has been addressed in 3 previous studies.^19-21 The results of these studies showed that the EndoVac system where apical negative pressure is used, avoids NaOCl extrusion in comparison to typical needle irrigation (positive apical pressure), sonic, or ultrasonic irrigation. This research led to the conclusion that the EndoVac system is able to provide a safe current flow of NaOCl to the apical terminus. These observations were confirmed clinically by a randomized clinical trial that showed that the EndoVac system allows a less painful endodontic treatment postoperatively.²²

IMPROVED CLEANING AND DECONTAMINATION OF THE CANAL SYSTEM
The ultimate goal of the microbial control phase is the elimination of necrotic tissue and biofilms in order to promote periradicular healing. To kill biofilms and dissolve necrotic tissue, NaOCl should be safely delivered to the apical one third of the root canal system. Failures to clean the apical one third can be explained by lack of NaOCl contact with the necrotic tissue or by an inadequate contact time. Three remarkable references show the advantage of the EndoVac system to improve the cleaning of the apical one third as observed by histological sections.^17,23,24 Nielsen and Baumgartner,²⁴ using extracted teeth with vital pulps, showed improved cleaning efficacy at one 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 at least 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 canals23 or necrotic teeth in an animal model.²⁵
Several studies have compared the antimicrobial efficacy of NaOCl delivered by positive or negative apical pressure.^26,27 Currently, no differences between the treatments were found. This result can be explained because microbiological in vitro studies are performed in single-rooted teeth that lack of complex apical anatomy. In addition, single-rooted teeth are more easily mechanically cleaned in comparison to posterior teeth. This can explain the absence of differences related for both irrigation techniques. However, cleaning of complex anatomies, such as the mesial root canals of mandibular molars, becomes the real challenge of endodontic treatment. The superiority of a negative apical pressure system, to address the cleaning of complex root canals and the difficulty that NaOCl has to reach and dissolve the tissue within the fins/isthmuses using conventional methods, has been shown in a previous study. Susin et al,¹⁷ using extracted mandibular molars, evaluated the area occupied by debris at the isthmus level between the one- to 3-mm apical level. Two irrigation methods were compared: the EndoVac system and the manual dynamic irrigation that uses a gutta-percha in a push-pull motion to increase the chance of NaOCl penetration into the apical third. The results again showed superior cleaning ability of the negative apical pressure system. At least 15% to 27% of isthmuses were filled with debris or pulp tissue in the manual dynamic irrigation in comparison to 0.7% to 2.5% in the EndoVac system. Elimination of debris of fins and isthmuses not only decreases the microbial load of the infected root canal, but also increases the quality of the obturation process, because the presence of debris decreases the chance to obtain a correct sealer/dentin interface.
Regardless of the aforementioned advantages and benefits of the EndoVac system, superior treatment outcome is the prime objective of all new endodontic methods, materials, and techniques. Recently, Paredes-Vieyra and Enriquez,²⁸ in a long-term clinical study, demonstrated that the negative apical pressure system produced a 96.57% success rate when nonvital teeth were treated in a single visit. This is remarkably higher success than the 80.1% rate reported by Su et al.²⁹

CONCLUSION
The literature has established that the use of the EndoVac system, based on negative apical pressure in conjunction with NaOCl delivery, has several advantages in comparison to conventional irrigation techniques. Simply put, the negative apical pressure system’s enhanced cleaning ability in complex anatomies results in remarkably improved treatment outcomes.

Acknowledgement
The work of Dr. Ronald Ordinola-Zapata is supported by the São Paulo Research Foundation (2010/16002-4).

References

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Dr. Ordinola-Zapata is a PhD student from the endodontic department at the Bauru School of Dentistry, University of São Paulo, Brazil. He has published more than 25 scientific papers in endodontic journals. Main research interest areas include dental anatomy, irrigant solutions, microcomputed tomography, and decontamination methods of the root canal system. Dr. Ordinola-Zapata is also author of the iBook, The Internal Anatomy of Human Teeth. He can be reached at ronaldordinola@usp.br.

Disclosure: Dr. Ordinola-Zapata reports no disclosures.

Dr. Glassman graduated from the University of Toronto, Faculty of Dentistry, in 1984 and was awarded the James B. Willmott Scholarship, the Mosby Scholarship, and the George Hare Endodontic Scholarship for proficiency in endodontics. A graduate of the endodontology program at Temple University in 1987, he received the Louis I. Grossman Study Club Award for academic and clinical proficiency in endodontics. The author of numerous publications, Dr. Glassman lectures globally on endodontics, is on staff at the University of Toronto, Faculty of Dentistry in the graduate department of endodontics, and is adjunct professor of dentistry and director of endodontic programming for the University of Technology, Jamaica. He is a Fellow of the Royal College of Dentists of Canada, and the endodontic editor for Oral Health. He maintains a private practice, Endodontic Specialists, in Toronto, Ontario, Canada. He can be reached at rootcanals.ca.

Disclosure: Dr. Glassman is a consultant for Axis|Sybron Endo.

Dr. Bramante is a full professor of the endodontic department at the Bauru School of Dentistry, University of São Paulo, Brazil. He has lectured during the past 30 years in several South and North American countries. He can be reached at bramante@fob.usp.br.

Disclosure: Dr. Bramante reports no disclosures.