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With the arrival of NiTi engine-driven files and mechanized instrumentation systems, root canal instrumentation became exponentially faster and somehow more predictable compared to manual instrumentation techniques. However, with the popularization of these files in recent decades, a breach between speedy preparation and infection control becomes evident, especially regarding the time and volume of irrigating fluid during the operation. Nevertheless, it is undeniable that the technical advance arising from NiTi engine-driven files has benefited the endodontic technique. The question is what is more important, speed or infection control? Naturally, the answer lies in infection control efficiency, without which the repair process will be impossible.
Rapid canal shaping impairs the quality of cleaning and disinfection processes based on the sodium hypochlorite (NaOCl) action principle. It does not provide the time necessary to accomplish either and gives even less time to guarantee the volume and renewal of the irrigating liquid, which can compromise the main objective of the treatment.1 This realization has triggered a change in the primary goal of root canal preparation. Mechanical instrumentation currently provides access to the apical morphology of the canal. It allows irrigants to flow, which is expected to accomplish most of the cleaning and disinfection. Therefore, the focus gradually shifted from types of irrigants to delivery methods and ways to enhance their effectiveness within the intricate root canal system.
The ideal mechanical preparation should uniformly debride and enlarge the entire perimeter of the root canal. However, micro-CT-based studies show inadequate canal preparation with engine-driven files (Figure 1) despite their design or kinematics (rotation or asymmetric reciprocation movements). As a result, microorganisms that remain on unprepared dentin walls may have the opportunity to recolonize the canal system, compromising the treatment outcome.2,3
Another critical point to be considered is to clean the canal space while preserving the maximum sound structure of the tooth. As already noted, current endodontic instruments have not evolved sufficiently to the point of adapting perfectly to the anatomy of the root canal during their use. Thus, the unnecessary removal of sound dentin tissue results from any preparation protocol currently available because of the need to reach all the root canal walls. Even performing more minor dentin-cutting maneuvers with small instruments—the minimally invasive preparation—does not seem to reduce unnecessary dentin removal significantly4 and can compromise proper cleaning and root canal disinfection.5
A side effect of mechanical preparation is hard-tissue debris accumulation,6 which carries the likelihood of being more clinically relevant than the smear layer. Engine-driven shaping produces and packs dentin debris into irregular areas of the canal space, with a sizable accumulation of debris in fins, isthmuses, and irregularities with ramifications of the complex canal network. In addition, non-removed debris could easily harbor bacteria biofilm from the disinfection procedures and reduce the antiseptic activity of the irrigating solution.7-9
THE QUINTESSENTIAL IMPORTANCE OF IRRIGATION
From a didactic point of view, the endodontic technique is divided into separate steps or phases so that the student can understand and practice systematically; from a clinical standpoint, the process is quite different. Cleaning and shaping should be seen as one single procedure. Mechanical instrumentation opens the way for the action of chemical disinfection through the use of irrigating solutions and intracanal medicaments. The initial cleaning of root canals is commonly performed by mechanical instrumentation, which removes the large bulk of the pulp tissue, the necrotic debris, bacterial biofilms, and/or a previous root filling. Unless this bulk material is first removed, no further cleaning is possible. Nevertheless, it is prudent to state that final intracanal disinfection mainly depends on the physicochemical activity of irrigation. Reaching areas not touched during the mechanical preparation with NaOCl solution is necessary to dissolve the biofilm and remaining necrotic tissues. When instruments fail to prepare the canal walls in complex canal systems, such as in molars or even oval canals, the chemical action of sodium hypochlorite is the last opportunity to gain control of the infectious contingent within the root canal system.
Recent histological and micro-CT analysis studies have determined that the instrumentation leaves around 40% of the walls untouched.10,11 Furthermore, the conventional irrigation technique with a syringe and an open-ended cannula does not solve the problem. There is conclusive evidence in the literature that more effective irrigation techniques related to accessing irrigation fluids in critical areas—for instance, the last apical millimeters and isthmuses and irregularities—should be used instead of the conventional irrigation technique.12,13
It seems paradoxical that, on the one hand, instrumentation techniques have undergone an extraordinary evolution with NiTi instruments capable of cutting significant amounts of dentin in short periods, yet on the other hand, the NaOCl solution requires a more extended time than that required by mechanical instrumentation to reach its full potential.14-16 The result is that the root canal is shaped but not properly cleaned and disinfected.
In the syringe-cannula classic irrigation method, irrigants are delivered deeply into the canal space using positive pressure irrigation through a cannula connected to a syringe via applying finger pressure on the syringe plunger, which pushes the irrigant solution through the cannula into the canal space by directly injecting the solution. In contrast, the suction cannula simultaneously aspirates it. The high flow rate used to introduce the irrigant into the canal results in technique-related factors that increase irrigant pressure at the apical portion of the canal when open-ended cannulas are used. This can sometimes extrude the solution periapically, resulting in tissue damage and postoperative pain. The proper frequency of such accidents is unknown as many of them may not be reported. Minor extrusion incidents might even remain undetected due to the absence of severe symptoms. A 2008 survey of endodontists in the United States indicated that nearly half of the respondents (42%) had experienced at least one NaOCl accident during their practice careers.17 The extrusion accident, and its consequences, explain why most clinicians often avoid a closer approach to the working length while irrigating with NaOCl solution. Regarding its efficacy, conventional syringes and open-ended cannula irrigation leave a large amount of debris clogged in the irregularities of the root canal system and do not efficiently deliver the irrigant solution into the apical third of the canal.18
Modifications in the distal end of the irrigation cannulas were carried out to minimize the problem of accidental NaOCl extrusion. This allowed the use of these closed-ended cannulas in a position closer to the working length, resulting in the development of lower liquid pressure at the apical foramen compared to open-ended cannulas. The modification directly affects the irrigant flow rate, the most frequently reported technique-related factor in extrusion accident case reports. Closed-ended, open-side cannulas (Figure 2, right) vary in diameter, length, and tip design. Because of its lateral laminar flow (and not contiguous along the long axis of the cannula), safety is improved. However, it has a negative impact on the effectiveness of irrigation in some areas of the canal, predominantly in the last apical millimeters.
Additionally, it is well documented that a “dead water” or stagnation zone is produced between the tip of the closed-end cannula and the apex, where no flow of irrigation occurs,19-21 resulting in the accumulation of debris in this region.22 This phenomenon, known as the “vapor lock effect or blockage” (Figure 3), has been confirmed in both in vitro and in vivo studies.23,24 Its origin is attributed to the interaction between the trapping of air bubbles caused by the irrigation distribution (positive pressure) and the gas production created by the chemical reaction of NaOCl with organic tissues. As a result, a vapor lock could theoretically block the irrigant from flowing toward the apical third. The phenomenon is described as the difficulty of irrigant solution dispersion in a narrowed space such as the root canal.
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