Re: IRRIGATION IN ENDODONTICS

THE SODIUM HYPOCHLORITE ACCIDENT
Fear of the sodium hypochlorite accident touches upon every aspect of endodontic treatment, from patient safety to dreaded years of litigation. What causes this phenomenon? All maxillary posterior teeth are capable of direct communication with the maxillary sinus, and sometimes even the Schneiderian membrane is absent (Figure 9). Direct communication between the maxillary sinus and the root canal systems of these teeth offers virtually no resistance to fluid escaping from the root canal space,16,17 and may be the most frequent cause of a sodium hypochlorite accident. However, these accidents also happen in the mandibular region, sometimes with even more devastating consequences18 (Figures 2 and 3).
Although the exact mechanism for the sodium hypochlorite accident has never been elucidated, the capillary blood pressure in the pulp is about 25-mm Hg or 0.48 psi,19 and the pulpal and periapical capillaries would seem to be a logical portal of entry to the immediate vasculature once endodontic irrigation pressure exceeds the normal regional blood pressure. In 2002, Bradford, et al20 explored several needle factors to determine which could produce the safest air pressure method to dry the root canal system. They explored open-ended versus side-venting needle designs, placement relative to binding point, and size. Although this study used positive air pressure in the root canal, the results apply equally to irrigation fluids, since “Fluid mechanics is the subdiscipline of continuum mechanics that studies fluids, that is, liquids and gases.”21 Bradford, et al20 concluded 2 disturbing facts: first, “No needle design proved safe to use in either round or ovoid canals, regardless of stage of instrumentation.” Second, “The clinical significance of these results is that there is no way to ensure complete safety when drying canals with pressurized air.” Given that the principles of fluid mechanics apply to both gases and liquids, there is also no way to ensure complete safety when delivering canal irrigants under positive pressure. Bradford, et al20 further concluded (again applicable to canal irrigation) that “Vacuum, rather than air under pressure, may be a superior means for canal drying.” The operative word is vacuum.
The EndoVac (endodontic vacuum) was designed to overcome the dangers of pushing irrigants into the capillary beds or the maxillary sinus by creating an apical negative pressure at full WL. The key component of the EndoVac system is a microcannula with an external diameter of 0.32 mm, a spherically sealed end used for guidance, and a population of 12 microholes radially arranged in the last 0.7 mm (Figure 10). The microholes serve 2 functions: to pull endodontic irrigants directly and abundantly to the last 0.2 mm of WL (Figure 11), and to serve as a micro filtration system to prevent clogging of the lumen (internal diameter) of the microcannula.
Other manufacturers also claim endodontic irrigation via “negative pressure,” but not “apical negative pressure.” Apical is the operative word. True ANP only occurs if the needle/cannula is used to aspirate irrigants from the apical termination of the root canal space (Figure 11). If the needle/cannula is used to discharge irrigants into the root canal system (Figure 4), it is a positive pressure device. Two simple metaphors best help describe the differences: the fire hose and the sewer pump. If irrigants are pushed out of the needle/cannula, which is how a fire hose discharges water, this is positive pressure. If the irrigants are sucked into the needle/cannula, which is how a sewer pump cleans a septic tank, this is apical negative pressure.
The apical suction effect of pulling (not pushing) endodontic irrigants down and along the walls of the root canal system creates a rapid turbulent cascading effect as the irrigants are forced to flow between the canal walls and the external surface of the microcannula. This turbulent action creates a current force, while the position of the microholes directs this fast-flowing stream of irrigant as close as 0.2 mm from full WL before reversing the irrigant’s direction up the microcannula