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ENDO tribuNEDental tribune Middle East & Africa Edition | January-February 2016 Irrigation dynamics in root canal therapy by Prof. Anil Kishen, Canada I rrigation dynamics deals with the pattern of irrigant low, penetration, exchange and the forces produced within the root canal space. Current modes of endodontic irrigation include the traditional syringe needle irrigation or physical methods, such as apical neg- ative-pressure irrigation or sonic/ultrasonically assisted irrigation. Since the nature of irrigation inluences the low of irrigant up to the working length (WL) and interaction of irrigant with the canal wall, it is mandatory to understand the irrigation dynamics associated with various irrigation tech- niques. Endodontic irrigants are liquid antimicrobials used to disin- fect microbial bioilms within the root canal. The process of delivery of endodontic irrigants within the root canal is called irrigation. The overall objec- tives of root canal irrigation are to inactivate bacterial bioilms, inactivate endotoxins, and dis- solve tissue remnants and the smear layer (chemical effects) in the root canals, as well as to allow the low of irrigant en- tirely through the root canal system, in order to detach the bioilm structures and loosen and lush out the debris from the root canals (physical ef- fects). While the chemical ef- fectiveness will be inluenced by the concentration of the an- timicrobial and the duration of action, the physical effective- ness will depend upon the abil- ity of irrigation to generate op- timum streaming forces within the entire root canal system. The inal eficiency of endodon- tic disinfection will depend upon both chemical and physi- cal effectiveness.[1–3] It is im- portant to realise that even the most powerful irrigant will be of no use if it cannot penetrate the apical portion of the root ca- nal, interact with the root canal wall and exchange frequently within the root canal system.[1] Syringe irrigation Irrigation methods are catego- rised as positive-pressure or negative-pressure, according to the mode of delivery em- ployed.[4] In positive-pressure techniques, the pressure dif- ference necessary for irrig- ant low is created between a pressurised container (e.g. a syringe) and the root canal. In negative-pressure techniques, the irrigant is delivered pas- sively near the canal oriice and a suction tip (negative-pres- sure) placed deep inside the root canal creates a pressure difference. The irrigant then lows from the oriice towards the apex, where it is evacuated. A detailed understanding of the irrigation dynamics associated with syringe-based irrigation would aid in improving its ef- fectiveness in clinical practice. Irrigant low during syringe irrigation The low of irrigants is inlu- enced by its physical charac- teristics, such as density and viscosity.[5] These properties for the commonly used end- odontic irrigants are very simi- lar to those of distilled water. [6, 7] The surface tension of endodontic irrigants and its de- crease by surfactants have also been studied extensively. The rationale of this combination is that it may signiicantly af- fect (a) the irrigant penetration into dentinal tubules and ac- cessory root canals[8, 9] and (b) the dissolution of pulp tissue. [10] However, it is important to note that surface tension would only inluence the interface be- tween two immiscible luids, and not between the irrigant and dentinal luid.[5, 11] Ex- periments have conirmed that surfactants do not enhance the ability of sodium hypochlorite to dissolve pulp tissue[12, 13] or the ability of chelating agents to remove the smear layer.[14, 15] The type of needle used has a signiicant effect on the low pattern formed within the root canal, while parameters such as depth of needle insertion and size or taper of the pre- pared root canal have only a limited inluence.[16–19] Gen- erally, the available needles can be classiied as closed-end- ed and open-ended needles. In the case of open-ended needles (lat, bevelled, notched), the ir- rigant stream is very intense and extends apically along the root canal. Depending upon the root canal geometry and the depth of needle insertion, reverse low of irrigant occurs near the canal wall towards the canal oriice. Inthecaseofclosed-endednee- dles (side-vented), the stream of irrigant is formed near the apical side of the outlet and is directed apically. The irrigant tends to follow a curved route around the needle tip, towards the coronal oriice. The low of irrigant apical to the exit of the needle is generally observed to be a passive luid lowing zone (dead zone), while the low of ir- rigant in the remaining aspect of the root canal is observed to be an active luid lowing zone (active zone; Figs. 1a–d & 2a–d). A series of vortices of lowing irrigant are generated apical to the tip. The velocity of irrigant inside each vortex decreases towards the apex. Large needles when used within the root canal hardly penetrate beyond the coronal half of the root canal. Current- ly, smaller-diameter needles (28- or 30-gauge) have been recommended for root canal ir- rigation.[20, 21] This is mainly because of their ability to ad- vance further up to the WL. This facilitates better irrigant exchange and debridement. [22–24] In addition, the use of a larger needle would result in decreased space being avail- able for the reverse low of ir- rigant between the needle and the canal wall. This scenario has been associated with (a) an increased apical pressure for open-ended needles and (b) decreased irrigant refreshment apical to the tip for closed-end- ed needles.[17, 19] The inlu- ence of tooth location (man- dibular, maxillary) on irrigant low has been observed to be minor.[16, 25] irrigant refreshment Irrigant exchange in the root canal system is a key prereq- uisite for achieving optimum chemical effect, because the chemical eficacy of the irrig- ants are known to be rapidly inactivated by dentine, tissue remnants or microbes.[24, 26, 27] Investigations have ex- plained the limitations in the irrigant refreshment apical to needles.[21, 28–30] Enlarging the root canal to place the nee- dle to a few millimetres from the WL and ensuring adequate space around the needle for reverse low of the irrigant to- wards the canal oriice allow effective irrigant refreshment coronal to the needle tip.[17, 19] Furthermore, increasing the volume of irrigant delivered could help to improve refresh- ment in such cases.[20, 31, 32] The effect of curvature on ir- rigant exchange has been studied indirectly by Nguy and Sedgley.[33] They report that only severe curvatures in the order of 24–28° hampered the low of irrigants. If the canal is enlarged to at least size 30 or 35 and a 30-gauge lexible needle placed near the WL is used, then irrigant refreshment can be expected even in severely curved canals. Wall shear stress The frictional stress that occurs between the lowing irrigant and the canal wall is termed “wall shear stress”. This force is of relevance in root canal irrigation because it tends to detach microbial bioilm from the root canal wall. Currently, there is no quantitative data on the minimum shear stress required for the removal of microbial bioilm from the ca- nal wall. Yet, the nature of wall shear stresses produced within the root canals during irriga- tion provides an indication of the mechanical debridement eficacy. In open-ended needles, an area of increased shear wall stresses develops apical to the needle tips, while in closed-ended needles, a higher maximum shear stress is generated near their tips, on the wall facing the needle outlet.[34] Thus, in open- and closed-ended nee- dles, optimum debridement is expected near the tip of the needle.[16, 34] Consequently, it is necessary to move the needle inside the root canal, so that the limited area of high wall shear stress involves as much of the root canal wall as possible. The maximum shear stress decreases with an increase in canal size or taper. Thus, over- zealous root canal enlargement above a certain size or taper could diminish the debride- ment eficacy of irrigation (Figs. 1a–d & 2a–d). Enhancing irrigation dynam- ics using physical irrigation methods Fluid dynamics studies on api- cal negative-pressure irriga- tion have demonstrated maxi- mum apical penetration of the irrigant, without any irrigant extrusion. This inding high- lights the ability of apical neg- ative-pressure irrigation to be safely used at the WL, circum- venting the issues of vapour lock effect.[35] Nonetheless, the apical negative-pressure irri- gation produced the lowest wall shear stress. This decrease in the wall shear stress could be attributed in part to the reduc- tion in the low rate with this irrigation system. Passive ultrasonically assisted irrigation, when compared with other irrigation methods, showed the highest wall shear stress along the root canal wall, with the highest turbulence intensity travelling coronal from the ultrasonic tip posi- tion. The lateral movement of the irrigant displayed by this method has important impli- cations with respect to its abil- ity to permit better interaction between the irrigant and the root canal wall, and to poten- tially enhance the interaction of irrigants with intra-canal bioilms[2, 3, 35] (Figs. 1a–d & 2a–d). Conclusion The requirements of adequate irrigant penetration, irrigant exchange, mechanical effect and minimum risk of apical extrusion oppose each other and a subtle equilibrium is re- quired during irrigation. Ide- ally, in a canal enlarged to size 30 or 35 and taper 0.04 or 0.06, an open-ended needle should be placed 2 or 3 mm short of the WL to ensure adequate irrigant exchange and high wall shear stress, while reducing the risk of extrusion. In the case of a closed-ended needle, placement should be within 1 mm short of the WL, so that optimum irrigant ex- change can be ensured. The apical negative-pressure irri- gation did not generate marked wall shear stress values, but allowed the low of irrigant consistently up to the WL. It was the safest mode of irriga- tion when used close to the WL. The passive ultrasonically as- sisted irrigation generated the highest wall shear stress. The use of combined methods to ob- tain optimum disinfection and to circumvent the limitations of one method is recommended. Editorial note: A list of refer- ences is available from the pub- lisher. With the side-vented needle tip (b), there was a much lower velocity than with the open-ended tip, and it extended only 0.5 mm. The side-vented needle tip (b) showed a localised region with a high amount of shear stress... With the apical negative-pressure irri- gation (c), there was a constant velocity slightly higher than the side-vented nee- dle irrigation that was constant as the irrigant moved coronally. ... while there was not an observable lev- el with the EndoVac irrigation (Kerr; c). The ultrasonically assisted irrigation (d) showed the highest magnitude of ve- locity, constant to at least 3 mm coronal to the tip placement.[35] The ultrasonically assisted irrigation (d) displayed the highest levels of shear stress over the greatest area of the canal wall.[35] Figs. 1a: Velocity magnitude of irriga- tion showing the extent of dead zone. With the open-ended needle tip (a), the velocity progressively decreased 1.5 mm apical from the tip. Figs. 2a: Time-averaged distribution of shear stress on the root canal wall showing a more uniform distribution on the canal wall with the open-ended needle tip (a).