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Endo TribuneAsia Pacific Edition | 12/201518 volume of irrigant delivered could help to improve refreshment in such cases.20, 31, 32 The effect of curvature on irrig- ant exchange has been studied in- directly by Nguy and Sedgley.33 They report that only severe cur- vatures in the order of 24–28° hampered the flow of irrigants. If the canal is enlarged to at least size 30 or 35 and a 30-gauge flexible 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 flowing 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 biofilm from the root canal wall. Currently, there is no quantitative data on the minimum shear stress required for the removal of microbial biofilm from the canal wall. Yet, the nature of wall shear stresses produced withintherootcanalsduringirriga- tion provides an indication of the mechanicaldebridementefficacy. Inopen-endedneedles,anareaof increased shear wall stresses devel- ops apical to the needle tips, while in closed-ended needles, a higher maximumshearstressisgenerated neartheirtips,onthewallfacingthe needle outlet.34 Thus, in open- and closed-endedneedles,optimumde- bridement is expected near the tip of the needle.16, 34 Consequently, it is necessarytomovetheneedleinside the root canal, so that the limited area of high wall shear stress in- volvesasmuchoftherootcanalwall as possible. The maximum shear stress decreases with an increase in canal size or taper. Thus, overzeal- ous root canal enlargement above a certain size or taper could diminish the debridement efficacy of irriga- tion(Figs.1a–d&2a–d). Enhancing irrigation dynamics using physical irrigation methods Fluid dynamics studies on apical negative-pressure irrigation have demonstrated maximum apical penetration of the irrigant, without any irrigant extrusion. This finding highlights the ability of apical nega- tive-pressure irrigation to be safely used at the WL, circumventing the issues of vapour lock effect.35 None- theless, the apical negative-pressure irrigation produced the lowest wall shearstress.Thisdecreaseinthewall shear stress could be attributed in part to the reduction in the flow rate withthisirrigationsystem. Passive ultrasonically assisted ir- rigation,whencomparedwithother irrigation methods, showed the highest wall shear stress along the rootcanalwall,withthehighesttur- bulenceintensitytravellingcoronal fromtheultrasonictipposition.The lateral movement of the irrigant displayed by this method has im- portantimplicationswithrespectto its ability to permit better interac- tion between the irrigant and the root canal wall, and to potentially enhance the interaction of irri- gants with intra-canal biofilms2, 3, 35 (Figs.1a–d&2a–d). Conclusion Therequirementsofadequateirri- gant penetration, irrigant exchange, mechanicaleffectandminimumrisk ofapicalextrusionopposeeachother and a subtle equilibrium is required during irrigation. Ideally, 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, whilereducingtheriskofextrusion. In the case of a closed-ended nee- dle, placement should be within 1 mm short of the WL, so that opti- mum irrigant exchange can be en- sured. The apical negative-pressure irrigation did not generate marked wall shear stress values, but allowed theflowofirrigantconsistentlyupto theWL.Itwasthesafestmodeofirri- gationwhenusedclosetotheWL.The passiveultrasonicallyassistedirriga- tiongeneratedthehighestwallshear stress.Theuseofcombinedmethods to obtain optimum disinfection and to circumvent the limitations of one methodisrecommended. Editorial note: A list of references is avail- ablefromthepublisher. ENDO NEWS Anil Kishen ob- tained his den- tal education in India and is Pro- fessor of Endo- dontics at the University of Toronto’s Fac- ulty of Dentistry in Canada. He can be contacted at anil.kishen@dentistry.utoronto.ca. The Dental Tribune International C.E. 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