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ePaper created 2016-03-31, 16:35:38 | version 1.38.0
| review irrigation 18 roots1 2016 needle tip.17, 19 Furthermore, increasing the volume of irrigant delivered could help to improve refreshment in such cases.20, 31, 32 The effect of curvature on irrigant exchange has beenstudiedindirectlybyNguyandSedgley.33 Theyre- portthatonlyseverecurvaturesintheorderof24–28° hamperedtheflowofirrigants.Ifthecanalisenlarged to at least size 30 or 35 and a 30-gauge flexible needle placed near the WL is used, then irrigant refreshment canbeexpectedeveninseverelycurvedcanals. Wall shear stress The frictional stress that occurs between the flow- ing irrigant and the canal wall is termed “wall shear stress”. This force is of relevance in root canal irriga- tionbecauseittendstodetachmicrobialbiofilmfrom therootcanalwall.Currently,thereisnoquantitative dataontheminimumshearstressrequiredforthere- moval of microbial biofilm from the canal wall. Yet, thenatureofwallshearstressesproducedwithinthe rootcanalsduringirrigationprovidesanindicationof the mechanical debridement efficacy. 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 needles, optimum debridement is expected near the tip of the needle.16, 34 Consequently, it is necessary to movetheneedleinsidetherootcanal,sothatthelim- itedareaofhighwallshearstressinvolvesasmuchof the root canal wall as possible. The maximum shear stressdecreaseswithanincreaseincanalsizeortaper. Thus, overzealous root canal enlargement above a certain size or taper could diminish the debridement efficacy of irrigation (Figs. 1a–d & 2a–d). Enhancing irrigation dynamics using physical irrigation methods Fluiddynamicsstudiesonapicalnegative-pressure irrigation have demonstrated maximum apical pene- tration of the irrigant, without any irrigant extrusion. This finding highlights the ability of apical negative- pressureirrigationtobesafelyusedattheWL,circum- ventingtheissuesofvapourlockeffect.35 Nonetheless, the apical negative-pressure irrigation produced the lowestwallshearstress.Thisdecreaseinthewallshear stresscouldbeattributedinparttothereductioninthe flow rate with this irrigation system. Passive ultrasonically assisted irrigation, when comparedwithotherirrigationmethods,showedthe highest wall shear stress along the root canal wall, withthehighestturbulenceintensitytravellingcoro- nalfromtheultrasonictipposition.Thelateralmove- mentoftheirrigantdisplayedbythismethodhasim- portant implications with respect to its ability to permit better interaction between the irrigant and the root canal wall, and to potentially enhance the interaction of irrigants with intra-canal biofilms2, 3, 35 (Figs. 1a–d & 2a–d). Conclusion Therequirementsofadequateirrigantpenetration, irrigant exchange, mechanical effect and minimum riskofapicalextrusionopposeeachotherandasubtle 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 shortoftheWLtoensureadequateirrigantexchange 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 opti- mum irrigant exchange can be ensured. The apical negative-pressureirrigationdidnotgeneratemarked wallshearstressvalues,butallowedtheflowofirrig- ant consistently up to the WL. It was the safest mode of irrigation when used close to the WL. The passive ultrasonically assisted irrigation generated the high- estwallshearstress.Theuseofcombinedmethodsto obtain optimum disinfection and to circumvent the limitations of one method is recommended._ Editorial note: A list of references is available from the publisher. contact Prof.Anil Kishen obtained his dental education in India and is Professor of Endodontics at the University ofToronto’s Faculty of Dentistry in Canada. He can be contacted at anil.kishen@ dentistry.utoronto.ca Figs.2a–d: 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).The side-vented needle tip (b) showed a localised region with a high amount of shear stress,while there was not an observable level with the EndoVac irrigation (Kerr; c).The ultrasonically assisted irrigation (d) displayed the highest levels of shear stress over the greatest area of the canal wall.35 Fig.2c Fig.2d