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Endo Tribune United Kingdom Edition

23 Endo Tribune United Kingdom Edition | 9/2016 TRENDS & APPLICATIONS to follow a curved route around the needle tip, towards the coronal orifice. The flow of irrigant apical to the exit of the needle is gener- ally observed to be a passive fluid flowing zone (dead zone), while the flow of irrigant in the remain- ing aspect of the root canal is ob- served to be an active fluid flow- ing zone (active zone; Figs. 1a–d & 2a–d). A series of vortices of flow- ing irrigant are generated apical to the tip. The velocity of irrigant inside each vortex decreases to- wards the apex. Large needles when used within the root canal hardly pen- etrate beyond the coronal half of the root canal. Currently, smaller- diameter needles (28- or 30-gauge) have been recommended for root canal irrigation.20, 21 This is mainly because of their ability to advance further up to the WL. This facili- tates better irrigant exchange and debridement.22–24 In addition, the use of a larger needle would result in decreased space being available for the reverse flow of irrigant be- tween the needle and the canal wall. This scenario has been asso- ciated with (a) an increased apical pressure for open-ended needles and (b) decreased irrigant refresh- ment apical to the tip for closed- ended needles.17–19 The influence of tooth location (mandibular, max- illary) on irrigant flow has been observed to be minor.16, 25 Irrigant refreshment Irrigant exchange in the root canal system is a key prerequisite for achieving optimum chemical effect, because the chemical effi- cacy of the irrigants are known to be rapidly inactivated by dentine, tissue remnants or microbes.24, 26–27 Investigations have explained the limitations in the irrigant refresh- ment apical to needles.21, 28–30 En- larging the root canal to place the needle to a few millimetres from the WL and ensuring adequate space around the needle for re- verse flow of the irrigant towards the canal orifice allow effective irrigant refreshment coronal to the 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 irri- gant exchange has been studied indirectly 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 flexi- ble 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 oc- curs between the flowing irrigant and the canal wall is termed “wall shear stress”. This force is of rele- vance in root canal irrigation be- cause 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 micro- bial biofilm from the canal wall. Yet, the nature of wall shear stresses produced within the root canals during irrigation provides an indication of the mechanical debridement efficacy. In open-ended needles, an area of increased shear wall stresses de- velops 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 nee- dle.16, 34 Consequently, it is neces- sary 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 pos- sible. The maximum shear stress decreases with an increase in canal size or taper. Thus, overzealous root canal enlargement above a certain size or taper could di- minish the debridement efficacy of irrigation (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 negative-pressure irriga- tion to be safely used at the WL, circumventing the issues of va- pour lock effect.35 Nonetheless, the apical negative-pressure irrigation produced the lowest wall shear stress. This decrease in the wall shear stress could be attributed in part to the reduction in the flow 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 high- est turbulence intensity travelling coronal from the ultrasonic tip po- sition. The lateral movement of the irrigant displayed by this method has important implications with respect to its ability to permit bet- ter interaction between the irrig- ant and the root canal wall, and to potentially enhance the inter- action of irrigants with intra-canal biofilms2, 3, 35 (Figs. 1a–d & 2a–d). Conclusion The requirements of adequate irrigant penetration, irrigant ex- change, mechanical effect and minimum risk of apical extrusion oppose each other and a subtle equilibrium is required during irrigation. Ideally, in a canal en- larged to size 30 or 35 and taper 0.04 or 0.06, an open-ended nee- dle should be placed 2 or 3 mm short of the WL to ensure ade- quate 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 exchange can be ensured. The apical nega- tive-pressure irrigation did not generate marked wall shear stress values, but allowed the flow of irri- gant 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 highest wall shear stress. The use of com- bined methods to obtain opti- mum disinfection and to cir- cumvent the limitations of one method is recommended. Editorial note: A list of references is available from the publisher. www.dental-tribune.com The Dental Tribune International Magazines Shipping Address Name Address Zip Code, City Country E-mail Date, Signature PayPal Credit Card Credit Card Number Expiration Date Security Code CAD/CAM Clinical Masters* cosmetic dentistry** implants laser ortho** roots Journal of Oral Science & Rehabilitation*** Fax: +49 341 48474 173 E-mail: subscriptions@dental-tribune.com EUR 44 per year (4 issues per year; incl. shipping and VAT for customers in Germany) and EUR 46 per year (4 issues per year; incl. shipping for customers outside Germany). * EUR 12 per year (1 issue per year; incl. shipping and VAT for customers in Germany) and EUR 14 per year (1 issue per year; incl. shipping for customers outside Germany). ** EUR 22 per year (2 issues per year; incl. shipping and VAT for customers in Germany) and EUR 23 per year (2 issues per year; incl. shipping for customers outside Germany). *** EUR 200 per year (4 issues per year; incl. shipping and VAT). Your subscription will be renewed automatically every year until a written cancellation is sent to Dental Tribune International GmbH, Holbeinstr. 29, 04229 Leipzig, Germany, six weeks prior to the renewal date. I would like to subscribe to SUBSCRIBE NOW! AD Anil Kishen ob- tained his den- tal education in India and is Professor of Endodontics at the University of Toronto’s Faculty of Dentistry in Canada. He can be contacted at anil.kishen@ dentistry.utoronto.ca. © robophobic Fax: +4934148474173

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