I trends _ re-treatment drop of chloroform into the canal at a time until the hand K-files just reach the MC. Once the estimated working length has been reached, the electronic apex locator can be used and the first determination of true working length can be obtained. When and where to stop attempts at achieving patency are common clinical concerns. In essence, when is it time to fill to the depth gained in the canal in the absence of patency? If repeated attempts to gain patency have failed using pre-curved hand K-files of the appropriate length and diameter, particularly if the clinician is sure that he or she has removed all of the previous obturation materials, the canal should be cleaned and shaped to an optimal diameter despite the blockage and then obturated. This recommendation notwithstanding, an experienced clinician can often gain patency in cases in which an inexperienced one cannot. This difference in skill level is usually related to the amount of pressure used, the correct curvature of the hand K-file, the correct diameter of the hand K-file, adequate irrigation and clinical experience. 8) Achievement of the optimal master apical diameter In the endodontic literature, the achievement of the correct apical diameter is correlated with enhanced cleanliness. Such larger apical diameters provide greater irrigant flows and the removal of necrotic dentine up to the MC. It is a common finding in failed endodontic cases that both the apical diameter and master apical taper are too small. One way to determine the ideal master apical diameter is through gauging; alternatively, the clinician can simply instrument the canal to the desired master apical diameter, keeping in mind that non-vital teeth have higher failure rates because they are harder to cleanse relative to vital teeth (for which the emphasis is on asepsis rather than disinfection of an already infected canal). 9) Obturation One benefit of creating larger apical diameters is the ease of cone fit and obturation, be it obturation with a master cone or obturator. Given that one of the most significant causes of clinical endodontic failure is the loss or lack of coronal seal, it makes intuitive sense to bond the obturation. In both in vitro and in vivo studies, RealSeal in the master cone and form of RealSeal 1 Bonded Obturator has been shown to resist the movement of bacteria in canals to a statistically significant degree relative to gutta-percha. In addition to placing a coronal seal in step 10 below, this provides an invaluable step in addressing one of the weaknesses of gutta-percha: it is a material that bonds neither to dentine nor to sealers, thus it is entirely dependent on the placement of a coronal seal for it to function clinically. Bonding obturation is simple; the clinician clears the smear layer with a liquid EDTA such as SmearClear and subsequently rinses with distilled water. After drying the canal, the RealSeal self-etching sealer is placed in the canal and obturation is achieved with either the aforementioned RealSeal master cones or RealSeal One Bonded Obturator. 10) Placement of a coronal seal A number of clinical principles and steps have been addressed that can streamline endodontic re-treatment procedures conceptually and clinically. Emphasis has been placed on optimal visual and tactile control, removal of crowns before re-treatment, passive removal of previous obturation materials and obstructions, repair and revision of previous treatment, achievement and maintenance of apical patency, and optimisation of master apical diameter. We welcome your feedback._ _about the authors roots Dr Richard E. Mounce lectures globally and is widely published. He has a private practice specialising in endodontics in Vancouver in Washington. Dr Mounce offers intensive customised endodontic single-day training programmes in his office for one or two doctors. For information, contact Dennis at +1 360 891 9111 or E-mail RichardMounce@MounceEndo.com. Dr Mounce is also the author of the non-fiction book Dead Stuck,“one man’s stories of adventure, parenting, and marriage told without heaping platitudes of political correctness”, by Pacific Sky Publishing (www.deadstuck.com). Dr Gary D. Glassman has authored numerous publications and is on the staff in the Faculty of Dentistry in the graduate Department of Endodontics at the University of Toronto. A renowned international lecturer on endodontics, Dr Glassman has presented at major dental conferences around the world. He is an endodontic editor for Oral Health and maintains a private practice in Toronto, Ontario. He can be reached through his website: www.rootcanals.ca. 28 I roots 4_ 2009