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Dental Tribune Asia Pacific Edition | 3/2017 TRENDS & APPLICATIONS 13 Using the operating microscope and ultrasonics for root canal preparation By Dr Anthony C.S. Druttman, UK Introduction The purpose of preparing the root canal system is well under- stood, and contemporary tech- niques involve the use of both hand and rotary instruments, in conjunction with an irrigation re- gime. However, the complexity and variability of root canal mor- phology can make effective prepa- ration very challenging, particu- larly in canals with irregular cross-sections. Current techniques are not always completely effective, and it has been well recognised that, while some parts of the root canal are over-prepared with ro- tary instrumentation, other sur- faces are not touched. One study concluded that at least 35% of the surface area of canals had re- mained untouched by rotary preparation.1 In another study, the results were even worse: 60– 80% of untreated surfaces were left in the distal canals of man- dibular molars, with 65–75% in the apical 4 mm after prepara- tion.2 Oval canals are particu- larly challenging, as the debris collects in the extensions and isthmuses (Figs. 1a & b). A review of preparation techniques states that “because of limited efficacy of irrigation in such recesses, de- bris and smear layer may accu- mulate and remain on these un- prepared root canals walls, de- crease the quality of obturation and jeopardise the long-term treatment success”.3 Preparation of root canal system The cause of failure of endo- dontic treatment has been attrib- uted to the presence of micro-or- ganisms persisting in the apical part of the root canal.4 Much at- tention has therefore been fo- cused on preparation and obtura- tion of the apical part of the canal, thereby depending on the apical seal to prevent toxins from leak- ing out into the periradicular tis- sue. While success rates of endo- dontically treated teeth without periradicular lesions are very high, there can be a significant re- duction in success in teeth with periradicular periodontitis and in those teeth in which endodontic treatment has failed.5 This is pre- dominantly due to the failure to remove microbes from the root canal system. The quest is to find more effective irrigants and irriga- tion techniques, as well as rotary files and preparation techniques, to overcome these difficulties. An ideal preparation shape with a rotary instrument can only be achieved in a canal with a matched cross-section. Many ca- nals are variable in shape. They may have irregular and oval cross-sections, and while much of the debris is captured within the flutes of the instruments, some is compacted into those spaces be- tween the instrument and the canal wall (Fig. 2). The incidence of isthmuses in both maxillary and mandibular first molars is very high.6 They are particularly liable to have an accumulation of com- pacted debris after preparation, and the inability to clean these areas effectively has been impli- cated as a major cause of failure of root canal treatment, particularly in mandibular and maxillary first molars (Fig. 3).7, 8 The more the debris is com- pacted, the more difficult it is for chemicals such as sodium hy- pochlorite and calcium hydroxide to penetrate through the inter- face. Paque et al. reported that ap- proximately half of the debris that accumulated during rotary in- strumentation of the mesial ca- nals of mandibular molars re- mained in the canal system after irrigation.2 Failure of endodontic treat- ment in maxillary molars has been attributed to the failure to locate and treat the second mesio- buccal (MB2) canal.9, 10 Various studies have shown the presence of the MB2 canal in up to 90% of maxillary first molars. Somma et al. showed that, in 58% of teeth, the MB1 and MB2 canals merge apically into one canal.11 In a pro- portion of these failed cases in which the MB1 canal has been lo- cated, cleaned, shaped and ob- truded well, the question should be asked whether the failure was due to inadequate treatment of the apical part of the MB1 canal or because the MB2 canal and isth- mus between the two canals had been missed. Identification and treatment of the MB2 canal with concomitant retreatment of the MB1 canal often lead to healing. This suggests that the seals are not always good enough to entomb the bacteria. Indeed, coronal mi- croleakage has been implicated as a major cause of failure of endo- dontic treatment.12 Undoubtedly, tracts of debris running alongside root fillings are conduits for bacte- ria to cause failure by this method. In an in vivo study, the mesial canals of sixteen mandibular mo- lars with infected root canals were root-treated by conventional tech- niques in a single visit and the api- cal portions removed by flap sur- gery and evaluated by corrective light and transmission electron microscopy.13 In the majority of cases, residual microbes were lo- cated in inaccessible recesses, un- instrumented areas of the main canals, accessory canals and inter- canal isthmuses. If the lateral extensions feed into the apical part of the canal, then removing bacteria and nutri- ents from these areas reduces the bacterial load and this has to be beneficial for the outcome of treatment. A variety of techniques have been proposed to overcome the inadequacies of mechanical preparation in non-circular ca- nals, including circumferential filing using both hand and rotary files and the use of a rotary self-ad- justing file (SAF) that adjusts to the shape of the canal. The SAF system has been shown to be more effective in cleaning oval canals than conventional rotary nickel–titanium instruments; however, in De Deus et al. using mandibular canines, even this technique did not render the canals completely clean.14 They showed that rotary files were una- ble to access the recesses of oval canals and that sodium hypochlo- rite had a “limited ability to com- pensate for the inadequacy of the file itself”. They further suggested that the common belief that “the file shapes; the irritant cleans” is based more on wishful thinking than on experimental facts. In a review article, it was recognised that SAF was unable to prepare the narrow isthmus of less than 0.2 mm.15 In the case of the narrow isthmus, the challenge is to de- liver sufficient quantities of irrig- ant effectively into a very small area in which debris has been compacted during preparation. Recently, new concept files XP-3D Finisher (Brassler) that change their shape with tempera- ture have been developed with the expectation that they can deal with canal irregularities. While these may be helpful in removing soft tissue in non-circular canals, they may be of limited value in sit- uations in which tissue or root fill- ing materials are strongly adher- ent to the root canal wall. 1a 3 1b 4 2 5 Fig. 1a: Debris left after root canal treatment of the distal canal of a mandibular molar. Fig. 1b: Radiograph of the failed root canal treatment shown in Figure 1a. Fig. 2: Oval-shaped canal in the apical third of the distal root of a mandibular molar. Fig. 3: Debris accumulated after preparation in the isthmus between the mesial canals of a mandibular molar. Fig. 4: Acoustic micro-streaming patterns produced by an ultrasonically energised K-type-file. Fig. 5: Inspection of a prepared oval-shaped distal canal of a mandibular molar reveals residual debris apically.