Traditional methods for determining WL have been the use of tactile sensation [43], the observation of bleeding points or moisture using a paper point technique [44, 45] and radiographic length determination [46].

The paper point technique used conventional absorbent paper points to determine WL. The technique is used as a method to determine final working length prior to obturation. It can only be used after initial shaping of the canal has been completed and where the patent canal is dry. WL is determined by the tip of the paper point and by the presence of either blood or tissue fluid indicating insertion beyond the foramen. Repeated points are used to determine the wet/dry points within the canal before final refinement of canal preparation [44, 45].

The radiographic apex is defined as the anatomical end of the root as seen on the radiograph [37]. Since this is visible radiographically, it has been widely used as a reference for determination of WL. Commonly 0.5–1 mm short of the radiographic apex has been used as a guide for determining termination of root canal preparation, and this is only an estimate. The radiographic apex and the anatomical canal terminus (where the canal exits the root) do not coincide. Frequently, canals exit short of the radiographic apex, and curvatures in the bucco-palatal/lingual plane will not be assessed using a two-dimensional image. Conversely when a file can be seen beyond the radiographic apex (such as the use of patency filing), we can be sure, without doubt, that we are beyond the confines of the canal. Radiographic determination of WL is subject to magnification, distortion, inter- and intra-observer variability and lack of three-dimensional representation. Despite inaccuracies, the use of radiographic length determination in combination with other techniques will give you the most accurate information as to the correct WL [53].

None of the aforementioned traditional techniques can truly satisfactorily determine the WL. The CDJ is a practical and anatomic termination point for the preparation and obturation of the root canal, and this cannot be determined radiographically. Modern electronic apex locators can determine this position with accuracies of greater than 90 % with some limitations. Knowledge of apical anatomy and the use of radiographs in combination with the correct use of an apex locator will assist practitioners to achieve predictable results [47]. The first electronic device for root canal length determination was constructed by Sunada using direct current based on the principle that the electrical resistance of the mucous membrane and periodontium registered 6.0kΩ in any part of the periodontium [48].

A number of electronic apex locator devices have been developed measuring resistance or impedance using frequencies (high, two or multiple) or low-frequency oscillation and/or a voltage gradient method to detect the canal terminus when an endodontic file penetrates inside the canal and approaches the apical constriction (minor apical foramen) [49]. Third-generation apex locators such as the Root ZX (J. Morita, Tokyo, Japan) were developed to calculate the ratio of two electrical impedances in the same canal using two different current frequencies to determine the canal length precisely even in the presence of electrolytes (root canal irrigants, serous intra-canal fluids, purulent exudate, haemorrhagic exudate) or vital pulp tissue in the root canal [50]. The Root ZX has been shown to be 90 % accurate to within 0.5 mm of the apical foramen and 100 % if accurate if 1.0 mm is accepted [51, 52]. Based on the majority of studies carried out in the endodontic literature, 0.5 mm should be subtracted from the length of the file at the point when the device suggests the file tip is in contact with the PDL (zero reading). This measurement does not indicate that the apical constriction has been located but rather that the instrument is within the canal and is close to the periodontal ligament [49]. Even though apex locators appear to be excellent tools for the determination of WL, they should be used as an adjunct and not as a substitute for radiographs. In fact, the combined use of electronic apex locators and radiographs of the initial or master apical file has been shown to be more accurate than the use of radiographs alone [53].

Apical patency is a technique in which the apical portion of the canal is maintained free of debris by recapitulation with a small file through the apical foramen [54]. Proponents of maintaining patency throughout the cleaning and shaping procedures are based on the belief that this helps to control debris accumulation in the apical portion of the canal preventing ledge formation, transportation and apical perforation [55]. In the teeth with necrotic and infected pulps, the use of patency filing ensures that mechanical and biological goals of cleaning and shaping are achieved by ensuring that cleaning and shaping procedures are carried out to the full extent of the canal (Fig. 3.3). In vital cases, apical patency is intended to prevent compaction of dentine chips into the apical region forming a plug that can interfere with WL [56]. The use of this technique is a matter of controversy, and antagonists claim this type of filing beyond the confines of the canal is biologically unacceptable with risks of over-enlargement of the apical foramen [57], damage to the periapical tissues [40] and inadvertent extrusion of contaminated dentine chips or bacteria leading to postoperative pain and sequelae [58].

Both stainless steel and nickel-titanium rotary instruments carry the risk of instrument fracture either as a result of flexural (fatigue fracture) or torsional (shear failure) stresses. Canal curvatures are thought to be the predominant risk factor for instrument failure through flexural stresses. To reduce the risk of file fracture when using rotary instrumentation techniques, coronal preflaring and the use of a manual or rotary glide path can help reduce stresses on the instrument [59]. The new PathFile NiTi rotary instruments (Dentsply Maillefer, Ballaigues, Switzerland) for mechanical preflaring have been recently introduced on the market and have demonstrated significantly less modification of canal curvature and fewer canal aberrations when compared to manual preflaring with stainless steel instruments [60].

The ideal final size of apical canal preparation remains controversial. The apical portion of the root canal system can harbour microorganisms that account for periapical pathosis and persistent inflammation; therefore, treatment interventions that allow for maximum reduction of pathogens should be indicated in the treatment of infected root canal systems [41]. Traditionally, final apical preparation techniques advocating enlarging the apical part of the root canal to three-four sizes larger than where the first file bound have been recommended [61]. However, this concept has been questioned as a result of recent studies, which indicate up to 60 % of untouched canal walls that could contain biofilm [28]. Furthermore, Wu and colleagues demonstrated that in 75 % of root canals, the first instrument to bind at the apex had contact on the canal wall on one side only, and in 25 % the instrument tip had no contact with the canal wall at all [62]. A second school of thought from North America aimed to keep the apical diameter and preparation as small as practically possible. This concept of root canal preparation included establishing and maintaining patency using a small #10 K file 1 mm through the foramen and finishing the apical one-third to at least a size #20 [1]. Corresponding to this technique, modern rotary instruments such as ProTaper Universal finishing instruments have apical diameters ranging from 0.20 to 0.30 mm.

Classical bacteriological studies indicated that mechanical instrumentation alone resulted in significant reduction of bacteria but could not predictably achieve bacteria-free root canals [63]. Even combining mechanical preparation, antibacterial irrigants and intra-canal antimicrobial dressings did not achieve bacteria-free root canals, demonstrating the limited antibacterial effect of chemomechanical preparations [64, 65]. Ørstavik and colleagues studied the effects of chemomechanical root canal preparation in 23 teeth with apical periodontitis. Following a step-back technique and irrigation with sterile saline, 14 teeth yielded positive cultures at the end of the first appointment. Following a 7-day dressing of calcium hydroxide, this number was reduced to eight teeth [66]. Dalton and colleagues studied the effects of both stainless steel and nickel-titanium instruments in a clinical study of 48 teeth with apical periodontitis. In terms of bacterial reduction, no difference could be found with either technique although increasing instrument size reported increased bacterial reduction. Nevertheless, it was not possible to achieve bacteria-free root canals [67].

Rollinson and colleagues compared the removal of radioactively labelled bacteria using two different rotary NiTi preparation techniques. Preparations to size #50 (using Pow-R instruments) resulted in reduced bacterial counts compared to size #35 (using GT rotary and profile) (see Figs. 3.4 and 3.5) [68]. A recent systematic review suggested that contemporary chemomechanical debridement techniques with canal enlargement techniques do not eliminate bacteria during root canal preparation at any size [69]. The same authors carried out a similar systematic review assessing healing outcomes related to small and large master apical file sizes. They concluded that with the limited information available, the best current available clinical evidence suggests that for patients with necrotic pulps and periapical lesions, enlargement of the apical size would result in an increased healing outcome in terms of clinical and radiographic evaluation [69, 70].

Irrigation type, volume and depth of penetration are all crucial factors related to effectiveness and hence microbial reduction. Irrigants commonly delivered by syringe and needle devices indicate that successful irrigation delivery is related to canal diameter and curvature, and contemporary needle gauges used (27 and 30 gauges) require 0.42 and 0.31 mm diameter preparations, respectively. Therefore, the minimum canal size preparation when using a 30-gauge needle is 0.31 mm for the needle and therefore irrigant to reach within 1 mm of the WL (for more detailed discussion, see Chap. 5).

The uses of antimicrobial inter-appointment intra-canal dressings such as calcium hydroxide are also regarded as important adjuncts to optimize cleanliness and disinfection within the root canal system. Cleaning and shaping procedures facilitate placement of a suitable dressing that can further reduce microbial flora by virtue of its antimicrobial effects combined with its ability to suppress the nutritional supply that may allow further growth and multiplication of any remaining organisms (see Chap. 6 for more detailed discussion).

In summary, the goal of effective cleaning and shaping procedures is to remove all necrotic and vital organic tissue as well as some infected hard tissue from the root canal system. Correct shaping procedures with maintenance of original anatomy (with reference to the apical constriction) without iatrogenic deviation (canal transportation) facilitate optimal and effective irrigant deposition and predictable placement of intra-canal medicament and facilitate permanent root fillings without risk of overfilling. Mechanical preparation techniques in terms of correct WL and master apical file size must be carried out with antimicrobial chemical treatments in mind (Fig. 3.6).