Summary

The main cause of post-treatment endodontic disease is inadequate biomechanical instrumentation of the root canal system. This can result from inadequate knowledge of root canal anatomy, as if one or more of the root canals remains undiscovered, the potential for failure increases. After access to the root canal system and localisation of the root canal orifices, the most difficult step during root canal treatment is the need to scout root canals and to create a safe and predictable glide path. This phase requires skills and clinical experience for its correct management but is often time-consuming and frustrating. In fact, complex cases in primary root canal treatments mainly depend on the initial ability to negotiate canals to their terminus and on the presence or absence of a natural glide path. Following these considerations, this chapter analyzes the various possible clinical scenarios, first considering cases in which an initial manual scouting is successful in negotiating the canal to its terminus, then describing why in some cases there are difficulties with negotiation caused by coronal or apical impediments and how to clinically deal with these situations predictably, safely, and efficiently, using the ideal instruments for each clinical situation. Several solutions will also be illustrated on how to predictably, safely, and effectively shape the most difficult and curved root canals, using the ideal instrument for each clinical situation and describing in detail the scientific background of the proper characteristics of an ideal instrument for root canal shaping. Clinical procedures on how to manage complex canals will be explained to simplify these procedures and ensure more predictable root canal treatments.

17.1 Introduction

The initial steps of contemporary root canal treatment after anaesthesia and isolation include an access phase and a root canal preparation phase. The access cavity preparation allows predictable detection of the root canal orifices, negotiation of the root canals up to their apical terminus, and proper shaping of the canals [1]. The latter phase includes preparation of a glide path, preflaring of the root canals, basic instrumentation of the body of the canal, and finishing the apical third with an apical preparation to anatomically driven dimensions, including diameter and taper [2–6].

Contemporary root canal treatment requires a dynamic approach in which each subsequent phase interplays with the previous ones to ensure greater visibility and accessibility to the entire canal system; thus, each phase must flow seamlessly into the subsequent one. For example, the concept of dynamic access cavity design is dictated by the anatomy, the location of carious lesions, the experience and skill of the clinician, and the available equipment [7], as well as any relevant patient factors that could have an impact on the procedure, e.g. ability to open mouth wide enough. Knowledge of the most common anatomical patterns of pulp chambers facilitates clinical exploration and negotiation of the deeper parts of the root canal system [3].

A dynamic access cavity design can take different forms according to the clinical scenario, assessment of root and canal anatomy and the evolution in materials and techniques used [3]. A dynamic access always prioritises the removal of carious and damaged tissues and restorations ahead of the negotiation into the pulp space. The approach to the pulp space always starts from a pinpoint exposure that is enlarged progressively to facilitate the removal of all irreversibly inflamed or necrotic pulp tissue, or existing root filling materials. Difficulties in the negotiation and disinfection of the anatomical extensions of the root canal system are managed by strategically extending the access openings towards the most convenient direction that will facilitate clinical management. The shapes of dynamic access cavities may well be modified with the aid of future technological developments that will take place in diagnosis, prevention, and management of apical periodontitis.

The cleansing phase of canal preparation includes root canal irrigation and the use of supplementary techniques that are available to improve debridement and disinfection [8–11]. In fact, the concept of chemo-mechanical preparation is based on these two phases and, in other words, canal shaping facilitates cleaning [12–16]. Filling the root canal system after shaping and cleaning and a high-quality coronal restoration completes the process of root canal treatment [17–19].

Modern endodontics is characterised by the introduction of new cutting-edge technological advancements that can help clinicians improve the quality of the treatments they provide, especially in the most difficult and complex cases and situations [3].

Considering the growing evidence base, this chapter provides clinicians with basic and advanced information about the detection of canal orifices, and the negotiation and subsequent management of calcified and curved canals. Such information will minimise tooth tissue loss and help maintain the original anatomy while providing the basis for good mechanical canal debridement and disinfection that will lead eventually to the long-term retention of natural teeth.

17.2 Detection of Canal Orifices

17.2.1 The Significance of Missed Anatomy on the Prognosis of Root Filled Teeth

Apical periodontitis is induced and maintained by microbial infection within the root canal system [20]. Numerous studies have reported that a substandard quality of treatment is one of the most important risk factors for post-treatment disease [21–26].

Detection of all root canals is critical, as untreated canals will harbour micro-organisms and their by-products, which are likely to have a negative effect on the development or persistence of apical periodontitis [27–29], as they serve as a reservoir for micro-organisms and their by-products that can replicate and overcome the periapical immune response [30] (Figure 17.1). Outcome studies have documented untreated canals as one of the causes of post-treatment disease [31–34]. Canals that are untreated may be a consequence of complexities in tooth configuration and/or a limited knowledge of tooth anatomy or procedural errors made by the operator [35].

Evidence is available on the incidence of missed canals in cases requiring root canal retreatment [35–37]. The prevalence rates of periapical lesions in root filled teeth with missed canals were high in all studies that analysed this topic, ranging from 82% [35, 38] to 98% [39]. Given these prevalence rates, a root filled tooth with an untreated canal has an odd ratio from 4.4 to 6.25 times more likely of being associated with a periapical lesion than root filled teeth without missed canals [35, 38, 39].

The incidence of missed canals is generally highest in the maxillary molars, in particular in first maxillary molar teeth, followed by mandibular molars, and in mandibular and maxillary premolars [35, 38, 39]. In particular, the mesiobuccal (MB) root of maxillary molars had the greatest frequency of untreated canals, and one of the most frequently missed canal was the second mesiobuccal (MB2) root canal [35, 38, 39]. This is easily explained because the anatomy of the MB root of maxillary molars is most often complex and variable [40–45], thus predisposing the operator to miss a canal during root canal treatment (Figure 17.2). In vivo studies reported a MB2 prevalence between 31% and 96% [46], whereas ex vivo analyses reported a proportion as high as 96% [47, 48], with two separate canals having two independent apical exits occurring as often as 43% of the time [49].

In mandibular first molars, missed canals were mostly found in the distal root (Figure 17.3), whereas in second molars the majority of untreated canals were in the mesial root [35, 38]. These results were probably due to the close approximation of the two mesial canals in the second mandibular molar, a situation in which both mesial canals may originate from a common orifice that divides into two separate canals deeper inside the root [50]. Another reason may also be the difficulty in accessing and visualising mesial canals when accessing mandibular second molars through indirect vision, given the distal position of the tooth in the dental arch. The high incidence of untreated canals in the distal root of root filled mandibular first molars might be correlated to a higher morphological variability of the root and root canal anatomy and the number of roots [46, 51 ].

Maxillary and mandibular incisors, canines, and premolars are associated with a lower frequency of untreated canals compared to molar teeth [35, 39]. The most frequently missed canal in mandibular incisors is the second lingual root canal [46], while a third buccal root canal may be more frequently missed in maxillary premolars [52] (Figure 17.4) and anatomical variations with two or three canals may lead to missed anatomy in mandibular premolars [39, 53] (Figure 17.5 ).

All these results clearly demonstrate that an untreated canal may act as an important predictive factor for post-treatment disease following root canal treatment [35], and detection of untreated canals has the potential to positively influence the outcome of retreatment or periradicular surgery [54]. Furthermore, the frequency of untreated canals in molars reinforces the difficulties encountered with these teeth during root canal treatment because of their complex anatomy [39]. Hence, clinicians should be fully aware of tooth anatomy, root canal configurations, and possible variations before the start of root canal treatment in order to minimise the possibility of missing canals during treatment.

17.2.2 Anatomical Landmarks for Detection of Root Canals

A thorough understanding of the most common anatomical landmarks of both the pulp chamber and the root canal system will facilitate the detection, exploration, and negotiation of all root canals and their associated anatomy. Knowing the internal and external dental anatomy and their relationships will help the clinician to develop a clear three-dimensional mental image of the inside of the tooth from pulp horns to the apical foramen before and during treatment.

A deep knowledge of basic tooth anatomy is fundamental to undertake the clinical procedures described in the present chapter. Before the anatomical study published by Krasner and Rankow [55], the indication of how to locate the root canal orifices was often given in a nonsystematic manner and mainly based on the average number of canals in a tooth [56, 57]. Unfortunately, when approaching an individual tooth, these ‘mean’ values are not of great value for the clinician, who needs to address the specific anatomy of that tooth. Essentially, in normal conditions, the pulp chamber is located in the centre of the tooth crown and its outline resembles the external shape of the crown [55]; thus, most advice has been to make an access in an appropriate position in the clinical crown so that it would normally allow the orifices to be visualised. Unfortunately, physiological and pathological events such as aging, deep carious lesions, and/or deep restorations or teeth affected by previous access cavity preparations often cause the root canal system to become obliterated. In these situations, normal anatomy is often severely distorted and detection of the root canal orifices is difficult.

Based on their observation of the anatomy of the pulp chamber floor of 500 teeth, Krasner and Rankow [55] identified consistent, anatomic configurations and landmarks in order to provide a more rational approach to root canal treatment, helping clinicians to locate more systematically and with greater certainty the pulp chambers and the number and position of root canal orifices on the pulp-chamber floor.

The authors described two categories of anatomic patterns: the relationships of the pulp chamber to the clinical crown and the relationships of orifices on the pulp-chamber floor. Consequently, they proposed several ‘laws’ for determining the pulp chamber position and the location and number of root canal entrances in each group of teeth (equally distributed between maxillary and mandibular anterior, premolar, and molar teeth) (Figure 17.6 ).

The anatomic laws formulated on the observation noted in the relationships of the pulp chamber to the clinical crown were the following:

• Law of centrality: The pulp chamber was always in the centre of the tooth, so the floor of the pulp chamber is always located in the centre of the tooth at the level of the cemento-enamel junction (CEJ).
• Law of concentricity: The walls of the pulp chamber are always concentric to the external surface of the crown of the tooth at the level of the CEJ, as the external root surface anatomy reflected the internal pulp chamber anatomy.
• Law of the CEJ: The CEJ is the most consistent, repeatable landmark for locating the position of the pulp chamber, as the distance from the external surface of the clinical crown to the wall of the pulp chamber was the same throughout the circumference of the tooth at the level of the CEJ.

The anatomic laws proposed on the observation noted regarding the relationships of orifices on the pulp-chamber floor were the following:

• Law of symmetry 1: Except for maxillary molars, the orifices of the canals are equidistant from a line drawn in a mesiodistal direction through the pulp chamber floor.
• Law of symmetry 2: Except for maxillary molars, the orifices of the canals lie on a line perpendicular to a line drawn in a mesiodistal direction across the centre of the floor of the pulp chamber.
• Law of colour change: The colour of the pulp chamber floor is always darker than the surrounding dentinal walls; this colour difference creates a distinct junction where the walls and the floor of the pulp chamber meet. Reparative dentine or calcifications are lighter than the pulp chamber floor and often obscure it and the orifices.
• Law of orifice location 1: The orifices of the root canals are always located at the junction of the walls and the floor.
• Law of orifice location 2: The orifices of the root canals are located at the angles in the floor-wall junction.
• Law of orifice location 3: The orifices of the root canals are located at the terminus of the root developmental fusion lines; the developmental root fusion lines are darker than the floor colour.

In that study, the CEJ was the most consistent anatomic landmark observed. Knowledge of the law of centrality will help prevent crown perforations in a lateral direction, regardless of how much clinical crown was lost or how extensive the crown restoration. The law of concentricity will help the clinician to extend the access properly, as the pulp chamber will extend in the direction indicated by the CEJ. The authors specified that an important requirement for proper use of the proposed laws was that the access should be completed so that the entire floor of the pulp chamber is visible without any overlying obstruction. There has been a trend for a minimally invasive approach to endodontic procedures, including a more conservative access cavity outline [7]. Extreme interpretation of this concept towards unnecessarily ultraconservative ‘ninja’ or truss cavities must always be avoided, with the visibility of orifices and accessibility to the pulp chamber floor remaining core principles, even if observed from different projections (angles) [7]. This will permit the clinicians to take advantage of the laws established by Krasner and Rankow [55], while also being as conservative as the tooth anatomy permits. The law of colour change provides guidance to determine when the access is complete. Because a distinct light-dark junction is always present, if it is not seen in one portion of the chamber floor, the operator knows that additional overlying structure that may be represented by restorative materials, reparative dentine or overlying pulp chamber roof must be removed.

After the junction where the walls and the floor of the pulp chamber meet is clearly seen, all of the laws of symmetry and orifice location can be used to locate the exact position and number of orifices. Even though the laws of symmetry can be invaluable in determining the exact position of canals and often indicate the presence of an additional unexpected canal, when the pulp chamber is obliterated by calcifications in the coronal portion of the root canal, the orifice may have moved to a different position (Figure 17.7). In this situation, it is often helpful to move the orifice back to its original position and regain better proportions to apply the laws of symmetry during root canal preparation of that canal. A classic clinical example may be a tooth with a deep buccal restoration at the level of the CEJ in which the buccal orifices beneath the restoration are likely to have moved more lingually than their original location because of dentine deposition. The laws of orifice location may be used to detect the number and position of the root canal orifices of the tooth, as they are located along the junction between the pulp chamber floor and the axial walls. Furthermore, the angles of the geometric shape of the dark chamber floor will specifically identify the position of the orifices. In case of calcified canals, this position at the vertex of the junction will be helpful to indicate where the operator should begin to penetrate to remove reparative dentine and proceed deeper inside the root (Figure 17.8).

When looking for a root canal, the age of the patient should also be taken into consideration because it is a common finding that as age advances, there are fewer chances of locating canals [58, 59]. It can be presumed that with age, the tooth is exposed to various insults such as caries, attrition, erosion, and so forth, leading to calcification of the orifice or canal itself. For example, mandibular molars in patients younger than 42 years old were four times are more likely to have a patent middle-mesial canal [59], whereas maxillary molars in patients with the age group 36–45 years had fewer patent MB2 canals than patients with 18–35 years [58]. This is in accordance with other studies reporting a significant inverse relationship between age and the occurrence of two patent canals in the MB root of maxillary molars, as they tend to be obliterated as one gets older [60, 61].

17.2.3 Clinical Detection of Canal Orifices

As specified by Krasner and Rankow [55], the two laws of symmetry cannot be applied to maxillary molars; thus, it is not surprisingly that maxillary molars have always been reported as the teeth with the greatest number of missed canals, especially the MB root and the MB2 canal. It is also not surprising that the clinical incidence of MB2 canals in maxillary molars has been reported to be lower than that of the laboratory-based reports and in some reports could only be detected in less than 40% of maxillary first molars [62, 63].

The law of orifice locations, in conjunction with the law of colour change, is often the only reliable indicator for the presence and location of a second canal in the MB root of maxillary molars. Along the floor-wall junction over the MB root, there is a line-angle in the floor/wall junction between the MB and palatal orifices that reflects the anatomy of the broad flat MB root. The MB root is always elongated from buccal to palatal parallel to the mesial wall of the mesial marginal ridge of the crown (Figure 17.9). Applying the laws of centrality and concentricity at this single root, the MB2 canals will lie on the line that runs palatally from the MB root canal being parallel to the mesial wall of the tooth. This orifice can be any distance from the MB orifice and at various depth, but it should be along this line-angle/junction, immediately mesial to the line connecting the MB and palatal canals [29]. In the study by Das [58], when located, the MB2 canal was either mesial to or directly on the line connecting the MB and palatal canals on an average within 3.5 mm palatally and 2 mm mesially from the main MB canal. Comparable landmarks have been described previously in many clinical reports [60, 64–66].

When attempting to locate the hidden orifice of the MB2 canal, it may be helpful to complete instrumentation of the main MB1 canal, as the repositioning of this root canal orifice in the centre of the buccal area of the MB root after preparation of the root canal may provide clues on where the MB2 canal orifice should be located. In fact, the orifice of the MB2 canal may be typically moved towards the centre of the pulp chamber floor, as most of the time it is covered by a dentine shelf that, if not removed, may guide the operator to search for this orifice too distally (Figure 17.10). As a consequence, when the MB1 canal has already been enlarged, the operator may have a better spatial visualisation where the orifice of the MB2 canal should be, on a straight line going palatally and parallel to the mesial wall of the tooth. This information may guide the clinician in the selective removal of the dentine shelf or calcifications in the small groove hiding the orifice of the MB2 canal, representing the initial key for obtaining straight-line access to the canal.

These findings reinforce the need for clinicians to appreciate that maxillary molars normally have four canals, and efforts should always be made towards locating the MB2 canal during treatment/retreatment. A clear and considerable increase in detection of MB2 canals [27, 42, 60, 65–69] may also be attributed to an improved awareness of its presence [70, 71] and the use of magnification and enhanced lighting.

The laws of symmetry, colour change, and orifice location described by Krasner and Rankow [55] can be applied to any tooth and may be especially valuable when unexpected or unusual anatomy may be present, as more than 95% of the specimens they included demonstrated all of the laws. Knowledge of the chamber floor anatomy laws helps the observer to appreciate if an additional canal, as for example in presence of the radix entomolaris and paramolaris of mandibular molars [72, 73] (Figure 17.11), or a tooth with fewer canals than normally expected may be present in a specific case [57] (Figure 17.12). There are, however, some exceptions to the laws described by Krasner and Rankow [55], as mandibular second and third molars were especially deviant and most often had a different anatomy, such as C-shaped canals [74–76] (Figure 17.13). Fan et al. [77] reported that 83% of mandibular second molars with C-shaped root canals possess an orifice at a level 2 mm below the CEJ, rising up to 98% at a level 3 mm below the CEJ. A basic knowledge of the patterns of pulp chamber anatomy is therefore important to prevent perforations. For example, some authors have reported a second lingual canal [78, 79]. In such a case, the conventional triangular access needs to be modified to a trapezoidal shape in order to improve access to the additional canal [75].

Furthermore, in some specific cases a hidden canal can be challenging to treat because it might share an orifice with an adjacent canal or can be harboured within, or just apical to the other canal orifice (Figure 17.14). Examples include the two buccal canals of three-canalled maxillary premolars [80] (Figure 17.15), the lingual canal of the mandibular incisors and canine [81] (Figure 17.16), the mandibular premolars containing two or three canals [53] (Figure 17.5), some MB2 canals of maxillary molars [41] (Figure 17.17), mesial canals of second mandibular molars [82], and middle-mesial (Figure 17.18) and middle-distal (Figure 17.19) canals of mandibular molars [59, 83]. In all these cases, the importance of using magnification and ultrasonic troughing to detect possible additional anatomy and eventually the use of cone-beam computed tomography (CBCT) imaging for teeth suspected of having extra canals and complex morphology must be emphasised.

17.2.4 Magnification and Ultrasonics: The Perfect Tools for Detection of Canal Orifices

In addition to the anatomical laws, the use of illumination and magnification and of ultrasonic (US) troughing will provide the best approach to explore all anatomic variations of the pulp chamber in order to locate all canal orifices. The operating microscope (OM), although not new, provides the operator with significantly improved visual feedback, depth perception, magnification, and coaxial illumination to enhance the view of the operative field [84]. US tips can enhance visibility and precision during operating procedures, thus promoting more precise and conservative treatments [85]. The troughing concept involves using a US endodontic tip in a back-and-forth brushing motion along a groove on the pulpal floor. The removal of the dentine shelf by troughing with high- or low-speed burs may be difficult and time-consuming, may result in unnecessary removal of the sound cervical dentine, and may increase the possibility of perforation. Even if the troughing approach with the help of a small round bur at low speed performed under magnification does not result in perforations [58], specific US tips are in general far more conservative and precise. In fact, especially in difficult cases, troughing to look for calcified root canals on the pulp chamber floor or deep inside the root is safer, more conservative, more predictable, and more successful if the combination of OM and US tips instead of high- or low-speed rotating burs is used [86] (Figure 17.20).

Traditionally, canal detection has been related to the tactile ability of the operator and the mental image provided by the knowledge of the root canal system, largely because visualisation of canal orifices with the naked eye is restricted [70]. The use of magnification has made canal location easier because it allows minute details to be visualised, for example, by magnifying and illuminating the grooves in the pulpal floor and differentiating the colours between the dentine of the floor and walls [87, 88].

It has been reported that without magnification, detection of the MB2 canal under direct vision is not predictable [82, 89, 90], while the use of magnification loupes increased the percentage of clinically located MB2 canals [91–93] and the use of an OM further increased this percentage [82, 91, 92]. Indeed, the use of an OM has been widely reported to considerably increase detection of MB2 canals [27, 47, 68, 90, 94, 95]. Stropko [68], in his clinical study over an 8-year period, reported that without magnification, the MB2 canal was located in 73% of first molars. When employing the OM routinely, this number increased to 93%. The clearing technique disclosed the presence of an MB2 canal in 94.7% of the teeth, while 5.3% of teeth had a single canal in the MB root [82]. This study further reported that the MB2 root canal was not detected with the aid of the OM [82] in only 4% of the cases. Das et al. [58], in an in vivo study, detected clinically the MB2 canal under direct vision running an endodontic explorer from the main MB canal towards the palatal canal but 1–2 mm mesially in 36% of the cases. This agrees with other studies conducted without magnification in which the importance of a modification in access cavity preparation from the historical triangular shape to a more rhomboidal shape for a better view and access to the area between the MB and palatal canals and to allow probing of the fissure or groove between these main canals was emphasised [64, 70]. The use of magnification under an OM (8–12X) with the help of an endodontic explorer increased the detection of MB2 canals to 54% of the cases [58], while 72% of the MB2 canals were detected [58] when troughing was carried under the same magnification removing dentine in the pulp chamber within 3 mm from the MB1 canal toward the palatal canal and 1–2 mm mesially and 2 mm deep.

Thus, it is important to emphasise that the use of OM and US troughing with specific tips to dislodge calcifications and debris over the canal achieves more predictable and minimally invasive detection of root canal orifices [47]. With the use of specialised technology and a modified approach in the treatment of MB2 canals, the clinicians may expect a high success rate in the identification and treatment of these canals [58].

Clinically, it is important to follow the indications given by the manufacturers of the US tips and US devices on the power at which any specific US tip should be used. This will reduce the risk for fracture of the tips, especially the smallest ones that are very delicate if used at the incorrect power. In general, it is advisable to start using any US tip at the lower value of the range usually suggested by the manufacturer and gradually increase the power towards the maximum value of this range to increase effectiveness of the tip [96]. This is also relevant to the specific US device used, as each is different in terms of the US power delivered.

US troughing for the detection of root canal orifices is generally performed without water to increase visibility and precision. Air cooling should always be used to avoid overheating of the tips and tooth tissue and also to facilitate removal of the dentine dust created by the action of the tip. For these reasons, the use of US energy for short periods is recommended (i.e. maximum 10 seconds). This will reduce heat transmission and will allow dentine powder to be removed that otherwise will reduce visibility and precision; it will also help visualisation of the colours of the pulp chamber floor that after rinsing will appear different when the same structures are dry (Figure 17.21). These techniques will allow the clinician to study the anatomy and the different colours of the pulp chamber floor and the root dentine. Root dentine is generally whiter, so when clinicians are looking for a root canal apical to the CEJ, they must be aware of this and look for a dentine with a different darker colour, because if white dentine is visualised it means the operator is proceeding into the root structure and not following the calcified canal, increasing the risk for perforation (Figure 17.22a). When looking for the MB2 canal in particular, it will be advantageous to follow the dark isthmus always present between the two MB canals (Figure 17.22b). When using US tips, however, the white dentinal dust produced is usually compacted in the isthmus area or inside the orifice (Figures 17.22c and 17.23a). This will be very helpful in recognising the anatomy and following the correct direction (Figure 17.23).

When looking for a calcified MB2 canal, the deeper the clinician proceeds down the root, the more the anatomical knowledge is important because the MB root always curves distally after a few millimetres apical to the CEJ. In these cases, if the clinician proceeds straight down, they will encounter the whiter root dentine and increase the risk for a mesial root perforation. This rule may be valid in the localisation of the entire calcified canal, in which it is important to proceed in the centre of the root following the darker calcified dentine surrounded by the whiter dentine of the root walls and to look for a white spot in the centre of the dark greyish calcified dentine provided by the dentine dust pushed into the virtual space of the orifice by the US energy. This spot may appear also darker than the greyish colour of the calcified dentine immediately surrounding it, if no dentine powder is pushed in it.

This strategy is also effective when detecting other root canals in the same root, such as the middle-mesial or middle-distal canals of mandibular molars [59, 83]. The middle-mesial (MM) canal is one of the anatomic complexities of the mesial root of mandibular molars, with an incidence ranging from 3%–46% [59, 97–99] (Figure 17.24). Akbarzadeh et al. [59] reported details of the anatomic characteristics of an MM canal with regard to the pulp chamber landmarks to propose clinical guidelines to help clinicians better predict the presence of an MM canal. They reported, in a CBCT study, a mean distance between the MB and mesiolingual (ML) orifices of 3.1 mm in samples in which an MM canal was identified, while this distance was 3.7 mm in cases in which an MM canal could not be identified. Thus, the MB-ML intraorifice distance was inversely associated with the presence of an MM canal, as the presence of an MM canal was two times less likely to be present in mandibular molars with every 1 mm increase in the distance of MB-ML orifices. The average MM orifice distance to the periodontal ligament on the distal side of the root (danger zone) was 1.7 mm. This value could help clinicians avoid mishaps while troughing deep in the mesial root looking for an MM canal. Furthermore, mandibular first molars with an isthmus between the MB and ML canals were almost five times more likely to have a MM canal in their CBCT scans.

17.2.5 Radiographic Techniques for Detection of Root Canals

Two-dimensional traditional radiographic images will help clinicians to identify the root canal anatomy of specific teeth by exposing periapical radiographic projections with different beam angulations that can allow parallax localisation. However, in most complex cases, two-dimensional techniques that can render interpretation of planar images difficult can be overcome by the three-dimensional nature of the CBCT analysis [100]. The introduction of CBCT has greatly improved the preoperative understanding of root and canal anatomy by providing three-dimensional information [36, 38, 54, 100–103]. The European Society of Endodontology (ESE) and the American Association of Endodontists (AAE) have updated their respective position statements on the use of cone-beam computed tomography in Endodontics [104, 105]. These documents suggest that CBCT should not be used routinely for endodontic diagnosis or for screening purposes and state that a small field of view (FOV) CBCT should be considered on a case-by-case basis where lower dose conventional radiography does not provide adequate diagnostic information, as it is essential to comply with the ALARA principle (as low as reasonably achievable) and, even though the effective dose is relatively low, CBCT must be used judiciously [106].

The list of indications given by these position statements includes the initial treatment of teeth with the potential for extra canals and suspected complex morphology and dental anomalies, the nonsurgical retreatment of cases with possible untreated canals, and the identification and spatial localisation of calcified canals, also taking into account the possibilities of guided endodontics.

These documents confirm that preoperative or intraoperative CBCT imaging, especially in the quality of the small field of view, has become an essential tool to help the visualisation of root canal anatomy in three dimensions for most configurations [107, 108] and reduce the incidence of missed canals [109]. However, it is still doubtful whether CBCT images will clearly show very narrow canals, especially in the apical third, which are usually located during careful clinical exploration using small hand files.

Two-dimensional traditional radiographic techniques may still be of importance especially in cases in which CBCT imaging is of limited value. The presence of beam hardening artefacts and high levels of scatter and noise may decrease CBCT diagnostic image quality and diagnostic yield [110, 111