Gaining Knowledge and Comprehension of Root and Canal Anatomy

Textbooks and courses in dental anatomy are ideal sources for teaching a dental student about normal human tooth anatomy. These study aids can present the dental student or dental practitioner with knowledge of the ideal coronal anatomy and its relation to occlusion, the anatomy of the human tooth root, and occasionally the morphology of the pulp and root canals. But the usual dental anatomy that is shown assumes the ideal (or most frequently encountered) tooth shape. Is this enough to perform endodontic treatment for our patients, when genetic variations may have produced roots and root canal systems that vary from the normal? For example, why is the maxillary first premolar inevitably depicted as having two roots (and two canals) whereas the second premolar is illustrated as a single but oval rooted tooth with one, or maybe two canals? Studies that compare the incidence of variation in root number in different populations have shown a significant variation in morphology of the human maxillary and mandibular premolars, based on ethnicity. Therefore, when it comes to performing the complex operation of root canal therapy, more detailed knowledge of root number and root canal system anatomy is required. The purpose of this chapter is to acquire that knowledge at a higher level.

The first task for a dental student, as stated, is to learn the normal anatomy of each tooth in the arch with respect to its complex root canal system. In principle, the shape of the external root will be reflected in the internal morphology of a root canal system. This is considered a tenet of the relationship of pulp-root anatomy. Each of the individual 16 types of teeth in the permanent dentition has its own individual root canal system morphology or shape. This is considered basic dental anatomy, which must then be matched clinically to what is interpreted from the two-dimensional (2D) shadow of the radiographic image.

The second task is to acquire detailed dental anatomy knowledge of possible variations from the norm. Thus one should realize that each human tooth type has a range of variation in its morphology. The shape of the root canal system is influenced by embryonic development and is controlled by each patient’s genetic background. To perform successful endodontic treatment, one must anticipate variation in chamber size and depth from the occlusal surface, the canal size, shape, length, curvature, branches, lateral canals, and apical accessory canals, to mention just a few variables. These variations may or may not be clearly seen in a standard periapical radiographic image.

Methods of Study to Learn Normal and Variations in Tooth Anatomy

In the first half of the 16th century, a set of seven books written by Andreas Vesalius (1514-1564) entitled De Humani Corporis Fabrica was published. This was a major advance in the history of anatomy over the long-dominant works of ancient Middle Ages writers. It is noteworthy that an important anatomic aspect of teeth that had been ignored by previous authors on which, centuries later, the endodontic specialty would be born, was highlighted for the first time in the literature. In Chapter XI, there is a drawing of a sectioned two-rooted mandibular molar showing its internal anatomy ( Fig. 12.1, A ). In 1563, Bartolomeo Eustachi (c. 1520-1574), in his treatise il Libellus de Dentibus, made very significant contributions toward the anatomy and physiology of the dentition, including the first descriptions of the dental pulp, the periodontal ligament, the dental follicles, the trigeminal nerve, and other oral structures, based upon extensive dissections of both human and animal specimens. In Chapter XVIII, Eustachi describes the pulp cavity and its contents, and shows accurate tables in which he specified the number of roots and the external morphologic variations of all groups of human teeth ( Fig. 12.1, B ). Eustachi’s book brought the macroscopic anatomy of teeth to a high degree of perfection that remained unsurpassed until the 19th century with the posthumous work of Georg Carabelli (1787-1842), who provided the most detailed description of the number and direction of the root canals at that time ( Fig. 12.1, C ). Nevertheless, it was only at the end of 19th century that some researchers finally realized the need for in-depth research on root canal morphology. In 1903, Gustav Preiswerk (1866-1908) performed a profound and comprehensive study on this subject. In his pioneering study, Wood’s metal, an alloy that melts at a low temperature, was molten and injected into the canal space. After complete decalcification of the teeth, 3D metal models of the internal anatomy were obtained for the first time ( Fig. 12.1, D ). Some years later, Guido Fischer (1877-1959) presented the challenging nature of the apical root anatomy. He obtained better results than Preiswerk by filling approximately 700 teeth with a collodion solution. This solution penetrated all the branches of the root canal system and hardened in 2 or 3 weeks, providing a full 3D replica of the root canal system ( Fig. 12.1, E ). The complexity and unpredictability of the root canal morphology led Fischer to coin the term Kanalsystem, which has been widely used nowadays as “root canal system.” It may be said that the innovative 3D anatomic studies of Preiswerk and Fischer resulted in huge advancements, adding new and significant knowledge to the dental literature and stimulating other researchers to undertake further investigations on this topic. In those later studies, large numbers of extracted teeth were collected, placed in a category and analyzed, or counted to compare their shape and size. Root numbers for the multirooted posterior teeth could even be tabulated. Data from these in vitro studies were then printed in the tables found in the earliest textbooks in dentistry. This was the first phase of research in human root anatomy.

Historical images of classical studies on root canal anatomy.

A, First drawing of a sectioned two-rooted mandibular molar showing its internal anatomy by Andreas Vesalius (1543); B, illustration depicting the number of roots and the external morphologic variations of all groups of human teeth by Eustachi (1563); C, illustration of the number and direction of the root canal in different groups of teeth by Carabelli (1842); D, 3D metal models of the internal anatomy of different teeth obtained by Preiswerk (1903); E, 3D models of the internal anatomy of teeth obtained by Fischer using collodion solution (1907); F, Canal shapes of mandibular molars obtained by injecting vulcanite into the canal systems by Hess (1925).

Courtesy Dr. Perrini. Published with permission.

Hess wrote a landmark publication on the morphology of root canals using some 3000 permanent teeth. Canal shapes were made visible by injecting vulcanite into canal systems and dissolving the surrounding roots. This process showed graphically how complex the canal system was for each tooth ( Fig. 12.1, F ). A similar method was used for primary teeth by Zürcher, and the results were combined and reprinted in the English literature in 1925. Many variable shapes and extra canals in single rooted teeth were shown to exist. For example, in the casts of mandibular incisor, mandibular premolar, and mandibular canine, the canals were complex and sometimes multiple in number. Illustrations in his publication are remarkable for their accuracy but were either overlooked or ignored as being abnormal variants at the time. Subsequent anatomic studies and an earlier published study have confirmed their observations. More recent studies on canal numbers, both ex vivo and in vivo, have shown that former endodontic techniques in root canal therapy using less flexible instruments only available at the time had inadvertently missed many of these extra and complex canal systems.

Clinical competence relies on the development of the widely recognized psychomotor aspect. Inextricably coupled with these psychomotor skills is the ability to self-evaluate the process of correction and the product itself compared with the desired outcome. Visual perception has been suggested as a prerequisite skill for determining the appropriate goals and strategy for a correction. Visual skill is required to observe normal 3D tooth morphology in detail, to differentiate normal tooth morphology from its variations. Further, motor skills are also required to execute clinically relevant dental procedures. In the development of psychomotor skills, the student must teach his or her hands to do that which his or her mind dictates as correct. Models that simulate teeth in which root canals can be visualized would serve as valuable teaching aids in offering direct visual information on the effects of instrumentation during endodontic procedures. The visual experiences afforded by these models must provide mental images, which can be transferred to the performance of endodontic procedures on actual teeth. Nowadays, most dental schools continue to present foundational knowledge of dental anatomy in lectures and to develop students’ psychomotor skills through a combination of 2D drawing projects, radiographs, and exercises to carve teeth from oversized wax blocks. As a result, neither knowledge nor psychomotor skills are learned in the context of clinical practice, thus potentially hindering the student’s ability to later recall and apply learning to actual patient care. On the other hand, practice on extracted teeth has been a universal method of teaching preclinical endodontics and gives students the opportunity of gaining expertise before moving to treating patients. However, infection control concerns, originated by the manipulation of extracted teeth, along with ethical factors are threatening such preclinical laboratory practice in some teaching institutions. These drawbacks have stimulated the development of alternative simulation methods for teaching root canal anatomy.

Root Canal Anatomy Since the Age of Specialization in Endodontics

One must not take away from dental practitioners who revived the popularity and viability of root canal therapy after the misinformed attitudes in the age of “Focal Infection Theory.” By the mid-20th century the attitudes and instruments in endodontics had changed; thus standardization of instruments and techniques led to saving many more teeth, while using a recognized and effective treatment rationale. However, it was recognized that if missed canals were leading to failure to seal the entire canal system, then new studies on the variability of root and canal anatomy had to be done. New methods of study were devised that used both radiographs and bench studies of extracted teeth. This has led to the second phase of research in dental and root anatomy, which began about the time of recognition of endodontics as a specialty branch of restorative dentistry in 1964 in the United States.

These tooth anatomy studies included methods such as conventional and laboratory radiography (with or without contrast media), resin injection, macroscopic and microscopic evaluations, tooth sectioning, root clearing techniques, and scanning electron microscopy. Kuttler in 1955 used a dissection microscope to show that the apical foramen in teeth varied considerably in diameter. Skidmore and Bjørndal in 1971 illustrated casts of mandibular first molars with multiple and complex canal systems. Another example of the practical aspect of studying root morphology was the paper by Davis et al. in 1972 that described the use of injected silicone into standard endodontic-prepared canals, with the resulting casts showing that some areas of the canal system had not been completely shaped. All of these canal anatomy studies and more, either in physical anthropology papers or in dental journals, built on a second wave of root and canal system knowledge.

Undoubtedly, these techniques have shown a great potential for endodontic research. However, although most of these methods required the partial or even full destruction of the studied samples rendering irreversible changes in the specimens and many artifacts, others provided only a 2D image of a 3D structure. These inherent limitations have repeatedly been discussed, encouraging the search for new methods with improved possibilities.

More recently, the third phase of studies in human root and root canal anatomy is well underway. Increased computer power of digital radiographs and advanced technology are producing studies of human teeth with conventional medical computed tomography (CT), magnetic resonance microscopy, tuned-aperture CT, optical coherence tomography, volumetric or cone beam CT (CBCT; used as a clinical enhancement of practice), and micro–computed tomography (micro-CT). CT-based training replicas produced using 3D printing technology have improved the use of artificial teeth for teaching purposes ( Fig. 12.2 ). Replicas with different root canal complexities can be printed as oversized models in a rapid prototyping printer, allowing the students to hold them in hand to observe details of the internal anatomy in different views. Additional applications of printed models in dental education also include the possibility to (1) scale the teeth for didactic purposes, (2) build a collection of 3D tooth models showing atypical or only regionally prevalent anatomies, (3) produce a large number of teeth for destructive analysis, (4) present the teeth in the form of individual substructures that need to be assembled correctly by the students, and (5) build an extensive collection of 3D models of healthy and diseased teeth using raw data made available online by researchers and dentists from all over the world.

Replicas of various teeth manufactured from corresponding real-tooth micro-CT scans using three-dimensional (3D) printing technology in assorted sizes for didactic or teaching purposes. A, True Tooth®; B, RepliDens®. ( A, images available at https://dentalengineeringlab.com/truetooth/ ; B, images available at https://www.smartodont.ch/replidens/ .)

As imaging has been adopted in modern dental education, it has benefited from the concurrent development of technologies that have allowed the material to be presented electronically. One of the technologies with the greatest effect has been the Internet. The Internet has increasingly been used as an educational tool as a result of its ability to provide a large volume of educational material in a single, readily accessible location and permitting flexibility in the material format. Images, text, interactive quizzes, and videos can be integrated seamlessly into a comprehensive educational resource. In this way, digital images acquired from micro-CT devices could be used to generate anatomic tooth data on a large scale and made freely accessible to the public through the Internet ( www.rootcanalanatomy.blogspot.com ), thus circumventing the problems of individual researchers requiring access to high-cost scanning devices. Therefore one of the most important aspects of the computer age of communication is the ability to find and access research from many more dental schools and dental researchers from all over the dental academic world. Although a computer search may seem quick and easy, one must rely only on reliable research publications from journals with a credible and high impact factor. A new understanding has been forthcoming on the importance of ethnicity and human dental anatomy.

The misconception of thinking about “one-root equals one-canal” in endodontic treatment has been shattered by a number of classic papers that demonstrate otherwise. In fact, a number of studies have classified and described the morphology of multiple canals in a broad diameter root. This multiple canal configuration may divide, combine, and separate as it forms in root closure toward the various morphologies of the apical foramen terminus. It is prudent to assume that any root that requires treatment may contain more than one canal system per root, until proved otherwise.

Multiple Canals Within a Single Root

Weine et al. in 1969 were the first authors to recognize and publish how commonly two canals in one root occurred, and then to classify the two canals in the mesiobuccal root of the maxillary first molar tooth as the “type specimen” ( Fig. 12.3, A ). Piñeda and Kuttler in 1972 used radiographs on 7275 extracted teeth to demonstrate multiple canal systems in three dimensions not usually seen in the clinical setting. Other researchers soon added observations that confirmed this morphology was not uncommon in many other broad labiolingual or buccolingual roots, as well as, the mesiobuccal root of maxillary molars.

A, Diagrammatic representations of Weine’s classification for root canal morphology. Type I: A single canal from pulp chamber to the apex (1-1 configuration); Type II: Two separate canals leaving the chamber, but merging short of the apex to form a single canal (2-1 configuration); Type III: Two distinct canals from pulp chamber to the apex (2-2 configuration); Type IV: A single canal leaving the chamber and dividing into two separate canals at the apex (1-2 configuration). B, Diagrammatic representations of Vertucci’s classification for root canal morphology. Type I: A single canal from pulp chamber to the apex (1-1 configuration); Type II: Two separate canals leaving the chamber, but merging short of the apex to form a single canal (2-1 configuration); Type III: A single canal that divides into two, and subsequently merges to exit as one (1-2-1 configuration); Type IV: Two distinct canals from pulp chamber to the apex (2-2 configuration); Type V: A single canal leaving the chamber and dividing into two separate canals at the apex (1-2 configuration); Type VI: Two separate canals leaving the pulp chamber, merging in the body of the root, and dividing again into two distinct canals short from the apex (2-1-2 configuration); Type VII: A single canal that divides, merges, and exits into two distinct canals short from the apex (1-2-1-2 configuration); Type VIII: Three distinct canals within one root from pulp chamber to the apex (3-3 configuration).

Vertucci et al. developed a more complex classification that is better adapted to research and applied in any other tooth that is wider in the labiolingual or buccolingual dimension ( Fig. 12.3, B ). Essentially, Weine and Vertucci’s configuration systems were based on the number of root canals that begin at the pulp chamber floor, arise along the course of the canal, and open through an apical foramen. Later, Versiani and Ordinola-Zapata expanded and adapted these classifications to 3D tomographic descriptions of at least 37 complex canal systems possible to be observed in a single root ( Fig. 12.4 ). The following tables of root numbers of tooth pairings will help one understand the variation in incidence of single and multiple canal numbers based on a large sample from multiple studies. The computer-generated figures will also show graphically some of the variations in anatomy that may be found in the human dentition. Other research studies have shown that furcation canals, lateral canals, and apical ramifications have developed all too commonly. Better cleansing and obturation techniques will more likely seal all portals of exit in the chamber and canal and lead to higher success rates in studies, based on evidence.

Thirty-seven most common canal configurations possible to be observed in a single root, according to Versiani and Ordinola-Zapata.

Success in root canal therapy can be achieved by first knowing the normal canal anatomy and then by being aware of the many variations that the path of the canal system can follow. One should be able to develop a 3D visualization, both in longitudinal and in cross-section while still using clinical tactile sense to guide a file toward the apical foramen or apical terminus. The following description and images will help provide that knowledge to aid one in honing that skill and expertise.

Pulp Chamber

The pulp chamber is a cavity normally situated in the center of the crown and, when there are no pathologic conditions, resembles the shape of the crown surface. In anterior teeth that have a single canal in one root, the pulp chamber and root canal are continuous whereas, in posterior teeth with multiple canals and more than one root, the pulp chamber floor separates these two components. In premolars and molars, the pulp chamber usually presents a square shape with six sides: the floor, the roof, and four axial walls identified as mesial, distal, buccal, or lingual (palatal). The pulp chamber roof usually presents projections or prominences associated with cusps, mamelons, or incisal ridges, denominated pulp horns. In teeth with physiologic wear or other irritation, continuous dentin formation (either physiologic or reactionary) by primary odontoblasts may lead to a decrease in the pulpal space dimensions which, in some cases, can compromise root canal treatment.

Based on the anatomic study of 500 teeth, Krasner and Rankow demonstrated that specific and consistent pulp chamber anatomy exists. Then, they proposed some general rules or laws ( Fig. 12.5 ) for aiding in the determination of the pulp chamber position and the location and number of root canal entrances in each group of teeth:

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  Law of centrality: The floor of the pulp chamber is always located in the center of the tooth at the level of the cementoenamel junction (CEJ).

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  Law of concentricity: The walls of the pulp chamber are always concentric to the external surface of the tooth at the level of the CEJ (i.e., the external root surface anatomy reflects the internal pulp chamber anatomy).

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  Law of the CEJ: The distance from the external surface of the clinical crown to the wall of the pulp chamber is the same throughout the circumference of the tooth at the level of the CEJ. The CEJ is the most consistent, repeatable landmark for locating the position of the pulp chamber.

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  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.

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  Law of symmetry 2: Except for the maxillary molars, the orifices of the canals lie on a line perpendicular to a line drawn in a mesiodistal direction across the center of the floor of the pulp chamber.

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  Law of color change: The color of the pulp chamber floor is always darker than the walls.

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  Law of orifice location 1: The orifices of the root canals are always located at the junction of the walls and the floor.

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  Law of orifice location 2: The orifices of the root canals are located at the angles in the floor-wall junction.

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  Law of orifice location 3: The orifices of the root canals are located at the terminus of the root developmental fusion lines.

Three-dimensional (3D) micro–computed tomography (micro-CT) images of posterior teeth demonstrating the (A) laws of centrality and concentricity, at the cementoenamel junction (CEJ) in a mandibular molar tooth; (B) laws of color change and orifice locations 1, 2, and 3 ( arrow : developmental fusion lines) in a four-rooted maxillary second molar tooth; and (C–D) laws of symmetry 1 (three-rooted maxillary molar) and 2 (two-rooted maxillary premolar).

In addition to knowing these laws, the use of better illumination and magnification sometimes associated with specific instruments, such as thin ultrasonic tips or special burs, would provide the best approach to explore the anatomic variations of the pulp chamber in order to locate all canal orifices and avoid missed canals.

Root Canal

The root canal is the portion of the pulp canal space within the root of the tooth limited by the pulp chamber and the foramen that follows the external outline of the root ( Fig. 12.6 ). The root canal can be subdivided into two components: the main canal, which is mostly cleaned by mechanical means, and lateral components composed by isthmuses, accessory canals (furcation, lateral and secondary canals), and some recesses of flattened- and oval-shaped canals.

Three-dimensional (3D) models of real (A) incisor and (B–C) molar teeth, obtained by micro–computed tomography (micro-CT) technology, showing the main components of the root canal system.

In longitudinal section, canals are usually broader faciolingually than in the mesiodistal plane. Traditionally, canal shape has been classified as round, oval, long oval, flattened, or irregular ( Fig. 12.7 ). Its geometric cross-sectional shape has been also quantitatively described by calculating the mean aspect ratio, defined as the ratio of the major to the minor canal diameters. The major diameter is the distance between the two most distant points of the canal in the buccolingual direction, whereas the minor diameter is the longest chord through the root canal that could be drawn in the direction orthogonal to that of the major diameter. Accordingly, an oval-shaped canal has an aspect ratio between 1 and 2, a long oval canal higher than 2 but lower than 4, and a flattened canal has a value higher than 4. It is interesting to point out that, in a same tooth, canal cross-sections may show different shapes at different levels of the root; however, at the apical third, it is more round or slightly oval in shape in comparison with the middle and coronal thirds. ^, Thus, as previously mentioned, the anatomy of the root canal systems is often complex and can vary greatly in number and shape.

Two-dimensional (2D) root canal cross-sections of three mandibular canines (A–C) showing that canals are usually broader buccolingually than in the mesiodistal plane, and may present different shapes at distinct levels of the root.

Isthmus

An isthmus, also called transverse anastomosis , is a narrow, ribbon-shaped communication between two root canals that may contain vital tissue, necrotic pulp, biofilms, or residual filling material. ^, Isthmuses (or isthmi) may present with different configurations ( Fig. 12.8 ), and their prevalence is dependent on the type of teeth, the root level, and the patient’s age. Hsu and Kim classified the isthmuses configuration into five types:

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  Type I—Two canals with no notable communication

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  Type II—A hair-thin connection between the two main canals

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  Type III—Differs from type II because of the presence of three canals instead of two

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  Type IV—An isthmus with extended canals into the connection

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  Type V—A true connection or wide corridor of tissue between two main canals.

Two-dimensional (2D) root canal cross-sections of four mesial roots of mandibular molars showing isthmuses with different sizes and shapes at different levels of the root.

It is noteworthy that experimental studies demonstrated the impossibility of obtaining a complete mechanical debridement or chemical disinfection of isthmuses with the current technology, mostly because of the presence of hard tissue debris packed into these areas during the mechanical preparation of the main root canal. Clinical studies have also shown that unfilled isthmuses can be commonly observed after root-end resection in cases referred for apicoectomy treatment. These limitations, however, can be surpassed in nonsurgical treatment by using chemical agents that have the ability to dissolve organic tissue at fins and isthmuses level, often associated with ultrasonic activation. In addition, with the advent of the operatory microscope, it is possible to identify and treat most of the isthmus areas with thin ultrasonic tips, in both surgical and nonsurgical endodontic procedures, to ensure their debridement and seal. ^,

Accessory Canals

An accessory canal is any branch of the root canal that communicates with the periodontal ligament, whereas a lateral canal is defined as an accessory canal located at the coronal or middle third of the root ( Fig. 12.9, A and B ). They are formed after a localized fragmentation of Hertwig’s epithelial root sheath develops, leaving a small gap, or when blood vessels running from the dental sac through the dental papilla persist as collateral circulation. Accessory canals represent potential pathways through which bacteria and/or their byproducts from the necrotic root canal might reach the periodontal ligament and cause disease.

Three-dimensional (3D) models of (A–B) two maxillary central incisors and (C) a two-rooted mandibular canine, obtained by micro–computed tomography (micro-CT) technology, showing the lateral components of the root canal system.

De Deus studied the frequency, location, and direction of the accessory canals in 1140 teeth and showed that 27.4% of the sample (n = 330) had accessory canals, especially in the apical area (17%), followed by the middle (8.8%) and coronal (1.6%) thirds. Similarly, Vertucci evaluated 2400 teeth and observed a lower occurrence of canal ramifications in the middle (11.4%) and coronal (6.3%) thirds compared with the apical level (73.5%). Lateral canals are not usually visible in preoperative radiographs, but their presence can be suspected when there is a localized thickening of the periodontal ligament or there is a lesion on the lateral surface of the root. Clinically, it is also relevant that lateral canals cannot be instrumented most of the time. In this way, their content can only be neutralized by means of effective irrigation with a suitable antimicrobial solution or with an additional use of intracanal medication. ^,

Canals connecting the pulp chamber to the periodontal ligament in the furcation region of a multirooted tooth are called furcation canals ( Fig. 12.9, C ). These canals are derived from entrapment of periodontal vessels during the fusion of the parts of the diaphragm, which will become the floor of the pulp chamber. In some cases, furcation canals have been associated with primary endodontic lesions in the interradicular region of multirooted teeth. Vertucci and Williams observed the presence of furcation canals in 13% of mandibular first molars, and in most of them the canal extended from the center of the pulpal floor, whereas in four and two specimens, respectively, the canals arose from the mesial and distal aspects of the floor. Later, Vertucci and Anthony observed the presence of foramina on both the pulp chamber floor and the furcation surface in 36% of maxillary first molars, 12% of maxillary second molars, 32% of mandibular first molars, and 24% of mandibular second molars. Recently, micro-CT studies have also demonstrated the presence of furcation canals in two-rooted mandibular canines and three-rooted mandibular premolars.

Apical Canal

The main root canal ends at the apical foramen (major foramen), which frequently opens laterally on the root surface, at a mean distance between 0.2 to 3.8 mm from the anatomic apex, despite larger distances have been reported recently. The anatomic apex is the tip or the end of the root as determined morphologically. Depending on the type of teeth, the apical foramen can coincide with the anatomic apex in a percentage frequency ranging from 6.7% to 46% of the cases. ^{, , ,} Its diameter has been described between 0.21 to 0.39 mm. The mesial roots of mandibular molars, the maxillary premolars, and the mesiobuccal roots of maxillary molars present the highest percentage of multiple apical foramina. A previous study on root apices of all groups of permanent teeth showed that the number of foramina on each root may vary from 1 to 16.

The apical portion of the root canal having the narrowest diameter has been called the “apical constriction” (minor foramen). From the apical constriction, the canal widens as it approaches the apical foramen. The topography of the apical constriction is not constant ^, and, when present, is usually located 0.5 to 1.5 mm from the center of the apical foramen. The cementodentinal junction (CDJ) is the point at which the cemental surface terminates at or near the apex of a tooth and meets dentin. At this histologic landmark pulp tissue ends and periodontal tissues begin ( Fig. 12.10, A ).

A, Anatomic landmarks at the apex of a cleared single-rooted tooth (CDJ: cementodentinal junction). B, Three-dimensional (3D) micro–computed tomography (micro-CT) model of a mandibular canine presenting apical ramification (arrow).

A, Courtesy Dr. Francisco Balandrano. Published with permission.

Another relevant variation of the root canal at or near the apex is an intricate network of ramifications, also called apical ramification of apical delta , which is defined as a morphology in which the main canal divides into multiple accessory canals ( Fig. 12.10, B ). In maxillary teeth, the percentage frequency of apical ramification ranges from 1% (central incisors) to 15.1% (second premolars), whereas in mandibular teeth its frequency varies from 5% (central incisors) to 14% (distal root of first molars). In the treatment of clinical cases, the infection of this tortuous and complex anatomic configuration with several portals of exit can be related as an etiologic factor of nonsurgical failures.

Canal Curvature and Size

Knowledge of the root curvature is an important factor in choosing the appropriate chemomechanical protocol for cleaning and shaping the root canal system. Before the introduction of nickel-titanium (NiTi) instruments, several iatrogenic procedures were associated with the preparation of curved canals including zips, separated instruments, ledges, and perforations. Nowadays, these iatrogenic complications are no longer a problem, except for instrument separation. Therefore this is one of the factors determining the difficulty of treatment and the likelihood of iatrogenic errors and shows that preoperative recognition of canal curvature is of utmost importance.

Nearly all root canals are curved in the apical third, particularly in a faciolingual direction, which is not evident on standard radiography. In general, the curvature may vary from gradual curvature of the entire canal, sharp curvature of the canal near the apex, or a gradual curvature of the canal with a straight apical ending. Numerous methods have been proposed to determine root canal curvature, ^, but the Schneider’s method has been the most widely used. Schneider classified single-rooted permanent teeth according to the degree of curvature of the root, which was determined by first drawing a line parallel to the long axis of the canal, then, a second line connecting the apical foramen to the point in the first line where the canal began to leave the long axis of the tooth. The angle formed by these two lines was the angle of curvature and its degree was classified as straight (≤5 degrees), moderate (10 to 20 degrees), or severe (25 to 70 degrees).

Another method was introduced by Weine that also relies on the definition of two straight lines, but it reflects the root canal curvature more accurately than Schneider’s method, especially in the apical part. A third proposal, geometrically equivalent to Weine’s method, was introduced by Pruett et al., but its major innovation was the concurrent measurement of the radius of curvature by the superimposition of a circular arc on the curved part of the root canal. Therefore, the Schneider angle, when used in combination with the radius and length of the curve, may provide a more precise method for describing the apical geometry of canal curvature.

Clinically, different angled views are necessary to determine the presence, direction, and severity of the root canal curvature. Schäfer et al. evaluated radiographically the degree of curvature of 1163 root canals from all groups of teeth. The degree of curvature ranged from 0 to 75 degrees and from 0 to 69 degrees in clinical and proximal views, respectively. The highest degree of curvature was observed in the clinical view of the mesiobuccal canal of maxillary molars and in the mesial canals of mandibular molars. In several cases, the angles of proximal curvatures were higher than those of the clinical view. Additionally, a secondary curvature (S-shaped canal) was observed in 12.3% and 23.3% of the maxillary and mandibular teeth, respectively.

Taurodontism

Taurodontism (or a “bull-shaped” tooth) is a dental morphologic variation in which the body of the tooth is enlarged and the roots are reduced in length. A taurodont tooth presents a large pulp chamber with apical displacement of the pulpal floor and furcation of the roots ( Fig. 12.12, A and B ). The etiology of taurodontism is unclear, but it also appears in certain genetic syndromes. It is thought to be caused by the failure of Hertwig’s epithelial root sheath diaphragm to invaginate at the proper horizontal level, resulting in a tooth with normal dentin, short roots, elongated body, and enlarged pulp. ^{, ,} The teeth involved are almost invariably molars or rarely premolars. It can be uni- or bilateral and may affect single or multiple teeth. The condition may also present rarely in the primary dentition molar teeth. Taurodontism is most famously reported to occur in high incidence by Keith in 1913 as found in Homo neanderthal from the Krapina archaeologic find in the early 1900s.

Root canal anomalies and embryologic malformations.

(A–B) Taurodont maxillary second molar presenting a large pulp chamber (yellow arrow) with apical displacement of the pulpal floor and furcation of the roots; C–F, Clinical and tomographic views of a maxillary lateral incisor with dens invaginatus (arrows) (Courtesy of Dr. Oscar von Stetten. Published with permission); G–I, Clinical and radiographic views of a mandibular second premolar with dens evaginatus (arrows).

Courtesy Dr. Daniela Bololoi. Published with permission.

Taurodontism was classified earlier by Shaw in 1928 and has been graded according to its severity: normal (cynodont), least pronounced (hypotaurodontism), moderate (mesotaurodontism), and most severe (hypertaurodontism). Clinically, the crowns of these teeth usually have normal characteristics. Therefore the diagnosis is entirely radiologic. Owing to the complexity of the root canal anatomy and the proximity of the orifices to the root apex, complete filling of the root canal system in taurodontism is challenging. Because the pulp of a taurodont is usually voluminous, control of bleeding in cases of pulpitis may take some time and effort compared to teeth with normal anatomy. Additional efforts such as application of ultrasonic instrumentation combined with sodium hypochlorite (NaOCl) as an irrigant solution should be made to dissolve as much organic material as possible. ^{, ,}

Dens Invaginatus and Dens Evaginatus

Dens invaginatus (dens in dente , dilated composite odontome, dilated odontome, gestant anomaly, invaginated odontome, dilated gestant odontome, tooth inclusion, dentoid in dente) is a developmental defect resulting from invagination in the surface of the tooth crown before calcification has occurred ( Fig. 12.12, C–F ). Clinically, it may appear as an accentuation of the lingual pit in anterior teeth and, in its more severe form, gives a radiographic appearance of a tooth within a tooth, hence the term dens in dente. Its etiology is controversial and remains unclear. The affected teeth radiographically show an infolding of enamel and dentin that may extend deep into the pulp cavity and into the root and sometimes even reach the root apex. The most common associated clinical finding is an early pulpal involvement, explained by the existence of a canal extending from the invagination into the pulp. The invagination also allows the entry of irritants into an area that is separated from pulpal tissue by only a thin layer of enamel and dentin and presents a predisposition for the development of dental caries. Therefore this condition must be recognized early and the tooth prophylactically restored. The variability of its root canal system configuration is unlimited. Clinically, however, it can only be speculated upon from radiographs. In this way, the most commonly referred classification was proposed by Oehlers, who categorized it into three types: Type 1—the invagination is confined to the crown and does not extend beyond the CEJ; Type 2—the invagination extends past the CEJ and does not involve the periradicular tissues, but may communicate with the dental pulp; and Type 3—the invagination extends beyond the CEJ and may present a second apical foramen, with no immediate communication with the pulp. In the literature, the reported prevalence of this anomaly varies from 0.25% to 10% and the most affected teeth are permanent maxillary lateral incisors, despite the fact that it may occur in any tooth. ^, This high range frequency of dens invaginatus has been associated with the study design, sample size and composition, and diagnostic criteria. ^,

Dens evaginatus is an anomalous outgrowth of tooth structure resulting from the folding of the inner enamel epithelium into the stellate reticulum with the projection of structure exhibiting enamel, dentin, and pulp tissue ( Fig. 12.12, G–I ). It arises most frequently from the occlusal surface of involved posterior teeth, mainly maxillary and mandibular premolars, and primarily from the lingual surface of associated anterior teeth (called talon cusps when in this location). ^, Its etiology remains unclear. However, it predominantly occurs in people of Asian descent with varying estimates reported at 0.5% to 15%, depending on the population group studied. The presence of pulp within the cusp-like tubercle has great clinical significance. Because the tubercle may extend above the occlusal surface, malocclusion or attrition with the opposing tooth may cause abnormal wear or fracture of the tubercle, and this is how pulp exposure occurs. Subsequent pulpal inflammation or infection will most likely ensue, at times when the root apex closure has not occurred in the young patient. It is important for the clinician to be able to recognize and treat the entity soon after affected teeth have erupted into the oral cavity in order to avoid the development of pathologic conditions.

Radix

Radix is a Latin word for “root” and is referred to additional roots of teeth, mostly molars. In radix molars, each root usually contains a single root canal.

In four-rooted maxillary molars, the palatal part of the root complex is made up of two macrostructures located mesially and distally, which are in principle cone-shaped and either separate or nonseparate in relation to each other. If the mesial of the two palatal root structures has direct affinity to the mesiolingual part of the crown, which is more pronounced, the mesial root structure is identified as radix mesiolingualis, whereas the distal structure is identical with the palatal root component. If the distal of the two palatal root structures has direct affinity to the distopalatal part of the crown, the distal root structure is identified as radix distolingualis, whereas the mesial structure is identical with the palatal root component ( Fig. 12.13 ).

Different views of a three-dimensional (3D) micro–computed tomography (micro-CT) models of the external and internal morphologies of a four-rooted maxillary molar showing radix mesiolingualis (RML) and radix distolingualis (RDL). DB , Distobuccal root; MB , mesiobuccal root.

In mandibular molars, additional roots have been identified as radix entomolaris and radix paramolaris . ^, Radix entomolaris has been defined as a supernumerary root on a mandibular molar located distolingually ( Fig. 12.14 ), whereas radix paramolaris is an extra root located mesiobuccally ( Fig. 12.15 ). The presence of these anatomic anomalies has been associated with certain ethnic groups such as Sino Americans, which include Chinese, Inuit, and American Indians. ^, Radix paramolaris is a very rare structure and its prevalence was found to be 0%, 0.5%, and 2.0% for the mandibular first, second, and third molars, respectively, whereas radix entomolaris occurs with a higher frequency, ranging from 0.2% to 32% of the studied samples. The orifice of the radix entomolaris is located disto- to mesiolingually from the main canal or canals of the distal root, whereas the orifice of the radix paramolaris is located mesio- to distobuccally from the main mesial canals. A dark line or groove from the main root canal on the pulp chamber floor leads to these orifices ; however, they provide a limited practical aid for its identification in clinical practice. These anatomic variations present definite challenges to therapy because of their orifice inclination and root canal curvature. In this way, preoperative periapical radiographs at different horizontal angles or a CBCT examination are required to identify this additional root, which will also result in a modified opening cavity. An accurate diagnosis of these anatomic variations is important to avoid missed canals.

Three-dimensional (3D) micro–computed tomography (CT) models of the external and internal morphologies of mandibular second molars showing radix entomolaris (yellow arrows) . In the lateral view, the curvature of the radix is depicted (black arrows).

Radiographic and tomographic views of a mandibular left second premolar showing a radix paramolaris (arrows) , before and after root canal treatment.

Courtesy Dr. Nuno Pinto. Published with permission.

C-Shaped Canals

The C-shaped configuration was first reported in the endodontic literature by Cooke and Cox in 1979, but this canal configuration has been well-known since the beginning of the 20th century. This anatomic variation is so named for the root and root canal cross-sectional shape of the capital letter “C.” Its main anatomic feature is the presence of one or more isthmuses connecting individual canals, which can change the cross-sectional and 3D canal shape along the root ( Fig. 12.16 ). Typically, this configuration is found in teeth with fusion of the roots either on its buccal or lingual aspect, and results from the failure of Hertwig’s epithelial root sheath to develop or fuse in the furcation area during the developing stage of the teeth. ^, Failure on the buccal side will result in a lingual groove, and the opposite cases would be possible. In such teeth, the floor of the pulp chamber is frequently situated deeply and may assume an unusual anatomic appearance. Below the orifice level, the root structure of a C-shaped tooth can harbor a wide range of anatomic variations, which make it a challenge with respect to disinfection. This variation may occur in different types of teeth ^, ; however, it is most commonly found in mandibular second molars ^, with a reported prevalence ranging from 2.7% to 44.5%. There is significant ethnic variation in the frequency of C-shaped molar teeth, which are much more common in Asian populations than in Caucasian populations. In population-based studies, the reported prevalence was 10.6% in Saudi Arabians, 19.14% in Lebanese, 31.5% in Chinese, and 44.5% in Koreans. To date, two studies have addressed the efficacy of different systems in the preparation of C-shaped mandibular molar canals showing a significant percentage of canal area unaffected by the instrumentation procedure. ^,

Three-dimensional (3D) micro–computed tomography (CT) models of the external and internal morphologies of mandibular second molars showing different C-shaped canal configurations.

In 1991 Melton et al. proposed the first classification for C-shaped canal configuration in mandibular second molars based on its cross-sectional shape. They fall into three categories: Category I—a continuous C-shaped canal running from the pulp chamber to the apex; Category II—a semicolon-shaped orifice in which dentin separates a main C-shaped canal from one mesial distinct canal; Category III—two or more discrete and separate canals which could join in the apical (subdivision I), middle (subdivision II), or coronal (subdivision III) thirds.

It is important to point out that mandibular molar teeth can present with irregularities in their canal systems throughout the root and the presence of these categories may vary from the pulp chamber to the apex. In this way, Fan et al. modified Melton’s method and recommended to classify each portion of the same tooth using five categories:

• •

  Category I—The shape was an uninterrupted “C” with no separation or division.

• •

  Category II—The canal shape resembled a semicolon resulting from a discontinuation of the “C” outline.

• •

  Category III—Two or three separate canals

• •

  Category IV—Only one round or oval canal in the cross-section (normally found near the apex)

• •

  Category V—No canal lumen (usually seen near the apex only)

Melton’s classification stated that categories II and III have separated canals, but no description was provided to differentiate them. In the modified classification, one of the canals in the category II would appear as an arc and would be more likely to extend into the “fused” area of the root where the dentin wall may be quite thin.

C-shaped canal anatomy has also been reported in third molars, lateral incisors, mandibular first premolars, ^{, , , ,} mandibular first molars, and maxillary first ^, and second molars. Recently the prevalence of C-shaped canal configuration in maxillary molars with root fusion was reported to be as high as 15% ( Fig. 12.17, A ). Mandibular first premolars present a variety of root canal configurations that include the presence of two or three root canals ^, and a C-shaped configuration system ( Fig. 12.17, B ). As in mandibular molars, C-shaped canal systems in the mandibular first premolars vary among different ethnic groups, with its prevalence being reported to range from 1% to 18%. This configuration has been highly associated with Vertucci’s type V configuration (i.e., a single canal that bifurcates at the middle third and with the presence of a groove or concavity on the external root surface). ^, Radicular grooves on mandibular first premolars usually begin 3 mm from the CEJ and present frequently on the proximal lingual area of the middle root, not always extending to the root apex. ^,

A, Three-dimensional (3D) models and two-dimensional (2D) cross-sections of maxillary second molars with fused roots. Note the complexity of the root canal system with the presence of canal interconnections, apical ramifications and C-shaped canals; B, 3D model and 2D cross-sections at different levels of the root of a mandibular first premolar with radicular groove and C-shaped canal configuration.

Preoperative diagnosis of C-shaped canals is complex, mainly because these unique anatomic features are not easily recognized on a traditional 2D periapical radiograph. With the increased use of CBCT scanning, clinicians may be able to detect C-shaped canals before endodontic treatment. Nevertheless, even when recognized, the disinfection procedure still remains a challenge, mostly because of the isthmus areas. Irregular areas in a C-shaped canal that may house soft tissue remnants or infected debris may escape thorough cleaning and may be a source of bleeding and severe pain. In this way, the use of a dental microscope associated with sonic or ultrasonic instrumentation techniques may make treatment outcome more predictable. Because of its challenging morphology, the C-shaped canal anatomy increases the difficulty in root canal therapy and may account for the frequent occurrence of endodontic failure on this tooth.

Other Anomalies

• (1) Fusion: It is commonly defined as the union of two distinct dental sprouts that occurs in any stage of the dental organ. They are joined by the dentin, whereas pulp chambers and canals may be linked or separated depending on the developmental stage when the union occurs. This process involves epithelial and mesenchymal germ layers resulting in irregular tooth morphology and occurs more frequently in anterior teeth ( Fig. 12.18 ).

  

  A, Clinical and B, tomographic views of a patient presenting fusion and gemination at the anterior teeth.

  Courtesy Dr. Antonis Chaniotis. Published with permission.

• (2) Gemination: It is a disturbance during odontogenesis in which partial cleavage of the tooth germ occurs and results in a tooth that has a double or “twin” crown, usually not completely separated, and sharing a common root and pulp space (see Fig. 12.18 ). The root and pulp are also irregular in morphology.

• (3) Hypercementosis: This refers to an excessive deposition of nonneoplastic cementum over normal root cementum, which alters the root morphology macroscopic appearance. Its pathogenesis is ambiguous. Most of the cases are idiopathic. Several local and systemic factors are also linked to this condition, such as Paget disease, acromegaly, or vitamin A deficiency ( Fig. 12.19 ).

  

  Coronal and sagittal cone beam computed tomography (CBCT) images from a distal root of a mandibular first molar presenting hypercementosis (arrow) .

  Courtesy Dr. Oscar von Stetten. Published with permission.

• (4) Radicular Groove: This is a developmental depression in the proximal aspect of the root surface. Radicular grooves have been reported as being widespread in Africans and native Australians and are relatively rare in Western Eurasians. It is relevant in clinic care because its depth may act as a reservoir for dental plaque and calculus, increasing the difficulty in the management of periodontal disease. ^, In mandibular premolar teeth, its presence has been associated with anatomic complexities of the root canal system, such as canal bifurcation and C-shaped configuration ( Fig. 12.20 ). ^{, ,}

  

  A, Three-dimensional (3D) micro–computed tomography (micro-CT) models of mandibular premolars showing the external anatomy with the presence of radicular grooves (arrows) ; B, Two-dimensional (2D) cross-section of the root of a mandibular premolar showing radicular groove (arrow) and a C-shaped canal configuration.

Aging

The root canal anatomy is susceptible to changes over the years because of physiologic or pathologic events. Natural physiologic aging tends to modify the root canal system morphology as a result of the deposition of secondary dentin, which starts to form once the tooth erupts and is in occlusion. Consequently, young patients tend to present with large single canals and pulp chambers, ^, whereas older patients tend to present with more sharply defined and narrow root canals. Other pathologic or iatrogenic factors can also modify the deposition of dentin including occlusal trauma, periodontal disease, carious lesions, or deep restorative procedures.

In the literature, CBCT imaging technology has been also used to address in vivo root canal morphologic changes caused by aging. Overall, results showed no significant difference between maxillary and mandibular anterior teeth groups regarding age, despite contradictory information can be found. While in mandibular anterior teeth most studies reported a lower prevalence of multiple canals in older patients, ^, in maxillary and mandibular premolars and in mandibular molars ^, it was observed that there was a progressive decrease of Vertucci’s Type I configuration with age. The prevalence of a second canal in the mesiobuccal root of maxillary first and second molars was also evaluated and most studies reported a lower prevalence of this configuration in older patients. ^,

Incisors

Morphologic aspects of the root and root canal anatomy of maxillary and mandibular incisors are detailed in Table 12.1 , Fig. 12.21 , and Appendices 1 to 4 (Summary of Root Numbers and Root Canal Systems of the Permanent Teeth).

Three-dimensional (3D) micro–computed tomography (micro-CT) models and two-dimensional (2D) cross-sections depicting the internal and external anatomy of maxillary and mandibular incisors.

The maxillary central incisors are centered in the maxilla, one on either side of the midline, with the mesial surface of each in contact with the mesial surface of the other. The pulp cavity follows the general outline of the crown and root. In this way, the pulp chamber is very narrow in the incisal region and wider in the mesiodistal dimension than in the labiolingual dimension. The maxillary lateral incisor supplements the central incisor in function, and the crowns bear a close resemblance. However, the lateral incisor is smaller in all dimensions except root length. The pulp chamber is narrow in the incisal region and may become very wide at the cervical level of the tooth, whereas pulp horns are usually prominent.

The mandibular central incisors are centered in the mandible, one on either side of the midline, with the mesial surface of each one in contact with the mesial surface of the other. The right and left mandibular lateral incisors are distal to the central incisors. The mandibular central and lateral incisors have smaller mesiodistal dimensions than any of the other teeth. The central incisor is somewhat smaller than the lateral incisor, which is the reverse of the situation in the maxilla. These teeth are similar in form and have smooth crown surfaces that show few traces of developmental lines. The mandibular central incisor is the smallest tooth in the mouth, but its labiolingual root dimension is large. This tooth usually has one canal. Two ribbon-shaped canals may be found, but not very frequently (15% and 20% of central and lateral incisors, respectively). The pulp horns are well developed in this tooth group. The mandibular lateral incisor tends to be a little larger than the mandibular central incisor in all dimensions, including the pulp chamber. The pulp canal may taper gently from the apex or narrow abruptly in the last 3 to 4 mm of the root canal.

Canines

Morphologic aspects of the root and root canal anatomy of maxillary and mandibular canines are detailed in Table 12.2 , Fig. 12.22 , and Appendices 1 to 4 (Summary of Root Numbers and Root Canal Systems of the Permanent Teeth).

Table 12.2

Morphologic Aspects of the Root and Root Canal Anatomy of Maxillary and Mandibular Canines

Adapted from Versiani MA, Pereira MR, Pécora JD, Sousa Neto MD: Root canal anatomy of maxillary and mandibular teeth. In Versiani MA, Basrani B, Sousa Neto MD, editors.: The root canal anatomy in permanent dentition . ed 1. Switzerland, 2018, Springer International Publishing, pp 181–240.

	Maxillary Canine	Mandibular Canine
Overall length	26.4 mm (20.0-38.4 mm)	25.9 mm (16.1-34.5 mm)
Root length	16.5 mm (10.8-28.5 mm)	15.9 mm (9.5-22.2 mm)
Number of roots	1 (100%)	1 (98.57%) 2 (1.43%)
Number of canals	1 (97%) 2 (3%)	1 (92.4%) 2 (7.3%) Other (0.3%)
Canal configuration	Types I (98.5%) III (1.2%) II (0.8%) V (0.7%) IV (0.2%) Other (0.1%)	Types I (92.4%) III (2.7%) II (1.9%) IV (1.5%) V (1.2%) Other (0.3%)
Accessory canals	3.4%-30% (coronal: 0%; middle: 10%; apical: 90%)	4.5%-30% (coronal: 4%; middle: 16%; apical: 80%)
Apical curvature	Straight (38.5%) Distal (19.5%) Labial (12.8%) Mesial (12%) Palatal (6.5%) Other (10.7%)	Straight (68.2%) Distal (19.6%) Labial (6.8%) Mesial (0.8%) S-shaped (1.5%) Other (3.1%)
Anomalies	2 canals Dens invaginatus	2 canals 3 canals 2 roots
Ethnic variations		Bifurcated roots in mandibular canines are most common in some Western Eurasian populations

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