1.3 MANDIBULAR CENTRAL INCISOR #41. The mandibular central incisor always has one root, but often (20 %) has two root canals. Usually (75 %), the two canals join before the apical foramen. The canal(s) is very flattened: wide in the bucco-lingual dimension and narrow in the mesio-distal dimension. Only the most apical part of the canal is more round. The long axis of the canal traverses the incisal edge or the labial surface of the crown. Because the access opening is made, for aesthetic reasons, in the lingual surface, there is always a risk that the lingual canal is missed unless it is specifically looked for with a pre-curved file. For the same reason there is a risk of unsymmetrical preparation of the labial side of the root canal. The canal(s) of the lower central incisor is almost always straight unlike in the lower lateral incisor, where the root tip and canal often curve sharply distally.

2. 5 MANDIBULAR LATERAL INCISOR #42. The lower lateral incisor is quite similar to the lower central incisor. However, the lateral incisor is approximately 2 mm longer and the apical root and canal often curve distally, which must be taken into consideration during instrumentation. 3. 6 MAXILLARY CANINE #13. The upper canine is the longest tooth, and occasionally longer files of 28 or 31 mm lengths are needed for the root canal treatment. It always has only one root canal, which usually has an oval cross-section. The root canal is typically quite large, but often the few most apical millimeters before the foramen are much narrower. This may lead to incorrect working length if the position of the apical constriction is determined only with tactile sensation with the file and fingertips. Like the upper lateral incisor, the apical canal in the upper canine may have a pronounced curve, usually either distally or labially, although not quite so frequently. Awareness of the possibility of apical curvatures and careful assessment of root canal anatomy are essential in order to avoid complications in therapy. 4. 8 MAXILLARY FIRST PREMOLAR #14. The upper first premolar normally has two roots and two root canals. Occasionally only one root is present, but even then two canals are still often found. The root tips are very fine which may result in perforation even in a straight canal if a large apical open size is attempted. The roots are often equally long but 1 - 2 mm differences may occur. The root tips and apical canals may curve in the mesio-distal or bucco-palatal dimensions. Rarely, the upper first premolar has three roots and three root canals (= molarization) as with upper molars, although the roots are much finer and smaller. 5. 9 MANDIBULAR FIRST PREMOLAR #44. All teeth in the lower jaw can have more than one root canal. Double canals are particularly frequent in the mandibular first premolars, with approximately 30% of these teeth having two root canals. First premolars with one canal are quite easy to instrument, the canal is oval in cross-section and seldom curves severely. When there are two canals, the files usually easily find the buccal canal, while the lingual canal often requires bending of the instrument tip. Molarization in the lower first premolar is very rare. 6. 11 MANDIBULAR SECOND PREMOLAR #45. The mandibular second premolar resembles the first premolar, but the lingual canal is present only occasionally. Instead, molarization is more frequent than in the first premolar, yet still quite rare. The root canal is oval in cross-section and rather straight with only a slight distal curvature in some canals. 7. 12 MAXILLARY FIRST MOLAR #16. Maxillary molars have from one to three roots and from two to four root canals. From an occlusal view the pulp chamber is situated rather mesially, which has to be taken into account when cutting the access cavity. The upper first molar is perhaps the most variable tooth when it comes to root canal morphology, and provides quite a challenge in endodontics. There are usually three roots with three or four root canals. Dentists are quite familiar with the mesiobuccal, distobuccal and palatal canals, but not with the fourth canal, which is known as the mesiocentric or mesiopalatal, mb2 or accessory mesiobuccal canal. This fourth canal is usually difficult to find just by clinical inspection and is not apparent in the radiograph. However, finding all canals is necessary for successful therapy. The distobuccal canal is often easy to locate and instrument. It is typically rather straight or curves only slightly mesially, or sometimes distally. The palatal canal always looks straight radiographically but often has a buccal curvature. If this curvature is not identified by careful exploration with files it can lead to perforation 2 - 4 mm before the apex. Moreover, in radiographs a file will still appear to be in the canal but in reality it is only superimposed onto the canal. The palatal canal is often 1 - 2 mm longer than the buccal canals. Two palatal roots in the upper first molar have been reported in the literature. The mesiobuccal root is the most challenging to treat. The root is usually curved all the way to the apex, which increases the risk of tip perforation and strip perforation. The distal surface of the root is concave which increases the risk of strip perforation. 8. 13 The mesiopalatal canal is present in well over half of cases, with some authors reporting over 90% incidence. The canal orifice is difficult to find because it is typically situated near the mesial wall of the pulp chamber. While the other three canals can readily be found, the fourth canal must always be actively looked for with suitable instruments. The orifice is usually located 1 - 3 mm palatally from the mesiobuccal canal. In most cases the mesiopalatal canal joins the mesiobuccal canal before the apex. MANDIBULAR FIRST MOLAR #46. The mandibular first molar is perhaps the most frequently endodontically treated molar. It is, however, often quite difficult to treat because of its root canal anatomy. It usually has 3 - 4 canals, two in the mesial root and one or two in the distal root. The Distal canal(s) is normally straight all the way to the apex, oval or flattened in cross- section, but quite large, which makes instrumentation easy. Often the most apical 1 - 2 mm of this canal curves up to 90 degrees distally, but this is seldom a clinical problem. The distal canal may also curve mesially, but the curvature is not sharp and usually remains easy to instrument. The mesial canals in the first molar are often a challenge for the dentist. Both the mesiobuccal and mesiolingual canals are usually curved along their whole length, and the curvature is typically greatest in the apical region. The canals curve distally, but they also curve buccally or lingually at the same 9. 14 time. Bucco-lingual curvatures are not readily seen in the radiograph, which emphasizes the importance of the dentist's knowledge of possible variations in canal morphology. One must routinely search for four canals in the lower first molar. The distal canals often start together and separate a few millimeters below the pulp chamber floor. Both distal and mesial canals can join before the apex. This is important to detect before obturation, to gain optimal results. Mandibular first molars with two canals are rare. Usually, finding only two canals indicates that the mesiobuccal canal has not yet been located. MAXILLARY SECOND MOLAR #17. The maxillary second molar closely resembles the first molar. However, the number of canals is usually three, sometimes two, but also four canals can be found (two canals in the mb root). A typical upper second molar resembles the first molar, the difference being that the orifices of the mb and db canals are closer together; sometimes almost forming a line (mb - db - pal). Sometimes the two buccal canals are side by side in the mesio-distal dimension. The apical part of the palatal and the mesiobuccal canals is not as curved as in the first molar. 10. 15 MANDIBULAR SECOND MOLAR #47. The lower second molar is much like the first molar but generally easier to instrument because the curvatures are milder. The occurrence of four canals in the second molar is more rare than in the first molar, and only two canals is a more frequent possibility than in the first molar. A small percentage of lower second molars have a special root canal anatomy; two or more of the canal openings in the pulp chamber floor join to form a C-shaped groove. This has occasioned the name "C- shaped canals". Usually the mb or ml canal joins the distal canal, sometimes both mesial canals join the distal canal. Deeper in the root there sometimes are further ramifications. 11. 16 MAXILLARY THIRD MOLAR #18. The upper third molar is often a "reduced version" of the second molar. There are usually two or three root canals, and the orifices of the buccal canals may be very close to each other. Some upper third molars have a root canal anatomy similar to first molars. Sometimes the buccal canals share the same orifice in the pulp chamber but then separate 1 - 4 mm below the chamber floor (this may also occur in the second molar). Some upper third molars have additional roots and/or root canals. 12. 17 MANDIBULAR THIRD MOLAR #48. The lower third molar resembles the first and second molars, but the probability of teeth with four canals is again less and of teeth with two canals greater. Third molars are shorter than the other molars, which makes instrumentation easier. However, many third molars have very curved canals and may be difficult to instrument. 13. 18 SPECIAL MORPHOLOGY FOR ROOT CANALS 14. 19 15. 20 Dentine structure Evaginations Evaginations are morphological anomalies where the pulp has made an extension towards the tooth surface. Dentine and enamel follow the pulpal extension which may be seen as an extra cusp or enamel pearl on the tooth surface. Evaginations are rare, and are usually seen in lower premolars. They typically cause occlusal interference. If eliminated by grinding in one appointment, pulpal exposure and damage will follow. Gradual grinding of 0.1 mm per month before occlusal contact is established may help to avoid pulpal inflammation. 16. 21 Invaginations Invaginations are shallow or deep developmental cavities in tooth crowns, covered partly or totally by enamel walls. Their frequency has been reported to be between 0.1 and 10%. They are most frequent in upper lateral incisors, but can be found in any tooth. Invaginations are divided into four main types (see drawing). Invaginations often increase the risk of pulp infection, and they should be well sealed with a permanent filling whenever found, in order to reduce the risk of infection in the pulp or in the periodontal tissues. Deeper invaginations (type 2) should be cleaned mechanically and by irrigation, and they should be filled to their whole depth if possible. Type 3 and 4 invaginations are problematic to treat if the infection penetrates to the tissues. Pulp stones Pulp stones are calcified structures that may form within vital pulpal tissue They are often oval or round, but they may also have an irregular shape. Sometimes pulp stone(s) may diffusely fill a major part of the pulpal chamber. 17. 22 Size and morphological features have been used for classification of intrapulpal calcifications, but classifications have little significance in endodontics. Previously, pulp stones were thought to be a sign of pulpal pathosis, but evidence for this is lacking. Nowadays pulp stones are not regarded as an indication for endodontic therapy. If endodontic treatment is, however, started for other reasons, pulp stones may complicate gaining access to the root canals or obtaining correct working length. Use of ultrasound often helps to remove pulp stones during root canal preparation. TABLES Table 1 The average length of teeth in the upper jaw varies from 19mm to 26 mm. The canine is the longest tooth in the upper jaw followed by the central incisor. The central incisor is the only tooth that is regularly straight to the root tip. The lateral incisor typically has a distal or buccal apical curvature. Upper canines may be straight but may also curve buccally or distally. Most teeth in the premolar and molar regions have curved roots. Double canals are practically never found in upper incisors or canines. Single-rooted premolars and mesiobuccal roots of upper molars often have double canals. As in the lower jaw, double canals are located in the bucco-lingual dimension. 18. 23 Table 2 The average length of teeth in the lower jaw varies from 19mm to 25 mm. The canine is the longest tooth in the lower jaw and only slightly shorter than the upper canine. The central incisor is usually straight, down to the root tip. Most lower premolars and canines are also quite straight, while lateral incisors and molars typically have curved roots. All teeth in the lower jaw can have double canals. Double canals are located in the bucco-lingual direction. In the molars, double canals are typically found in mesial canals, but may be also found in distal canals, particularly in the first molar. Terminology 19. 24 20. 25 Apical canal Apical preparation assumes a key role in successful therapy of apical periodontitis, because it is the bacteria, particularly in this area of the root canal, that are responsible for the development of the periapical lesion. The technical goal of treatment of apical periodontitis is to reach the apical constriction and all regions of the root canal system with preparation instruments, intracanal medicaments and the root filling. If this can be done successfully, prognosis of the therapy is good. Variations in apical root canal morphology, however, may complicate treatment, as in the case of an apical delta, which may offer areas of concealment for micro-organisms. Details of apical root canal morphology often cannot be seen in radiographs. Changes in morphology Ageing and various irritants, such as deep caries lesions, cause several changes in teeth. Pulp chambers and root canals become narrow and more obliterated because of secondary dentine produced by odontoblast cells in the pulp. Also the crown becomes shorter because of occlusal wear. It is important to understand the effects of these changes on endodontic treatment. 21. 26 Canal cross-sections Thorough knowledge and understanding of the cross-sectional shape of root canals in different teeth and tooth groups is essential for successful endodontic treatment. Optimally, the canal should be round or only slightly oval to allow easy access for preparation instruments to all parts of the root canal system. In practice, however, many root canals are flattened and asymmetric in shape. The cross-sectional shape of the root canal also changes during its course from the pulp chamber towards the apex. In the apical 1 - 4mm, most canals become oval or round. This again facilitates cleaning of the apical canal, which is essential for control of the infection and helps to give the canal a shape that can be tightly filled with a root filling. 22. 27 Curved canals Up to 90% of all root canals are curved to some degree. Canal curvatures are a challenge to preparation and can cause different kinds of technical complications (preparation of curved canals). Canals that curve in the mesio-distal dimension are usually easily detected in radiographs. However, many canals also curve in the bucco- lingual dimension, which can only occasionally be detected in radiographs. For optimal clinical results it is important to detect all curvatures in order to select the correct instruments and avoid complications. The type of curvature dictates the ease or difficulty of instrumentation. Even curvatures with a long radius are easy to prepare with the right choice of instruments and techniques. Sharp curves with a short radius and S-shaped curvatures are always very demanding and easily result in transportation, ledges and even perforations. Even up to 90% of all root canals are more or less curved. Canal curvatures are a challenge to preparation and can cause different kinds of technical complications (see preparation of curved canals). Canals that curve in the mesio-distal direction are usually easily detected in radiographic pictures. However, many canals curve also in the bucco-lingual direction, which can only occasionally be detected in radiographs. For optimal clinical results it is important to detect all curvatures in order to select the correct instruments and avoid complications. The type of curvature dictates the ease or difficulty of instrumentation: even curvatures with a long radius are easy to prepare with the right choice of instruments and techniques, sharp curves with a short radius and S-shaped curvatures are always very demanding and easily result in transportation, steps and even perforations. 23. 28 Double canals Double canals means two canals in one root. Double canals can be separate from the pulp chamber down to the apex, both having their own apical foramen. However, the canal may also begin as one canal, divide into two canals, and join again before the apex. Double canals are almost always situated as buccal and lingual canals in the root, which makes their detection in radiographs difficult. However, knowing the possibility of their existence together with careful analysis of radiographs and clinical examination helps to find double canals. From the clinical point of view it is important to be aware of the possibility of double canals. Double canals can be present in most roots. Maxillary incisors and canines are the only teeth where double canals are practically never found. Also the palatal and distobuccal roots of upper molars usually have only one root canal. Double canals are most frequent in mesial roots of mandibular molars, followed by the mesiobuccal root of the maxillary first molar, upper second premolar and lower first premolar. Roughly one fifth of lower incisors and canines also have double canals, but most of these join shortly before the apex. Analysis of radiographs 24. 29 Double canals are almost always located bucco-lingually, so that they may be difficult to detect in radiographs. However, a reliable way to identify double canals is to follow the radiographic shadow of the canal; if the shadow suddenly almost disappears, it is a strong indication of canal ramification. Taking the radiograph at a different horizontal angle also helps to find double canals in many teeth. In looking for double canals it is important to identify the periodontal ligament space that often projects on the tooth and may resemble a canal. Molarization Sometimes premolars have a root morphology similar to that of molars, a phenomenon known as molarization. Thus lower premolars will have a mesial and a distal root just like lower molars, and upper premolars have two buccal roots and one palatal root just like upper molars. The crowns in these premolars with molarization usually look quite normal, particularly in the upper premolars. Sometimes there may be an extra cusp present and the crown may be slightly longer mesio-distally. The frequency of molarization in premolars is approximately 1%. In the maxilla it is more frequent in the first premolar whereas in the mandible it is more frequent in the second premolar. These teeth usually have three root canals, but mandibular premolars can sometimes have only two. 25. 30 C-shaped canals The C-shaped canal is a special feature of some lower second molars. Approximately 1% of lower second molars have C-shaped canals. The name comes from the appearance of the pulp chamber floor when viewed from above. Some or all of the canal orifices are joined in the form of a groove or isthmus with a shape of the letter C. In teeth with three canals the mesiobuccal canal usually joins the distal canal. In some teeth both mesial canals join the distal canal at the cervical area near the pulp chamber floor. The canals may later, closer to apex, separate again to leave the tooth via separate foramina. Taurodontism 26. 31 Taurodontism is a special anatomic variation occasionally seen in molars. The pulp chamber continues apically far beyond the normal height: often the root canals start only a few millimeters before the apex. Taurodontism makes root canal treatment more difficult because localization of canal orifices is more complicated. In cases of pulpitis, control of bleeding can also take a lot of time and effort compared to teeth with normal anatomy. SELF ASSESSMENT Morphology Self Assessment Maxillary teeth True False The only teeth with always one root canal are maxillary central incisors The only teeth with always one root canal are maxillary incisors The only teeth with always one root canal are maxillary incisors and canine The root tip of maxillary lateral incisor often bends mesially The root tip of maxillary lateral incisor often bends distally The average length of an intact maxillary lateral incisor is ca 23 mm The average length of an intact maxillary canine is ca 24 mm The root tip of maxillary canine may bend distally and labially Mandibular canine is the longest tooth First maxillary premolar is the shortest tooth 27. 32 Molarization may occur in all front teeth and premolars Maxillary "molarization" premolars have two buccal roots and one palatal root The roots in three-rooted maxillary premolars are easy to detect in the radiographs Maxillary second premolar with two root canals has one mesial and one distal canal Two root canals in maxillary second premolar usually join 1 - 5 mm before apex Maxillary first molar has usually three (3) root canals Maxillary second molar has usually three (3) root canals MB1 and MB2 canals of upper molars often join before apex Sometimes maxillary second molar has only one root canal MB2 canal is located in the distobuccal root The openings of MB1 and MB2 canals in the same root are of same size and equally easy/difficult to find MB2 canal in first maxillary molar is located on the straight line between MB1 and palatal canal MB2 canal in first maxillary molar is located mesially to the straight line between MB1 and palatal canal There is always only one palatal canal in maxillary molars Palatal canal in maxillary molars is the narrowest canal Mesiobuccal root of maxillary molars in flattened mesio-distally Palatal canal of maxillary molars often curves palatally at the apical end Palatal canal of maxillary molars often curves buccally at the apical end The apical curvature of maxillary molar palatal canal is readily visible in the radiographs 28. 33 Mandibular teeth True False Mandibular incisors and canines have always one root canal 20 % of mandibular incisors have two canals in the same root (= double canals) Double canals in mandibular incisors usually join 1 - 5 mm before apex The root tip of lower lateral incisor often curves distally Lower incisors of the same patient are always equally long Lower central incisor is usually longer than the lateral incisor Mandibular canine has always only one root Mandibular canine may have two root canals that often join before apex First mandibular premolar can have one canal First mandibular premolar can have two canals First mandibular premolar can have three canals Two canals are more usual in lower second than in lower first premolar When two canals are present in lower premolars, the files typically have easier access to the lingual canal Molarization is more frequent in second than in first lower premolar First mandibular molar has usually three or four root canals First mandibular molar can have five root canals Double canals in molar roots (except upper palatal roots) are always buccal and lingual 29. 34 Curved root canals in lower molars curve only in mesio-distal direction When an extra root is present in lower molars, it is usually mesial Double canals in molar roots typically have anastomoses Lower third molar can have up to four root canals Evagination True False Evaginations are more frequent than invaginations Evagination can increase the risk for pulpal infection Invagination True False Invagination can increase the risk for pulpal infection Invaginations occur only in maxillary lateral incisors Invagination has always a connection to the root canal A tooth with an invagination cannot be saved from pulpal necrosis Invaginations cannot occur in mandibular teeth Type III (three) invagination opens into periodontal tissue in mid-root Type I invagination is the deepest of the four invaginations Pulp stone True False A pulp stone is not an indication for endodontic treatment Pulp stones are found only in the pulp chamber Pulp stones are found only in the root canal 30. 35 Pulp stones are not always round Once diagnosed, pulp stones are always easy to remove Pulp stones are much softer than dentine Apex True False Apical foramen can be located at the radiographical apex Apical foramen can be located at the lateral root surface One root canal has always only one apical foramen Lateral canals end at the dentine-cement border Root surface cement can be found a few micrometers inside the apical canal Changes in morphology True False Reduction of pulpal space is always a consequence of a pathological phenomenon Calcification/obliteration of the pulp is an indication for endodontic treatment Pulp chamber space reduction occurs mainly by the floor "growing up" Pulp chamber space reduction occurs mainly by the roof "growing down" In the apical 1 - 4 mm most canals are oval or round in cross section Curved canals True False Ca. 10% of the canals are curved Canals curve only in mesio-distal direction 31. 36 Normal radiographs can detect mesio-distal and bucco-lingual curvatures equally easily Sharp curves with a short radius are more difficult to instrument than even curves with long radius S-shaped canal curves two times to the same direction Palatal canal of upper molars often curves buccally Double canals True False Double canals can join and separate again before apex Double canals always join before apex Difficulty to see double canals in radiographs is because they are located bucco-lingual Sudden disappearance of canal shadow in mid-root in the radiograph in a strong indication of a double canal Depending on the angulation, periodontal ligament space can cause canal-resembling vertical shadows on the root in the radiograph Molarization True False Is equally common/rare in all premolars The frequency of molarization is ca 1% Maxillary "molarization" premolars have two buccal and one palatal root C-shaped canals True False C-shaped canal is a special feature of lower second molar In C-shaped canals the mesial canals join forming a C-shaped orifice in the pulp chamber Ca. 5 % of lower second molars have a C-shaped canal system Taurodontism 32. 37 True False In taurodontism, the pulp chamber is exceptionally deep Taurodontic teeth are generally difficult to instrument Taurodontic teeth are easier to root fill than normal teeth