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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.
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.
#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.
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
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.
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.
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.
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
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.
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
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.
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
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.
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 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.
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
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.
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.
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
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.
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.
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
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.
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
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
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.
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.
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 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.
Morphology Self Assessment
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
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
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
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
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
Evaginations are more frequent than invaginations
Evagination can increase the risk for pulpal infection
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
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
Pulp stones are not always round
Once diagnosed, pulp stones are always easy to remove
Pulp stones are much softer than dentine
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
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
Ca. 10% of the canals are curved
Canals curve only in mesio-distal direction
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 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
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 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
Ca. 5 % of lower second molars have a C-shaped canal system
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