Preface
The goal of root canal therapy is to remove the contents of the root canal space and then fill it. Proper treatment requires knowledge of both the external and internal anatomy of the tooth to reduce the risk of failure and the possibility of iatrogenic biological damage.
Understanding the coronal morphology of the tooth allows us to make endodontic access in the most conservative manner; The shape of the access cavity is described for all teeth. Studying the morphology of the root canal system and its several variations makes it obvious why there are operational difficulties during instrumentation, as shown in the iconographic part. From the morphological and histological tables of Hess, the complexity of the entire root canal system is visible, which confirms the difficulty of completely removing pulp tissue from the endodontic space and encourages the search for new methods and technologies in endodontics.
The microscopic anatomy section summarizes the interaction of the root dentin wall structure with mechanical (files, ultrasound), chemical (irrigants) and physical (lasers) factors during therapy. In particular, in order to understand the different effects of lasers on tissue depending on the wavelength, it is very important to carefully study the ultrastructure and histology of dentin in the canal.
Anatomy of primary teeth
The macroscopic anatomy of primary teeth is very similar to that of permanent teeth, with some differences between them outlined in this section.
Ernst Zurcher (1922), Walter Hess school in Zurich, carried out the first scientific work on pulp morphology in primary teeth.
The endodontic morphology of primary teeth is very similar to the endodontic morphology of permanent teeth, but is smaller in size. Primary teeth are usually shorter and smaller than permanent teeth; the roots are narrow, while the roots of permanent teeth are thicker, especially in the cervical third. However, the width of the crowns of primary teeth is more pronounced compared to their height. The roots of temporary molars, in addition to being thinner than permanent roots, diverge to the sides to ensure the eruption of permanent premolars, first of all during their formation, and then during eruption.
Primary upper and lower molars often have a fourth canal in the mesiobuccal root of the upper molar and in the distal root of the lower molar.
As the child grows, the length of the roots of primary teeth decreases due to physiological resorption (exfoliation) (Fig. 1.1 and 1.2). Sometimes resorption of the floor of the pulp chamber at the furcation precedes apical root resorption (Fig. 1.3ad).
Rice. 1.1 Primary upper molar: root resorption starting in the apical region (arrows)
Rice. 1.2. Primary lower molar: root resorption affecting all parts of the root from apex to crown
Rice. 1.3. Primary upper molar: radicular resorption is more obvious at the floor of the pulp chamber in the area of the furcation, which in this case precedes apical resorption; (a) root apices are intact; (b) resorption in the area of the furcation of the pulp chamber; (c) palatal root resorption (arrows); (d) mesial view of furation resorption (arrows)
Unlike permanent teeth, the loss of primary teeth explains why canal filling at the end of endodontic treatment requires the use of resorbable materials that allow gradual root resorption.
Tooth numbering schemes in dentistry
Currently, tooth numbers in dentistry are systematized according to several schemes:
- Universal numbering system.
- Square-digital or Zsigmondy-Palmer system.
- Haderup system.
- American numbering or alphanumeric system.
- European numbering Viola or WHO system.
Let's take a closer look at the features of each of them.
Universal system
This numbering scheme is based on assigning each permanent tooth a specific number from 1 to 32. In primary occlusion, each tooth has its own letter. In this case, the count is carried out from the right half of the upper jaw clockwise from the wisdom tooth.
The dental formula of a permanent set according to the universal system looks like this:
Milk teeth are marked according to the same principle, but only using letters of the Latin alphabet:
Zsigmondy-Palmer system
This numbering is the most imperfect, since it still indicates the teeth under numbers without a more precise indication of their location. At the same time, Arabic numerals from 1 to 8 are used for the permanent set, and Roman numerals (IV) are used for numbering dairy products.
The digital system does not exclude errors when carrying out diagnostic or therapeutic measures, therefore, today it is used only by orthodontists (for example, when installing and marking braces) or maxillofacial surgeons. And entries on it in the patient’s medical record are made only on a special diagram.
Haderup system
Numbering according to the Haderup system also applies to digital numbers. To designate the teeth of an adult, Arabic numbers 1-8 are used with a plus or minus sign in front of them. The “+” sign is used to number the upper ones, and the “-” sign to indicate the lower ones.
The numbers of children's teeth are similarly written in Arabic numerals with the signs “+” or “-”, but at the same time the number “0” is placed in front of them.
The inconvenience of such a system is the need to indicate the location of the tooth on the left or right side of the jaw.
American alphanumeric system
This numbering is widespread in the USA. The alphanumeric system is based on assigning a letter value to each group of teeth (capitals for a permanent set, and capitals for a primary set), as well as a digital code that indicates the location of the tooth in a correct bite.
Letter meanings of teeth:
- I (i) - permanent (deciduous) incisors;
- C (s) - permanent (deciduous) fangs;
- P - premolars (absent in primary dentition);
- M (m) - permanent (deciduous) molars.
American numbering also does not take into account the left- or right-sided position of the tooth, which can cause certain difficulties.
European international numbering Viola
Today, this is the newest and most advanced teeth numbering system. Its essence lies in the fact that the jaws are divided into segments (2 on top and 2 below), each of which is assigned its own number. For adults, numeric values are used 1-4, and for children - 5-8. As a result, each tooth receives a two-digit number, the first digit of which indicates a specific segment, and the second - a serial number.
The convenience of the Viola system lies in the absence of cumbersome names while accurately indicating the location of the required tooth and the minimal risk of error. This numbering is indispensable when sending a patient for an x-ray, as well as when identifying teeth on a panoramic image.
Permanent teeth
Macroscopic anatomy
Human permanent teeth consist of a crown and one or more roots. The endodontic space, created by the dentin of the root and pulp chamber, is very complex and is classified differently depending on the crown-root relationship and canal morphology.
Weine (1996) classified the root canal system into four types when considering the relationships between the pulp chamber, root canals and their apical termination (Fig. 1.4).
Rice. 1.4. Weine's classification of the root canal system takes into account the relationship between the pulp chamber, root canals and their apical termination.
Vertucci (1984) identified eight main types. Later, the Vertucci classification was expanded to include other morphological classifications by Gulabivala et al. (2001 and 2002) and Sert and Bayirili (2004) (Figures 1.5, 1.6 and 1.7).
Rice. 1.5 and 1.6 Graphic display of the morphological classification of Gulabivala endodontic system
Rice. 1.7 Graphic display of the morphological classification of Sert and Bayirili endodontic system
Schneider analyzed single-rooted human teeth and classified them according to the degree of root curvature as straight, with a curvature less than or equal to 5°, with a moderate curvature between 10° and 20°, and with a strong curvature between 25° and 70°. Lautrou [1987] also described and classified various morphologies of root cross sections (Fig. 1.8).
Rice. 1.8 Graphical representation of Lautrou classification of morphology in root cross sections
Zidell (1985) and Ingle and Taintor (1985), in addition to the degree of root and apical curvature, also considered the complexity of the anatomy, including the presence of bifurcations, the presence of accessory canals, and the presence of lateral and accessory canals.
Various studies and texts later described the anatomy and morphology of permanent human teeth and their countless possible shapes.
In paragraphs about a particular tooth, the age of eruption considered is according to Logan and Kronfeld, slightly modified by Schour and included in the text by Ash. The dimensions of human teeth described in this textbook are instead taken from various texts.
Wisdom teeth
They are “eights”, third molars. They erupt by the age of 25-28 and are considered a vestigial organ. The number of wisdom teeth varies between 1-4, some people are missing even in their infancy. The eruption of third molars occurs after the final formation of the facial skeleton and can be very difficult due to lack of space in the jaw. Impacted and dystopic wisdom teeth must be removed.
A special branch of medicine – dentistry – is devoted to the study of the structure, function, and pathologies of teeth. Dentists diagnose, treat and correct various pathologies of the teeth, oral cavity and jaws. In modern dentistry, there are several specialized areas (see Dentistry).
Upper incisors
Central upper incisor
The central upper incisor erupts (one per quadrant) between the ages of six and seven years, and the complete formation of the apical third occurs after 2 or 3 years. The average tooth length is 22-23 mm.
The crown has a triangular shape, about 10.5 mm long, the base extends in the mesial-distal direction, corresponding to the anterior edge of the tooth, up to 9 mm in size, and its buccal-palatal size is 7 mm (Fig. 1.9).
Rice. 1.9 Upper central incisors: palatal view of the crown
The root is usually straight (75%), but according to Ingle, a slight curvature may be present in a small percentage of cases. Lateral canals may be present in more than 20% of cases, and an apical delta is also common (35%).
The coronal pulp space also has a triangular shape, especially in the region of the cervical radicular third, with the base facing the vestibular wall and the apex located palatally, after which it gradually becomes a round canal until the apex.
The access cavity is triangular and follows the shape of the pulp space (Fig. 1.10).
Rice. 1.10 Upper central incisor: graphical representation of the access cavity
Lateral upper incisor
The eruption of the upper lateral incisor (one in the quadrant) occurs 1 year after the central one, and its complete formation takes approximately 3 years.
It is approximately 1 or 2 mm shorter than the central incisor, its mesial-distal size is smaller - about 7 mm, and the vestibular-palatal size is only 5.5-6 mm (Fig. 1.11).
Rice. 1.11 Upper lateral incisor: palatal view of the crown
Normally there is only one root canal, straight in 30% of cases, and often it is curved distally (53%), in a small percentage of cases there is curvature in other directions. The shape of the canal is ovoid in the cervical region, with a tendency to become more rounded in the apical region; the access cavity has a similar structure (Fig. 1.12).
Rice. 1.12 Upper lateral incisor: graphical representation of the access cavity
Lower incisors
The lower incisors, two per quadrant, are very similar to each other, with some peculiarity retained.
Lower central incisors
The lower central incisors are the first permanent teeth to emerge in children's mouths, usually between 6 and 7 years of age, and are fully formed by 9 or 10 years of age. The lower central incisor is approximately 21.5 mm long. The apical third of the root is straight in 60% of cases and curves distally in 23% of cases.
The crown has a trapezoidal shape with a large base corresponding to the cutting edge and a smaller base that continues into the cervical third of the root. Its mesio-distal size is 5.5 mm at the greatest width of the incisal edge and gradually decreases to 3 mm at the cervical level (Fig. 1.13).
Rice. 1.13 Lower incisors: lingual view of crowns
The lower central incisor has a buccal inclination. In the root, it is possible to have a second canal located lingually in relation to the main canal in 18-23% of cases (Fig. 1.14).
Fig 1.14 Lower central incisor: graphical representation of the access cavity
Lower lateral incisor
The lower lateral incisor is similar to the central incisor, but slightly larger - about 1 mm, usually erupts 1 year after the lower central incisor and completes its formation after 3 years.
Root examination confirms the presence of two canals in 15% of cases.
The access cavity of the lower incisors is formed in a buccal-lingual direction to search for a second canal located lingually (Fig. 1.15a-c).
Rice. 1.15 (a - c) Lower central incisor: buccal, lingual and proximal views
Other counting systems
It’s easier to represent the location of teeth by numbers - the diagram is called the 2-digit Viola system. It has been operating since 1971. Its advantage is the convenience of calculation without the need to map the series. But in international dental practice, other systems are also used: Haderup, Palmer-Zsigmondy (used by orthodontists and surgeons), alphanumeric (in the USA). Each of their schemes has its own system for counting teeth in children and adults.
Fangs
Upper canine
The upper canine, one per quadrant, erupts at approximately 11-12 years of age and completes its formation after 3-4 years.
This is the longest tooth in the arch (about 27 mm or more).
The crown has a diamond shape with a peculiar sharp cusp, which on the vestibular side divides the mesial and distal sides of the crown. The crown is 10 mm high (length) and has a maximum mesial-distal diameter of 7.5 mm at the point of proximal contact and decreases to 5.5 mm cervically. The buccal-palatal diameter is wider (8 mm), and it remains up to 7 mm at the cervical level due to the presence of a prominent marginal ridge (Fig. 1.16).
Rice. 1.16. Upper canine: palatal view
The long root, like the crown, is narrow in the mesial-distal direction and more pronounced in the buccal-palatal direction. There is almost always only one root canal with lateral canals at different levels in 24% of cases. The apical third is straight in 40% of cases, and is often curved distally (32%) and/or vestibularly (13%) (Fig. 1.17).
Rice. 1.17 Upper canine: graphical representation of the access cavity
The pulp space at the level of the cervical third and the root middle third has an oval shape in the buccal-palatal direction; often two canals are present with a tendency to merge into one common round canal in the apical third (Fig. 1.18a, b).
Rice. 1.18 (a, b) Upper canine: buccal and proximal view
The endodontic access cavity is oval-shaped, extending from the cusp to the marginal ridge of the coronal cervical third to gain access to the radicular space without obstruction that could cause instruments to create steps or transposition of the apical foramen (see Fig. 1.19).
Rice. 1.19. Anatomical pictures from Hess and Keller: the endodontic space of the upper canine at the level of the cervical third of the canal is represented by one oval canal, and in the middle third it is divided into two different canals and often merges into one round canal in the apical third
Lower canine
The lower canine, one per quadrant, erupts approximately 8-10 months before the upper one and completes its formation after 3-4 years.
The crown has a length of 11 mm, and its mesiodistal diameter is narrower than the upper one (7 mm), and decreases to 5.5 mm at the cervical level. The maximum buccolingual diameter is 7.5 mm wide and decreases to a minimum at the cervical level. The cervical lingual marginal ridge is less pronounced than that of the upper canine (Fig. 1.20).
Rice. 1.20 Lower canine: lingual view of the crown
The lower canine is about 27 mm long, and has two separate roots in 6% of cases, or two canals in one root, connected by isthmuses, with a common or two separate apexes (Fig. 1.21). In 20% of cases, the apical third has a distal slope (Fig. 1.22).
Rice. 1.21 Lower canine: graphical representation of the access cavity
Rice. 1.22. Anatomical pictures from Hess and Keller: the endodontic space of the lower canine has two separate canals connected by an isthmus in the coronal third; the canals merge into one opening in the apical third
The endodontic space is oval in shape in the bucco-lingual direction for two-thirds of the root, and then gradually becomes rounded.
The endodontic access cavity should be oval in shape in a buccolingual direction, from the cusp to the marginal ridge (Fig. 1.23a, b).
Rice. 1.23 (a, b) Lower canine: buccal and proximal view
Anatomical structure of the tooth
We figured out the order of the teeth. Further - more interesting! After all, a tooth is a unique organ, designed by nature to perform very important functions, primarily chewing. To be firmly held in the jaw bone, you need a root. The longest root is at the canine. Teeth can be single-rooted or multi-rooted. The structure of the upper teeth (molars) is somewhat more complex than that of the lower ones, since they have 3 roots. There are especially many variations in the number and type of roots in “wisdom teeth.” But the visible part of the tooth protruding above the gum is called the crown. It has different shapes in the incisors, canines and molars to perform the functions of biting, crushing or chewing food. The boundary between the crown and the root is the neck of the tooth. It is usually not visible, but it can become exposed due to certain diseases (periodontal disease, etc.). An x-ray is a kind of photo of the structure of the tooth and will give you an idea of what the state of affairs is in the oral cavity.
Upper premolars
Two per quadrant, premolars erupt between 10 and 12 years of age, and root formation is completed after 3 years. They replace temporary molars.
The first and second premolars have a similar crown but different morphology.
Upper first premolar
The first upper premolar erupts when the child is about 10-11 years old. It has a length of about 21-22 mm, and its buccal-palatal dimensions are 9 mm and mesial-distal dimensions are 7 mm. It has two cusps, buccal and palatine, which are slightly shorter (about 1 mm). The pulp space is determined by the shape and size of the outer crown (Fig. 1.24). In approximately 72% of cases, two roots with two different apical foramina are present (Fig. 1.25). Premolars may also have only one root with two canals (13%) (Fig. 1.26a, b), and in some cases three roots (6%) (Fig. 1.27). In addition, in 37% of cases we found a distal root bend in the apical third.
Rice. 1.24 First upper premolar: occlusal view
Rice. 1.25 Upper premolars: graphical representation of the access cavity; note the mesial position of the second premolar access cavity (right in the figure) to facilitate passage of the distally curved canal
Rice. 1.26 (a, b) Upper single-root premolar: buccal and proximal view
Rice. 1.27 Anatomical pictures from Hess and Keller: complex pulp space of an upper single-rooted premolar; two different channels have multiple messages along the root and two separate outputs
The access cavity to the pulp chamber should have an oval shape in the buccal-palatal direction, from one tubercle to another Fig. 1.28.
Rice. 1.28 Anatomical pictures from Hess and Keller: an upper premolar with three roots, two buccal and one longer palatal
Upper second premolar
The second upper premolar erupts between the ages of 10 and 12 years. It is very similar to the upper first premolar, both in size and crown shape. But there are some main differences between the roots, presented in three versions:
One root with one canal in 75% of cases (Fig. 1.29a, b)
Rice. 1.29 (a, b) Upper first premolar: mesial and distal view
One root with two canals and one or two separate apical foramina (12%) (Fig. 1.30 and 1.31)
Rice. 1.30. Anatomical pictures from Hess and Keller: an upper single-rooted premolar having two canals merging into one exit; lateral canals are also present in the apical and middle third of the canal
Rice. 1.31 Anatomical pictures from Hess and Keller: upper single-rooted premolars have two canals with separate exits, apical and lateral
Two separate roots and canals (12%)
Three separate roots (usually two of them buccal) with three canals (1%)
On the buccal side, the roots have a distal curvature in 27% of cases, and a vestibular curvature in 12% of cases, and in 20% of cases they have two sharp curvatures.
The endodontic access cavity to the pulp chamber has an oval shape in the buccal-palatal direction. If there is significant distal apical curvature, the approach should be moved closer to the mesial marginal ridge, maintaining an oval shape (Fig. 1.28).
Lower premolars
Two in each quadrant, they erupt between 10 and 12 years of age, with full root formation after about 3 years. They replace primary molars. In contrast, the upper premolars are distinct from each other (Fig. 1.32).
Fig 1.32 Lower premolars: occlusal view
Lower first premolar
The lower premolar erupts sometimes several months before and sometimes after the lower canine and is the smallest of all premolars. The crown has two cusps, one very large and similar to the cusp of a canine tooth. The other is shorter by about 2 mm, similar to the lingual marginal ridge of the canine (Fig. 1.33).
Figure 1.33 Lower premolars: graphical representation of the access cavity
Length is about 21-22 mm, usually has one root with one canal (73-74%). Sometimes one root with two canals is possible (19%) (Fig. 1.34a, b), two roots with two canals (6%) (Fig. 1.35a, b) or three canals (1-2%).
Rice. 1.34 (a, b) First lower premolar: occlusal and proximal view
Rice. 1.35 (a, b) Second lower premolar: (a) coronal view; (b) Proximal view showing the presence of a single root
The root is often distally curved in 35% of cases and with a sharp curve in 7% of cases. The shape of the access cavity to the pulp chamber is oval, extending from the main tubercle to the tip of the minor lingual tubercle (Fig. 1.36).
Figure 1.36 Anatomical pictures from Hess and Keller: a mandibular premolar with a complex canal system that has two main canals connected by several fins and two separate exits
Lower second premolar
The second lower premolar erupts at the age of 11-12 years and is larger than the first premolar by 1 or 2 mm. The crown has two cusps (a larger buccal and a smaller lingual), but the lingual cusp is often divided into two parts (Fig. 1.37).
Rice. 1.37 Anatomical pictures from Hess and Keller: lower premolar with two roots and two canals; several short lateral canals present in the apical third
It has only one root with one canal in 85% of cases, but we can also find two separate canals in one root (11.5%) or two canals that merge into one apical foramen (1.5%) (Fig. 1.37 ). Rarely three channels are possible (0.5-1%). In approximately 40% of cases the root is straight, while in approximately 40% the apical third is distally curved. Severe curvature (7%) and vestibular curvature (10%) are possible.
The access cavity has an oval shape, also in the buccal-lingual direction, located in the center of the occlusal surface (Fig. 1.36).
Teeth of ancient people
The dentofacial apparatus of prehistoric and modern humans differs significantly. Ancient people had more than 36 teeth, protruding fangs and a massive jaw. This was explained by the need to chew rough food and raw meat. With the addition of thermally processed foods to the diet, the dentition began to change. The canines were the first to transform, becoming aligned with the bite line. Then the jaw arch narrowed, the interdental spaces disappeared, and the teeth themselves decreased in size. Currently, 32 teeth in humans are the norm, but third molars are considered to be an atavism.
Interesting fact!
The teeth of ancient man cannot be called aesthetic, but they were healthy. According to scientists, cavemen never suffered from caries and other oral diseases.
Upper molars
Three molars in a quadrant without corresponding primary teeth.
Upper first molar
It erupts between 6 and 7 years behind the second primary molar. The formation of the apex is completed between the ages of 9 and 10 years.
The crown has four cusps on the occlusal surface and three roots, one palatal and two vestibular (Fig. 1.38).
Rice. 1.38 First upper molar: occlusal view
The length of the tooth is about 21-22 mm and there is one canal in the palatal and disto-buccal roots
The mesiobuccal root has two separate canals in 60-70% of cases (MB1 and MB2). The second canal in the distobuccal root (DB2) is quite rare (2-3%) (Figs. 1.39, 1.40 and 1.41).
The access cavity to the pulp chamber has a triangular shape with the base facing the buccal side and the apex facing the mesial-palatal tubercle; the cavity is always located mesial (in front) of the transverse ridge, which unites the mesiopalatine tubercle with the distal buccal tubercle. The mouths of the canals are located at the vertices of the triangle, which form the access cavity (Fig. 1.42); if we combine the orifice of the MB1 canal with the palatal orifice, we can find a second mesiobuccal root canal (MB2), which is often closed by calcification (Fig. 1.43).
Rice. 1.39 Anatomical pictures from Hess and Keller: the upper molar has a very complex canal anatomy
Rice. 1.40 Anatomical pictures from Hess and Keller: the upper molar has a very complex canal anatomy; MB1 and MB2 visible in the mesiobuccal root
Rice. 1.41 Anatomical pictures from Hess and Keller: the upper molar has a very complex canal anatomy; MB1 and MB2 visible in the mesiobuccal root
Rice. 1.42 Upper first molar: triangular access cavity with three canals
Rice. 1.43 Upper first molar: access cavity extended to the mesial ridge to locate the MB2 canal on the line connecting the mesiobuccal and palatal canal
Second upper molar
Eruption occurs at the age of 12 years with the completion of the apical third at approximately 14 years.
It is smaller and shorter than the first molar by about 1-2 mm, is about 19-20 mm high and has four occlusal cusps. There are three roots and three canals, just like the first molar, and the MB root also has a second canal (37-42%). The shape of the pulp chamber is rhomboidal rather than triangular (see Figs. 1.42 and 1.43).
Upper third molar
This tooth erupts after age 17. It has a very unique anatomical morphology; the crown has three or five tubercles (Fig. 144). The average height of the third molar is about 18 mm. There may be one root or more often than one - three or four roots, usually curved distally (Figs. 1.45, 1.46, 1.47 and 1.48).
Figure 1.44. The upper third molars have a very distinctive occlusal morphology with three or four cusps
Rice. 1.45. Upper single root third molar
Rice. 1.46. Upper third molar with four distally curved canals
Rice. 1.47. Upper third molar with atypical anatomy
Rice. 1.48. Upper third molar with strong 3D root curvature
Lower molars
Three molars in a quadrant without corresponding primary teeth.
First lower molar
This is the first permanent tooth that erupts at the age of 6 years and completes its formation at approximately 9-10 years. The crown has five cusps, two lingual and three buccal (Fig. 1.49). 22-23 mm long. There are usually two roots, one mesial and one distal (97%), and we rarely find a third distolingual root (3%). In the mesial root we find two canals in 65% of cases: they may have only one apical foramen (40%) or two separate ones (60%). The distal root has one canal (70%) or two canals (30%); there may also be two separate apical foramina (38%) or only one (62%) (Figs. 1.50, 1.51 and 1.52) (Figs. 1.53 and 1.54).
Rice. 1.49. Lower first molar: occlusal view
Rice. 1.50 Anatomical pictures from Hess and Keller: lower molar with very complex canal anatomy
Rice. 1.51 Anatomical pictures from Hess and Keller: lower molar with very complex canal anatomy
Rice. 1.52 Anatomical pictures from Hess and Keller: lower molar with very complex canal anatomy
Rice. 1.53. Micro-computed tomography scan of the roots of lower molars: note the complex morphology of the pulp space
Rice. 1.54. Micro-computed tomography scan of the roots of lower molars: modern digital techniques reproduce the subtle and complex morphology of the pulp space previously illustrated by Hess histological images
All roots on the buccal side have a moderate distal slope. The pulp chamber is triangular or square, depending on the number of canals (3 or 4), located in the center of the crown in the mesial-lingual area. Because of this feature, during the formation of the pulp chamber access cavity, it is important to be careful not to remove precious tooth structure in the distal area.
The shape of the access cavity should be triangular or trapezoidal with the larger base facing the mesial ridge and the smaller (or triangular apex if there is only one distal canal) slightly distal to the central occlusal fossa (Figs. 1.55 and 1.56).
Rice. 1.55 Lower molar: access cavity to three canals
Rice. 1.56 Lower molar: trapezoidal access cavity to four canals
If there is a third root, its mouth can be found along the bottom of the pulp chamber in the corners of the base of the trapezium.
Second lower molar
The tooth erupts at the age of 11 years and is very similar to the lower first molar with some differences.
It is smaller than the first molar by 1-2 mm in each direction. It has four cusps on the occlusal surface and usually two roots, one mesial and one distal. The mesial root has only one canal in 13% of cases; most often there are two channels that end in only one opening (49%), or there are two channels with two independent openings (38%). The distal root has only one canal in 92% of cases. Rarely there are two canals with one apical foramen (5%) or two separate foramina (3%).
The access cavity to the pulp chamber is located in the middle of the mesial-lingual region and has a trapezoidal shape. Due to the size of the pulp chamber and its proximity to the mesial-lingual wall, we advise careful design of the access cavity to avoid unnecessary excision of tooth structure (see Figures 1.55 and 1.56).
Third lower molar
Erupts after 17 years, usually at the age of 25. Also called "wisdom teeth", they have an atypical and varied morphology. It can have from three to five tubercles (Fig. 1.57). Often there is a fusion of roots from two or three roots. And, as a consequence, the anatomy of the canals cannot be schematized, as with the upper molar. Experience in root canal treatment will help the clinician perform the endodontic procedure correctly (Figs. 1.58, 1.59, and 1.60).
Rice. 1.57. The varied occlusal morphology of mandibular third molars may have four to five cusps
Rice. 1.58 (a) Lower third molar with a curved mesial root fused to the distal one. (b) Lower third molar: distal root
Rice. 1.59 (a) Lower third molar with four roots, two mesial and two distal: the mesiobuccal root is a very curved canal in the middle third. (b) Lower third molar: distal view showing graceful curvature of the distolingual root
Rice. 1.60 (a) Lower third molar: on the buccal side, two separate mesial roots are visible with extremely severe curvature, according to Schneider. The distal root has a moderate 3D curvature in the mesial and lingual direction. (b) Lower third molar: from the distal side, a 3D curvature of the distal root is visible in the lingual as well as mesial direction
Igor Lukinykh
How many teeth does a person have?
The number of teeth a person has depends on age and anatomical features. The child has a set of 20 primary teeth, which are replaced by a permanent bite of 28 teeth. Third molars erupt, as a rule, after twenty years or do not grow at all, which is not a pathology.
In dentistry, a single numbering of human teeth is adopted. Doctors classify teeth as lower and upper and distinguish the right and left segments of the jaws. Each of them includes two incisors, a canine, two premolars and three molars. The countdown starts from the first front tooth and ends, accordingly, with a figure eight. Sometimes a number is added to the serial number indicating the location zone. For example, the right canine of the top row is numbered 13. This order in the schematic representation is called the formula of human teeth.
Polyodontia
In rare cases, an anomaly such as polyodontia is observed - supernumerary, or extra teeth in a person. Dental units can appear in the primary and permanent dentition anywhere in the jaw, separate from or fused with the main teeth. The defect affects not only the aesthetics of the smile, but also leads to the formation of incorrect occlusion, impairs the quality of chewing food and diction. Most often, supernumerary teeth are removed in childhood or built into the dentition.
Edentia
There is also a deviation of the opposite meaning called edentia - congenital or acquired absence of dental units. The causes of the phenomenon include heredity or improper development of the embryo in the womb. People without teeth cannot fully eat and speak, have a deformed facial contour and weakened immunity.
