Molars are divided into canines, incisors, premolars, and molars. The first three types of teeth erupt in place of similar milk teeth. In turn, molars do not have predecessors and appear behind the temporary ones, therefore their second name is accessory.
Each jaw has 16 teeth - four incisors, two canines, four premolars and six molars. In some cases, third molars, better known as wisdom teeth, do not erupt because they do not have buds in the jaw - then instead of 16, a person has 14 teeth above and below. Experts explain this phenomenon by a reduction, or simplification, of the dental system caused by changes in dietary style.
Central lower incisor
Average age of eruption: 6-7 years
Average age of root formation: 9 years
Average length: 20.7 mm
Narrow and flat in the labial-lingual direction, the lower central incisor is the smallest adult tooth. Radiologically it is visible in only one projection and thus appears more accessible than it actually is. The crown, narrow in lingual projection, has a limited area for access. For gentle access formation, a fissure bur and a spherical bur No. 2 are used. The access cavity should be oval and performed on the lingual side.
The lower central incisor often has two canals. One study reported that 41.4% of mandibular central incisors studied had two separate canals, of which only 1.3% had two separate apical foramina [1]. After completing the access formation, the doctor must examine the tooth cavity to identify an additional canal. Endodontic treatment failures in the lower incisor area are most often associated with an unidentified canal, usually in the lingual location. To create conditions for the straight entry of endodontic instruments into the additional canal, access can be expanded in the incisal direction.
There is a great danger of perforation of the vestibular wall, but it can be avoided if the doctor remembers that it is almost impossible to perforate in the lingual direction, since the bur axis is in contact with the incisal edge. The slit-like lumen of the canal is so common that it can be considered a normal variant, and this requires special attention when cleaning and shaping.
Lateral perforations and pulp anatomy will be discussed in an illustration of the lower second molar.
Proportional ratio of teeth sizes
In most cases, specialists determine the proportions of a tooth by correlating its height and width. A result of about 0.75 is considered ideal. The most accurate diagnosis is carried out through the use of formulas.
- Gerlach's formula.
The method is based on the proportional ratio of the sizes of the front teeth and dental units of the chewing zone. The width of the crowns of the upper central incisors should correspond to the width of the four lower incisors. The canine, two premolars and one molar of both jaws are normally equal to each other. The width of the lateral part of the dentition is 10 mm greater than the width of the anterior segment. - Pon's formula.
The distance between the first premolars is equal to the sum of the widths of the four incisors multiplied by 100 and divided by 80, and the distance between the first molars is the sum of the widths of the four incisors multiplied by 100 and divided by 64. - Corkhouse formula.
The length of the segment from the midline to the first molar of the upper jaw should be 2 mm greater than the same distance on the lower jaw.
Lateral lower incisor
Average age of teething: 7-8 years
Average age of root formation: 10 years
Average length: 21.1 mm
Very similar to the lower central incisor, so access cavity preparation follows the same principles.
Their similarity can cause rare but serious errors. Hasty installation of a rubber dam, identical fillings and carelessness can lead to preparation of the access cavity on the wrong tooth. This error can be prevented by marking the vestibular surface of the tooth with a felt-tip pen before applying the rubber dam.
Trauma, periodontal disease, carious lesions and malocclusion can lead to obliteration of the canal. When moving apically to identify the orifice, great care must be taken to prevent unnecessary destruction of the crown and root. Labial perforations were discussed in Illustration IX. If the bur is not oriented along the long axis of the tooth, there is a risk of lateral perforation. The situation becomes more complicated with the traumatic loss of an anatomical crown. Without anatomical landmarks, a lateral perforation can be easily made when moving in a coronal direction. To prevent this, access is carried out without a rubber dam so that the root can be palpated.
Lateral perforation with endodontic files and Gates-Glidden burs is facilitated by the presence of slit-shaped canals with a narrow, hourglass-shaped cross-section. To avoid vertical fracture along the approximal root wall, minimal expansion and preparation of the space for the abutment pin is indicated.
Apical curves and accessory canals are common in the lower incisors.
How are tooth sizes determined?
To determine the size of a person’s permanent teeth, the calculation tables of Wetzel and V.L. Ustimenko with average standards and permissible deviations are used. However, an experienced dentist is able to independently identify the anomaly during a visual examination or by applying the formula for the ratio of the height and width of the natural crown. During diagnosis, the specialist takes into account the patient’s face shape and height. For example, with a wide jaw, the size of teeth exceeding the norm is not a pathology.
To determine the size of the root canal, there are also tables that indicate the average distance from its apex to the cutting edge or cusp of the tooth surface. The longest is the root of the canines - about 26 millimeters, the root canal of the incisors is 21 - 23 millimeters, and the size of the roots of the chewing zone and premolar teeth ranges from 19 to 22 millimeters.
Mandibular canine
Average age of teething: 9-10 years
Average age of root formation: 13 years
Average length: 25.6 mm
The canine of the lower jaw is more powerful and significantly wider than the incisors in the mesial-distal direction. It rarely causes treatment problems. The atypical form with two roots can be problematic, but is rare.
The access cavity is oval and can be expanded in the incisal direction to facilitate vestibular-lingual access. In the cervical region the canal is oval, in the middle third it is rounded. To completely clean its walls, directed instrumental action is necessary.
If there are two roots, one of them will always be easier to instrument. The other canal must also be opened and funnel-shaped in accordance with the first to prevent dentine filings from entering it and impairing access. Pre-bending the instruments during the initial approach will allow the clinician to walk along the walls of the buccal or lingual root until the tip of the instrument enters the orifice. Once a difficult-to-reach canal has been identified, every effort must be made to shape and create a funnel-shaped orifice to keep access open.
Microdentia
The size of teeth in microdentia is less than anatomical standards. The list of reasons for deviation includes exposure to radiation, premature removal of a baby tooth, narrow jaw, and infectious diseases. There are several types of anomalies:
- isolated - a single violation affecting the lateral incisors;
- relative - the teeth are of normal size, but look smaller due to the enlarged jaw, as a result of which interdental spaces and diastema are formed;
- generalized - the defect covers a group of teeth.
First lower premolar
Average age of teething: 10-12 years
Average age of root formation: 12-13 years
Average length: 21.6 mm
Often considered a mystery to endodontists, the mandibular first premolar, with its two canals separating at different levels of the root, can be very difficult to machine.
The crown consists of a well-developed buccal cusp and a small or almost non-existent lingual enamel protrusion. The approach is performed buccally from the central sulcus and directed along the long axis of the root to the central cervical region. The oval-shaped pulp chamber is opened using fissure burs with a cutting apex and elongated spherical burs No. 4 or 6. In teeth with one canal, the pulp cavity in the neck area has an almost circular cross-section, and in teeth with two canals it is oval.
One study reported that “at least 23% of first mandibular premolars have a second or third canal”[17]. The canals can split almost anywhere in the root. Due to the lack of direct access, cleaning, shaping and filling these teeth can be extremely difficult.
In a recent study, Vertucci [13] showed that the first lower premolar has one canal at the apex in 74.0% of cases, two canals in 25.5% and three canals in the remaining 0.5% of cases.
Classification and frequency (%) of canal types in first and second lower premolars
By Vertucci, F. J. Am. Dent. Assoc. 97:47, 1978.
Size of modern human teeth
Over more than two million years, the size of teeth gradually decreased due to the transition to eating soft and processed foods. The first transformations affected the fangs - in primates they were larger and more strongly advanced relative to the rest of the row. The interdental spaces have disappeared, the size of the front teeth has become smaller. Refusal of raw meat led to a decrease in the chewing load and a narrowing of the jaw, as a result of which there was no room left for the “eights”. Now wisdom teeth are considered atavism and are often subject to removal.
Second lower premolar
Average age of teething: 11-12 years
Average age of root formation: 13-14 years
Average length: 22.3 mm
The second lower premolar, which is very similar in crown shape to the first premolar, has a less complex root.
Its crown has a well-developed buccal cusp and a much better formed lingual cusp than on the first premolar. The access is made slightly oval, wider in the mesial-distal direction. They begin to form access in the central sulcus with a fissure bur with a cutting apex, and then expand and form the contour of the burr hole with spherical burs No. 4 and 6.
Researchers reported that only 12% of mandibular second premolars studied had a second or third canal [17]. Vertucci [13] also showed that second premolars had one apical foramen in 97.5%, while only 2.5% of the teeth examined had two foramina.
An important circumstance that should not be forgotten is the anatomical location of the mental foramen and the vessels and nerves passing through it. Due to the proximity of these structures, an acute inflammatory process in the area of the lower premolars can cause temporary paresthesia. The exacerbation of the pathological process in this area is more severe and resistant to conservative treatment than in other areas.
First lower molar
Average age of teething: 6 years
Average age of root formation: 9-10 years
Average length: 21.0mm
The mandibular first molar erupts earlier than other permanent teeth and most often requires endodontic treatment. It usually has two roots, but sometimes three roots are found, with two canals in the mesial root and one or two canals in the distal root.
The distal root is easily accessible for endodontic cavity preparation and mechanical treatment.
The doctor can directly see the opening(s) of the canal. The distal root canals are wider than those of the mesial root. Sometimes the mouth is wider in the buccolingual direction. This indicates the presence of two channels or a slit-like channel with a complex network-like configuration that can complicate cleaning and shaping.
The mesial roots are usually curved, with the greatest curve in the mesiobuccal canal. The orifices of the canals at the bottom of the pulp chamber are usually clearly separated from each other and located buccally and lingually relative to the upper tubercles.
The tooth often undergoes extensive filling. It almost always experiences a strong chewing load, so the coronal pulp cavity can be obliterated. It is easiest to identify the mouths of the distal canals.
Then the mouths of the mesial canals are found, which will be located in the above locations in the same horizontal plane.
Since the mouths of the mesial canals lie under the mesial tubercles, they may not be detected during normal cavity preparation. In this case, to determine their location, it is necessary to remove the hard tissue of the tubercle or filling. During access preparation, overhanging molar cusps need to be ground down [15]. Remember that this tooth, like all other lateral teeth, after endodontic treatment requires complete restoration of the entire occlusal surface area. Therefore, to identify anatomical landmarks and orifices, it is better to make a wider access cavity than to skip one or more channels for the sake of “sparing” preparation, which may cause failure.
Skidmore and Bjoradal [11] found that approximately one third of the mandibular first molars examined had four root canals. If there are two canals, “they either remain separate with separate apical foramina, or unite and form a common apical foramen, or communicate with one another by transverse anastomoses partially or completely... If the tooth, instead of the usual triangular shape, had a more rectangular shape, this would allow better vision and explore a possible fourth canal in the distal root.”
In the area of bifurcation of the roots of the lower molars there are several orifices of additional canals [9]. They are usually impossible to clean and shape, and are rarely visible except incidentally on radiographs when they are filled with root cement or heated gutta-percha during treatment. It would be correct to assume that if irrigation solutions tend to clean the canal from protein decomposition products, then the area of root bifurcation in the pulp chamber must be thoroughly cleaned (remove denticles, etc.) so that the solutions can reach the small mouths of the canals.
All infected dentin, leaking fillings, and pulp denticles must be removed before endodontic treatment begins. It is recommended to completely cover the cusps with a restorative structure after endodontic treatment.
Functions of the dental system:
- Chewing is the main function of the teeth and dentofacial system. Chewing is the first stage of the digestion process.
- Aesthetics. The dental system affects the appearance of the face. Teeth also play an important aesthetic role. The position, shape and shade of teeth have a significant impact on the formation of a person’s personality.
- Pronunciation. Teeth help us pronounce words correctly as they play an important role in the pronunciation of consonants. Missing teeth (especially the upper front teeth) or an incorrect bite can somewhat impair normal speech.
Second lower molar
Average age of teething: 11-13 years
Average age of root formation: 14-15 years
Average length: 19.8 mm
The crown is somewhat smaller in size than the first molar, and the more symmetrical second lower molar is characterized by a close arrangement of roots. The roots gradually converge distally and have closely spaced apices.
Exposure is made at the mesial portion of the crown with access extending only slightly distal to the central sulcus. After trephination of the cavity with a fissure bur with a cutting apex, an elongated ball-shaped bur is used to expand it until free access is achieved. The distal root angles often allow for a smaller opening than the lower first molar.
Particular attention should be paid to the shape of the mouth of the distal canal. A narrow oval orifice indicates the presence of a slit-like lumen in the distal canal, which requires more careful treatment.
All carious tissue, leaking fillings and denticles should be removed and replaced with a suitable temporary filling prior to endodontic treatment.
The lower second molar is most susceptible to vertical fractures. After preparing the access, but before starting endodontic treatment, the clinician should examine the floor of the pulp cavity using fiber optics. For mesial-distal crown or root fractures, the prognosis is extremely unfavorable.
To avoid vertical fractures after endodontic treatment, it is necessary to completely cover the cusps with a restorative structure.
