Introduction: The teeth are located in the mouth in the form of two lower and upper jaw arches. The lower jaw is movable and the upper jaw is fixed.

Components of a tooth:

1. Anatomical crown: Part of the tooth that is covered by the enamel.
2. Anatomical root: Part of the root that gets covered by cementum.
3. Enamel: A very hard calcareous tissue that coats the dentin of the crown.
4. Dentin: The main part of tooth is made of dentin. It is covered by enamel in the crown and by cementum in the root.
5. Cementum: Hard bone-like layer that covers the dentin in the root section.
6. Pulp: The soft tissue that fills the cavity inside the tooth and contains the arteries and nerves.
7. Pulp cavity: The entire cavity inside the teeth is called the pulp cavity, which includes: 
7.a. Pulp canal: The section of the pulp that is inside the root.
7.b. Pulp-chamber: A big cavity in the center of the crown.
7.c. Pulp-horns: Hair-like protrusions from the pulp-chamber that are separated in the crown environment.
8. Cervical line: It is a clear line on the outer surface of the tooth right between the enamel and the cementum, also called the CEJ, which stands for Cemento-Enamel Junction and separates the anatomical crown from anatomical root.
9. Dentino-Enamel Junction: The connection point between dentin and enamel in the anatomical crown. 

. The stage of permanent teeth is between 6 and 21 years old.

Teeth Form Categorization:

There are 8 teeth in each half-jaw, which are divided into 4 types:

A. Incisors: there are 2 in each half-jaw, right next to the midline. The first one next to the midline is called the central incisor and the second one is called the lateral incisor. These teeth are tasked with cutting. Therefore, in each jaw there are 4 incisors and in total there are 8 incisors in the mouth. In the figure below the incisors are marked blue around the orange midline.

B. Canine teeth: It is the third tooth from the midline, tasked with cutting, tearing, perforating and holding a thing. These teeth are also called the cuspid teeth. In each jaw arch there are two canines and in total, there are 4 canines in the mouth. 

C. Premolars: the fourth and the fifth teeth from the midline are called the first and the second premolars. They are tasked with cutting chewing and holding; and they are also called the bicuspids. In each jaw, there are 4 premolars; and in the mouth, 8 in total.  

D. Molars: The sixth, seventh, and eighth teeth form the midline are called molars. The third molar is called the sixth molar. They are tasked with chewing and grinding food. There are six molars in each jaw and 12 molars in the mouth.  

• The central and lateral incisors, and the canine teeth are called the anterior teeth.
• The rest of the teeth that are behind the anterior teeth are called the posterior teeth.

Teeth Surfaces:

The crown of the anterior teeth have one edge and 4 surfaces. And they are:

A. Mesial: It is the surface closer to the midline. 
B. Distal: It is the surface that is furthest from the midline.
C. Labial: It is the external surface of the lips that faces the lips.
D. Lingual: It is the internal surface that is facing the tongue.
E. Incisal edge: It is the cutting edge.

All posterior teeth have five surfaces. These surfaces are: 

A. Mesial, distal and lingual like anterior teeth.
B. Buccal: It is the cheek-side of the tooth.
C. Occlusal: It is also called the occlusal surface.
D. The mesial and distal surfaces of the two adjacent teeth is called proximal.

Morphology and Other Parts of the Tooth:

A. Contact points: Areas from proximal surfaces of the teeth that are in contact with each other.
B. Protrusions of the crown:

1. Cusp: It is the main protrusion on the occlusal surface or the incisal edge of the canine tooth.
2. Tubercle: Round and small protrusions that are often seen on the lingual surface of the upper anterior teeth.
3. Cingulum: Large and completely round protrusion on the lingual surface of the anterior teeth.
4. Ridge: A type of protrusion in the teeth that consists of two ranges that meet in one line. There are different types of ridges:

4.a. Marginal ridge: It is linear and exists on the mesial and distal occlusal surface of the posterior teeth.
4.b. Triangular ridge: It is linear and goes from the top of the cusp of posterior teeth to the center of the occlusal surface.
4.c. Transverse ridge: Consisted from the union of two triangular ridges and connects the top of a buccal cusp to the top of a lingual cusp.
4.d. Oblique ridge: Completely exclusive to upper molars, it is stretched from mesiolingual to distobuccal cusp.
4.e. Cusp ridge: Each cusp has four ridges that are stretched in mesial, distal, facial and lingual direction towards the base of the cusp.
4.f. Mamelon: Small and round protrusions on the incisal surfaces of the anterior teeth. 

C. Depressions of the crown:

1. Fossa: They are irregular and rounded depressions. On the lingual surface of the anterior teeth, there is a relatively large fossa, and on the occlusal surface of the posterior teeth, there are 2 or more fossae with different sizes.
2. Sulcus: They are narrow, long, v-shaped depressions on the occlusal surface of the posterior teeth.
3. Developmental groove: A groove that represents the boundaries connecting the cusps of the tooth crowns to each other.
4. Transverse ridge: Separates from the developmental grooves and is located in the slope of the cusps.
5. Pit: Where the main developmental grooves meet, they form a pit and it is the deepest part of the fossa.

Numbering the teeth:

The most practical system is the palmer system displayed below.

In this system the horizontal and vertical midlines of the teeth divide the mouth into 4 half-jaws. Then, from the centrals towards the back, the teeth are numbered from 1 to 8.

Teeth Introduction Section :

Permanent incisor teeth: 

• The upper incisors are generally larger than their counterparts in the mandible. The upper central incisors are larger than the upper laterals, while the lower lateral is larger than the lower central.
• The incisors are important because of their location, color, form and aesthetic; and they support the lips and maintain the general form of the face. 

Permanent Upper Central Incisors:

• They are located on either side of the midline and have mesial contact with each other and have distal contact with the lateral.
• The overall shape of the crown is trapezoidal from the labial and lingual point of view, and is triangular from the proximal point of view. Generally the largest tooth is the incisor.
• From the labial point of view in the cervical 1/3, it has the maximum convexity.
• In the mesial and distal view, it is slightly convex and the convexity in the distal side is slightly larger.
• On the incisal ridge, three mamelons are seen and they might probably be grinded and gone.
• From the lingual view, the mesial and distal surfaces converge towards the lingual surface, so the mesiodistal width of this surface is less than the labial surface. At half to 2/3 of the incisal, this surface has a shallow concavity called lingual fossa, and at the cervical 1/3, it has a cingulum. 
• It has a conical, straight root, ends in a perfectly rounded apex, and the root is slightly inclined toward the mesial, and in the buccal it is wider than the lingual, and the root length is 1.5 times the length of the crown.

Permanent Upper Lateral Incisors: 

• This tooth is in contact with the distal of the central. 
• The lateral incisor cooperates with the central in terms of function, but in a smaller and more delicate scale. It has less convexity, but the length of its root is equal to that of the central incisor. The lateral incisor is more rounded in all sides.
• Its mesial ridge is similar to the central incisor but the distal ridge is more convex.
• Incisal ridge: Due to the rounding of the incisal angles, this ridge is not as straight as the central ones. Sometimes there are 3 mamelons.
• Lingual view: The marginal ridges of mesial, distal, and cingulum are more prominent than the central ridges and the lingual fossa is also deeper. The linguogingival groove is more abundant in this tooth, and the lingual pit is located in the center of the same groove, and decaying often starts in this area.
• Root: It has a single root and it is narrower than the root of the central tooth and from the mesiodistal view it is wider and from the labiolingual view it is thicker. The apex tends to the distal surface and is sharp.

Mandibular incisors: 

• The lower incisors are usually the smallest of permanent human teeth, and the central is slightly smaller than the lateral one, while the upper lateral is smaller than the central. The lower incisors are the simplest human teeth regarding their form and they look very much alike. 

Permanent lower Central Incisors:

• The lower central incisor has the smallest permanent tooth crown. Its mesial and distal half are almost symmetrical. 
• From the labial view, the maximum convexity is in the one-third of the incisal.
• Incisal ridge: After the mamelons are grinded, this ridge is straightened and is perpendicular to the longitudinal axis of the teeth. Also, the mesial and distal surfaces are symmetrical.
• Lingual surface: It is a clear surface without any anatomical protrusions, and sometimes there is a shallow lingual fossa seen on this surface. There are no grooves, pits or seams on this surface. Although the cingulum is prominent, but it’s not as prominent as the protrusion on the upper incisors.
• Root: This tooth has a single straight root. This root is broad in the faciolingual direction. The middle of the mesial and distal surfaces may be convex or flat. It can also be concave, which in that case, it will be called longitudinal groove concavity.

Permanent Lower Lateral Incisors::

• The lateral tooth is slightly larger than the central tooth in all dimensions, but it is very similar to it, in terms of shape and function. 
• Form the labial view, the incisal ridge is slightly sloped towards the distal surface.
• From the lingual view, it looks completely like the labial view. Properties of this surface, including cingulum, marginal ridge, and fossa are generally less prominent than the central incisors.
• The cementoenamel junction has a smaller arch and the mesial arch is smaller than the distal.
• The incisal edge is not straight like the central incisor, and in the distal surface has a lingual inclination. 
• Root: the root of this tooth is longer than the central and it is slightly thicker and wider. As it can be seen by the longitudinal grooves on the root.

Canine teeth

• There is one canine or cuspid in each half-jaw. They are relatively similar to each other and are also called the foundation of human mouth. These teeth are located at the jaw arch angle in the transition phase between the incisal teeth and the posterior teeth.
• Canines perform task that are in the middle of anterior and posterior teeth. Meaning, they are not tasked with cutting or chewing. They are tasked with tearing.
• From the proximal view, canines are similar to incisors, but from the facial view they look like premolars. Canines have the incisal edge, cingulum, marginal ridge and other features that exist in the anterior teeth. Meanwhile, they have a cusp similar to posterior teeth.
• The positioning of the canines is between the incisors and the posterior teeth, which means, the incisors are in the latitudinal direction and the posterior teeth are in the longitudinal direction; but, the canines are in a position in between the aforementioned states. Therefore, this positioning renders canine teeth important regarding aesthetics, functions and their role in supporting the facial muscles.
• Canine teeth are the longest of human teeth and firmly places in the jaw bone. 
• Experience had shown that whenever a periodontal disease loosens the teeth, canines are the last ones that get affected.
• The labial view: Its geometric shape is a polygon with 5 sides. The labial surface is convex in all sides. The apex of the cusp is usually more protruded than the occlusion surface of other teeth in the jaw arch. The labial surface has a labial ridge, which is stretched towards the incisogingival direction. This ridge is a sign of significant growth in the middle section of the teeth compared to the mesial and distal section. Its maximum protrusion is in the one-third of the labial surface. The length of the lower canine crown is equal or even bigger than the upper canine. The cusp of the lower canine is not as prominent and sharp as the upper canine. 
• Lingual view: the mesiodistal width of this surface is less than the labial surface, since the mesial and distal surfaces incline towards the lingual surface. Cingulum is very prominent but flat and there is shallow fossa present in this section. Lower canine is more flat in this surface and the cingulum and marginal ridges are less protruded. 
• The mesial surface is convex in all of its dimensions and from the labiolingual view, it is wider than the incisors and the distal surface is smaller than the mesial surface. The mesial view of the lower canine is completely triangular but its lateral sides are longer than the lower canine. The distal surface of the lower canine is smaller than its mesial surface in all its dimensions.
• Incisal view: In general, it is convex and due to its significant labiolingual thickness, it is stronger than the incisors. From the incisal view, the lower canine is asymmetrical when halved buccolingually.
• Root: This tooth has a single root, and it is the longest root among the other teeth. In all dimensions, it is conically inclined to the apex and ends with a sharp tip. The lower canine also has a single root and it is the longest among the teeth in the lower jaw. Both roots, in the labiolingual direction are thicker than the mesiodistal direction. The figure on the right is the upper canine and the figure on the left is lower canine.

Permanent Upper Premolar Teeth:

• Premolars and molars together form the posterior teeth. The posterior teeth have drastic structural differences compared to the anterior teeth in terms of the number of cusps, the marginal ridge location, the ridges, the number of roots, and the existence of occlusal surfaces.

• Generally, premolars are called bicuspids, which points out the two cusps in these teeth. Their main task is chewing. Upper premolars, in contrast with lower premolars, are more similar in terms of size, form and function.

• All premolars have one buccal cusp and one or two lingual cusps. Generally, premolars have two cusps, but they might also have an extra cusp. All the upper premolars have two sharp and prominent cusps that are almost the same size. The second premolar is generally smaller and more rounded in all dimensions. Plus, the lingual and buccal cusps are also the same size. 

• The buccal view: The first premolar is similar to the upper canine and the second premolar. The CEJ arch is less than the anterior teeth. Its occlusal margin is similar to that of a canine tooth, but it is less prominent, and its maximum convexity is at the cervical 1/3. In the second premolar, the buccal cusp is not the same size as the second premolar.

• Lingual View: The lingual part of the crown is perfectly convex and spherical in all dimensions. No specific grooves or concavities are seen on this surface, and no mesial or distal convergence is seen in this area. In fact, the lingual surface is smaller than the buccal surface in all dimensions. The tip of both cusps are visible from the lingual point of view. The tip of the lingual cusp has a clear mesial inclination. In the second premolar, the lingual cusp is taller than the second premolar.

• Mesial and distal view: The geometric shape of these surfaces is trapezoidal, which has two collars in the large base. This margin is generally convex. In the second premolar, in the mesial and distal view there are two cusps with the same size.

• Occlusal view: The geometric shape of the crown is a polygon with 6 sides, in which the buccolingual width is more than the mesiodistal width. This occlusal table contains two buccal and lingual cusps and is surrounded by marginal ridges. The triangular fossa of the mesial and distal and the two pits of the mesial and distal surfaces are also seen. Some grooves are seen on the occlusal surface. In the second premolar, the angles of the crown are more rounded and the central groove is shorter and more irregular.

• Root: The first upper premolar in most cases has one straight and conical root, which ends with a rounded apex. The buccal and lingual surfaces are convex and the buccal surface is wider than the lingual. The root of the second premolar is not very different from the first premolar, but it is slightly lighter.

The figure in the right displays the maxillary first premolar and the figure in the left compare the first and second premolars. 

Lower Second Premolars: 

• These teeth usually have two cusps, but it is possible for them to have more. The second premolar is generally bigger than the first and has a longer root. Also, it has a bigger role in chewing. The second premolar usually has 3 cusps. Lingual cusps of this tooth have had a more significant growth and play a role in teeth function. 

• Buccal view: This view is five-sided, the CEJ arch is uniform and less convex. Its maximum convexity is at 1/3 of the middle. The second premolar is slightly larger, but the apex of the buccal cusp is slightly longer and sharper. 

• Lingual view: It is convex in all dimensions but smaller than the buccal view. CEJ is uniformly convex towards the apex. The lingual cusp in the second premolar has had a significant growth. The buccal cusp is higher than the lingual cusp. In the three-cusped premolar there is a mesiolingual cusp (wider and taller) and a distolingual cusp and between them there are two lingual grooves. In the two-cusped premolar, there is a single cusp without any grooves.

• Mesial and distal view: The geometric shape of these surfaces is generally parallelogram. The distal surface is slightly lower than the mesial. In the second premolar, from the mesial and distal view, the lingual cusp is more prominent. In the three-cusped premolar form the distal view, both cusps can be seen.

• Occlusal view: It generally has a rhombic shape and the crown has a strong lingual tendency. There are buccal and lingual cusps, transverse ridge of the marginal ridges, 2 triangular mesial and distal fossae, 2 mesial and distal pits, and a central groove that connects 2 the 2 pits. In the second premolar, the geometric shape of this part is square. The buccal cusp is higher than 2 lingual cusps. Also there is a Y groove and a central pit present.

• Root: It has a single, straight and conical root. In the buccolingual direction the root is wider than the mesiodistal direction. In the second premolar, the root is slightly wider and linger than the first premolar. 

Permanent upper molars:

• Molars are an essential element in the formation of occlusion. Their main function is mastication because they have a wide occlusal surface and numerous separate roots. They also support the facial muscles. 

• Buccal view: The upper first molar has the largest crown in the human mouth. It is trapezoidal and the larger base is at the occlusal surface. The buccal growth groove divides this surface into two parts of mesial and distal, and ends in the buccal pit. The mesial cusp is larger. The CEJ is irregularly arched toward the apex. In the upper second molar, the crown is similar to the upper first molar, but it is generally smaller. In the upper third molar the crown is so varied in shape that we can hardly set a standard shape for the third molar. The most common is the heart shape and its distobuccal cusp is very small.

• Lingual view: The upper first premolar is trapezoidal and more convex compared to the buccal view. CEJ has a slight arch towards the apex. In the lingual part of the mesiolingual cusp, there is a protrusion or mini-cusp called the Carabelli cusp. In the upper second molar, the distolingual cusp is much smaller the same cusp in the first molar.

• Mesial and distal view: generally, it is trapezoidal and has a wider cervical dimension than the occlusal. In the upper second molar, the height of the occlusocervical crown is less than the first molar.

• Occlusal view: The general form of this surface is a parallelogram, with a mesial and a distal marginal ridge, four main cusps, one Carabelli cusp, an oblique ridge, a transverse ridge, a mesial, a distal and a central fossa and a mesial and a central pit. In the upper second molar, two geometric forms of the crown can be seen: 1- parallelogram 2- heart shape

• Roots: The upper first molar has 3 large and grown roots. Two of the roots are in the buccal and one of them is in the lingual part. The lingual root is the largest, tallest and strongest root of this tooth. In the upper second molar, the number, contour and the direction of the roots are similar to the first molar. Root length is equal to or higher than the first molar. In the roots of the mandibular third molar, the roots have a severe distal deviation and the lingual root is very narrow.

Mandibular molars:

• The mandibular molars, generally have two roots and 4 cusps. Unlike the maxillary molars, the crown of these teeth is wider in the mesiodistal direction than the buccolingual.

• Buccal view: It is generally trapezoidal with the largest side in the occlusal surface. At least, some parts of the 5 cusps of this tooth can be seen from this view. This surface is very wide. On the occlusal margin, there are two growth grooves, the mesiobuccal and the distobuccal. The maximum convexity is in the 1/3 of the middle. In the second and third molar, 2 cusps and one groove can be seen from this view. 

• Lingual view: In general, it is trapezoidal and the mesial and distal surfaces converge towards it. Also, a very shallow lingual groove can be seen. Unlike second and third molars, from this view, the buccal margin and the proximal surfaces are visible.

• Mesial and distal view: The shape of the crown is usually rhombic and, like other lower posterior teeth, tends towards the lingual surface. The distal view appears smaller than the mesial view. In the second and third molars, the mesial and distal surfaces are not as wide.

• Occlusal view: it is usually a five-sided polygon. It includes: mesiolingual, distobuccal and mesiobuccal cusps (the biggest), distolingual and the smallest cusp (distal cusp), distal and mesial triangular fossae, central fossa and central, distal and mesial pits. In the second molar, this surface is rectangular and the mesial and distal margins have no lingual convergence. In the third molar, the marginal and distal margins are highly convex.

• Roots: The mandibular first molar has a root trunk that is divided into two branches of mesial and distal, with the mesial root being wider and stronger. In the second molar, the roots are usually shorter and attached together. In the third molar, in terms of size and tendency, the roots are greatly varied.

Source: The Book of Tooth Anatomy and Morphology

Dr. Nikoo Kahvandi