BIOLOGY OF THE HUMAN DENTITION I

I. Introduction. The Jaws and Dental Arches.

The teeth are arranged in upper and lower arches. Those of the upper are called maxillary; those of the lower are mandibular.

A. The maxilla is actually two bones forming the upper jaw; they are rigidly attached to the skull.

B. The mandible is a horseshoe shaped bone which articulates with the skull by way of the temporomandibular joint the TMJ.

C. The dental arches, the individual row of teeth forming a tooth row attached to their respective jaw bones have a distinctive shape known as a catenary arch.

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II. Dentition is a term that describes all of the upper and lower teeth collectively.

Clinically, there are three dentitions.

A. The primary dentition consists of 20 teeth in all: ten upper and ten lower teeth. Primary teeth may also be called 'baby' teeth, deciduous, 'milk', or lacteal teeth. Primary teeth begin to appear at about age six months and are entirely replaced by about ages 12 - 13.

B. The mixed dentition is composed of both primary and permanent teeth. It commences with the eruption of the first of the permanent teeth at about age six, and ends with the loss of the last of the deciduous teeth at about the age of 12-13 years

C. The permanent dentition is composed of 32 teeth in all, 16 upper and 16 lower. Half of a dental arch (primary or permanent) is called a quadrant. The permanent teeth that replace deciduous teeth are call succedaneous teeth. (Succedaneous means literally, to replace. In dental science, permanent teeth that replace deciduous teeth are called successional teeth. Permanent molars, which replace nothing are called accessional teeth.)

FACTOID: A person aged 70 years will have spent 91% of his/her chewing career with permanent teeth and only 6% with deciduous teeth. Yet, the deciduous teeth are very important!

D. Types of dentitions:

1. Diphyodont. Most mammals--humans included--typically develope and erupt into their jaws two generations of teeth. The term literally means "two generations of teeth."

2. Monophyodont. Some mammals--such as the manatee, seals, and walruses have only a single generation of teeth.

3. Polyphyodont. Most reptiles and fishes develope a lifetime of generations of successional teeth--as if on a conveyer belt. Such teeth have a brief functional life and are anatomically simple in design.

4. Homodont. In many vertebrates, all of the teeth in the jaw are alike. They differ from each other only in size. The alligator is an example of homodontism.

5. Heterodont. Most mammals, humans included, develope distinctive classes of teeth that are regionally specialized. We will discuss classes of teeth in the next unit.

6. Anodontia is the developmental absence of teeth. Among mammals, the whalebone whale and the anteater are toothless; their ancestors had teeth. In humans, anodontia is a pathological condition. Partial anodontia is one or a few teeth missing.

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III. Classes of Teeth.

A. Anterior teeth

1. Incisors: Four uppers and four lowers. There are central incisors and lateral incisors. These function in cutting food, articulating speech, appearance, and for support of the lips.

2. Canines: Two upper and two lower. In dogs and cats, the long canine teeth are used for catching food, tearing the food, and for defense. In some primates, I large canines are used in threatening gestures. In humans, they function along with the incisors for support of the lips, cutting or shearing of food, and as guideposts in occlusion. In traditional re storative dentistry, they are the cornerstones of the dental arch.

B. Posterior teeth (Cheek Teeth)

1.Premolars: Also known as bicuspids. They are four number in the upper arch and four in number thelo arch. They are designated as first or second bicuspi their order in the dental arch. They function wi molars in the mastication of food and in maintaini vertical dimension of the face.

2. Molars: There are six upper and six lower, designated as first, second, or third. Permanent molars are important in the chewing and grinding of food, and in maintaining the vertical dimension. Important: Upper molars have three roots and low molars have two roots.

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IV. Dental Formula, Dental Notation, Universal Numbering System

A. Dental Formula. The dental formula expresses the type and number of teeth per side. It is used by comparative anatomists and zoologists. It appears on National Boards, our exams, and scholarly articles.

1. Primary teeth.

It is said as: incisors, two upper and two lower; canines, one upper and one lower; molars two upper and two lower equals ten per side.

2. Permanent teeth.

It is said as: incisors, two upper and two lower; canines, one upper and one lower; premolars, two upper and two lower; and molars, three upper and three lower. Comment: You won't use it clinically, but you should be aware of it.

A description of that system is provided to you for reference only. In this course we will use proper anatomical terms and for notation, the Universal Numbering System.

 

B. Dental Notation: Several systems of dental notation have been used overthe years. Many orthodontists, for example, retain an older system that uses just numbers or letters for the teeth with lines or brackets to designate upper or lower. The American Dental Association has approved use of the International ISO T106 System which is suitable for computer entry.

 

C. The Universal Numbering System. The rules are as follows:

1. Permanent teeth are designated by number, beginning with the last tooth on the upper right side, going on to the last tooth on the left side, then lower left to lower right. There are thirty-two pemanent teeth, correct?

Hint: Look at this diagram as you would be looking at a patient. You are numbering the teeth counting clockwise. You will use this numbering system in this course, in the clinics, and in practice when you graduate.

2. Deciduous teeth are designated by letter, beginning with the last tooth on the upper right side and proceeding in clockwise fashion as described above We will cover this again in our unit on deciduous teeth.

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V. Parts of the Tooth.

A. Crown. The term can be defined in two ways.

1. The anatomical crown is covered with enamel.

2. The clinical crown is the portion of the anatomical crown that is visible clinically. It is what you see when you look in the mouth.

B. Root. The term can be defined in two ways.

1.The anatomical root is the portion of the tooth that is covered with cementum, a bone-like substance that facilitates anchorage of the tooth in its bony socket (the alveolus).

2. The clinical root is that part of the anatomical root that is actually embedded in the jaw. In a patient with advanced bone loss, the clinical root may be reduced in size.

C. Cervical line. This is the line that separates the anatomic crown from the anatomic root. It is the junction between two tissues--the enamel and the cementum. It is also called the cemento-enamel junction or simply the CEJ. This region of the tooth is also called the cervix of the tooth. The cervical line is important in your laboratory drawings.

D. Pulp cavity. This is the space in the tooth that in life contains the pulp or 'nerve' of the tooth. In your specimens, the pulp will be withered or absent. It has a coronal (crown) portion and a radicular (root) portion, usually called the rooth canal.

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VI. Dental tissues.

A. Enamel. The protective outer surface of the anatomic crown. It is 96% mineral and is the hardest tissue in the body.

B. Dentin. Located in both the crown and root, it makes up the bulk of the tooth beneath the enamel and cementum. It lines the pulp cavity.

C. Cementum. This substance covers the surface of the anatomic root.

D. Pulp. The central, innermost portion of the tooth. It has formative,sensory, nutritive, and functions during the life of the tooth.

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VII. Descriptive anatomy (points of reference)

Median sagittal plane: the imaginary plane in the center that divides right from left.

Median line: an imaginary line on that plane that bisects the dental arch at the center.

Mesial: toward the center (median) line of the dental arch.

Distal: away from the center (median) line of the dental arch.

Occlusal plane: A plane formed by the cusps of the teeth. It is often curved, as in a cylinder. We will speak often of the occlusal surface of a tooth.

Proximal: the surface of a tooth that is toward another tooth in the arch.

Mesial surface: toward the midline.

Distal surface: away from the midline.

Facial: toward the cheeks or lips.

-Labial: facial surface of anterior teeth (toward the lips).

-Buccal: facial surfaceof anterior teeth (toward the cheeks).

Lingual: toward the tongue.

Occlusal: the biting surface; that surface that articulates with an antagonist tooth in an opposing arch.

Incisal: cutting edge of anterior teeth.

Apical: toward the apex, the tip of the root.

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VIII. Dental terminology.

Cusp: a point or peak on the occlusal surface of molar and premolar teeth and on the incisal edges of canines. Wheeler's text also defines it as an elevation on the crown of a tooth making up a divisional part of the occlusal surface.

Contact: a point or area where one tooth is in contact (touching) its neighbor.

Cingulum: a bulge or elevation on the lingual surface of incisors or canines. It makes up the bulk of the cervical third of the lingual surface. Its convexity mesiodistally resembles a girdle (L. cingulum = girdle) encircling the lingual surface at the cervical.

Lobe: one of the primary centers of formation in the development of the crown of the tooth.

Mamelon: A lobe seen on anterior teeth; any one of three rounded protuberances seen on the unworn surfaces of freshly erupted anterior teeth.

Ridge: Any linear elevation on the surface of a tooth. It is named according to its location or form. Examples are buccal ridges, incisal ridges, marginal ridges, and so on. Four specific types are defined below. See cusp ridge at right.

Marginal ridges are those rounded borders of enamel which form the margins of the surfaces of premolars and molars, mesially and distally, and the mesial and distal margins of the incisors and canines lingually.

Triangular ridges are those ridges which descend from the tips of the cusps of molars and premolars toward the central part of the occlusal surface.

Transverse ridges are created when a buccal and lingual triangular ridge join. Listen up! It is the union of two triangular ridges crossing transversely across the surface of a posterior tooth.

Oblique ridges are seen on maxillary molars and are a companion to the distal oblique groove.

Cervical ridges are the height of contour at the gingival, on certain deciduous and permanent teeth.

Fossa: An irregular, rounded depression or concavity found on the surface of a tooth. A lingual fossa is found on the lingual surface of incisors. A central fossa is found on the occlusal surface of a molar. They are formedby the converging of ridges terminating at a central point in the bottom of a depression where there is a junction of grooves.

Pit: A small pinpoint depression located at the junction of developmental grooves or at the terminals of these groops. A central pit is found in the central fossa on the occlusal surfaces of molars where developmental grooves join. A pit is often the site of the onset ofdental caries.

Developmental groove: A sharply defined, narrow and linear depression formed during tooth development and usually separating lobes or major portions of a tooth. Major grooves are named according to their location.

A supplemental groove is also a shallow linear depression but it is usually less distinct and is more variable than a developmental groove and does not mark the junction of primary parts of a tooth.

Buccal and lingual grooves are developmental grooves found on the buccal and lingual surfaces of posterior teeth.

Tubercle: A small elevation produced by an extra formation of enamel. These occur on the marginal ridges of posterior teeth or on the cingulum of anterior teeth. These are deviations from the typical form.

Interproximal space: The triangular space between the adjacent teeth cervical to the contact point. The base of the triangle is the alveolar bone; the sides are the proximal surfaces of the adjacent teeth. Note well: the interproximal space is normally filled by the gingival papillae.

Embrasures: When two teeth in the same arch are in contact, their curvatures adjacent to the contact areas form spillway spaces called embrasures. There are three embrasures:

(1) Facial (buccal or labial)

(2) Occlusal or incisal

(3) Lingual (Special note: there are three embrasures; the fourth potential space is the interproximal space defined above.) Study the diagrams.

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IX. Location and position.

A.For purposes of description, the crowns and roots are arbitrarily divided into thirds. This is helpful in describing features such as height of contour or any other anatomic feature. A glance at the diagram to the right will clarify this for you.

B.The teeth present us with rounded surfaces, yet the intersection of these surfaces can clearly be described as line angles. Line angles are used as descriptive terms to indicate a position.

A line angle is the intersection of two planes. Carefully examine the diagrams at the right to clarify this concept in your mind. Line angles derive their names from the combination of the two surfaces that interesect (along a line).

C. A point angle is formed by the junction of three surfaces. The point angle also derives its name from the combination of the names of the surfaces forming it. Take a look at the diagram to the right to clarify this concept in your mind.

Final note: Line angles and point angles can be used to describe internal features of the tooth, such as cavity preparations or the pulp chambers.

CJ '98