III. The Articulatory System Dentition

Teeth The teeth are accessory structures of the digestive system. Each tooth is located in a dental alveolus, or socket, in the tooth-bearing alveolar processes of the mandible and maxillae.

Alveolar Processes The alveolar processes are covered by the gingivae or gums, which extend into the each socket forming the gingival sulcus.

Tooth Sockets Moreover, the sockets are lined by the periodontal ligament or membrane, a dense fibrous connective tissue that attaches to the socket walls and the cementum, a thin layer of bone-like tissue covering the tooth root.

Dental Anatomy Thus, the ligament anchors the teeth in position and also acts as a shock absorber for the forces of chewing. A typical tooth consists of three principal portions: the crown, the root, and the neck.

Dental Anatomy The crown is the exposed portion of the tooth above the level of the gums. The root consists of one to three projections embedded in the socket. The neck is the constricted junction line of the crown and root near the gum line.

Dental Composition Teeth are composed primarily of dentin, a calcified connective tissue that gives the tooth its basic shape and rigidity. The dentin of the crown is covered by enamel that consists primary of calcium phosphate and calcium carbonate.

Dental Composition Enamel is the hardest substance in the body and protects the tooth from the wear of chewing. It is also a barrier against acids that easily dissolve the dentin. The dentin of the root is covered by cementum, another bone-like substance.

Dental Composition The dentin of the tooth encloses a cavity. The enlarged part of the cavity, the pulp cavity, lies in the crown and is filled with pulp. Pulp is a connective tissue containing blood vessels, nerves, and lymphatic vessels.

Dental Composition Root canals are narrow extensions of the pulp cavity that run through the root of the tooth. Each root has an opening at its base, the apical foramen. It is through the foramen that blood vessels bearing nourishment, lymphatics affording protection, and nerves providing sensation enter.

Dentitions Everyone has two dentitions, or sets of teeth. The first of these dentitions, the deciduous or primary teeth, begin to erupt at about 6 months of age. About each month thereafter, one pair appears until all 20 are present.

Primary Teeth The incisors, which are closest to the midline, are chisel-shaped and adapted for cutting into food. Based on their position, they are referred to as either central or lateral incisors.

Primary Teeth Moving posteriorly, the canines or cuspids are next to the incisors. These teeth have a pointed surface called a cusp. Cuspids are used to tear and shred food. The incisors and cuspids have only one root apiece.

Primary Teeth Behind them lie the first and second molars, which have four cusps. Upper molars have three roots; lower molars have two roots. The molars crush and grind teeth.

Secondary Dentition Between 6 and 12 years of age, all the deciduous teeth are lost and are replaced by the permanent (secondary) dentition. The permanent dentition contains 32 teeth. They resemble the deciduous dentition with the following exceptions.

Secondary Dentition The deciduous molars are replaced with the first and second premolars (bicuspids), which have two cusps and one root (the upper first bicuspid has two roots). Premolars are used for crushing and grinding.

Secondary Dentition The permanent molars erupt into the mouth behind the premolars. They do not replace any deciduous teeth and erupt as the jaw grows to accommodate them. The first molars appear at age 6 years, the second molars at age 12, the third molars (wisdom teeth) after age 18.

Secondary Dentition The human jaw has become smaller through time and often does not afford enough room behind the second molars for the eruption of the third molars. In this case, the third molars remain embedded in the alveolar bone “impacted.” Most often they cause pressure and pain and must be surgically removed. In some individuals, third molars may be dwarfed in size or may not develop at all.

Dental Surfaces Because of the curved nature of the dental arches, dental anatomists have given special names to the five free surfaces of the teeth. One surface is the biting or masticatory surface. It is in contact with opposing teeth of the opposite jaw and is called the occlusal surface.

Dental Surfaces In the incisors, the occlusal surface is reduced to a chisel-like incisal edge or margin. One surface of each tooth faces the oral cavity while the other faces either the vestibule of the oral cavity or the buccal cavity.

Dental Surfaces That portion of the tooth facing the oral cavity is referred to as the lingual surface because of its relation to the tongue. If the opposite surface faces the vestibule, as in the case of the incisors and canine teeth, it is called the labial surface.

Dental Surfaces If the opposite surface faces the buccal cavity, as in the case of the premolars and molars, the surface is called the buccal surface. Except for the last molar, the other two free surfaces of each tooth are in contact (or nearly so) with adjacent teeth.

Dental Surfaces These are approximal surfaces. The approximal surface that faces toward the midline is the mesial surface. The approximal surface that faces away from the midline is the distal surface.

Dental Anomalies In adequate tooth development or excessive tooth development may result in the following anomalies. Anodontia, or the absence of teeth, results from failure of the tooth buds to form. Partial anodontia may be seen at normal sites of development in children with cleft palate.

Dental Anomalies Supernumerary teeth result when the dental lamina produces an excessive number of tooth buds. These teeth most often develop in the region of the upper central incisors.

Dental Anomalies Amelogenesis imperfecta is a dominant genetic trait that results in a thin layer of abnormal enamel, through which the yellow color of the dentin can be seen. The condition usually affects both primary and secondary dentition.

Dental Anomalies Dental fluorosis in found in persons whose drinking water has an excessively high fluoride content. The mottled enamel varies from small white patches to severe brownish discoloration.

Spatial Relationships of the Teeth In the normal skull, the maxillary arch has a slightly larger diameter and is longer than the mandibular arch. Given this difference, the normal relationship between the upper and lower teeth is that the upper arch overlaps and confines the lower arch in what is termed a maxillary overbite. The upper incisors and canines, and to a lesser extent the premolars, bite outside the lower teeth.

Spatial Relationships of the Teeth The normal relationship between the teeth of the upper and lower arches is such that each tooth is opposed by two teeth of the opposite arch. The lower arch molars are positioned one cusp (one-half tooth) ahead of the upper-arch molars.

Occlusion It is this relationship that forms the basis for classification of occlusion. Occlusion is the full meeting or contact, in a position of rest, of the masticating surfaces of the upper and lower teeth.

Centric Occlusion Centric occlusion refers to the mandible being central to the maxilla with full occlusal contact of the upper and lower teeth. With centric occlusion, the forces generated by the muscles of mastication are distributed over a large area of the alveolar bone.

Malocclusion In malocclusion, only a few teeth may touch, and the forces generated by the muscles of mastication are distributed over a much smaller area. In adulthood, malocclusions are the leading cause of loss of teeth.

Angle’s Classification In 1899, Angle proposed a system of classification of three main types of occlusions determined by observing the teeth in centric occlusion. A Class I (normal) occlusion has the upper teeth biting slightly ahead of the lower teeth.

Angle’s Classification In a Class I malocclusion, the molar relationship is normal, but the variation lies in the anterior portion of the dental arch where the teeth can be crooked, crowded, or turned.

Angle’s Classification A Class II malocclusion is the most common of the occlusal discrepancies and is found in 45% of the population. It occurs when the cusps of the first mandibular molars are behind and inside the opposing molars of the maxillary arch.

Angle’s Classification A Class II malocclusion results in an increased overjet, the appearance of a receding chin, and a decrease in lower facial height.

Angle’s Classification In a Class III malocclusion, the cusps of the first mandibular molar interdigitate a tooth or more ahead of the opposing maxillary incisors.

Angle’s Classification A Class III malocclusion results in the appearance of a prognathic jaw and an increase in facial height.

Crossbite Normally the mandibular teeth are overlapped by the maxillary teeth. A reversal of this relationship is called a crossbite, and it may involve a single tooth or an entire arch.

Open Bite Sometimes during the course of development, the anterior teeth fail to erupt sufficiently to reach the line of occlusion, or the posterior teeth have erupted past the normal line of occlusion. In either case, the condition known as open bite results. The anterior teeth are unable to approximate and a persistent space exists between them.

Closed Bite Conversely, a closed bite occurs when the upper teeth overlap the lower teeth too much, causing the lower incisors to press into the roof of the mouth when chewing