Teeth

Learning Objectives

Describe the components that comprise the periodontium.
Describe the structural components of the tooth.
Predict the possible functional outcome to a patient following extraction of the four maxillary incisors.
Compare the nerve supply of the gingiva to that of the pulp for the mandibular dental arch.
Explain which nerves should be anesthetized for a maxillary third molar extraction.
Explain the types of sensory receptors found in the PDL and their functional role.
Describe the types of afferent nerve fibers found in the pulp.
Explain the trigeminal path followed by the low-threshold mechanoreceptor (Aβ and Aδ) following mechanical or thermal stimulation to the pulp.

Anatomical and Structural Components of Teeth

Knowledge of the specific distribution of the nerves supplying the dental arches and oral cavity is important for understanding the basis of orofacial pain, assessing nerve damage of the oral tissues, and determining the appropriate nerve to anesthetize for different dental procedures. The following chapter describes the anatomical and structural components of the teeth along with the innervation to the pulp and periodontium. The trigeminal pathway for pain, temperature, mechanosensory, and proprioceptive input is briefly reviewed. A detailed description of central trigeminal pathways is covered in Chapter 13.

Overview of Teeth and Dental Arch

Humans have two sets of teeth: the primary (deciduous) dentition and secondary (permanent; succedaneous) dentition.
- A total of 20 deciduous teeth develop prenatally within the bony sockets (alveoli) of the mandibular and maxillary dental arches. The eruption of the primary teeth begins postnatally in the sixth month and is complete by 3 years of age.
- The complete primary dentition consists of 10 teeth in each dental arch, with each side of the arch (quadrant) containing 3 basic tooth forms: 2 incisiform (incisors), 1 caniniform (canines), and 2 molariform (molars). The incisors, canines, and molars function in biting, tearing, and grinding, respectively.
- The primary teeth that function between ages 2 through 6 years are gradually shed (exfoliated) and replaced by the permanent teeth between 6 and 12 years of age ().
The complete permanent dentition contains 32 teeth in total, with 16 in each jaw. The teeth are sequentially numbered, 1 to 32, in a clockwise direction, beginning with the upper right third maxillary molar as tooth 1 and continuing to the lower right third mandibular as tooth 32 ().
Each quadrant contains a similar arrangement of the incisors, canines, and molars as the primary dentition with two notable exceptions ().
- Each half of the maxillary and mandibular dental arch contains two premolar (bicuspids) teeth that function in piercing foods. The premolars are unique to the permanent dentition and replace the two deciduous molars.
- There are three permanent molars in each quadrant. The permanent molars are considered accessional teeth that lack deciduous predecessors and develop distal to the primary dentition.
For reference, several terms are used to describe the external surface of the tooth and the regions of the tooth relative to the arch ( and ).

Fig. 25.1 Deciduous and permanent teeth of a 6-year-old child. The anterior bone was removed above the roots of deciduous teeth. Frontal view of the (a) maxillary and (b) mandibular arch illustrating the erupted deciduous teeth and the position of the developing crowns of the anterior permanent teeth and permanent premolars. The permanent incisors and canines are positioned palatal and lingual to the primary teeth. Note the crowns of the two permanent premolars in both arches are visible in the mandibular arch between the roots of the primary molars. By 6 years of age, all deciduous teeth have erupted and are present, along with the first permanent tooth, the first permanent molar. Note the erupted permanent mandibular molar (6-year molar) with a portion of the root developed is visible. Root formation continues after tooth eruption and growth of the jaw. (Reproduced with permission from Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy Second Edition, Vol 3. ©Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)

Fig. 25.2 Universal numbering system of adult teeth; numbering shown from the perspective of the viewer. The universal numbering system is commonly used in the United States to designate the position of the permanent teeth within the dental arch. Teeth in both arches are numbered sequentially, 1 to 32, in a clockwise direction, beginning with the upper right maxillary third molar as number 1 and continuing to the left maxillary molar (tooth 16). The numbering of the lower arch continues clockwise from 17 to 32. (Reproduced with permission from Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy Second Edition, Vol 3. ©Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)

Fig. 25.3 Arrangement of permanent teeth within the maxillary (upper) and mandibular (lower) dental arches. (a) Inferior view of the maxilla. (b) Superior view of the mandible. Chewing surfaces are shown on the left side. Each arch contains 16 teeth arranged bilateral and symmetrical within two quadrants. Each quadrant contains 2 incisors, 1 canine, 2 premolars, and 3 molars. The alveolar process after removal of teeth can be seen on the left side. Support for the tooth is provided by the periodontal ligament and the components of the alveolar process, which include an outer cortical plate, intervening spongy bone, and the alveolar bone proper, which lines the dental alveoli (tooth sockets). The interalveolar septum lies between the adjacent alveoli, and an inter-radicular septum is present in the alveoli anchoring multirooted teeth. (Reproduced with permission from Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy Second Edition, Vol 3. ©Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)

Surfaces of tooth and regions of dental arch

Surfaces of tooth and regions of dental arch
Surface of tooth within jaw	Description of region
Occlusal	Part of tooth that is in contact with tooth of the opposite jaw; serves as the chewing/grinding surface In proper occlusion, every tooth contacts two opposing teeth Teeth are offset from each other so that the cusp of one posterior tooth fits into the fissures of the two opposing teeth
Incisal	The cutting edge of the anterior teeth
Lingual	Tooth surface adjacent or closest to the tongue
Palatal	Region adjacent to the palate of the maxillary arch
Buccal	Adjacent closest to the cheek (vestibule); term used for posterior teeth (premolar and molars)
Labial/facial	Adjacent or closest to the mucosal surface of the lip used for anterior teeth (canines and incisors)
Distal and mesial	Side of the adjacent teeth within the same jaw The mesial surface of the tooth is oriented anteriorly toward the midline of the arch; distal surface is farthest from the midline of the arch

Fig. 25.4 Designation of tooth surfaces and directions of the dental arches. (a) Inferior view of the maxillary dental arch. (b) Superior view of the mandibular dental arch. Buccal, distal, and occlusal views of the right mandibular first molar (tooth 30). The coronal, apical, and cervical directions of a tooth and the approximal surfaces, which contact, or face, the adjacent teeth are also depicted. These designations are used to describe the precise location of small carious lesions. (a: Reproduced with permission from Baker EW. Anatomy for Dental Medicine. Second Edition. © Thieme 2015. Illustrations by Markus Voll and Karl Wesker. b: Reproduced with permission from Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy Second Edition, Vol 3. ©Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)

Anatomical and Structural Components of the Tooth

All teeth consist of two anatomical regions, a crown and root, and are anchored similarly within the dental arch by the supportive structures of the periodontium (; and ).
The crown is the portion of the tooth visible in the oral cavity and may be subdivided into three regions: an incisal one-third (anterior teeth) or occlusal one-third (posterior teeth), a middle one-third, and a cervical one-third.
- Enamel, a highly mineralized layer comprised of tightly packed, inorganic hydroxyapatite crystals, covers and protects the external surface of the crown.
The root lies below the oral surface, within a bony socket (alveolus) of the maxillary and mandibular jawbones. The root also contains three subdivisions, the cervix, the body, and the root apex.
- Cementum, a mineralized layer similar in composition to bone tissue, covers the external surface of the root.
The cervix (neck) of the tooth refers to the junction between the crown and the root of the tooth. The body of the tooth contains dentin, a mineralized tissue layer composed of numerous fluid-filled dentinal tubules. Dentin provides structural support to the overlying enamel and cementum and surrounds the pulp cavity.
The mineralized regions of the crown and the root surround and protect an inner core of richly innervated and well-vascularized loose connective tissue (CT), known as the dental pulp. The pulp tissue extends from the crown to the root and resides within an anatomical space known as the pulp chamber and root canal, respectively.

Structural components of the tooth

Structural components of the tooth
Structural components: external coverings of the tooth
Enamel	Hardest mineralized tissue in body covers the external surface of tooth crown Thin, translucent layer comprised of 95% inorganic hydroxyapatite crystals; crystals arranged parallel to each other and perpendicular to tooth surface Produced during crown stage of tooth development prior to tooth eruption Cellular layer of formative which synthesize enamel degenerate following crown formation Devoid of neurovascular and cellular structures Enamel is incapable of repair Remineralization possible due to ionic composition and buffering capacity of saliva
Cementum	Mineralized tissue covering the external surface of the root Principal function is to connect the alveolar bone with tooth via the gingival and periodontal ligament Similar in composition to bone tissue; comprised of (45–55%) inorganic hydroxyapatite crystals Produced initially during root formation (prefunctional eruption); continued deposition over life of tooth to maintain tooth position; cellular layer of formative cementoblasts that lines the external root surface Devoid of neurovascular structures Limited repair; no remodeling
Structural components: body of the tooth
Dentin	Mineralized layer of tissue that forms the bulk of the tooth; provides structural support to the overlaying enamel and cementum Surrounds and protects the centrally located pulp tissue; defines the boundary of pulp cavity Dentin production starts in the crown stage, continues throughout the life of the tooth; a cellular layer of formative odontoblasts remains along the pulpal border throughout the life of the tooth and contributes to the dentin–pulp complex Comprised of 70% inorganic hydroxyapatite crystals; characterized by numerous fluid-filled dentinal tubules that run through the entire thickness of the dentin Dentinal tubules contain dentinal fluid and the cellular part of odontoblast known as the odontoblastic process. Aδ afferent nerve fibers extend into the initial portion of the dentin tubules Odontoblasts may deposit dentin in response to dental caries or other potentially damaging insult to the pulp Exposed dentin caused by loss of the overlying enamel or cementum leads to dentin sensitivity
Pulp cavity	Central space of the tooth containing neurovascular loose connective tissue (CT) known as dental pulp tissue Pulp follows the contour of tooth and consists of two regions: a pulp chamber located in the crown and a root (pulp) canal located in the root Areas of pulp tissue within pulp chamber extend into cusps to form pulp horns The number of root (pulp) canals varies based on tooth type Apical foramen is the distal opening of pulp cavity at the root apex; permits passage of neurovascular structures from the periodontium to the pulp
Dental pulp	Neurovascular loose CT) required to maintain vitality of the tooth; fibroblasts are the primary cell type; pulpal stem cells present; odontoblasts present in the periphery of the pulp and comprise the pulpal border Pulp tissue in the region of the crown or the pulp chamber is known as the coronal pulp; pulp tissue in the root or root canal is known as the radicular pulp Intradental nerve fibers include myelinated low-threshold mechanoreceptors (type IIAβ, and type IIIAδ fibers), nociceptors (myelinated type IIIAδ fibers and unmyelinated type IVC fibers), postganglionic unmyelinated sympathetic C fibers Communicates with the periodontal ligament (PDL) in the periodontal space through the apical foramen; potential mechanism for spread of infection

Summary of anatomical regions of the tooth

Summary of anatomical regions of the tooth
Anatomical regions of tooth
Crown	Anatomic crown: portion of the tooth covered by enamel; extends from the occlusal or incisal surface to the cervix of the tooth: Divided into three regions: incisal one-third (anterior) or occlusal one-third (posterior), middle one-third, and cervical one-third Clinical crown: portion of the crown that is visible in the oral cavity
Root	Clinical root: portion of the tooth that lies below the oral surface, within a bony socket (alveolus) of the maxillary and mandibular jaw bones Anatomic root: the portion of the tooth covered externally with cementum: Divided into three regions: the cervix, the body, and the root apex Apical foramen located at the root apex transmits the neurovascular structures into pulp cavity; forms open communication between pulp cavity and periodontal connective tissue
Cervix (cervical line)	Represents the anatomical boundary between the crown and the root Cervical line demarcates the cementoenamel junction (CEJ), a point where the enamel and the cementum meet. Areas in the tooth cervix may be devoid of cementum, resulting in the exposure of the underlying dentin and may cause dentin sensitivity, and increased risk of caries

Fig. 25.5 Structures and anatomical regions of the tooth. The longitudinal section of the mandibular incisor. Enamel covers the crown of the tooth from the cusp to the cervix (neck). The cementum covers the external surface of the root and anchors the tooth to the alveolus via the periodontal ligament. The dentin, which supports the overlying enamel and cementum, and surrounds the pulp cavity, forms the body of the tooth. The apical foramen is the opening at the tooth apex that permits the passage of the neurovascular structures from the periodontal tissue to the pulp cavity. (Reproduced with permission from Schuenke M, Schulte E, Schumacher U. THIEME Atlas of Anatomy Second Edition, Vol 3. ©Thieme 2016. Illustrations by Markus Voll and Karl Wesker.)

Periodontium

The periodontium refers to the tissue that supports the teeth in the upper and lower dental arches. The structures of the periodontium include cementum, periodontal ligament (PDL), alveolar bone, and gingiva ().

Overview of periodontium

Overview of periodontium
Periodontium
Periodontal ligament (PDL)	Dense connective tissue (CT) organized as distinct fiber bundles that are embedded in the cementum matrix and the portion of the alveolar bone forming the osseous tooth socket Prevents force applied to the tooth from being directly transmitted to the alveolar process Provides mechanosensory input concerning masticatory bite force, jaw position, along with the direction, intensity, and velocity of the occlusal forces applied to the crown of the tooth. Comprised primarily of fibroblasts that continually remodel the PDL; abundant neurovascular structures run through the PDL: Interdental nerves fibers and receptors include myelinated type IIAβ (mechanoreceptors) and type IIIAδ (nociceptors) fibers and unmyelinated type IVC (nociceptors) and C (postganglionic sympathetic) fibers
Cementum	Mineralized tissue covering the external root surface. Cementum matrix embeds bundles of collagen fibers (Sharpey’s fibers) of the PDL and anchors the tooth
Alveolar process (alveolus)	Components of the alveolar process include an outer cortical plate, intervening spongy bone, and the alveolar bone proper (alveolus), which is a region of compact bone lining the tooth socket The alveolus (osseous tooth socket) is the portion of the mandible and maxilla in which the dental roots are embedded. Socket lined with periosteal CT (periosteum) containing neurovascular structures and stem cells Fibers of the PDL extend from the cementum of the root and insert into the alveolus via Sharpey’s fibers to anchor the tooth It contains neurovascular structures that anastomose with PDL, gingiva and pulp Bone capable of extensive repair and remodeling, and functions to maintain the tooth in the occlusal position The presence of a tooth is necessary to maintain the alveolar process
Gingiva (gums)	A region of oral mucosa that surrounds and protects the tooth. Subdivided into an attached gingiva and free gingiva. Free (marginal) gingiva comprises a 1-mm cuff of unattached tissue surrounding the tooth. The medial surface of the free gingiva is anchored to the cervical enamel through the junctional epithelium Attached gingiva binds the underlying alveolar periosteum and supports the crown of the tooth Neurovascular structures similar to PDL; extensive anastomoses with vessels of PDL, and alveolar bone and pulp Gingival nerve fibers and receptors similar to oral mucosa and include: myelinated type IIAβ (low-threshold mechanoreceptors) and type IIIAδ fibers (nociceptors) and unmyelinated type IVC fibers (nociceptors) and type C fibers (postganglionic sympathetic) Gingival sulcus: space between the enamel of the tooth and gingival margin. The depth of the sulcus varies, but it is normally 0.5 to 1.5 mm above the base of the sulcus