Teeth

Introduction and Key Concepts for Teeth

Teeth are prominent structures in the oral cavity. They can be divided into maxillary (upper) and mandibular (lower) teeth. The root of the tooth is surrounded by the alveolar bone (alveolar process or alveolar arch), which forms a socket to hold and support each tooth. The alveolar bone of the maxillary teeth is a part of the maxilla (upper jaw); the alveolar bone of the mandibular teeth is a part of the mandible (lower jaw). There are two sets of teeth: primary (baby) and permanent teeth. The primary teeth are eventually replaced by permanent teeth. In adults, there are 32 permanent teeth including two incisors, one canine, two premolars, and three molars on each of the four quadrants in the maxillary and mandibular arches (Fig. 14-7). Each tooth can be divided into three parts: the crown, the cervix (neck), and the root. The crown of the tooth is covered by enamel and extends above the gingiva (gum); the shape of the crown is unique in different types of teeth and is adapted to their functions. The cervix is also called the neck of the tooth and is the junction between the crown and root. The region where the enamel and cementum meet is called the cementoenamel junction (CEJ). The root of the tooth is covered by cementum and surrounded by the alveolar bone. A tooth has one or more roots; the apex is the end of the root. The apical foramen is a small opening where nerves and blood vessels enter and exit the dental pulp (Figs. 14-1 and 14-7).

TOOTH DEVELOPMENT (ODONTOGENESIS) stems from two different origins: ectodermal and neural crest–derived mesenchymal tissue. The oral epithelium derives from ectodermal tissue, which gives rise to the dental lamina, and later becomes the enamel organ. The dental papilla derives from mesenchymal tissue and gives rise to the future dental pulp. The enamel organ, dental papilla, and dental sac (follicle) as a whole are described as a tooth germ, which eventually forms a tooth. The initiation of tooth germs occurs along the oral epithelium of the maxillary and mandibular prominences (processes). Tooth germs undergo initiation phases (development of the dental lamina and initiation of tooth germs), morphogenesis phases (cell movement and formation of the shape of the tooth), and histogenesis phases (formation of the hard tissue and development of the tooth root). Although tooth development is a continuous process, odontogenesis can be divided into several stages based on the shape of the tooth germ and formation of the tooth structure. These stages include initiation, bud, cap, bell, and apposition (crown) stages. When the primitive oral cavity is forming, the first pharyngeal arch gives rise to the maxillary and mandibular prominences or processes (developing dental arches), which lead to the future upper and lower jaws (Fig. 14-9A).

1.Initiation stage: At about 6 to 7 weeks into development, some oral epithelial cells on the surface of the maxillary and mandibular prominences increase proliferation activity, become thicker, and invaginate into the underlying mesenchymal tissue to form the primary epithelial band (Fig. 14-9A).

2.Bud stage: At about 8 to 9 weeks, the primary epithelial band gives rise to the vestibular lamina and dental lamina. The vestibular lamina forms a cleft that becomes the vestibule between the cheek and tooth; the dental lamina forms a U-shaped structure and develops into tooth buds (enamel

organs and mesenchymal tissue), which become primary deciduous teeth (Fig. 14-9A). The development of permanent teeth comes from the secondary tooth buds, which sprout from the primary tooth buds.

3.Cap stage: At about 10 to 11 weeks, the enamel organ appears as a cap shape and the condensed mesenchymal cells beneath the enamel organ form the dental papilla (Fig. 14-9B).

4.Bell stage: At about 12 to 14 weeks, the enamel organ continues to grow into a bell shape, and cells of the enamel organ differentiate into four distinguishable cell layers: the outer enamel epithelium, inner enamel epithelium, stellate reticulum, and stratum intermedium. At the same time, the dental papilla grows and helps to form the shape of the tooth crown. Cells in the dental papilla are differentiated into outer and inner cell groups. The outer cells of the dental papilla will develop into odontoblasts that produce future dentin; the inner cells will develop into future dental pulp tissues (Fig. 14-10A,B).

5.Apposition (crown) stage: At about 18 to 19 weeks, cells of the tooth germ continue to differentiate; the inner enamel epithelial cells have become preameloblasts which induce the outer cells of the dental papilla to become odontoblasts and begin to produce the dentinal matrix at the tooth crown region. This material is called predentin, and after undergoing calcification will become dentin. When dentin is formed, it induces preameloblasts to differentiate and become active ameloblasts, which produce enamel. Thus, the production of both the enamel matrix and the dentin for the tooth crown has begun (Fig. 14-11A,B). When the two hard tissues, dentin and enamel, have formed and the shape of the crown is completed, the root of the tooth begins to develop. The tooth then starts to erupt into the oral cavity in order to make room for the tooth root to grow (Fig. 14-12A,B).

ENAMEL, DENTIN, AND DENTAL PULP are components of teeth. Enamel is the hardest tissue in the body. The basic morphological unit of enamel is the rod, also called the prism, which is composed of a head and a tail. The rods are arranged in a three-dimensional complex, perpendicular to the dentinoenamel junction (DEJ). They extend from the DEJ to the surface of the enamel. Enamel provides a seal for the dentin and makes a strong surface for chewing (Fig. 14-14A,B). The crown of the tooth is covered by enamel and the root of the tooth is covered by cementum. Dentin surrounds and forms the walls of the dental pulp. It is composed of numerous dentinal tubules, odontoblastic processes, and the dentinal matrix and is the second hardest tissue of the body (Fig. 14-15A,B). The central core of the tooth is occupied by dental pulp, which is made up of loose mucous connective tissue containing blood vessels and nerve fibers (Fig. 14-17A,B).

THE PERIODONTIUM refers to the structures surrounding and supporting the tooth root and includes the cementum, the PDL, and the alveolar bone. The cementum is a thin layer of hard tissue that covers the root dentin (Fig. 14-18A,B). The PDL is the dense fibrous connective tissue that attaches the cementum to the alveolar bone (Figs. 14-18C and 14-19A). The alveolar bone, also called the alveolar process, is part of the maxilla and mandible. The alveolar bone supports and protects the tooth root. It includes the alveolar crest, the alveolar bone proper, and supporting bone (Figs. 14-18C and 14-19A).

DEJ

Gingival

sulcus

Molars Premolars Canine Incisors

Alveolar

mucosa

Dentin

Neonatal line of dentin Predentin

Cementoenamel junction

Acellular cementum

Dental pulp

Nerve fibers and blood vessels

Periodontal ligament (PDL)

Alveolar bone

Cellular cementum

Apical

foramen

Alveolar bone (alveolar process)

Bone

Enamel

organ

dental papilla

1

Dental papilla

3

Cervical

loop

Bone

B

Figure 14-10B. Bell stage, cell layers. H&E, 37; insets 472

During the differentiation process of the enamel organ at the bell stage, four cell layers are distinguishable. (1) The inner enamel epithelium is a row of columnar cells that contain rich RNA and are much taller in the tip region where initial enamel formation will take place at the apposition stage. The inner enamel epithelium differentiates and later becomes ameloblasts. (2) The stratum intermedium is composed of two to three layers of squamous or cuboidal cells and is located between the stellate reticulum and inner enamel epithelium. Cells in this layer are rich in alkaline phosphatase, which assists in the inner enamel epithelium production of enamel. (3) The stellate reticulum is located within the enamel organ. It may also be present in the cap stage, but is more developed in the bell stage. The cells of the stellate reticulum are star shaped and have many cellular processes interconnected with one another to form a network within the enamel organ. The extracellular spaces of the stellate reticulum are large and filled with a fluidlike, fluffy material. These cells contain glycosaminoglycans and alkaline phosphatase and have desmosomes and gap junctions between the cells. The stellate reticulum plays a role in maintaining tooth shape and protecting underlying dental tissue. (4) The outer enamel epithelium is composed of cuboidal cells and is the outermost layer of the enamel organ. This cell layer separates the enamel organ from the nearby mesenchymal tissues. It serves as a protective barrier and also helps maintain the shape of the enamel organ. The inner enamel epithelium and outer enamel epithelium meet to form the cervical loop.

Figure 14-11B. Apposition (crown) stage, amelogenesis. H&E, 19; insets (right) 104; inset (left) 354

The formation of crown dentin induces preameloblasts to differentiate and become ameloblasts. Active ameloblasts are columnar cells, and their nuclei are located toward the stratum intermedium. They actively secrete enamel matrix with assistance from the stratum intermedium. The newly secreted enamel matrix, in contact with the dentin matrix, forms the DEJ. At the same time, the ameloblasts and the odontoblasts retreat from the DEJ as deposition of the matrix proceeds. Enamel formation moves outward (toward the enamel organ), and dentin formation moves inward (toward the dental pulp). The process of enamel formation is called amelogenesis. During amelogenesis, conical processes, known as Tomes processes, are developed at the secretory (apical) surface of the ameloblasts. In the apposition stage, the two types of hard tissues (dentin and enamel) begin to form at the tooth crown. These two types of hard tissues are formed in a regular rhythm.

Figure 14-12A. Tooth root development. H&E, 29; (upper) inset 160; (lower) inset 90

Root development begins after the formation of the crown has been completed. It involves three structures: the epithelial root sheath, the dental sac (follicle), and the dental papilla. Dentin formation proceeds from the crown to the root. The formation of the root begins at the epithelial root sheath (Hertwig epithelial root sheath), which develops from the cervical loop (Fig. 14-11A). The inner and outer enamel epithelia of the cervical loop extend to form the root sheath. The epithelial root sheath grows around the dental papilla. It bends at a 45-degree angle and is called the epithelial diaphragm (Fig. 14-11B). The epithelial diaphragm gradually encloses the dental papilla with the exception of the apical foramen. The root sheath is important in forming the shape of the root. It induces the outer cell layer of the dental papilla to differentiate into odontoblasts, which produce the root dentin. When the root dentin has formed, the mesenchymal cells from the dental sac come in contact with the surface of the root dentin and induce these cells to differentiate into cementoblasts, which produce cementum. Fibroblasts, which differentiate from the dental sac, begin to form the PDL.

Figure 14-12B. Tooth eruption. H&E, 14; inset 5

As root length increases, additional space is needed for root growth, and the tooth gradually moves and erupts into the oral cavity. Tooth eruption and root growth, therefore, occur at the same rate. The formation of the tooth root includes root dentinogenesis (formation of root dentin), cementogenesis (formation of cementum), pulp formation, formation of the PDL, and alveolar bone development. The crown of the tooth passes through the bony crypt, dental epithelium, and oral epithelium to emerge into the oral cavity. As the tooth moves vertically, the overlying bone gradually becomes absorbed by osteoclasts. The dental epithelium (outer enamel epithelium, stellate reticulum, stratum intermedium, and ameloblasts) is compressed into a thin layer called the reduced enamel epithelium (REE). The oral epithelium fuses with the REE and gradually degenerates, enabling the tooth crown to erupt into the oral cavity. The initial junction epithelium is created during tooth movement and eruption; later it becomes the junction epithelium (Fig. 14-1), forming a seal between the gingiva and enamel. In a regular H&E stain, highly calcified enamel disappears because of the decalcification process during tissue preparation.

C

Tooth crown

Abrupt bend Tooth root

Figure 14-12C. Dilaceration.

Dilaceration is a developmental anomaly of the tooth in which there is an abrupt bend between the crown and the root of a tooth. Trauma to the primary predecessor tooth and developmental disturbances to the primary tooth germ are the possible causes of this condition. Dilaceration can happen in any tooth, but is more common in mandibular third molars, maxillary second premolars, and mandibular second molars. Visual examination can find a crown dilaceration, but the x-ray is the most appropriate diagnostic tool to diagnose a root dilaceration. Dilacerations may be associated with some syndromes, such as Smith-Magenis syndrome, a chromosomal disorder that produces a set of characteristic physical, behavioral, and developmental features. Dilacerations can complicate cavity preparation, root canal preparation, and other treatments.

A

Figure 14-13A. Gemination, Incisor.

Gemination is a variation in tooth structure that occurs when two teeth develop from the same tooth bud, resulting in a larger than normal tooth crown, but with a normal number of teeth. The gemination results from a disturbance that occurs at the cap stage. During the cap stage, the tooth germ is undergoing early morphogenesis and beginning to form the shape of the tooth. Gemination represents the unsuccessful division of a single tooth germ into two tooth germs. There is a deep cleft on

Cleft the surface of the tooth, giving the appearance of two crowns. The radiograph at left shows two crowns (arrows) sharing one root. Occasionally, x-ray images may show two independent pulp chambers and root canals. The cause of gemination is unknown.

B

Figure 14-13B. Amelogenesis Imperfecta.

Amelogenesis imperfecta is a group of hereditary disorders that affect the dental enamel formation at the apposition and maturation stages. The teeth are covered with a thin, defective enamel or lack enamel covering. Mutations in different genes may affect enamel proteins, such as amelogenin, which is involved in enamel mineralization. The most common forms of amelogenesis imperfecta include hypoplastic and hypocalcified types. In the hypoplastic form, the teeth do not have a sufficient amount of enamel, whereas in the hypocalcified form, the quantity of enamel is normal, but it is soft and easily worn. Affected teeth are most often yellow-brown and blue-gray in color, and are vulnerable to dental caries, damage, and loss. Full crowns will improve the appearance of the teeth and protect the teeth from damage.

C

Enamel pearl

Figure 14-13C. Enamel Pearl.

Enamel pearl is an ectopic formation of enamel that is usually found close to the CEJ between tooth roots. Localized failure of

Hertwig epithelial root sheath to separate from the dentin may be the cause of this anomaly. The enamel pearl results from a disturbance during the apposition or maturation stages during the tooth development. An enamel pearl may lead to periodontal disease because of the extension of a periodontal pocket. Histologically, enamel pearls have reduced enamel epithelium and are surrounded by normal cementum. X-rays can reveal enamel pearls. Enamel pearls cannot be removed by scaling, but they can be ground away to restore the normal shape of the tooth. An enamel pearl on a molar is shown here.

DEJ

Enamel

B

Figure 14-14B. Enamel structures, tooth. Ground specimen, 68; inset (upper) 13; inset (lower) 62

The incremental growth lines of the enamel, also called the striae of Retzius, can be found in either cross (bands) or longitudinal sections (arcs) of the mature enamel. These patterns reflect the changes in enamel secretory rhythm. The neonatal line (see Fig. 14-7) is much more prominent than the striae. It is a landmark that indicates the transition from enamel produced before birth and enamel produced after birth. This line is produced by a metabolic change that occurs at birth. There are three defects of enamel: (1) Enamel tufts are hypomineralized areas filled with organic material at the DEJ and toward the surface; (2) enamel lamellae are hypomineralized, thin, sheetlike defects that can run through the entire enamel and are commonly caused by cracks; and

(3) enamel spindles are thin, needlelike lines extending from the DEJ to the enamel and are because of odontoblast processes trapped in the enamel during early amelogenesis.

C

White streaks

Figure 14-14C. Enamel Fluorosis.

Enamel fluorosis is the general term for enamel changes caused by excessive fluoride intake during tooth development (before age 8 years). Fluoride helps to prevent and control tooth caries, but too much fluoride intake during tooth development can result in hypomineralization of the enamel surface. These changes are characterized by diffuse, opaque, and white streaks that run horizontally across the enamel. Stains with rough, irregular enamel surfaces are common. A fluoride-induced toxic effect on ameloblasts during enamel formation is believed to be the mechanism of the condition. Prevention is through controlling the fluoride intake, and treatments include bleaching and enamel microabrasion.

B

Figure 14-15B. Dentin and dentinal tubules, tooth. H&E, 35; inset

359

Dentin is produced by odontoblasts, and the formation of dentin is continuous throughout life. During the dentinogenesis process, the cell bodies of the odontoblasts retreat, but their cytoplasmic processes remain and are embedded in the mineralized dentinal matrix. Each odontoblastic process resides in a narrow channel, the dentinal tubule, which is lined by highly calcified peritubular dentin and traverses the entire thickness of the dentin layers. The dentinal matrix, between the dentinal tubules, is called intertubular dentin and is less mineralized than the peritubular dentin. The dentinal tubules run from the dental pulp surface to the DEJ. These tubules are larger close to the pulp and more branched near the DEJ. Each dentinal tubule contains an odontoblastic process about halfway toward the DEJ; the rest of the space is filled with fluid. Nerve fibers for pain extend from the dental pulp to the inner part of the dentinal tubule.

Under certain circumstances, such as in sensitive teeth, the exposure of dentin to air blasts or drinking or eating cold, hot, or sweet substances will cause movement of the fluid in the dentinal tubules. This movement produces a mechanical disturbance that activates the nerve endings of pain fibers, inducing intense pain (hydrodynamic theory).

CLINICAL CORRELATION

Irregular dentinal tubules

Figure 14-15C. Dentinogenesis Imperfecta. H&E, 329; inset (lower) ×2

Dentinogenesis imperfecta is an autosomal dominant genetic disorder of tooth development caused by mutations in the dentin sialophosphoprotein gene. Affected teeth are often blue-gray or yellow-brown in color and translucent. Dentinogenesis imperfecta is classified as types I–III. Type I is associated with osteogenesis imperfecta, in which bones are brittle and easily fractured and pulp chambers are diminished. Type I collagen defects are also associated with type I dentinogenesis imperfecta. Type II is characterized by diminished pulp chambers and may be associated with progressive hearing loss. Type III has large pulp chambers. Patients suffer frequent enamel fractures and enamel attrition. Full crowns will improve the appearance of the teeth and protect the teeth from damage. Histologically, dentinal tubules are irregular and are larger than normal in diameter, and uncalcified matrix may be present.

CLINICAL CORRELATIONS

A

Calcification defect in dentin

Figure 14-16A. Vitamin D–Resistant Rickets. H&E, 476 Vitamin D–resistant rickets, commonly known as X-linked hypophosphatemic rickets, is characterized by resistance to conventional vitamin D treatment, decreased reabsorption of phosphate by the renal tubules, abnormalities in bones and teeth, osteomalacia, and hypocalcemia. It is an X-linked autosomal dominant disorder. Patients with these rickets disorders do not respond to high-dose vitamin D treatment. Signs include rickets, short stature, bowing of lower limbs, seizures, congestive cardiac failure, and tooth defects including calcification defects in dentin, enlarged pulp chambers, pulpitis, and pulp necrosis. Treatment with 1, 25-vitamin D (which is not dependent on normal hormonal mechanisms or organ systems to activate) plus controlled phosphate therapy may improve the condition.

B

Globules of dentin

Dentin

Broad

root

Figure 14-16B. Dentin Dysplasia. H&E, 35; inset 7

Dentin dysplasia is an autosomal-dominant tooth disorder. Teeth in this disorder are sometimes called rootless teeth, because they often have very short and conical roots. Dentin dysplasia can be classified into types I and II. Type I, also called the radicular type, is characterized by short roots and pulp obliteration. Type II, also called the coronal type, is characterized by a “thistle tube” pulp. Radiographically, extension of the pulp chamber into the root is usually observed in dentin dysplasia. Pulp stones and sudden constriction of the pulp chamber are common. Dentin dysplasia type II shares some similarities with dentinogenesis imperfecta type II, but in dentin dysplasia the permanent dentition has normal color or only slight discoloration. Histologically, globules of dentin and disorganized dentinal tubules are characteristic of this condition. The left image shows globules of dentin; the right image shows a low-power photograph of a dentin dysplasia.

A Dentin

Odontoblasts

Predentin

Figure 14-17A. Dental pulp. H&E, 136

Dental pulp is a specialized loose, cellular mucous connective tissue that fills the pulp chamber in the central core and root canals of the tooth. Fibroblasts are the most numerous cells in the pulp; they produce connective tissue fibers (mainly type I and III collagen, fibronectin, and elastin) and ground substance. They maintain the pulp matrix. The second most numerous cells are odontoblasts (producing dentin). Other defense cells, such as macrophages and lymphocytes, may be found in the pulp. No mast cells or adipocytes (fat cells) are found in the pulp. The pulp contains many blood vessels and nerve fibers, which enter and leave at the apical foramen. Most of the nerve fibers are afferent fibers; they are either C fibers or Ad fibers, and both carry pain information through the trigeminal system to the brain. The efferent fibers are autonomic nerve fibers and innervate the smooth muscle of the blood vessels. Dental pulp plays an important role in producing dentin and providing nutrients and sensory input for the dentin.

Dental pulp

1. Odontoblast

layer

3. Cell-rich zone

Dentin

Figure 14-17B. Dental pulp. H&E, 136; inset 190

Dental pulp is derived from mesenchymal tissue that forms the dental papilla during tooth development. The papilla becomes pulp in the mature tooth. The pulp can be divided into four zones from the periphery to the center. (1) The odontoblast layer forms a single cell layer along the peripheral edge of the pulp. These cells have processes extending into the dentin. (2) The cell-free zone, also called the zone of Weil, is directly under the odontoblast layer. This layer has fibers, cellular processes, axons, and capillaries running through it but contains no cell nuclei. (3) The cell-rich zone is beneath the cell-free zone, and has many cells and nuclei of cells densely packed in rows. This layer has fibroblasts, undifferentiated mesenchymal cells, neural plexuses, and capillaries. If some odontoblasts die, the undifferentiated mesenchymal cells in this layer will differentiate into new odontoblasts. (4) The pulp proper (pulp core) is the central part of the pulp and contains blood vessels and nerves within the loose, mucous connective tissue. The layers of both cell-free and cell-rich zones are more visible in the coronal than the radicular pulp region (Fig. 14-17A).

CLINICAL CORRELATION

C

Pulp cavity

Abscess with neutrophils

Figure 14-17C. Pulp Abscess. H&E 23; inset 168

Pulp abscess refers to an abscess involving the pulp tissue of a tooth. Pulp abscesses are usually sequelae of dental caries, but they can also develop in teeth showing no detectable lesions. Pulp abscesses can also occur after restoration work has been performed. They are characterized by severe, intermittent pain that may intensify when a patient reclines. Sharp pain may also be triggered by cold liquids, tapering off to a dull, pulsating pain. Periapical tissues may not be involved, and the affected tooth may not show any difference from other healthy teeth in percussion or pressure tests. Treatment includes using antibiotics and undergoing root canals and even tooth extraction.

Figure 14-18A. Acellular cementum, cervical region of tooth root. H&E, 181; inset 158

The periodontium includes the cementum, the PDL, and the alveolar bone. These structures surround and support the tooth in the tooth socket. Cementum is a thin layer of hard tissue (calcified matrix) that does not have a direct blood supply. It covers the dentin and seals the dentinal tubules at the root region. Cementum is thicker at the root apex than at the cervix region and can be divided into acellular and cellular cementum. Acellular cementum has no cementocytes embedded within the matrix and is often found at the cervical two thirds of the root, attached to the dentin. The junction between the dentin and cementum is called the dentinocemental junction (DCJ), and the junction between the cementum and enamel is the CEJ. There are three forms of the CEJ: (1) the cementum overlaps the enamel (60%); (2) the cementum meets the enamel end-end (30%); and

(3) there is a gap between the cementum and the enamel (10%).

Figure 14-18B. Cellular cementum, apical region of tooth root. H&E, 181; inset 408

Cellular cementum is often found at the apical third of the tooth root and is similar to bone with a calcified intercellular matrix. During the process of formation of cementum some cementoblasts are trapped in the matrix, and each one is surrounded by a lacuna. These cementoblasts then become cementocytes. The matrix of cellular cementum is deposited more rapidly than that of acellular cementum. Acellular cementum has a slower formation rate and formation continues throughout life. The slower growing cementum allows fibers (Sharpey fibers, left inset) from the PDL to become trapped in the matrix of the cementum to form the tooth attachment. Cementum is much more resistant to reabsorption than bones. This characteristic helps maintain root integrity and prevents exposure of the root dentin, which is important as teeth are moved around during orthodontic procedures.

Figure 14-18C. PDL and alveolar bone. H&E, 34; inset 72

The PDL is a dense fibrous connective tissue with a direct nerve and blood supply. It is located between the cementum and the alveolar bone, which surrounds the tooth root. Fibroblasts are the main cells responsible for the formation of the PDL. The PDL supports the tooth root by forming a strong attachment between the cementum and alveolar bone by Sharpey fibers (Fig. 14-19A). The functions of the PDL are to provide sensory information regarding pain and pressure, to provide signals to the alveolar bone regarding bone remodeling, and to maintain tooth position in the dental arch. The alveolar bone (also called the alveolar process) surrounds and supports the tooth root and is part of the maxilla and mandible. It is constantly remodeled and is built up by osteoblasts and absorbed by osteoclasts. Alveolar bone has the general characteristics of bones. It consists of both compact and cancellous bones (see Chapter 5, “Cartilage and Bone”).

CLINICAL CORRELATION

B

Alveolar bone

Junction between alveolar bone and dentin (absence of PDL)

Dentin

Figure 14-19B. Tooth Ankylosis. H&E, 68

Tooth ankylosis is the fusion of a mineralized root surface to its surrounding alveolar bone. It is characterized by a typical metallic percussion sound and loss of the PDL space in radiographs. Periapical inflammation and subsequent tissue repair are believed to be the cause of ankylosis. Ankylosis is more common in deciduous teeth and usually results in impaction of a subjacent permanent tooth. Impaction of incisors is much less common than it is in the mandibular third molars. This slide shows impaction of a central incisor with focal loss of the PDL the incisor.