Cysts of Jaw and Oral Cavity Nonepithelialized cysts of the mandible or maxilla may result from trauma with intermedullary hemorrhage, or they may be manifestations of monostotic and polyostotic fibrous dysplasia (disseminated or localized osteitis fibrosa) and generalized osteitis fibrosa, also called cystic osteodystrophy (von Recklinghausen disease). Because the latter conditions are systemic disorders of the bones or endocrine system (primary or secondary hyperparathyroidism), they will not be discussed in this volume. The lesions are more often solid than fluid in content and are recognized as cysts chiefly by their radiographic appearance. The epitheliated cysts of the jaws are etiologically divided into radicular, follicular, and facial cleft cysts. The periapical or radicular cyst has an inflammatory basis and evolves from a granuloma at the root apex of a tooth devitalized by caries or trauma. Bacteria and toxins of the infected pulp canal stimulate proliferation of epithelial remnants left in the periodontal membrane from Hertwig sheath, after it has been ruptured and fragmented by the developing tooth. Eventually, this epithelium lines the necrotic center of the granuloma and thickens, tending to isolate the inflammatory process. A fibrous capsule develops outside the epithelial sac, and the lumen increases by transudation of fluid. Round cell infiltration of the cyst membrane, including the adjacent connective tissue and cellular debris, pus, macrophages, and cholesterol crystals are usually found histologically. Even if the tooth with the granuloma is removed, the cyst remains and may even expand more rapidly as a sterile lesion. The dentigerous or follicular cyst arises from the enamel epithelium of the dental follicle forming around the crown of an unerupted tooth. It is a benign, noninflammatory cyst thought to be developmental in origin. Infection is present only when contiguous teeth are coincidentally abscessed. The pathogenesis of follicular cysts begins with retrograde changes and edema in the enamel organ, which, by expansion, assumes various shapes above the developing crown. A simple follicular cyst forms before enamel is excreted, arresting tooth maturation or growing entirely separate from the tooth. A dentigerous cyst arises at a later phase, after amelogenesis, and gradually envelops the crown, thus interfering with eruption. The tooth may be forced by the cyst fluid to a site remote from its normal position in the jaw. A follicular cyst is usually unilocular, but multilocular forms occur. The most frequent location is the mandibular third molar region. Until the cyst attains a large size and expands the cortical plate of bone, the cyst may remain unrecognized. The buccal, palatal, or alveolar bone may then bulge outward, or the maxillary sinus or nasal cavity may be invaded. The wall of the cyst becomes parchment thin and yields a crackling sound on palpation. Due to the pressure of the cyst and the crowding of roots, the adjoining teeth are tilted when viewed on radiographic imaging and unerupted teeth are displaced in total. Teeth are not devitalized and resorption of roots is not encountered. A smooth, encapsulating layer of cortical bone, a unilocular shape, and the absence of root erosion distinguish a cyst from an invasive neoplasm, ameloblastoma, benign giant cell tumor, and osteitis fibrosa localisata or generalisata. However, a cyst secondarily infected from an adjacent tooth shows an obliterated capsule and appears infiltrative. Also, a few cysts may be multilocular and cannot be completely distinguished from ameloblastoma without biopsy examination. A layer of compact bone borders the cyst sac, which is composed of fibrous connective tissue lined with epithelium. The latter is stratified squamous, ranging from thin to a considerable thickness; in some cysts, it may be simple columnar. The fluid content is clear and straw colored, with an iridescent sheen imparted by cholesterol crystals, or thick and cheesy from epithelial and hemorrhagic debris. Facial cleft cysts, also called fissural cysts, form at the junction of the embryonic segments that fuse to make up the jaws. They may be found at the median fissure of the palate, maxillary bones or mandible (midline or median cyst), or nasoalveolar junction. The nasopalatine cyst, because it is formed in the incisive canal or the incisive foramen from remnants of the fetal nasopalatine communication, may be grouped with the facial cleft cyst. Irritation or mucous secretions likely play an etiologic role in the more superficial type. Ordinarily, a nasopalatine cyst presents no clinical swelling, unless it is very large, and is detected only on radiographic examination. A radiolucent area appears in the midline between the apices of the central incisor teeth and is often misinterpreted as a radicular cyst. The size varies from slight enlargement of the incisive fossa to one of a few centimeters, involving a higher part of the incisive canal. If retention of serous or mucoid secretion produces swelling in the region of the interincisive papilla, discharge from the two tiny orifices on the side of the papilla may be expressed. Secondary infection may occasionally ensue, producing a fluctuant swelling of the palatal mucosa, resembling a dentoalveolar abscess. Mucopurulent discharge then appears at the incisive outlets on pressure. In edentulous mouths, resorption of the alveolar ridge may expose a nasopalatine cyst that escaped previous detection. Cysts of the oral mucous membranes are retention cysts of the mucous glands or their ducts or, occasionally, the sublingual salivary gland. The mucocele may appear on the inner surface of the lips or cheeks, especially at a level parallel to the occlusal plane, where chewing injuries cause obstruction of the mucous ducts. A round, translucent, sometimes bluish swelling may range from a very minute size to a centimeter. In the tongue, the glands of Blandin and Nuhn may form more deeply seated mucoceles, which reach a considerable size, presenting a swelling on the anterior ventral surface. The ranula is named for its resemblance to a frog’s belly; the term is a rather loose one applied to cystic swellings of the floor of the mouth. A common error is to attribute a ranula to obstruction of the submandibular (Wharton) duct, which in the presence of a typical ranula is found patent. The most frequent cause is the occlusion of an excretory duct of the sublingual gland. In a few cases, a ranula may arise from the incisive glands in the midline or from the ciliated cysts that carry epithelium (Bochdalek “glands”), which are derived from the primitive thyroglossal duct. A typical ranula begins in the lateral anterior floor of the mouth immediately beneath the mucous membrane. It usually grows slowly, but rapidly developing cystic swellings (acute ranulas) are also known. The color is bluish gray, with numerous small vessels well outlined. Palpation gives a decided impression of fluid confined by an elastic membrane; the wall may rupture spontaneously but soon refills. As it reaches large proportions it crosses the midline and shows a division marked by the frenulum, also displacing the tongue and impeding speech. A ranula will sometimes bulge downward toward the chin because a portion of the sublingual gland perforates the mylohyoid muscle. Dermoid cysts, owing their origin to inclusion of ectodermal structures by the mesoderm during the embryonic development of the head, usually become visible only in the second or third decade of life. They are located in the midline beneath the chin or between the geniohyoid muscles deep in the floor of the mouth, or in a lateral position beneath the angle of the jaw. The cyst consists of a fibrous capsule lined with a stratified, squamous epithelial membrane, with a cheesy or semisolid mass containing sebaceous material and hair filling the lumen. Theoretically, teeth and other appendages may be present. The dermoid may reach large proportions and protrude in the neck or beneath the tongue as a soft or semifirm doughy lump that pits on pressure and is not fluctuant. The color tends to be waxy or yellowish when the mucosa is thin enough to reveal it.