Thomas L. Slovis

Embryology

At birth, the mandible consists of two lateral halves united in the midline at the symphysis by a bar of cartilage (Fig. 22-1, e-Fig. 22-2, and Figs. 22-3 and 22-4). Bony fusion of the symphysis usually occurs before the second year, but segments of the fissures may persist beyond puberty. The body of the mandible is large at birth compared with the relatively short rami and poorly differentiated coronoid and condylar processes. The rami form an angle of about 160 degrees with the body at birth.

Formation of the dental buds is visible by 20 weeks’ gestation. Beginning mineralization in the first molar tooth occurs between gestational weeks 33 and 34, and in the second molar tooth, it occurs between gestational weeks 36 and 37. In contrast to bone age, dental age is little affected by endocrine aberrations.

All or some of the teeth may be missing developmentally, as in persons with anhidrotic ectodermal dysplasia (Fig. 22-4).¹ Because infants and children with normal dentition seem to have a plethora of teeth in radiographs (deciduous and permanent dentitions), the condition may be recognized in a newborn by the lack of dental buds. Many diseases cause loss of dentition, and recognition of premature loss of teeth can lead to important diagnoses (Box 22-1).²

The opposite condition—too many teeth with little alveolar bone—can be seen in persons with cleidocranial dysplasia, who have marked delay in shedding of the deciduous teeth. Early tooth extraction has no effect on the subsequent eruption of the permanent dentition and may result in a lengthy edentulous period for the child.³

Anatomy

The mandible is the only freely movable bone of the face; it articulates with the temporal bone in the temporomandibular fossa anterior to the external auditory canal (see Fig. 22-3). The range of motion is free in all directions, and the condyle moves downward and forward in the articular fossa upon opening of the jaw.

The temporomandibular joint (see Fig. 22-3) is a complex joint in which a biconcave fibrous disk divides the articular space into upper and lower compartments.^4–6 Gliding movements occur in the upper compartment, whereas the lower compartment functions as a true hinge joint. The articulating bony surfaces are not covered by hyaline cartilage as in other joints, but by an avascular, fibrous tissue that is separated from the underlying bone of the condyle by growth cartilage.⁷

Diseases of the Mandible

Significant congenital malformations of the mandible are rare, with the most important being hypoplasia (micrognathia), which may be a cause of congenital stridor. The short, small mandible apparently causes a retrodisplacement of the tongue and obstruction to airflow (Fig. 22-5).

Micrognathia occurs in a variety of dysmorphic syndromes. The combination of cleft palate and hypoplasia of the mandible defines the Pierre Robin sequence radiographically. The Pierre Robin association is nonspecific and occurs with several genetic and drug-induced syndromes and some loosely associated anomalies, as well as an isolated symptom complex.⁸ In the cerebrocostomandibular syndrome, it is associated with posterior rib defects, cleft palate, and, occasionally, mental retardation.⁹

Elongation of the body of the mandible and widening of its arc occur in conditions associated with an enlarged tongue, including lymphatic malformations and Beckwith-Wiedemann syndrome. Relative enlargement of the mandible is observed in several craniofacial syndromes. The angle of the mandible is markedly increased in persons with pyknodysostosis. Rare cases of hyperplasia of the coronoid process are observed; the enlargement impinges on the zygomatic arch and interferes with normal opening of the jaw. The temporomandibular joints frequently are involved in persons with juvenile rheumatoid arthritis. Rarely, the mandible may be partially duplicated.

Temporomandibular Joint Disorders

Disorders of the temporomandibular joint are infrequent but not rare.10,11 Adventitious sounds on mandibular movement, muscle tenderness or pain, and deviation of the mandible during movement are the most common signs and symptoms. Magnetic resonance imaging (MRI) is the most precise imaging modality.^12,13

Fractures

Direct trauma is the usual cause of fractures, although pathologic fractures occur in association with cysts, destructive inflammations, and neoplasms. More than half of traumatic fractures are found in the body of the mandible near the canine fossa. Fractures high in the ramus frequently are overlooked on standard radiographs but are clearly defined by computed tomography (CT) (Fig. 22-6) and panographic tomography (e-Fig. 22-7). Direct trauma is the usual cause, although pathologic fractures occur in association with cysts, destructive inflammations, and neoplasms. More than half of traumatic fractures are found in the body of the mandible near the canine fossa.

Vehicular trauma is responsible for 35% of mandibular fractures in children, and falls cause 28%. More condylar fractures are found in younger children (<9 years). During childhood, the mandible contains teeth in different stages of eruption, and the pattern of the alveolar bone predisposes it to fractures along the lines of the developing dental crypts. The lack of cortical bone and the relative excess of cancellous bone are responsible for the frequency of incomplete, greenstick fractures with minimal destruction of the bone or cartilage. The condylar neck usually breaks before the condyle itself because of the vascularity and thickness of the latter. During childhood, rapid union of mandibular fractures should be expected because of the rich blood supply, the high osteogenic potential of the periosteum, and the high local metabolic rate.¹⁴ The radiographic criteria for healing of fractures in other bones are not applicable to mandibular fractures because strips of rarefaction may persist at the margins of the fracture long after satisfactory union has occurred.

Osteomyelitis

Osteomyelitis rarely occurs in the mandible because it has no metaphyseal-epiphyseal junction structures. Osteomyelitis generally occurs as a result of dental infection or local trauma (e-Fig. 22-8). The immature mandibular structure allows inflammation to extend easily and rapidly. Bone necrosis and sequestra formation are common; destructive changes may not be visible by conventional radiography until weeks after the onset of infection.¹⁵ Hyperplastic changes usually predominate and have been confused with the manifestations of infantile cortical hyperostoses (Caffey disease).

Cysts, Neoplasms, and Neoplasm-like Disorders

Most primary mandibular cysts and neoplasms are of dental origin (Figs. 22-9 through 22-11, e-Fig. 22-12, and Box 22-2).¹⁶ Cystic changes of the jaw may be a manifestation of the basal cell nevus syndrome, in which basal cell carcinomas, skeletal anomalies, ovarian and falx calcification, ovarian fibromas, and pits in the palms and soles also are present (see Fig. 22-9). The jaw cysts may manifest in childhood; the basal cell carcinomas develop only later. Dental root cysts result from proliferation of paradental epithelial cells in the periapical granulation tissue of local infection. Dentigerous cysts are formed by the excessive accumulation of fluid between the enamel and the dental capsule and show a well-formed dental crown without roots with its base adjacent to a large cystic structure that surrounds its upper portion (see Fig. 22-10).

Cystic adamantinomas or ameloblastomas result from the proliferation of ectopic ameloblasts derived from the enamel organ (see Fig. 22-11).¹⁷ Composite odontomas have increased radiodensity (see e-Fig. 22-12). Osteomas and giant cell tumors resemble similar lesions elsewhere, but tissue diagnosis is necessary in all cases. Osteomas of the mandible and other cranial bones may be a manifestation of Gardner syndrome, which is associated with other connective tissue tumors and particularly polyposis of the large bowel. Benign osteoblastoma has been reported in the mandible. Giant cells, usually osteoclasts, are found in a variety of bone lesions in the jaws.¹⁸ Radiographic findings alone do not provide adequate differential characteristics because radiographic, CT, and MRI patterns may be found in association with several different microscopic patterns (e-Fig. 22-13, Fig. 22-14, and e-Fig. 22-15).

The radiographic “floating teeth” of Langerhans cell histiocytosis (Fig. 22-16) are not pathognomonic, having been observed in metastatic neuroblastoma, lymphosarcoma, reticulum cell sarcoma, and Ewing sarcoma. Neuroblastoma is the most common metastatic disease to the mandible.¹⁹ The common pathologic factor is a destructive process that affects the supporting alveolar structures of the teeth. Melanotic neuroectodermal tumor of infancy occurs most frequently in the maxilla; however, occasionally it may arise in the mandible. In native children of equatorial Africa, Burkitt lymphoma of the jaw accounts for 30% to 50% of all lymphomas, but this frequency is rare in the non-African form of the disease.

Desmoblastic fibroma, chondrosarcoma, pseudotumors, and chondroid tumors also may occur in the mandible (see Fig. 22-14 and e-Fig. 22-15).

Familial Fibrosis of Jaws (Cherubism)

Cherubism is an autosomal-dominant condition characterized by asymptomatic facial swelling beginning about the third year of life. The mandible and maxilla are generally involved with multilocular cystic changes, but other facial bones are spared. Rarely, cystic lesions in the anterior ends of the ribs have been reported. Stretching of the skin as a result of maxillary involvement tends to pull the lower eyelids down, revealing a band of white sclerae between the eyelid and the iris (Fig. 22-17). This cherubic “eyes raised to heaven” expression has led to the term “cherubism.” Dentition is seriously disturbed; deciduous and permanent teeth are shed prematurely and irregularly, and some of the molar and premolar teeth are congenitally absent.

During the first 2 to 3 years after onset, the swellings increase rapidly in size, then remain stationary until puberty, when spontaneous regression begins and continues through the second and third decades. Not all patients have the typical features and course; massive jaw swelling and similar dental abnormalities usually seem to be variants of the condition, because no other satisfactory diagnosis can be made.

Radiographs show diffuse swelling and multiloculated rarefaction of the mandible (see Fig. 22-17 and e-Fig. 22-18). The maxilla is also enlarged, with at least partial obliteration of the sinuses. CT can detect lesions of the upper jaws not observable in conventional radiographs, including involvement of the head of the condyle (e-Fig. 22-18).^20,21 Mandibular enlargement and some sclerosis may persist in later life.

Microscopic examination shows diffuse fibrosis of the affected bones, with replacement of bone by fibrous tissue that contains varying numbers of giant cells and blood vessels. Submandibular lymph nodes are usually enlarged, showing nonspecific hyperplasia on biopsy. No definite relationship with polyostotic fibrous dysplasia has been shown.

Mandibular Involvement in Generalized Diseases of the Skeleton

Rheumatoid arthritis affects the mandible in about 15% of cases because of the retardation of growth ventrally and caudally after temporomandibular arthritis. Deformity of the condyles and a prominent antegonial notch may be present. Because of the small size of the mandible, these patients have birdlike facies, and movements of the mandible are limited.

In persons with thalassemia, the changes in the mandible are similar to those in the calvarium and long bones and include expanded external contours, dilation of the medullary spaces, cortical atrophy, and generalized osteoporosis.

Fibrocystic changes in the mandible may be a feature of polyostotic fibrous dysplasia.

The disappearance of the lamina dura in persons with hyperparathyroidism was considered a pathognomonic radiographic sign of the disease. More experience has shown that the lamina dura also may be resorbed (in part or in toto) in persons with many other disorders, including rickets, leukemia, Cushing disease, Paget disease, idiopathic osteomalacia, idiopathic chronic familial hyperphosphatemia, and primary and metastatic malignancies, as well as after dental extractions. The bone of the alveolar processes is always affected when the lamina dura has disappeared; the lamina dura does not disappear in the presence of normal alveolar bone.²² Mandibular involvement in infantile cortical hyperostosis is an important diagnostic feature of this condition.

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References

References

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