Benign Fibrous Bone Lesions

Avulsive Cortical Irregularity

Avulsive cortical irregularity (cortical desmoid) refers to the common radiographic irregularity over a short segment of cortical surface along the posterior aspect of the medial femoral condyle, just superior to the adductor tubercle. The radiographic appearance is similar to that of a fibrous cortical defect. The margins are variably sclerotic (Figure 63-1). Small osseous fragments in the immediately adjacent soft tissues are sometimes visible on lateral radiographs or CT. Avulsive cortical irregularity is often bilateral and is more common in boys. The typical age range is 3 to 17 years. This “lesion” is apparently related to repetitive stress at the site of attachment of the medial head of the gastrocnemius or adductor magnus muscles.¹

The only clinical importance of avulsive cortical irregularity is when it is mistaken for a neoplasm, infection, fracture, or other pathology. Patients with this finding typically have no related symptoms. When the diagnosis is in doubt, radiographs of the contralateral knee typically show an identical appearance. Bone scintigraphy is normal or shows minimal increased uptake. On T1-weighted MR images, there is a rim of low signal intensity at the origin of the medial head of the gastrocnemius muscle; the area is sometimes hyperintense on T2-weighted images.²

Fibrous Cortical Defect

A fibrous cortical defect is a common, benign fibrous lesion of bone that is histologically identical to a nonossifying fibroma. This lesion is, by definition, confined to the cortex; most are less than 2 to 3 cm in diameter. The nonossifying fibroma is a closely related lesion that is at least 2 cm in diameter and extends into the medullary portion of the bone. Fibrous cortical defect is composed of fibrous tissue; the fibrous tissue induces a mild adjacent osteoblastic response. Developmentally, the ectopic fibrous tissue likely originates from the physis, with subsequent bone growth causing the lesion to “migrate” into the metadiaphyseal region. Most fibrous cortical defects eventually resolve spontaneously, either by reparative ossification or by gradual extrusion from the cortex.³

Most fibrous cortical defects occur in the extremities, especially the femur and tibia. This common, asymptomatic lesion can be considered a developmental variation of normal; approximately 30% to 40% of otherwise normal children have 1 or more fibrous cortical defects. They are rarely demonstrated in boys younger than 2 years of age or girls younger than 4 years of age. Fibrous cortical defects occur more commonly in boys, with a male-to-female ratio of 1.6:1.

When viewed en face, the radiographic appearance of a fibrous cortical defect is that of a round, oval, or multiloculated radiolucent lesion that is cortically based. There are well-defined sclerotic margins. When viewed in tangent, the cortical location is confirmed; there is no extension into the medullary cavity (Figure 63-2). When oval, a fibrous cortical defect is oriented longitudinally relative to the long axis of the bone. With a small fibrous cortical defect in a young child, the sclerotic margin is thin or lacking. There is usually aggressive marginal sclerosis as the lesion expands. With regression, sclerosis eventually fills the radiolucent defect. Remodeling occurs to the point that there is usually no radiographically visible residual lesion in the mature skeleton.

Nonossifying Fibroma

A nonossifying fibroma (fibroxanthoma) is a benign fibrous lesion of bone that is very common in children. Most are asymptomatic, although a large nonossifying fibroma is susceptible to a pathological fracture. Nonossifying fibroma is more common in males (M:F = 2:1). The tibia and femur are the most common locations. The lesion arises in the metaphysis, and “migrates” into the diaphysis as the child grows. The lesion is cortically based, but frequently extends into the medullary cavity; a feature that distinguishes this lesion from the histologically similar fibrous cortical defect. By definition, the lesion is at least 2 cm in diameter; smaller lesions are classified as fibrous cortical defects. There is a predilection for involvement of the posterior and medial aspects of the metaphyseal cortex. Multiple lesions or coexistent fibrous cortical defects are common.³

The histological features of nonossifying fibroma consist of whorled, spindle-shaped fibroblasts that form a focal eccentric mass in the cortex. Hypercellular fibrous septa may be present. Other potential histological findings include hemorrhage, hemosiderin, collagen, and foamy histiocytes. The lesion is bordered by sclerotic or normal bone, which has a scalloped or serpentine configuration. Bony septations are common.

Radiographs show a nonossifying fibroma as an eccentric, slightly expansile radiolucent lesion of the metaphysis or metadiaphysis of a long bone (Figure 63-3). There is extension from the cortex into the medullary cavity. There is often thinning and slight outward bulging of the overlying cortex. A nonossifying fibroma frequently has a “bubbly,” scalloped, or multiloculated appearance. Occasionally, a pathologic fracture is present, which may be accompanied by adjacent soft tissue swelling. In the absence of an acute fracture, there should be no soft tissue abnormality.⁴

The diagnosis of nonossifying fibroma is usually definitive on standard radiographs. In atypical cases, CT or MR can provide confirmatory information; surgical biopsy is rarely indicated. The fibrous matrix of the lesion produces low attenuation on CT (Figure 63-4). The margins are well defined. There is a thin sclerotic border. The cortex is slightly expanded and thinned, but otherwise intact. There is no extraosseous component.⁴

MR shows the matrix of a nonossifying fibroma to be hypointense to marrow and hypointense or isointense to skeletal muscle on T1-weighted images (Figure 63-5). The appearance on T2-weighted images is more variable; most are hypointense, but an isointense or hyperintense pattern can also occur. Those that are hypointense on T2-weighted images usually have a predominance of hypercellular fibrous tissue, whereas those that are hyperintense consist of foamy histiocytes and scant fibrous tissue. Hemosiderin can produce areas of marked T2 hypointensity. Contrast enhancement of variable intensity usually occurs throughout the lesion with gadolinium administration, often with a heterogeneous pattern (Figure 63-6). A common feature of nonossifying fibroma is contrast enhancement of a thin hypervascular zone at the periphery and along septal surfaces. This zone is sometimes hyperintense relative to the matrix on fat-suppressed T2-weighted images as well.⁵

Nonossifying fibroma is a metabolically active benign tumor. Therefore, there is uptake on FDG-PET ([₁₈F]-fluorodeoxyglucose positron emission tomography) imaging. Uptake ranges from mild to marked. There is usually minimal uptake on bone scintigraphy (Figure 63-7). Marked uptake occurs in the presence of a pathologic fracture.⁶

A variety of patterns can occur on sequential radiographs of nonossifying fibroma. If the lesion arises in a young child, subsequent radiographs demonstrate migration into the diaphysis as bone elongation occurs. Many of these lesions slowly fill in with normal bone and eventually disappear (Figure 63-8). Residual sclerosis may be present in the mature skeleton. A period of enlargement occasionally occurs. Most nonossifying fibromas require no treatment. Pathologic fractures are usually managed nonoperatively. Those rare instances of nonossifying fibroma that are accompanied by pain or repeated fractures can be treated with curettage.⁷

Jaffe-Campanacci syndrome includes the presence of multiple nonossifying fibromas and a variety of congenital abnormalities. The extraskeletal abnormalities dominate the clinical presentation; these include mental retardation, ocular anomalies, café-au-lait spots, cryptorchidism, and hypogonadism.⁸

Fibrous Dysplasia

Fibrous dysplasia is a mesodermal developmental abnormality in which the medullary space is replaced by a mixture of fibrous tissue and small abnormally arranged regions of trabecular bone. Mutations affecting the stimulatory α subunit of G protein (Gsα) have been found in the dysplastic bone lesions. Histological analysis reveals abnormalities in cells of the osteoblastic lineage and therefore in the bone matrix. The histological features include disorganized collagen fibers, woven bone formation, and immature cytology of the osteoblasts and preosteoblastic cells. The abnormal deposition of immature bone matrix in fibrous dysplasia results from decreased differentiation and increased proliferation of osteoblastic cells.⁹

The etiology of fibrous dysplasia is unknown. It is considered a developmental skeletal anomaly. It is not familial or hereditary. Fibrous dysplasia occurs in polyostotic and monostotic forms. A solitary lesion is present in 70% to 80% of patients with fibrous dysplasia; the polyostotic variety occurs in 20% to 30%. Males and females are affected with similar frequencies. Malignant degeneration to osteosarcoma, fibrosarcoma, or malignant fibrous histiocytoma is rare, particularly in children. Two syndromic forms of fibrous dysplasia include McCune-Albright syndrome and Mazabraud syndrome.

The monostotic form of fibrous dysplasia is frequently asymptomatic. The lesion may be discovered as an incidental radiographic finding. A monostotic lesion of sufficient size can present as a palpable mass or with acute symptoms due to a pathologic fracture. Progressive limb deformity can occur with a large lesion. Facial deformity is common in patients with craniofacial involvement. The lesion is usually discovered during the second or third decade of life. Monostotic fibrous dysplasia involves the femur in 36% of patients, the tibia in 19%, the craniofacial structures in 17%, and the ribs in 10%. Craniofacial involvement is most often in the maxillae or mandible. Monostotic fibrous dysplasia typically does not convert to the polyostotic form, the lesions tend to remain stable in size, and the disease becomes inactive around the age of puberty.

The polyostotic form of fibrous dysplasia typically results in more profound clinical manifestations than, and has an earlier presentation than, the monostotic form. McCune-Albright syndrome is characterized by the triad of polyostotic fibrous dysplasia, pigmented skin lesions, and endocrinopathy with precocious puberty. It is an inherited disorder due to a somatic activating mutation of the gene (GNAS1) that encodes the α-subunit of the Gs protein. A major function of the G(s) protein is to couple hormone receptors to the effector enzyme adenylyl cyclase; it is therefore required for hormone-stimulated intracellular cAMP generation.¹⁰ Clinical recognition of McCune-Albright syndrome usually occurs early during childhood.

The most commonly involved osseous structures with polystotic fibrous dysplasia are the craniofacial bones, pelvis, femurs, shoulder region, and spine. Polyostotic fibrous dysplasia is often unilateral, and may be monomelic. Craniofacial structures most often involved are the ethmoids (71%), followed by the sphenoid (43%), frontal (33%), maxillary (29%), temporal (24%), parietal (14%), and occipital bones (5%).¹¹ The most common presenting features include facial pain and headache, complaints referable to the sinuses, proptosis and diplopia, hearing loss, and facial numbness. Craniofacial fibrous dysplasia involving the orbital region can present with deformity, pain, paresthesia, and visual compromise. Clinical manifestations of long bone involvement include pain and deformity. Pathologic fractures can occur. The disease tends to become quiescent at puberty, although symptoms may persist and existing deformities sometimes progress.

Clinical sequelae of fibrous dysplasia such as deformity, pain, and pathologic fracture are largely related to osteolysis. Bisphosphonates are now used to treat many patients with fibrous dysplasia. These drugs decrease bone resorption, and increase bone mineral density; the therapeutic response can be quantified with dual x-ray absorptiometry.¹² Surgical options for the treatment of symptomatic circumscribed lesions include curettage, cryosurgery, and bone grafting. More extensive lesions (usually in the polyostotic form) may also require corrective osteotomies and internal fixation to manage deformities.¹³

The radiographic appearance of fibrous dysplasia consists of a radiolucent bone lesion that is typically diaphyseal; metaphyseal extension often occurs, but an epiphyseal location is rare. The lesion is usually intramedullary and slightly expansile (Figure 63-9). The borders are irregular, but well defined. A characteristic leading edge of sclerosis or thick cortex around the periphery of the lesion is sometimes present; this is the rind sign. The expansile growth of the lesion results in scalloping of the endosteal cortex. The imaging appearance of the matrix of fibrous dysplasia varies according to the relative proportions of the fibrous and osseous components. When present, a ground glass pattern (due to randomly distributed small trabeculae) is diagnostic; this finding may be visible in only a portion of a lesion. Radiographs frequently show manifestations of weakening of the affected bone: bowing of weight-bearing segments, remodeling, and pathologic fracture (Figure 63-10). In the proximal femur, this results in the classic shepherd’s crook deformity, characterized by varus bowing.⁴

Cross-sectional imaging with CT and MR is sometimes helpful in the diagnosis and characterization of fibrous dysplasia. As with standard radiographs, the appearance varies according to the specific composition of the lesion. Most often, the fibrous component predominates; therefore, T1-weighted MR images demonstrate intermediate-to-low signal intensity and T2-weighted images show intermediate-to-high signal intensity. Contrast enhancement tends to be heterogeneous. Lesions with greater proportions of woven bone have diminished MR signal intensity. Foci of cartilage can result in a heterogeneous pattern. Necrosis can result in a cystic appearance.¹⁴

The CT evaluation of fibrous dysplasia is most beneficial in areas of complex anatomy, such as the skull base (see Chapter 26). The affected bone is thickened. Most often, there is homogeneously increased attenuation (Figure 63-11). These lesions occasionally have attenuation values that approximate those of soft tissue, however.

The lesions of fibrous dysplasia uniformly cause avid accumulation of bone-seeking radiopharmaceuticals. Bone scintigraphy in these patients is most useful for surveying the entire skeleton to determine the number and distribution of lesions.

McCune-Albright syndrome is a rare syndrome characterized by the triad of polyostotic fibrous dysplasia, pigmented skin lesions (café-au-lait spots) and multiple endocrinopathies that often include precocious puberty. This is a nonfamilial disorder due to a GNAS1 gene mutation. McCune-Albright syndrome is sometimes complicated by hypophosphatemia. The cutaneous lesions are flat, pigmented macules with margins that are likened to the coast of Maine, due to an irregular contour. The bone lesions in patients with McCune-Albright syndrome tend to be more severe and disabling than those of nonsyndromic polyostotic fibrous dysplasia.

Mazabraud syndrome is a rare sporadic disorder that includes fibrous dysplasia of the bone and intramuscular myxomas. Most of these patients have the polyostotic form of fibrous dysplasia, although there are uncommon instances of monostotic disease. There is a female predilection. This rare disorder predominantly occurs in middle-aged adults. Most affected pediatric patients are teenagers.

Myxomas are benign tumors that tend to develop in muscles near the most extensively involved bones. On CT, these lesions appear as homogeneous soft tissue masses that produce relatively low attenuation when compared to normal muscle. The myxomas are hypointense to slightly hyperintense on T1-weighted MR images and markedly hyperintense to muscle on T2-weighted images. There is a complex pattern of enhancement due to the combination of solid myxoid tissue and fibrous septa. Rim enhancement is common. Mazabraud syndrome is associated with an increased risk of malignant degeneration of bone lesions to osteosarcoma.¹⁵

Osteofibrous Dysplasia

Osteofibrous dysplasia (intracortical fibrous dysplasia; ossifying fibroma of the long bones) is a rare, benign fibroosseous lesion of cortical bone that most often involves the lower leg. Osteofibrous dysplasia is more common in males. The presentation is usually during the first 2 decades of life. Osteofibrous dysplasia is histologically similar to fibrous dysplasia, although active osteoblasts are present in this lesion. Osteofibrous dysplasia is composed of fibroblast-like spindle cells and osseous tissue. Characteristic histological features include osteoblastic rimming and bone zonation. Cartilage differentiation is not present. Proliferating cell nuclear antigen expression can be demonstrated in the nuclei of osteoblasts adjacent to the bone trabeculae in these lesions; this finding may be helpful in the histologic differentiation from fibrous dysplasia. Osteofibrous dysplasia is radiographically similar to adamantinoma; histologic differentiation is by identification of epithelial elements in the latter.¹⁶

Osteofibrous dysplasia typically occurs in the tibia (90%) or fibula. There are a few case reports of ulnar involvement.¹⁷ The lesion is usually diaphyseal. Those located in the tibia tend to involve the anterior cortex of the middle to proximal third of the shaft. Multiple lesions in the same bone are sometimes present. Synchronous involvement of multiple bones is rare. Although some patients suffer progression of the lesion or deformity requiring surgery, spontaneous regression with little or no permanent deformity is commonly identified on followup radiographs.¹⁸

Radiographs show osteofibrous dysplasia as a focus of intracortical osteolysis, often with a relatively characteristic adjacent sclerotic band (Figure 63-12). The matrix is lucent or has a ground glass character. The involved cortex is thinned and expanded. Central expansion into the medulla is common. There often is irregular sclerosis adjacent to the lucent lesion. CT confirms the cortical origin of the lesion and the well-defined margins (Figure 63-13). As with fibrous dysplasia, the attenuation of the matrix varies with the proportions of fibrous tissue and dysplastic bone. On MR, the matrix is moderately hypointense to normal marrow on T1-weighted sequences, heterogeneously hyperintense on T2-weighted sequences, and undergoes mild-to-moderate contrasts enhancement. Osteofibrous dysplasia is often multiloculated and occasionally is multicentric. The bone is usually enlarged and bowed. Pathological fracture or pseudarthrosis can occur.

Osteofibrous Dysplasia-Like Adamantinoma

The “classic” form of adamantinoma of the appendicular skeleton is a rare, locally aggressive or malignant bone tumor of adults and adolescents (see the Adamantinoma section latter in this chapter).

Osteofibrous dysplasia-like adamantinoma is a “differentiated” form of adamantinoma (sometimes termed “juvenile adamantinoma”) that occurs almost exclusively in children. This lesion is intracortical and occurs in the tibia or fibula. Aggressive growth and metastasis do not occur with this lesion. The radiographic features of differentiated adamantinoma are identical to those of osteofibrous dysplasia: a sharply marginated radiolucent lesion of cortical origin in the diaphysis of the tibia or fibula. Histological differentiation is based on the presence of epithelial elements in adamantinoma.¹⁵

Gnathodiaphyseal Dysplasia

Gnathodiaphyseal dysplasia is an unusual generalized skeletal syndrome characterized by fibroosseous lesions of the mandible (with a prominent psammomatoid body component), bone fragility, and bowing/sclerosis of tubular bones. These patients lack the GNAS1 mutation of true fibrous dysplasia.¹⁹

Gigantiform Cementoma

Gigantiform cementoma is a rare, benign fibro-cementoosseous lesion of the mandible (see Chapter 26). It occurs most frequently in young girls. The radiographic appearance is that of an expansile, mixed radiolucent and radiopaque lesion that crosses the midline. A familial pattern is occasionally identified. The term gigantiform cementoma has been used interchangeably with designations of other fibroosseous entities.²⁰

Cherubism

Cherubism refers to fibrous lesions of the mandible that can occur in young children and infants. Many cases have autosomal dominant inheritance and variable penetrance. The defective gene in familial cases is SH3BP2. Clinically, children with cherubism have obvious bilateral jaw fullness, without pain. Radiographs show expansile, multilocular, radiolucent mandibular lesions. These sometimes have a ground glass appearance that is similar to that of fibrous dysplasia. The overlying cortex is expanded and thinned. Concomitant maxillary involvement occurs in some patients. As with most other fibrous bone lesions, there is avid tracer accumulation with skeletal scintigraphy. Cherubism is usually a self-limiting disorder, although resolution frequently has a time course of years.^21–23

Desmoplastic Fibroma

Desmoplastic fibroma of bone is a rare skeletal system fibrous tumor that is histologically similar to desmoid tumors that arise in the soft tissues. The lesion contains fibroblasts that produce well-formed collagen. This lesion is also termed desmoid tumor of bone. Desmoplastic fibroma can occur in patients of any age; adolescents and young adults are most commonly affected. There is a slight male predilection. Desmoplastic fibroma of bone can arise in any portion of the skeleton; those within the tubular bones most frequently involve the metadiaphyseal regions.

The radiographic appearance of desmoplastic fibroma of bone usually is that of an osteolytic lesion, frequently with coarse ridge-like trabeculae due to uneven bone destruction. Minimal areas of sclerosis in the matrix are occasionally visible on standard radiographs. Evidence of cortical disruption is present in about half of patients with desmoplastic fibroma. The margins are usually relatively well defined. The margins are sclerotic in half of patients.^24,25

On CT examination, the matrix of desmoplastic fibroma is most often osteolytic; a mixed osteolytic and mildly sclerotic matrix is present in about one-third of patients. Evidence of cortical destruction is visible on CT in most patients. An extraosseous soft tissue mass is sometimes present extending beyond the destroyed cortex.²⁶

T1-weighted MR images show desmoplastic fibroma to be isointense or hypointense to muscle. The fibrous nature of the lesion is reflected by low-to-intermediate signal intensity on T2-weighted MR sequences. At least a portion of the matrix is isointense or hypointense to muscle on conventional T2-weighted spin-echo or fat-saturated sequences. A pathologic fracture may alter this signal pattern, however.²⁴

Benign Bone-forming Tumors

Osteoid Osteoma

Osteoid osteoma is a benign lesion of enchondral bone. It consists of a highly vascular nidus and surrounding reactive new bone formation. Osteoid osteoma usually arises in the cortex; subperiosteal lesions have also been reported. Although nearly any bone can be involved, osteoid osteoma occurs most commonly in a metaphysis or diaphysis of a lower extremity long bone; 50% to 60% of all osteoid osteomas arise in the femur or tibia. Epiphyseal locations are rare. The hand and foot account for 10% to 20%; the most common location of osteoid osteoma within the foot is in the talus. Spinal locations account for 10%, with 90% in the posterior elements.^27–29

Osteoid osteoma usually presents during the second decade of life; occurrence in children younger than 5 years is rare.³⁰ There is a 3:1 male-to-female ratio. The classical symptoms are localized bone pain that flares nocturnally, but is promptly relieved with nonsteroidal anti-inflammatory drugs (aspirin). Less common, but potentially misleading, clinical findings include fever, leukocytosis, erythrocyte sedimentation rate elevation, and adjacent soft tissue swelling. An intraarticular or periarticular osteoid osteoma (such as in the proximal femur) can lead to a joint effusion, joint pain, and limitation of motion. Osteoid osteoma of the spine commonly presents with painful scoliosis.

The pathological appearance of osteoid osteoma is a round or oval nodule of granular bone that is sharply demarcated from the surrounding bone. The lesion is composed of osteoid and woven bone with interconnected trabeculae. There is a background of highly vascularized fibrous connective tissue. About three-quarters of these lesions are located in cortical bone and one-quarter in cancellous bone; a subperiosteal location can also occur, but is rare.

The nidus of an osteoid osteoma is radiolucent but often contains variable amounts of dense calcification. The nidus is less than 1.5 cm in size. The adjacent reactive sclerosis is usually the dominant radiographic feature, and can obscure the underlying small nidus that represents the true lesion (Figures 63-14 and 63-15). The periosteal reactive bone is usually solid, but a laminated pattern can occur. Disuse osteopenia may be present in the involved extremity. In the spine, radiographs often show localized scoliosis and a radiodense pedicle (Figure 63-16).³¹

Skeletal scintigraphy is highly sensitive for the detection of osteoid osteomas, due to the prominent vascularity of the lesion, new bone formation within the nidus, and the adjacent reactive bone formation. Prominent uptake on both blood pool and delayed images is a characteristic feature of osteoid osteoma. Most often, there is a “double-density” appearance on scintigraphy, with markedly increased uptake in the nidus and a surrounding zone of prominent but less avid uptake in the reactive perilesional bone (Figure 63-14). An osteoid osteoma adjacent to a joint can cause a reactive synovitis, in which case blood pool scintigraphic images show prominent periarticular uptake. When pronounced, the synovial uptake can obscure a small underlying nidus.³²

The CT appearance of osteoid osteoma is often pathognomonic. CT frequently allows documentation of a radiolucent nidus that is too small for accurate visualization on standard radiographs or is obscured by overlying reactive bone. CT examination is also useful for evaluation of osteoid osteomas in complex skeletal structures, such as the spine (Figure 63-17). The low-attenuation intracortical nidus is surrounded by medullary sclerosis and periosteal reaction. The inner aspect of the nidus has a smooth appearance. Although usually of low attenuation, a round central calcification is sometimes present within the nidus (Figure 63-14). The peripheral fibrovascular envelope that surrounds the nidus is nearly always visible as a thin, low-attenuation ring. As with standard radiographs, the nidus of an osteoid osteoma is occasionally obscured on CT due to associated changes such as marrow edema, periosteal edema, periosteal new bone formation, marrow sclerosis, cortical thickening, and nidus calcifications.³³

Although CT is the usual cross-sectional imaging technique utilized for patients with suspected osteoid osteoma, MR can provide valuable information in select cases. T1- and T2-weighted fat-saturated imaging sequences should be performed. The nidus produces low or intermediate signal intensity on T1-weighted images and high signal intensity on T2-weighted images, but adjacent edema in the periosteum and bone marrow also appears bright. Additional imaging utilizing a dynamic gadolinium-enhanced method provides the best MR visualization of the nidus, and allows distinction from edema in the adjacent bone.³⁴ Most osteoid osteomas undergo arterial phase enhancement and rapid partial washout.

Edema within the bone marrow and adjacent soft tissues can occur in patients with osteoid osteoma, appearing on MR as prominent signal intensity on fat-suppressed T2-weighted sequences. This can lead to a mistaken clinical and imaging diagnosis of osteomyelitis or fracture. The mechanism for this edema is uncertain. Production of prostaglandin within the lesion has been implicated.

Osteoblastoma

Osteoblastoma is a rare, benign, osteoid-producing neoplasm. Pathologically, this tumor resembles a large osteoid osteoma. Osteoblastoma is predominantly a lesion of older children and young adults; the median age is 18 years. The male-to-female ratio is 2-3:1. Osteoblastoma accounts for less than 1% of all bone tumors and 3% of all benign bone tumors. Patients may present with pain (usually dull and insidious in onset) or a palpable mass. Typically, the pain associated with osteoblastoma is less than that of osteoid osteoma, nocturnal exacerbation of pain is less common, and aspirin may or may not produce symptomatic improvement. Vertebral osteoblastomas can cause symptoms due to spinal cord or nerve root compression; scoliosis due to muscle spasm is common.^35–37

The most common location of osteoblastoma is the spine (40%). Thirty percent occur in the long bones (most commonly in the femur and tibia), 15% in the skull and face, 15% in the pelvis, and 10% in the hands and feet. Those located in the long bones typically arise in the metadiaphyseal region. Histological examination demonstrates numerous osteoblasts that produce trabeculae, osteoid, or bone. In comparison to the histologically similar osteoid osteoma, osteoblastoma is larger (nidus at least 1 to 2 cm in diameter), there is less reactive sclerosis, and there is a greater degree of bone expansion. Approximately 15% of osteoblastomas have an aneurysmal bone cyst component.

The radiographic appearance of an osteoblastoma is that of a lytic, expansile lesion that often contains a variable quantity of dense, amorphous bone or stippled, ring-like calcifications. The radiolucent areas correspond to unmineralized osteoid; the dense areas represent osteoid that has become at least partially mineralized. Most osteoblastomas are round or ovoid in shape. Larger lesions are associated with bone expansion, usually with a thin peripheral cortical shell that is derived from the periosteum. Occasionally, there is insufficient periosteal bone formation for radiographic visualization of this shell, although this tumor rarely violates the periosteum. In other patients, there is extensive reactive bone formation, similar to that of osteoid osteoma (Figure 63-18). Osteoblastomas occasionally undergo rapid growth.

Approximately 60% of osteoblastomas of the spine are confined to the posterior elements and 25% have involvement of the posterior elements as well as the vertebral body. Radiographs often show some degree of scoliosis, with the lesion usually located at the apex of the convex portion of the curve. More than half of spinal osteoblastomas are radiographically lucent; others have a variable quantity of intralesional trabecular ossification. Adjacent reactive sclerosis is usually minimal; many of these lesions have only a thin, reactive marginal rim.

Osteoblastoma in the appendicular skeleton is equally distributed between origins in the cortex and medulla. About three-quarters of these lesions arise in a diaphysis and 25% and a metaphysis. Most common is a well defined, spherical, or ovoid predominately radiolucent lesion, with or without bone expansion (Figure 63-19). Those that arise in a tubular bone often are eccentrically located. About half of these lesions of the appendicular skeleton have radiographically visible tumor matrix mineralization and half are associated with solid periosteal reaction. Cortical disruption is visible in less than 20% of these lesions.

The CT findings of osteoblastoma are similar to those of standard radiographs. The lesion is expansile and has well-marginated borders. There is a thin, bony shell that defines the tumor margin (Figure 63-20). The tumor matrix usually has attenuation values in the range of 80 Hounsfield units or higher, although there is substantial variation between patients. CT is most beneficial for evaluating lesions in areas of complex anatomy, such as the spine (Figure 63-21).

The matrix of an osteoblastoma produces low-to-intermediate signal intensity on T1-weighted MR images and intermediate-to-high signal intensity on T2-weighted images. Focal signal voids are usually present within the matrix due to calcifications. There is a low signal intensity rim at the margin of the tumor, representing the pseudocapsule or bony shell (Figure 63-22). There sometimes are manifestations of reactive edema in adjacent soft tissues, with hyperintensity on T2-weighted images. This inflammatory response may result in findings that mimic those of a malignant tumor. There is contrast enhancement of the soft tissue components of the tumor, sometimes with concomitant enhancement of the adjacent inflammatory reaction.^38,39

The hypervascular character of osteoblastoma and the active bone formation result in avid accumulation of bone-seeking radiopharmaceuticals. There is prominent uptake on blood pool images and delayed images (Figure 63-23). Contrast angiography also shows a hypervascular appearance of osteoblastoma (Figure 63-24).

Osteoma

An osteoma is a benign, slow-growing hamartomatous lesion that is composed of well-differentiated mature bone. This lesion arises beneath the endosteum from the inner surface of the cortex. Elevation of the periosteum causes surrounding reactive bone formation. Histologically, the lesion has similar characteristics as normal cortical bone. Most osteomas are discovered in adults, although adolescents can also develop this lesion. There is no gender predilection or racial predilection. Most sporadic osteomas arise within the walls of a paranasal sinus (75%); other relatively common locations include the mandible and the skull. Multiple osteomas occur in patients with Gardner syndrome (colonic polyposis, osteomas, dental lesions, soft tissue tumors).

The radiographic appearance of an osteoma is that of a densely sclerotic mass that arises from the surface of the bone. The margins are well defined. There is no associated osseous destruction. Minimal adjacent periosteal new bone formation is sometimes present. The lesion appears homogeneous on CT; there is no contrast enhancement. The osteoma produces low signal intensity on all MRI sequences. An actively growing osteoma has a variable degree of increased radiotracer uptake with bone scintigraphy; a quiescent lesion has similar uptake as normal adjacent bone.⁴⁰

In patients with Gardner syndrome, the most common sites of osteomas are the skull and facial bones (Figure 63-25). Lesions can also arise in the ribs and long bones. A tubular bone osteoma in these individuals sometimes appears on radiographs as a wavy area of cortical thickening, rather than a well-defined bony mass. These can occur along the metacarpal shafts. Potential dental lesions in Gardner syndrome patients include supernumerary teeth, odontoma, dentigerous cyst, and hypercementosis. The soft tissue tumors associated with Gardner syndrome are desmoid tumor, sebaceus cyst, epidermoid cyst, subcutaneous fibroma, and subcutaneous lipoma.⁴¹

A parosteal osteoma is an uncommon variant in which a large osteoma arises from the cortical surface of a bone. The most common sites are the clavicle, scapula, ilium, or a tubular bone. The surface of the densely ossified lesion is smooth. There is no soft tissue component.

Enostosis

An enostosis, or bone island, is a focus of dense histologically normal bone within the medullary portion of any bone. This lesion is of no clinical significance. The presence of innumerable enostoses is the radiographic hallmark of osteopoikilosis. The radiographic appearance of an enostosis is an intraosseous sclerotic focus that has discrete margins. There often are thin radiodense spicules that extend from the surface. There is no associated cortical expansion. Those that are oval tend to be aligned with the long axis of the bone (Figure 63-26). Most are a few to several millimeters in size. Larger enostoses occasionally occur in the pelvis or lower extremity long bones.

Benign Cartilage Tumors

Chondroblastoma

Chondroblastoma (Codman tumor) is a rare, benign bone neoplasm that typically arises in the epiphysis of a long bone. This lesion accounts for 9% of primary bone neoplasms in children. Approximately 75% of chondroblastomas are discovered during the second decade of life. This lesion is more common in males; M:F = 2:1. The clinical presentation is often insidious; the epiphyseal location can be associated with joint pain, tenderness, and swelling. Pathological fracture is rare with this lesion. Up to 30% of patients have a joint effusion.^42–45

Chondroblastoma likely arises from cartilage germ cells in the physis. The lesion consists of nodules composed of mature cartilage matrix, surrounded by undifferentiated tissue with primitive chondroblast-like cells. Multinucleated giant cells are present in the lesion. A secondary aneurysmal bone cyst (ABC) occurs in up to 10% of these lesions. Foci of calcification and hemorrhage may occur within this soft tissue mass. The most common sites of chondroblastoma are the femur, tibia, and humerus. About one-third of chondroblastomas occur around the knee and two-thirds in the lower extremity. This lesion can also arise in an apophysis or an epiphyseal-equivalent site of a flat bone (e.g., iliac crest).

The radiographic appearance of a chondroblastoma is that of a lucent epiphyseal lesion (Figure 63-27). Most are located eccentrically within the epiphysis. Extension into the metaphysis sometimes occurs in skeletally mature patients with a closed growth plate. The lesion may have an oval or round configuration. The margins are smooth (60%), scalloped (30%), or lobulated (10%). There is usually a sclerotic rim that develops as a reactive phenomenon. Most often, the rim is thin and well defined. Expansion of the adjacent cortex is uncommon with this tumor. Bone expansion in conjunction with secondary ABC formation occasionally occurs. Although uncommon, chondroblastoma can have an aggressive pattern of growth, with cortical penetration and invasion of the adjacent soft tissues or joint.⁴⁶

The matrix of a chondroblastoma sometimes (in about half of these tumors) contains mottled or stippled calcifications. These are occasionally visible on high-quality standard radiographs, but are best demonstrated with CT. The lesion usually has low-to-intermediate signal intensity on T1-weighted MR images and heterogeneous intermediate-to-low signal intensity on T2-weighted images, due to the presence of chondroid matrix, calcifications, hemosiderin, and cellular stroma. This appearance is distinct to the uniform T2 hyperintensity of bone lesions that are composed of hyalin cartilage, such as enchondroma. The matrix enhances with IV contrast. The margin of a chondroblastoma is well defined and has low signal intensity on all imaging sequences. Short tau inversion recovery (STIR) images show edema in soft tissues and bone marrow adjacent to the tumor. The presence of large multiloculated cysts that contain fluid–fluid levels suggests ABC formation.^47–50

Skeletal scintigraphy of chondroblastoma shows prominent tracer accumulation on blood pool and delayed images. This is due to the peripheral osteoblastic reaction that is stimulated by the tumor. There is also some degree of extended uptake beyond the tumor, due to periosteal reaction and hyperemia. Therefore, scintigraphy may overestimate the extent of this lesion (Figure 63-28).

Enchondroma

A solitary enchondroma is a benign bone neoplasm that is composed of mature hyaline cartilage. The lesion arises from the medullary portion of the affected bone. The pathogenesis likely involves displacement of cartilage tissue from the adjacent physis, with the tumor arising from cartilage cells within this tissue. An enchondroma can arise in any bone that forms by enchondral ossification. More than half of these lesions occur in the hands (proximal phalanges > metacarpals > middle phalanges > distal phalanges); other potential locations include the humerus, femur, tibia, and foot. In tubular bones, the metaphysis is the most common site of origin. Enchondromatosis (Ollier disease) is a rare bone dysplasia in which there are multiple enchondromas (see Chapter 57).

Most solitary enchondromas are discovered in young or middle-aged adults; in the pediatric age group, most patients with this lesion are adolescents. There is no gender predilection. These lesions are frequently asymptomatic, discovered as incidental radiographic findings. Occasionally, bone expansion leads to palpable swelling. A pathological fracture may also occur.

An enchondroma consists of lobules of hyaline cartilage, often encased peripherally by woven or lamellar bone. Foci of calcium are frequently present. The lesion arises in the medullary portion of the bone, and causes a variable degree of cortical thinning and expansion. The major cellular component on microscopic examination consists of chondrocytes.

The predominantly cartilaginous composition of an enchondroma results in a radiolucent appearance on radiographs. Calcifications are often present, particularly within lesions of the long bones. Calcifications may be flocculent, popcorn-like, stippled, or punctate in character. Circular, ring, or arc densities can occur due to enchondral ossification. Calcifications are not invariably present in enchondromas, however. The lesion arises from the medullary cavity, and may have a central or eccentric location. There is lobulated erosion of the endosteal margin of the adjacent cortex, sometimes progressing to complete cortical destruction (Figure 63-29). Cortical expansion is common. Thin osseous septa are often visible, sometimes producing a fine multiloculated character. A longitudinally oriented channel-like or columnar appearance is common, due to columns of cartilaginous tissue that extend along the shaft of the bone between longitudinal bony septa (Figure 63-30). When located in the metaphysis of a growing bone, the lesion usually interferes with normal bone modeling. The epiphysis is not involved as long as the adjacent growth plate is open.^51,52

An enchondroma usually is associated with mildly increased uptake on skeletal scintigraphy. The predominant mechanisms of radiopharmaceutical accumulation with this tumor are hyperemia, enchondral ossification, and adjacent reactive new bone formation; the cartilage itself does not substantially accumulate tracer. The chondroid matrix of an enchondroma results in CT attenuation values that most often are approximately isoattenuating to soft tissue. CT is quite sensitive for the detection of calcification or ossification within the matrix, which is a helpful characterizing feature. Contrast enhancement is lacking or is minimal.

The predominantly cartilaginous matrix of enchondroma produces T1-weighted MR signal intensity that is similar to that of muscle. The cartilage is markedly hyperintense on T2-weighted images, whereas osseous septa appear in as thin relatively hypointense structures. Calcifications result in signal voids on both sequences. There is little or no enhancement with contrast; occasionally, thin enhancing arcs and rings are visible. Typically, the margins are well defined and lobulated.⁵³

Rarely, an enchondroma expands through the cortex to produce an exophytic mass, enchondroma protuberans, that can radiographically mimic an osteochondroma or juxtacortical chondroma. However, this lesion lacks a cartilage cap and does not contain trabecular bone. The typical radiographic appearance of enchondroma protuberans is that of an osteolytic intramedullary lesion with a cortical defect and a round or oval exophytic component. The margins of the lesion are well defined. As with other enchondromas, matrix calcifications are sometimes visible radiographically. MR evaluation is sometimes required to define the extraosseous component. The lesion is predominantly hyperintense on T2-weighted images, sometimes with foci of low intensity within the matrix due to calcification. A thin periosteal hypointense shell encircles the extraosseous component.⁵⁴

Osteochondroma

Osteochondroma (osteocartilaginous exostosis) is the most common benign bone “tumor.” This is a benign exophytic osseous lesion that has a cartilaginous cap. Osteochondromas develop in bones that form through enchondral ossification; the metaphyses of long tubular bones are the most common sites. These lesions can arise in any portion of the skeleton adjacent to an epiphyseal or apophyseal growth plate. The most commonly involved bones are the femur (30%), humerus (20%), tibia (15%), small bones of the hand and foot (10%), and ilium (5%). The clinical presentation is during childhood in about three-quarters of patients with an osteochondroma. The estimated prevalence of osteochondroma in the general population is 1%; many are asymptomatic. Small osteochondromas can arise at sites of radiation therapy. Multiple osteochondromas occur in patients with the bone dysplasia multiple osteochondromatosis (see Chapter 57).⁵⁵

Osteochondroma most likely represents a developmental defect, rather than a true neoplasm. Injury to the perichondrium of a growing bone may result in isolation or rotation of a small segment of the physis, thereby leading to lateral bone growth from the margin of the metaphysis. With progressive longitudinal enchondral growth of the parent bone, the enlarging osteochondroma migrates away from the adjacent growth plate. Interference with normal metaphyseal modeling occurs as part of this process. Alternative hypotheses concerning the origin of osteochondroma include cartilaginous metaplasia within the periosteal membrane and displacement of cells from the growth plate to the bone surface.

Many osteochondromas are asymptomatic. Those that produce symptoms usually present in a adolescent or older child as they grow to a sufficient size to be palpable or cause fullness due to displacement of adjacent tissues. Growth deformities sometimes occur in adjacent osseous structures, particularly in the forearm or lower leg. Clinical manifestations in these patients may also be related to effects on adjacent nerves or vessels; spinal cord or nerve root compression can occur with a vertebral osteochondroma. Occasionally, a bursa forms over an osteochondroma, and symptoms may be produced by inflammation or hemorrhage of this structure. Acute fracture is an occasional presentation of a pedunculated lesion. Malignant degeneration of a solitary osteochondroma (usually into a chondrosarcoma) is rare; the major clinical warning signs are new onset of pain or sudden increase in size of a previously stable lesion.

An osteochondroma is composed of histologically normal bone. The cortex and medullary portion of the osteochondroma blend imperceptibly with the parent bone on microscopic examination. The lesion may have a pedunculated or sessile configuration. A cartilaginous hyaline cap, which serves as the major site of growth, covers the peripheral aspect of the lesion. Growth of the lesion ceases concomitant to closure of the adjacent normal physis in the parent bone.

Radiographs show an osteochondroma as a sessile or pedunculated exostosis that arises from the external surface of a bone (Figure 63-31). In the long bones, an osteochondroma in a young child is metaphyseal, whereas “migration” during normal bone growth results in a metadiaphyseal or diaphyseal location in teens and adults. The lesion contains normal-appearing spongiosum and cortex, which are contiguous with the parent bone. The cortex covering the lesion is usually somewhat thin and irregular. With the pedunculated variety, the cortex and medullary bone of the stalk are relatively normal, whereas the mushroom-shaped head has thin irregular cortex. The metaphyseal locations of these lesions frequently are sites of tendinous or ligamentous attachments; therefore, growth usually occurs in the direction of the tensile forces created by these attachments, such that a pedunculated osteochondroma points away from the adjacent joint (Figure 63-32). The metaphysis of the parent tubular bone is variably widened due to failure of normal modeling at the site of the osteochondroma. The tip of an osteochondroma is covered by a hyaline cartilage cap, which sometimes contains flocculent densities on radiographs.⁵⁵

Cross-sectional imaging with CT or MR is helpful for selected patients with an osteochondroma to demonstrate the effects of the lesion on adjacent structures. These techniques also show the thickness of the cartilage cap and detect the presence of an overlying bursa. The cartilage has high signal intensity on T2-weighted MR images (see image 57-35 in Chapter 57). The perichondrium results in a thin low signal band covering the cap. The cartilage cap usually is less than 5 mm in thickness; 10 mm is generally considered the upper limits of normal for the cartilage cap of a typical osteochondroma. Findings suspicious for malignant degeneration include a thick cartilage cap (>1.5 cm on MR) and a soft tissue mass.^56,57

Subungual exostosis is a variant of osteochondroma in which an irregular exostosis arises under the nail bed from the distal phalangeal tuft of a finger or toe. The great toe is the most common site. Radiographs show a broad-based osteochondroma deep to the nail. This lesion usually lacks the medullary continuity that is characteristic of osteochondromas at other locations. The pathogenesis of this lesion likely involves trauma.⁵⁸

Dysplasia Epiphysealis Hemimelica (Trevor Disease)

Dysplasia epiphysealis hemimelica (Trevor disease) refers to an osteocartilaginous lesion that arises from an epiphysis or a tarsal or carpal bone. The lesion is histologically identical to an osteochondroma. The most common sites are the femur, tibia, and talus. Although most often solitary, there is occasionally involvement of multiple sites in one extremity. These patients present during childhood or early adolescence with joint deformity or a painless palpable mass. About three-fourths of patients with Trevor disease are male.^58–61

The radiographic appearance of dysplasia epiphysealis hemimelica is that of an intraarticular exostosis or irregular enlargement of one side of an epiphysis. The mass is largely cartilaginous in infants and young children; there is progressive ossification with increasing patient age. The pattern of ossification is usually irregular. The pattern of ossification is such that the mass often appears separate from the underlying bone on standard radiographs (Figure 63-33). MR is particularly helpful for characterizing predominantly cartilaginous lesions such as this in young children (see Figure 58-37). CT or MR is appropriate in older children.

Chondromyxoid Fibroma

Chondromyxoid fibroma is a rare benign bone neoplasm that is predominantly composed of chondroid tissue. The peak presentation is during the second and third decades of life. This tumor is slightly more common in males than in females. The usual clinical presentation is local swelling and pain; pathologic fracture is uncommon.^62–65

Chondromyxoid fibroma can arise in any bone. Sixty to 70% occur in the long bones of the extremities, with the tibia the most common site. In the long bones, the metaphysis is the usual (95%) site of origin. This tumor is composed of a mixture of chondroid, myxoid, and fibrous tissues. There is reactive sclerosis in adjacent bone. Hemorrhagic and cystic components can occur. Chondromyxoid fibroma can cause cortical expansion. Extension into the adjacent soft tissues occurs in some patients, but the overlying periosteum usually remains intact.

The classic radiographic appearance of chondromyxoid fibroma is an expansile lytic metaphyseal lesion with a geographic pattern of bone destruction and a well-defined sclerotic margin. Those that are oval-shaped have the long axis of the lesion oriented parallel to the shaft of the bone. Thin calcified septa are sometimes visible within the mass. Matrix calcification is rarely visible on standard radiographs. Skeletal scintigraphy shows nonspecific increased radiopharmaceutical uptake within the lesion.

Matrix calcification within a chondromyxoid fibroma is often visible on CT; this serves as a differentiating feature of this lesion. On MR, the matrix of this lesion produces low-to-intermediate signal intensity on T1-weighted images and intermediate-to-high signal intensity on T2-weighted images. The appearance on T2-weighted images is heterogeneous due to the mixed composition of the lesion; hemorrhagic foci may also be present. A hypointense rim represents cortical bone or a thin layer of periosteal new bone.

Bizarre Parosteal Osteochondromatous Proliferation

Bizarre parosteal osteochondromatous proliferation (BPOP) is a rare benign, but locally aggressive, lesion of the hands and feet. The peak age range is in the third and fourth decades, although cases have been reported in children as young as 2 years. Patients report a painful enlarging mass. Radiographs show a bony mass with well-defined borders (Figure 63-34). It has a wide base at the surface of the bone and, in many respects, resembles an osteochondroma. However, the medullary and cortical blending with the underlying bone that is typical of an osteochondroma is lacking with this lesion. The tumor is hypointense on T1-weighted MR images and hyperintense on fat-suppressed T2-weighted images; a cartilage cap is lacking.

Other Benign Focal Bone Lesions

Unicameral Bone Cyst

Unicameral bone cyst (simple bone cyst) is an idiopathic, nonneoplastic, fluid-filled intramedullary bone lesion. This lesion is most often discovered in adolescents and older children; approximately 80% present between the ages of 3 and 14 years. The male-to-female ratio is approximately 2.5:1. About two-thirds of these lesions present acutely with a pathologic fracture; most others are detected incidentally on imaging studies performed for another indication. Approximately 95% of unicameral bone cysts are located in the long bones; the proximal humerus and proximal femur are the sites of about three-quarters of these lesions. The calcaneus is an occasional site.^66–68

The wall of a unicameral bone cyst consists of very thin fibrous tissue. The cavity contains clear or yellow fluid. These lesions most often arise within a metaphysis adjacent to the physis. Bone growth “carries” the lesion away from the physis; at the time of presentation, most unicameral bone cysts are located in the metaphyseal or metadiaphyseal region.

Replacement of normal cancellous bone with a fluidfilled cavity produces the radiolucent appearance of a unicameral bone cyst on standard radiographs (Figure 63-35). The overlying cortex is expanded and thinned, but otherwise intact. There often is a thin sclerotic rim that completely surrounds the cyst. Many unicameral bone cysts are wider at the metaphyseal end of the lesion than at the diaphyseal end, resulting in the shape of a truncated cone. Ridges along the inner surface of the cyst wall may produce a trabeculated or multilocular appearance radiographically. Fibrous septa occasionally form compartments in the cyst; these can be demonstrated with MR or with radiography and intralesional contrast injection at the time of treatment.

The thinned cortical bone adjacent to a unicameral bone cyst is susceptible to fracture. This can occur as a simple linear fracture, cortical buckling, or a comminuted fracture. A comminuted fracture can lead to separation of a fragment of cortical bone that “falls” into the pendent portion of the cyst. This radiographic finding is termed the “fallen fragment sign,” which is essentially pathognomonic of this lesion (see Figure 65-50). Following a pathological fracture, a solid layer of periosteal new bone formation occurs and the adjacent cortex becomes thickened.

CT shows a unicameral bone cyst to be an intramedullary lesion. The cyst contents typically have attenuation values between 15 and 20 Hounsfield units. Higher attenuation occurs in the presence of a pathologic fracture, due to hemorrhage. Most often, a unicameral bone cyst produces low-to-intermediate signal intensity on T1-weighted MR images and high signal intensity on T2-weighted images. The presence of blood products related to recent or remote trauma alters these characteristics (Figure 63-36). Fluid–fluid levels are occasionally visible on MR or CT (Figure 63-37). Cross-sectional imaging may also show evidence of septations.⁶⁹

Many unicameral bone cysts are not detectable on skeletal scintigraphy. High-quality images may show mild peripheral uptake due to osteoblastic activity in adjacent bone. A large cyst may be visible as a photopenic focus. A pathologic fracture of a unicameral bone cyst is leads to intense accumulation of tracer.⁷⁰

The conventional surgical technique for the treatment of a large unicameral bone cyst is curettage and bone grafting through a small cortical window. The recurrence rate with this technique, however, is up to 40%. Subtotal resection, without or with bone grafting, is an alternative surgical treatment. There are various percutaneous treatment techniques for unicameral bone cyst as well. These include steroid injection, trephination, and injection of autogenous bone marrow into the cyst. Imaging studies serve to assess the results of therapy and to detect recurrence.^71–73

Aneurysmal Bone Cyst

ABC is a nonneoplastic, osteolytic, expansile bone lesion. The lesion consists of intraosseous blood-filled spaces, separated by connective tissue that contains trabecula of bone or osteoid tissue as well as osteoclast giant cells. Although the pathogenesis is incompletely elucidated, the lesion may represent a hemodynamic disturbance that is due to a primary or secondary venous malformation of the bone. The lesion contains fibrous tissue septa that include trabeculae of reactive new bone. The cyst is partially lined by endothelium. There are dilated efferent veins.^68,74–78