Enchondroma is the second most common benign tumor of bone, constituting approximately 10% of all benign bone tumors and representing the most common tumor of the short tubular bones of the hand. This benign lesion is characterized by the formation of mature hyaline cartilage. When it is located centrally in the bone, it is termed an enchondroma (Fig. 18.1); if it is extracortical (periosteal) in location, it is called a chondroma (periosteal or juxtacortical) (see Figs. 18.10 and 18.11). Although occurring throughout life, enchondromas are usually seen in patients in their second through fourth decades. There is no sex predilection. The short tubular bones of the hand (phalanges and metacarpals) are the most frequent sites of occurrence (Fig. 18.2), although the lesions are also encountered in the long tubular bones (Fig. 18.3). According to recent investigations, enchondromas result from the continued growth of residual benign cartilaginous rests that are displaced from the growth plate. They are often asymptomatic; a pathologic fracture through the tumor (Figs. 18.4 and 18.5) often calls attention to the lesion.

Enchondroma protuberans is a rare variant. It is a lesion that arises in the intramedullary cavity of a long bone and forms a prominent exophytic mass on the cortical surface. This lesion must be distinguished from an osteochondroma or central chondrosarcoma that penetrates the cortex and forms a juxtacortical mass.

In most instances, radiography suffices to demonstrate the lesion. In the short bones, the lesion is often entirely radiolucent (Fig. 18.6), whereas in the long bones it may display visible calcifications. If the calcifications are extensive, enchondromas are called “calcifying” (Fig. 18.7). The lesions can also be recognized by shallow scalloping of the inner (endosteal) cortical margins because the cartilage in general grows in a lobular pattern (see Fig. 18.1).

Computed tomography (CT) and magnetic resonance imaging (MRI) may further delineate the tumor and more precisely localize it in the bone. On spin-echo T1-weighted MR images, enchondromas demonstrate intermediate to low signal intensity, whereas on T2-weighted images they exhibit high signal intensity. The calcifications within the tumor will image as low-signal-intensity structures (Figs. 18.8 and 18.9). It must be stressed, however, that most of the time neither CT nor MRI is suitable for establishing the precise nature of a cartilaginous lesion, nor can CT or MRI distinguish benign from malignant lesions. Despite the use of various criteria, the application of MRI to the tissue diagnosis of cartilaginous lesions has not brought satisfactory results, although preliminary results of recent trials with fast contrast-enhanced MR imaging showed that this technique might assist in differentiation between the benign and malignant cartilaginous tumors.

Skeletal scintigraphy usually reveals mild to moderate increased uptake of the tracer in uncomplicated enchondromas, whereas the presence of a pathologic fracture or malignant transformation is revealed by marked scintigraphic activity.

Intracortical chondroma is a very rare variant of conventional enchondroma. The lesion is located in cortical bone and is surrounded by sclerosis of the medullary bone and periosteal reaction. Some of these lesions may actually represent periosteal chondroma with an atypical radiographic appearance, as reported by Abdelwahab and associates. Intracortical chondroma can occasionally simulate an osteoid osteoma.

Periosteal chondroma is a slow-growing, benign cartilaginous lesion that arises on the surface of a bone in or beneath the periosteum. It occurs in children as well as adults, with no sex predilection. There is usually a history of pain and tenderness, often accompanied by swelling at the site of the lesion, which is most commonly located in the proximal humerus. As the tumor enlarges, it is seen radiographically eroding the cortex in a saucer-like fashion, producing a solid buttress of periosteal new bone (Fig. 18.10). The lesion has a sharp sclerotic inner margin demarcating it from the buttress of periosteal new bone. Scattered calcifications are often seen within the lesion (Fig. 18.11).

CT may show to better advantage the scalloped cortex and matrix calcification (Fig. 18.12). It also may demonstrate the separation of a lesion from the medullary cavity, an important feature
in differentiation from osteochondroma. MRI findings correspond to radiographic findings, depicting the cartilaginous soft-tissue component. If periosteal chondroma affects the medullary canal, MRI may be useful in depicting the extent of involvement (Fig. 18.13). Fat suppression or enhanced gradient-echo sequences may improve tumor-marrow contrast. The potential pitfall of MRI is marrow edema mimicking tumor invasion or vice versa. Unlike enchondroma and osteochondroma, periosteal chondroma may continue to grow after skeletal maturation. Some lesions may attain a large size (up to 6 cm) and may resemble osteochondromas (Figs. 18.14 and 18.15). Some lesions may mimic an aneurysmal bone cyst. Very rarely the lesion may encase itself intracortically, thus mimicking other intracortical
lesions (such as intracortical angioma, intracortical fibrous dysplasia, or intracortical bone abscess).

Histologically, enchondroma consists of lobules of hyaline cartilage of varying cellularity and is recognized by the features of its intracellular matrix, which has a uniformly translucent appearance and contains relatively little collagen. The tissue is sparsely cellular, and the cells contain small and darkly staining nuclei. The tumor cells are located in rounded spaces known as lacunae. On histologic examination of periosteal chondroma, the findings are identical to those of enchondroma, although the lesion sometimes exhibits higher cellularity, occasionally with atypical cells.

Enchondromatosis is a condition marked by multiple enchondromas, generally in the region of the metaphysis and diaphysis (Fig. 18.19). If the skeleton is extensively affected, with predominantly unilateral distribution, the term Ollier disease is applied. The clinical manifestations of multiple enchondromas, such as knobby swellings of the digits or gross disparity in the length of the forearms or legs, are frequently recognized in childhood and adolescence; the disease has a strong preference for one side of the body. The disorder has no hereditary or familial tendency. Some investigators claim that it is not a neoplastic lesion but rather a developmental bone dysplasia.

The pathogenesis of Ollier disease is unknown. There are two hypotheses for the mechanism of enchondroma formation—one attributes formation to ectopic nests of chondroblasts and the other to the failure of chondrocytes and the growth plate to mature.

Conventional radiography is usually sufficient to demonstrate the typical features of enchondromatosis. Characteristically, interference of the lesion with the growth plate causes foreshortening of the limbs. Deformity of the bones is marked by radiolucent masses of cartilage, often in the hand and foot, containing foci of calcification (Fig. 18.20). Enchondromas in this location may be intracortical and periosteal. They sometimes protrude from the shaft of the short or long tubular bone, thus resembling osteochondromas (Fig. 18.21). Linear columns of cartilage in the form of radiolucent streaks extend from the growth plate to the diaphysis, and a fan-like pattern is common in the iliac bones (Fig. 18.22).

Histologically, the lesions of enchondromatosis are essentially indistinguishable from those of solitary enchondromas, although on occasion they tend to be more cellular.

Also known as osteocartilaginous exostosis, this lesion is characterized by a cartilage-capped bony projection on the external surface of a bone. It is the most common benign bone lesion, constituting approximately 20% to 50% of all benign bone tumors, and is usually diagnosed in patients before their third decade. Osteochondroma, which has its own growth plate, usually stops growing at skeletal maturity. The most common sites of involvement are the metaphyses of the long bones, particularly in the region around the knee and the proximal humerus (Fig. 18.25). Variants of osteochondroma include subungual exostosis, turret exostosis,
traction exostosis, bizarre parosteal osteochondromatous proliferation, florid reactive periostitis, and dysplasia epiphysealis hemimelica (also called intraarticular osteochondroma).

The radiographic presentation of osteochondroma is characteristic according to whether the lesion is pedunculated, with a slender pedicle usually directed away from the neighboring growth plate (Fig. 18.26A), or sessile, with a broad base attached to the cortex (Fig. 18.26B). The most important characteristic feature of either type of lesion is uninterrupted merging of the cortex of the host bone with the cortex of the osteochondroma; additionally, the medullary portion of the lesion and the medullary cavity of the adjacent bone communicate. CT scanning can establish unequivocally the lack of cortical interruption and the continuity of cancellous portions of the lesion and the host bone (Fig. 18.27). These are important features that distinguish this lesion from the occasionally similar looking bone masses of osteoma, periosteal chondroma, juxtacortical osteosarcoma, soft-tissue osteosarcoma, and juxtacortical myositis ossificans (Fig. 18.28). The other characteristic feature of osteochondroma involves calcifications in the chondro-osseous portion of the stalk of the lesion (see Fig. 18.26) and cartilaginous cap. The thickness of the cartilaginous cap ranges from 1 to 3 mm and rarely exceeds 1 cm. On MRI, the cartilaginous cap shows high signal intensity on T2-weighted and gradient-echo sequences. A narrow band of low signal intensity surrounding the cap represents the overlying perichondrium (Fig. 18.29).