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November 2000

Pathological Case of the Month

Author Affiliations


Arch Pediatr Adolesc Med. 2000;154(11):1163. doi:

Diagnosis and Discussion: Osteochondroma (Exostosis)

Figure 1. Clinical deformity of the forefoot with splaying and displacement of the toes.

Figure 2. Plain x-ray films show a bony outgrowth arising from the metaphysis of the proximal phalanx of the toe and growing away from the adjacent joint.

Figure 3. Gross appearance: a broad-based lesion covered with a cauliflower-shaped cartilaginous cap.

Figure 4. Histological findings: bony trabeculae with mature osteoblasts and osteoclasts and cartilage with mature lacunar chondrocytes arranged in rows or clusters (hematoxylin-eosin, original magnification ×250).

Osteochondroma is the most common benign primary tumor of bone.1 It has been reported in every bone in the body that develops by enchondral ossification.2 The sites affected most in descending order of frequency are the distal femur, the proximal tibia, the proximal humerus, the scapulae, pelvis, clavicles, and vertebrae.3 In the foot osteochondroma is commonly located in the phalanges and metatarsal bones.4 A particularly common site is the distal phalanx of the toe where it is a subungual exostosis.5,6 Most such cases involve the great toe. Other locations such as the talus and calcaneus have also been reported though less frequently.4,7,8

It is unclear whether osteochondroma is a true neoplasm or an aberration of development in endochondral bone formation. Uncertainty exists regarding the timing of its first appearance, whether it is congenital or evolves sometime later in life. Eighty percent of cases are diagnosed before age 21 years.2 Plain radiographs are usually diagnostic and no further workup is necessary.1 Differential diagnosis should include periosteal and parosteal osteogenic sarcoma.3

Osteochondroma is present in 2 distinct forms: (1) the solitary lesion occurring sporadically and (2) the hereditary familial osteochondromatosis transmitted to half the offspring and characterized by a single individual affected by numerous lesions throughout the skeleton that may cause severe deformities and growth disturbances of the bones.1-3 A third type of entity also exists: postradiation osteochondroma. The lesions occur in 12% of children with irradiated malignant neoplasms at the edges of the treatment fields.3 Osteochondroma in the long bones is a metaphyseal lesion arising from the cortex near the epiphyseal plate junction. It tends to grow away from the adjacent joint. It macroscopically consists of a mushroom-shaped bony outgrowth with a broad base that is either sessile or pedunculated and covered by a cartilage cap with a cauliflower appearance. The marrow cavity of the osteochondroma is continuous with that of the normal bone. Microscopy reveals mature cancellous (trabecular) bone in which foci of endochondral ossification may be observed.1-3 The cap is made of hyaline cartilage with mature lacunar chondrocytes often arranged in rows or clusters.1,2

Spontaneous regression is rare but has been reported.3 Malignant transformation may take place with a risk of 1% to 2% for solitary lesions and 5% to 25% for multiple lesions.1 Proximal lesions located in the axial skeleton and pelvic and shoulder girdles are at higher risk than average.1 The tumor has a tendency of turning into a low-grade chondrosarcoma but may also develop into an osteogenic sarcoma or a fibrosarcoma in rare cases.3 The osteochondroma grows in size until maturity and remains static throughout adulthood.1 It may be asymptomatic unless it disturbs the surrounding soft tissues such as tendons, nerves, and blood vessels. It can therefore cause tendinitis or bursitis, neural entrapment syndromes, and pseudoaneurysms.3,9

Asymptomatic quiescent lesions require no treatment. Those that are disturbing or suspected to be malignant (due to either rapid growth or sudden onset of pain) should be surgically removed by simple resection. The periosteum covering the lesion should be excised as well to avoid recurrence.1,3 The prognosis is excellent for isolated lesions and depends on the number and location of lesions, the degree and severity of joint and bone deformities, and malignant transformation in the familial type.1-3

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