Osteochondromas accounted for 33.4% of the benign bone tumors and 10.1% of all tumors in the Mayo Clinic series. Of all tumors in the chondrogenic series, 32.8% were osteochondromas. Most osteochondromas are asymptomatic and are never found, and many of those that are discovered are never excised, so that the actual incidence is much greater than these figures for surgical cases indicate. Approximately 86% of the patients (884) had solitary lesions. One patient with a lesion of the proximal fibula had received radiation treatment for a desmoid tumor previously (Fig. 2.1).

Approximately 62% of the patients in both the solitary and the multiple exostoses groups were males. The literature indicates little sex predilection.

At the time of the first excision of the osteochondroma, approximately 60% of the patients were younger than 20 years, and 49% were in the second decade of life. The ages of patients with multiple exostoses closely paralleled the ages of patients with single exostosis.

Osteochondromas may occur on any bone in which enchondral ossification develops. They usually occur in the metaphyseal region of the long bones of the limbs. Rarely, they are near or in the middle of such bones. The lower end of the femur, the upper end of the humerus, and the upper end of the tibia, in that order, were most frequently involved. The ilium contributed 53 of the 70 osteochondromas arising from the innominate bone. Only 26 patients with solitary lesions had involvement of the small bones of the hands and feet. Involvement of the small bones is not uncommon in patients with multiple hereditary exostoses. An osteochondroma-like lesion of a small bone is much more likely to be a reactive process. The bones of the skull and jaw were not involved with osteochondroma.

The patient’s symptoms are generally related to the size of the tumor. The most common complaint is that of a hard swelling, usually of long duration. Pain may result from impingement of the tumor on neighboring structures and from weight-bearing or other activity. Pain or mass effect may be caused by an overlying bursa. An overlying bursa was significant enough to be described in 13 of the cases. The bursa may suggest a soft tissue mass and hence the possibility of a secondary chondrosarcoma. Ultrasound examination has been suggested to be of use in recognizing the bursa. The bursa may contain osteocartilaginous loose bodies or even nodules of synovial chondromatosis. One instance of nodules of chondrosarcoma situated in a bursa overlying a secondary chondrosarcoma and simulating chondromatosis has been described. Pain may also result from a fracture through the stalk of the osteochondroma.

A palpable mass is ordinarily the only finding. Secondary effects may occur, especially when the tumor impinges on the spinal canal.

The characteristic radiographic appearance is a projection composed of a cortex continuous with that of the underlying bone and spongiosa, similarly continuous. The adjacent cortex often flares to become the base of the tumor. The projection may have a broad base or be distinctly pedunculated. Irregular zones of calcification may be present, especially in the cartilaginous cap, but extensive calcification with radiolucent irregularities of the cap should arouse the suspicion of malignant change. Osteochondroma commonly arises at the site of tendon insertions, and the direction of growth is often along the line of the tendon’s pull. Pedunculated osteochondromas characteristically point away from the nearest joint. The affected bone is often abnormally wide at the level of an osteochondroma because of failure of normal tubulation. Such widening is especially likely in patients with multiple exostoses. In some instances, computed tomographic images and magnetic resonance images may be useful in demonstrating the continuity between the osteochondroma and the underlying bone, especially in lesions of flat bones. These modern techniques are also helpful in accurately delineating the thickness of the cartilage cap (Figs. 2.2, 2.3, 2.4, 2.5 and 2.6).

The gross pathologic features confirm the radiographic pattern. Sessile exostoses may be flat, whereas pedunculated ones are somewhat long and slender, and variations between these exist. Many are cauliflower-shaped, with or without a stalk (Fig. 2.7).

The cortex and periosteal covering of the tumor are continuous with those of the underlying bone. A bursa may develop over the exostosis. The marrow of the tumor may be fatty or hematopoietic, often mirroring the status of the spongiosa of the underlying bone with which it merges (Figs. 2.6 & 2.8).

The gross specimen should be cut perpendicular to the bony stalk so that the true thickness of the cartilage cap can be measured. This cap may cover the entire external surface of a sessile tumor, but it covers only the rounded end of a stalked exostosis. The cap is ordinarily 2 to 3 mm thick and has a smooth surface. The cartilage may be 1 cm or more in thickness in the actively growing benign exostosis of adolescents (Fig. 2.7). Irregularity and thickening of the cap, especially in an adult, demands careful histologic study because of the likelihood of secondary chondrosarcoma. Although a rare thinner cap may be associated with malignancy, secondary chondrosarcomas are usually at least 2 cm in thickness. Cystic change within a cartilage cap also is a cause for concern. If the cartilaginous rim is thin and regular and the underlying spongiosa appears to be normal, the tumor is always benign. If the exostosis is arrested, as may occur in adults, there may be practically no cartilaginous cap. This appearance is especially likely with the rare osteochondroma associated with an overlying bursa. The osteochondroma giving rise to the bursa may be tiny and overlooked.