Osteochondromas (exostoses) arise in approximately 3% of the population. They are most common in the second decade of life, but may present at nearly any age, and have a slightly higher incidence in males (male:female ratio of 1.4:1). Most osteochondromas occur in the extremities (95%), and the region of the knee is the most common location.

These tumors are an outgrowth from normal underlying bone, without a host response and are continous with the medullary canal and cortex as seen on the radiographs, CT, and MR imaging. The lesions contain a cartilaginous cap, not visible on radiographs unless calcified. They arise at the metaphysis, and migrate toward the diaphysis as normal bone growth occurs. Growth ceases at skeletal maturity.

Although most lesions are solitary, they may be several, or even multiple. There is an inherited, autosomal dominant disorder of multiple exostoses, referred to as multiple hereditary exostoses, which may arise sporadically, in addition. Patients with the disorder, are at higher risk of malignant degeneration of lesions to chondrosarcoma (approximately 10% risk versus approximately 1% risk in individuals without the disorder). Persistent pain or increasing size of osteochondromas, after a patient reaches skeletal maturity, warrants further workup for malignant degeneration. Malignant degeneration is more common in the proximal tumors.