Osteochondroma radiographs show the projection that is continuous with what structures?

• A. The periosteum only
• B. The cortex and medullary canal
• C. The joint space
• D. The epiphysis

Answer: (B)

Osteochondroma is a benign bone outgrowth that arises from the metaphysis and grows away from the joint. Its radiographic hallmark is that the bony projection is continuous with both the cortex and the medullary canal of the parent bone. In other words, the outer surface and the internal marrow space of the lesion are the same as those of the host bone, typically connected by a stalk. This cortical and medullary continuity distinguishes it from other lesions and explains why the projection looks like an extension of the bone itself.

The periosteum is the surrounding membrane, not the internal bone structure, so a lesion limited to periosteal involvement wouldn’t show continuous medullary canal continuity. The joint space is not involved because the growth originates from the metaphysis rather than the joint itself. The epiphysis is the end portion of a bone; osteochondromas arise from the metaphysis, again reflecting this continuity with the cortex and medullary canal rather than epiphyseal or intra-articular involvement.