Findings: Image #1 is an axial CT through the L-3 spinous process demonstrating an exophytic expansile lesion extending from the right aspect of the L-3 spinous process. The lesion demonstrates a corticomedullary continuity. In addition, there is an adjacent region of decreased density which likely represents adjacent fat.

Images #3 and 4 again demonstrate the aforementioned lesion associated with a region of increased signal on T-2 weighted images which demonstrates peripheral enhancement as seen on image #4. These findings are consistent with a well-marginated cartilaginous cap measuring approximately 1.5 cm. These findings are highly characteristic for osteochondroma.

Diagnosis: Osteochondroma of the L-3 spinous process.

Discussion: Osteochondromas represent a developmental growth defect where there is lateral growth of the epiphyseal cartilage plate instead of the normal downward growth toward the metaphysis. Osteochondromas are most often located in the long tubular bone but may on occasion be located in other unusual places such as small bones of the hands or the spine. Vertebral osteochondromas predominate in posterior osseous elements. All osteochondromas have a direct communication with the cortex as well as narrow cavity. The majority of osteochondromas are solitary in nature, but there is a hereditary form in which patients have multiple exostoses.

Osteochondromas generally are nontender, painless, and slow-growing to the time of epiphyseal closure, after which time they undergo little if any growth. However, when an osteochondroma becomes large, or is in certain anatomic sites, it can lead to symptoms with potential complications. An osteochondroma is characterized radiographically by an osseous protuberance arising from the external surface of a bone demonstrating corticomedullary continuity. The tip of the osteochondroma is often covered by a cap composed of hyaline cartilage. In children and adolescents, in whom there is active bone or growth, the cap may be as thick as 3 cm. However, in adults, the cap may be entirely absent. In addition, if the cartilage cap is thicker than 1 cm. in an adult, the suspicion for malignant transformation into a chondrosarcoma should be raised.

In the presence of mechanically significant friction or impingement upon adjacent soft tissues, a bursa may form. In addition, vascular complications such as impingement upon the popliteal artery secondary to osteochondroma of the distal femur or proximal tibia may occur. Although uncommon, rib and vertebral osteochondroma may result in neuropathic symptoms including radiculopathy, spinal stenosis, and cauda equina syndrome.

With a solitary osteochondroma, malignant degeneration is seen in approximately 1% of cases, however, there is an approximately 10% chance of malignant transformation in the hereditary form of multiple exostoses. Because osteochondromas are derived from the epiphyseal plate and generally stop growing on epiphyseal plate closure, any growth or pain in adulthood should raise the suspicion for malignant transformation. Generally in adulthood, the cartilaginous cap measures less than 1 cm, although somewhat larger cartilaginous caps may be seen in completely benign osteochondromas.

Reference:
Karosec D: Symptomatic Osteochondromas: Imaging Features. AJR; 1997;168:1507-1512.

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