Osteochondroma - extraosseous

Case contributed by Jini P Abraham


Swelling in left knee since 2 years with mild restriction of movements of knee joint. A hard swelling was palpated over the anterior aspect of knee joint.

Patient Data

Age: 45 years
Gender: Female

Lateral and frontal radiographs of the knee joint revealed a lobulated calcified mass lesion in the anterolateral aspect of the knee joint in the infrapatellar region. No adjacent bone destruction noted.

Axial CT sections (bone window) showed a lobulated calcified mass lesion in the lateral aspect of the left knee joint.


Evidence of a lobulated altered signal intensity lesion noted in infrapatellar region, anterolateral to the patellar tendon, showing T1 and T2 isointensity and PD hyperintensity with T2 hyperintense foci within. Fluid collection noted surrounding the lesion posteriorly showing T1 and T2 hypointensity. Post contrast study shows peripheral enhancement of the lesion.

Impression: Extraosseous osteochondroma with bursa formation.

The patient proceeded to excision of the lesion.


Section studied from the bone shows a benign hyaline cartilaginous capped bony growth. Focally the cartilaginous cap is less cellular thinner and replaced by bone. Perichondrium is seen overlying the cartilaginous cap with the trabeculae of lamellated bone underlying it. Intervening areas between the bony trabeculae shows adipose tissue admixed with few blood vessels. Enchondral ossification is seen focally at the cartilage bone interface.

Morphological features are those of Extraosseous osteochondroma.

Case Discussion

This is a pathologically proven case of extraosseous osteochondroma. 

Osteochondromas found within soft tissue without osseous/intra-articular involvement are called extraskeletal osteochondromas. X-ray and CT can confirm the extraskeletal location of the mass and may show foci of calcification. MRI helps to delineate the characteristics of the lesion, assess cartilage thickness and presence of edema in bone and adjacent soft tissues.

Complications include malignant transformation, fracture of bony stalk, nerve impingement syndromes, mass effects that produce mechanical complications and bursitis or bursa formation. A reactive bursa formation is a relatively common complication of osteochondroma and arises from friction between osteochondroma and overlying soft tissue, which was noted in this case.

In this case, on MRI, there was no irregularity or thickening of the cartilaginous cap (>1cm), hence malignancy was ruled out. Size of the lesion and few areas of chondroid tissue made synovial chondromatosis unlikely. 

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