Presentation
Progressing pain over a long-standing knee bump, worse with movement.
Patient Data
A large cauliflower exostosis is observed at the posteromedial aspect of the proximal tibial metaphysis. There are no definite erosions along the outer surface of the exostosis to suggest malignant degeneration.
There is a heterogeneous mass arising from the posteromedial aspect of the proximal tibia. The mass demonstrates medullary continuity with the tibial metaphysis, and measures 6.8 x 4.2 x 6.1 cm (AP, TV, CC). The mass demonstrates heterogeneous STIR and T1 signal centrally, with a peripheral rim of increased STIR signal representing a cartilaginous cap. The maximal thickness of this cartilaginous cap is 9 mm. Post contrast images demonstrate mild heterogeneous enhancement of the mass as well as the fibrovascular tissue cover the cartilaginous cap, with noenhancement of the cartilaginous cap itself.
The mass encircles the distal semitendinosus tendon, and the tendon has a small fluid collection medially to it after it passes through the lesion. Within the lesion, the semitendinosus tendon demonstrates linear intrasubstance high STIR signal. There is mild edema within interstitial areas of the fat superior to the mass near the semitendinosus tendon before it enters the mass.The sartorius and gracilis tendons passed immediately anterior to the mass. The semimembranosus tendon and its expansions, as well as the medial collateral ligament pass medial and deep to the mass. The popliteal neurovascular structures are separate from the mass.
No meniscal tears are seen. The anterior and posterior cruciate ligaments are intact. The medial and lateral supporting structures of the knee are intact. The extensor mechanism is intact.
The bone marrow is normal in signal intensity all pulse sequences. There is no joint effusion.
Case Discussion
The most worrisome complication of osteochondromas is malignant degeneration, which should be suspected with any new onset of pain or growth after skeletal maturity. In this case, the lesion is symptomatic because it is causing a tendinosis as it encircles and entraps the semitendinosus tendon, and substantially deviates the surrounding musculotendinous structures.
Case courtesy of Dr. Aaron Schein.
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