Aneurysmal bone cyst is not encountered frequently and only one previous instance of this lesion of the patella has been reported in the literature. This case, a twenty-two years old R.O.K. army soldier, complained of pain and swelling in the knee joint. Radiologically, the patella was involved, showing a large oval, lytic decreased density with a sharply circumscribed expanded thin margin within the entire patella with an irregular margin at the middle of the articular surface. The patella was excised.
Pathological examination revealed grossly and microscopically an aneurysmal bone cyst.
The patient made an uneventful recovery, and, two years later, he had a stable knee no complaints.
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