What is the most common primary malignant neoplasm involving the joints?
Synovial sarcoma. This tumor constitutes 5% to 10% of all soft-tissue sarcomas. It is a highly malignant neoplasm and generally occurs in the lower extremities (70%) of young people, with peak age of occurrence being 15 to 29 years. The tumor usually arises in the periarticular tissues of the knee or ankle. Only 10% of these malignancies arise directly within the joint itself.