May Be Caused by

 Common

Chronic hematoma

Dermatofibrosarcoma protuberans

Ewing sarcoma

Fibromatosis

Fibrous dysplasia

Giant cell tumor

Gout

Lymphoma

Osteosarcoma

Pigmented villonodular synovitis

Primitive neuroectodermal tumor

 Uncommon

Amyloid

Brown tumor of hyperparathyroidism

Clear cell sarcoma

Extensively mineralized mass

Fibrosarcoma

Giant cell tumor of tendon sheath

Granular cell tumor

Granuloma annulare

Leiomyoma

Malignant fibrous histiocytoma

Metastases

Multiple myeloma

Nodular synovitis