A hard bulge in the knee may be caused by bone cysts, chondrosarcoma, osteochondromatosis and other diseases, which need to be treated with surgery according to the cause of the disease. 1. Bone cysts: a hard bulge in the knee may be caused by bone cysts, patients can take minimally invasive surgical treatment, such as hormone injection therapy, autologous red bone marrow injection and transplantation therapy, drilling and drainage therapy, or open surgery, such as lesion scraping and bone grafting, sub-total resection and bone grafting. 2. Chondrosarcoma: knee long a hard bulge may also be due to chondrosarcoma, patients can take extensive or radical resection, or limb preservation surgery. If the degree of malignancy is high, amputation or joint disarticulation can be taken. 3. Osteochondromatosis: the knee long a hard bulge is also considered to be caused by osteochondromatosis, the patient can take the complete surgical resection of the cartilage cap. Most patients can improve their prognosis after surgery. There are many reasons that lead to the bulging of a hard bag on the knee, and it is recommended that patients identify the cause of the disease in time and actively take relevant treatment measures.