A sudden mass in the knee joint may be a soft tissue mass, such as a lipoma, sebaceous cyst, or meniscal cyst. It may also be a bony mass, such as a bone cyst, osteochondroma, or even osteosarcoma, and should be examined promptly. The first step is to see an orthopedic surgeon to determine the origin of the mass in conjunction with a preliminary physical examination. If the mass is considered to be a soft tissue mass, local ultrasound or magnetic resonance examination can be improved. If the mass is considered to be a bony mass, a local x-ray can be completed and the corresponding treatment can be done according to the results of the examination. For soft tissue masses, such as lipomas or sebaceous cysts, small ones can be followed up and observed, while large ones can be surgically removed. If the mass is bony, such as bone cyst or osteochondroma, it can be removed locally. In the case of osteosarcoma, prosthetic replacement or amputation should be considered.