There are many reasons for the presence of a hard mass on top of the knee, and the condition can be initially determined by a knee x-ray. If it is a free mass in the knee that can slide, it can be treated by minimally invasive knee arthroscopy to remove the mass. In addition, there is a localized anatomical bump on the bone itself, which exists bilaterally and symmetrically and does not require special treatment. Other causes are localized skeletal tumor lesions, such as osteochondroma, bone cysts, and osteosarcoma. In this case, the mass cannot be pushed and is sometimes accompanied by local vasodilation. If it is a benign mass such as osteochondroma, it can be treated with local scraping surgery. In the case of malignant masses such as osteosarcoma, treatment such as extended surgery or even amputation is required. If there is a history of previous traumatic fracture, it may also be a localized bone scab, avulsed bone mass, or ossifying myositis.