A sudden swelling of the knee should be promptly examined by local color ultrasound, as the most common lesions are benign, including benign lipomas or fibroids. In addition, some patients may have a localized hematoma as a result of trauma or a history of hemangioma, which can easily rupture and lead to an enlarged lesion. Many patients may also experience pain or numbness in the lower extremities, as well as soreness and swelling, which can be further confirmed by ultrasonography. The most effective treatment is surgery, where the lesion can be removed under local anesthesia. Post-operative pathology is also required, and due to the special anatomy of the knee, the wound healing is easily affected by activity. Therefore, it is also important to pay attention to regular medication changes after surgery. During the recovery period, it is best to be less active and to elevate the lower limbs to promote venous blood flow and to relieve local swelling.