There are many conditions that can cause a patient to develop a bulge and tenderness on the outside of the knee. For example, overexertion and strain in the knee area, resulting in a subcutaneous cyst on the outside of the knee, can cause a bulge and tenderness on the outside of the knee. Local tenderness to touch, pressure pain, and good mobility can lead to an ultrasound examination or a local MRI film, which can make a definitive diagnosis. In terms of treatment, most of them can be treated conservatively by giving local hot compress treatment and using Chinese herbal fumigation, which can promote the absorption of the cyst. If conservative treatment is ineffective, surgery can be performed to remove the cyst by incision under anesthesia, during which the cyst wall needs to be removed to avoid postoperative recurrence.