Most bruises in the knee are caused by trauma or impact. Since the subcutaneous tissue of the knee is thin and the patella is below it, after an impact, it tends to cause bleeding or oozing of blood under the skin, and a bruise can be found on the surface. In the early stages of bruising, ice can be applied to the surface for cold compresses, which can effectively reduce bleeding, or reduce the exudation of tissue fluid. After 1-2 days, hot towels can be applied to the surface to accelerate local blood circulation, which can dissipate the bruise as soon as possible. It is also important to check the patient’s blood routine, as well as coagulation routine, if the thrombocytopenia or PT and APTT are significantly prolonged, which can also lead to unexplained subcutaneous bleeding.