If the patient has a sudden bruise on the thigh, it means that the leg may have developed some bruising reaction. Bruising can be caused by either trauma or localized vascular abnormalities. In terms of trauma, it may be a crush or impact injury that causes local subcutaneous bruising, while vascular abnormality may be due to local vascular blockage or the formation of local cutaneous vasculitis, resulting in a certain hemorrhagic reaction. Of course, the bleeding is mainly a subcutaneous hemorrhagic reaction. Some people appear thighs suddenly green a piece may be related to certain physical stimuli, such as into the cold environment, and the formation of certain capillary contraction, thus producing subcutaneous hemorrhage phenomenon.