First, the joint may have been traumatized around the joint, resulting in hematoma, which may lead to subcutaneous petechiae and swelling, and then the skin may darken as the swelling is gradually absorbed. Secondly, some patients may have spontaneous bleeding at the knee, and if the joint is impacted at this time, localized bleeding may occur, which may lead to darkening of the joint when it heals. Finally, it may be due to the individual’s physical condition that the individual’s own skin tone is dark, and the joints may show darkening of the joints due to the seriousness of the skin folds, which does not affect the health of the body. There is also a more serious condition, which is a skin disease that causes melanin deposits in the skin, which in turn makes the joints dark. In this case, it is recommended that the patient go to the hospital for an examination to determine the cause of the dark joints and then cooperate with the doctor for treatment.