There are many causes and conditions that lead to red patches on the skin of a patient’s knee. For example, in rheumatoid knee osteoarthritis, rheumatic nodules are formed on the local skin after rheumatism has affected the knee joint, and the patient will have red patches, which will gradually worsen after getting cold. This phenomenon will gradually be relieved with the application of heat or anti-rheumatic treatment. If the patient has localized contact dermatitis or allergic reactions in the knee joint, this clinical manifestation will also appear in appearance and will gradually disappear with the right symptomatic treatment. Therefore, the patient needs to be seen promptly after this clinical manifestation occurs, and the correct treatment by the doctor will provide relief.