Pain in the area under the ear when you open your mouth indicates pain in the temporomandibular joint, which is usually caused by mumps, but can also be caused by a dislocation of the temporomandibular joint. If the pain is caused by mumps, it is usually caused by a viral infection after immune deficiency or overexertion, and is usually manifested by swelling and pain under the joint below the ear. The pain is obvious when pressure is applied and is accompanied by difficulty in opening the mouth and is more pronounced when the mouth is opened, so oral antiviral medication or intravenous antiviral medication such as ribavirin should be administered. At the end of the treatment cycle, the inflammation of the parotid gland is usually controlled, the pain is relieved, and the lumps subside. Pain at the jaw joint when opening the mouth is mainly caused by uneven force on the bilateral temporomandibular joints. This action is mainly in the usual chewing without bilateral force, habitually using one side to force. The habitual force on one side and excessive force on the temporomandibular joint on the same side cause the synovial membrane inside to proliferate and the cartilage to wear down, resulting in temporomandibular arthritis. If the pain is caused by TMJ dislocation, usually the patient cannot open and close the mouth, usually the jaw is dislodged downward, and at the same time there will be a significant depression when pressing the TMJ, so go to the hospital to perform TMJ reset.