The pain in the maxillomandibular joint when opening the mouth may be caused by trauma, mumps, temporomandibular joint dysfunction and other reasons. 1. Trauma: It may be due to the local soft tissue injury caused by trauma, accompanied by the symptoms of local tissue swelling and bruising, and the symptoms of jaw joint dislocation. 2. Mumps: pain in the maxillomandibular joint when opening the mouth may be caused by mumps. The main symptoms of mumps include redness, swelling, fever, swelling and pain, which may be accompanied by pain in the maxillomandibular joint when opening the mouth. 3. Temporomandibular joint dysfunction: If the maxillomandibular joint hurts when you open your mouth and is accompanied by symptoms such as popping when you open your mouth, headache, tinnitus, etc., it may be caused by temporomandibular joint dysfunction. Clinical patients who have pain in maxillomandibular joint when opening mouth may also be caused by joint tumor, and should go to the stomatology department of regular hospitals in time for detailed examination to clarify the cause of the disease and then treat it correctly.