Inability to open the mouth, i.e. restricted opening, may be caused by oral cancer, and may also be caused by joint disease and oral inflammation, etc. The details are as follows. 1. Oral cancer: for example, mandibular gingival cancer can invade the floor of mouth and buccal area, and when it develops backward to posterior molar area and pharynx, it can cause restricted opening; when buccal mucosa cancer affects the soft palate and pterygomandibular ligament, it can cause restricted opening. 2. Joint diseases: including different types of joint disk displacement, synovitis, arthrogryposis, septic arthritis, joint tuberculosis, traumatic arthritis, rheumatoid arthritis and so on, which may also cause mouth restriction. 3. Oral inflammation: Temporal space infection, infratemporal space infection, occlusal space infection, and pericoronitis of wisdom teeth can all cause different degrees of mouth restriction. There may be other reasons for restricted opening, so it is recommended to go to the hospital in time to clarify the cause of the disease and then carry out targeted treatment.