A normal person’s mouth can be opened up to three fingers, often with their index finger, middle finger and ring finger in a horizontal row, vertically placed in the mouth between the upper and lower incisors. Anyone who cannot fit three fingers into the mouth is said to have restricted mouth opening. The degree of mouth opening restriction is clinically divided into 3 degrees: those who can put in two fingers are considered to have one degree of restriction; those who can put in one finger are considered to have 2 degrees of restriction; those who have less than one finger or cannot open their mouth at all are considered to have 3 degrees of restriction or are called to have closed teeth. The mandible is the only movable bone in the maxillofacial region, with the masticatory muscle attached to it, which governs the opening and closing of the mouth. The mandible is divided into two parts: the mandibular body and the mandibular branch. There is a condyle on the upper posterior part of the mandibular branch, which forms the temporomandibular joint with the temporal joint concavity. Restriction of mouth opening often indicates the involvement of the masticatory muscles or the temporomandibular joint; it can also be caused by fracture displacement obstruction or scar contracture. The distance between the incisive edges of the upper and lower incisors is used as the standard when examining the degree of mouth opening. The mouth opening of a normal person is about the width of the three fingers of the index finger, middle finger and ring finger when they are brought together, which is about 3.7cm on average. Mouth opening limitation is often considered the following diseases: 1, temporomandibular joint disorder: before the onset of the disease, there may be biting hard objects, joint popping, injury, mental and emotional impact, etc. There are pressure points in the temporomandibular joint and masticatory muscles, and sometimes there is a popping sound on the affected side. X-rays and arthroscopy can assist in the diagnosis. 2.Temporomandibular joint ankylosis: Most of them have a history of infection and injury, and the course of the disease is long. The opening of the mouth becomes smaller and smaller, or even completely unable to open the mouth. If the patient has a true temporomandibular joint ankylosis at a young age, it is often accompanied by a deformity of the jaw development, which affects the appearance of the lower part of the face – the affected side is full, the healthy side is flat; bilateral dysplasia, the mandible is too small, the chin is retracted, then it has a beak-like deformity Medical Education|Education.com collects and organizes. The condylar movement is reduced or completely lost. In patients with pseudo-articular ankylosis, adhesive scar strips can be palpated inside and outside the mouth, and X-ray radiography can help to clearly diagnose articular ankylosis. 3, oral and maxillofacial interstitial infection: including acute pericoronitis, maxillofacial interstitial infection, mandibular osteomyelitis, etc. The course of the disease is short, with localized acute inflammatory symptoms such as redness, swelling, heat, pain, and elevated body temperature. However, chronic inflammation can be without acute attack, and the degree of mouth opening restriction caused by infection varies, with the most serious mouth opening restriction caused by infection in the pterygomandibular gap, temporal gap and occlusal gap. 4. Tumors in special areas: When malignant tumors in the buccal mucosa, posterior molar area, lateral side of soft palate and posterior maxillary sinus affect the masticatory muscles, especially the internal pterygoid muscle in advanced stage, mouth opening restriction can occur, accompanied by pain, malodor and cachexia. Masses and ulcers can be examined in the oral cavity. Nasopharyngeal carcinoma and malignant tumors of the pterygopalatine fossa may also present with severe mouth opening restriction with ear symptoms. X-ray and CT examination are needed to help the diagnosis. After radiation therapy for oral and nasopharyngeal malignancies, mouth opening restriction can also be caused due to fibrosis of the masticatory muscles. Such patients have a history of radiation therapy, and the skin of the face and neck has pigmented radiolucent spots. 5, injury: masticatory muscle spasm caused by mandibular fracture, zygomatic arch and zygomatic bone fracture displacement compression rostral prominence can cause mouth opening restrictions. X-rays and CT examinations can assist in the diagnosis. 6. Tetanus: History of surgery, injury or middle ear infection. Onset is quicker. The muscles are in continuous, tonic spasm. In addition to difficulty in opening the mouth, there are facial muscle twitching, bitter smile, difficulty in swallowing or paroxysmal convulsions. 7, hysteria: more common in women. The onset is rapid and the trigger can be asked. There are various spasmodic and flaccid paralysis, clear consciousness, but crying and laughing unpredictably.