Difficulty in opening the mouth refers to the inability to open the mouth to a normal degree. According to clinical studies, generally speaking, the normal degree of mouth opening is about 4.0 cm. If, for various reasons, it is not possible to open the mouth to about 4.0 cm, it is called difficult mouth opening. Local factors 1. Acute mandibular third molar pericoronitis Most often seen in young people. In the early stage, mild localized gingival pain is felt. If the inflammation further develops and affects the masticatory muscles, different degrees of mouth opening restriction may occur and may be combined with systemic symptoms, such as fever, chills, leukocytosis, etc. 2. Osteomyelitis of the mandible is often caused by odontogenic infection, and patients may have numbness of the lower lip on the diseased side and various degrees of systemic symptoms, along with symptoms of mouth opening restriction. 3.Acute purulent parotitis Patients have painful redness and swelling in the parotid area centered on the earlobe, and the pain increases when eating. The symptoms of restricted mouth opening are due to the protective inhibition of local pain when opening the mouth. 4, peri-tonsillar abscess often due to acute purulent tonsillar inflammation. Patients have painful swallowing, salivation, different degrees of mouth opening restriction, and a history of upper respiratory tract infection. 5.Maxillofacial injury Fracture of maxillofacial maxilla and mandible, zygomatic bone and zygomatic arch, displacement of fracture fragment compressing masticatory muscle group and mandibular rostral process, affecting the movement of mandible and causing mouth opening restriction. 6.Maxillofacial tumor Most commonly seen in the posterior wall of maxillary sinus invading the pterygoid fossa after the cancer of maxilla, causing damage to the muscles inside and outside the pterygoid fossa or the cancer of oral cheek invading the buccal muscle; or malignant tumor in parotid gland or nasopharynx invading the masticatory muscles, etc.; all can cause mouth opening limitation. 7.Temporomandibular joint dysfunction disease, especially the irreducible anterior displacement of the temporomandibular joint disc, mostly seen in young adults, is often related to psychosocial factors, abnormal dental relationship, masticatory muscle problems, etc. Patients often have a history of recurrent attacks, and there are various clinical types. The three major symptoms include abnormal jaw movements, joint popping and murmurs, pain, and restricted mouth opening. 8.Temporomandibular joint ankylosis Temporomandibular joint ankylosis refers to those who have long-term difficulty in opening their mouths or cannot open their mouths at all due to organic pathology. It can be divided into intra-articular ankylosis and extra-articular ankylosis. Intra-articular ankylosis is caused by a lesion in one or both joints, which eventually results in fibrous or bony adhesions within the joint, also known as true joint ankylosis. Extra-articular ankylosis is a lesion located in the skin, mucosa or deep muscle tissue between the upper and lower jaws outside the joint, forming an intermaxillary scar contracture, also known as pseudo-articular ankylosis 9. Most commonly seen in children with otitis media, mumps, etc. In addition, it can also be caused by systemic infectious diseases such as measles and scarlet fever. 10.Radiation muscle fibrosis Because the most common treatment for oral and maxillofacial malignant tumors, especially nasopharyngeal cancer, is radiation therapy. And while treating the disease, radiotherapy will also affect the normal tissue structure, such as causing fibrosis of muscles, which leads to difficulty in opening mouth. 11.Oral mucosal disease The most common one is oral mucosal fibrosis, especially seen in Hunan Xiangtan area, people who often chew betel nut. Long-term betel nut chewing will lead to fibrosis of the oral mucosa and loss of muscle elasticity, which leads to difficulty in opening the mouth. Smoking will also enhance the development of fibrosis. 12, temporomandibular joint tumor, including benign and malignant temporomandibular tumor, with the development of tumor growth, will affect the degree of mouth opening, resulting in mouth opening difficulties. This condition is treated with tumor, and the effect is objective. Systemic factors 1, tetanus Generally, there is a history of trauma, and the chewing muscle group spasm causes the teeth to close, and also the expression muscle tightness forms “bitter smile face”. In addition, it is accompanied by generalized muscle paroxysmal convulsions, back muscle contraction. 2, hysteria Hysterical dental closure is easier to diagnose if it is accompanied by other muscle spasms or twitching symptoms throughout the body. This disease occurs mostly in young women with a previous history of hysteria and unique personality traits, usually with psychiatric factors at the onset, followed by sudden onset of difficulty in opening or closing the teeth. This disease is often effective with verbal suggestion or indirect suggestion (using other therapeutic methods combined with verbal suggestion). 3.Scleroderma is a collagen disease, if the masticatory muscles and jaw joints are involved, muscle and joint ankylosis will occur and cause mouth opening restriction. In addition to pain and abnormal sensation in the pharynx during swallowing, it often causes pain in the posterior condylar region as well as pain in the posterior joint region, posterior ear region and neck involvement during opening and chewing. 5. Otogenic diseases External ear canal boils and inflammation of the middle ear also often cause pain in the joint area and affect opening and chewing. 6. Intracranial pathology Various causes of intracranial pathology can cause the patient to be unable to complete the directive mouth opening action, and the patient is unable to open the mouth voluntarily. Especially after the damage of higher brain function, the release of the primitive reflex can make the patient’s mouth and lips immediately appear reflexively closed when touching food.