1, wisdom tooth pericoronitis: wisdom tooth pericoronitis inflammation swelling, can spread to the edge of the occlusal muscle and pterygoid muscle, which leads to the tension spasm of the two muscles appear mouth opening difficulties. 2, gap infection: molar periapical infection further leads to gap infection, such as the occlusal gap, pterygomaxillary gap, temporomandibular gap, etc., which can lead to spasm of the adjacent muscles by inflammatory invasion and mouth opening difficulties. 3, malignant tumors: malignant tumors located in the medial and lateral occlusal muscles and deep facial area will cause difficulty in opening the mouth as long as they invade the occlusal muscles, and malignant tumors located in the inferior temporal space and temporal space invade the temporal muscles, pterygoid muscles and pterygoid muscles can cause difficulty in opening the mouth. (1) Intra-articular ankylosis: mainly due to trauma or inflammatory invasion of the joint, especially if the trauma is not properly treated when young, it can lead to intra-articular ankylosis or even bony ankylosis; (2) Inter-maxillary contracture: mainly due to the inelastic scar tissue formed after the ulceration of the inter-maxillary soft tissue, which restricts the movement of the temporomandibular joint. (3) Displacement of the articular disc: the displacement of the articular disc will prevent the sliding of the condyle and thus intermittent difficulty in opening the mouth. (5) Trauma: (1) Fracture of zygomatic arch of zygomatic bone, especially the fracture piece or displacement of the broken end may compress and stimulate the temporal muscle leading to difficulty in opening the mouth. (2) Fracture of the condylar process of the temporomandibular joint leading to inability of the articular process to move and thus difficulty in opening the mouth. (3) Rupture of the articular disc and tearing of the articular ligament may lead to difficulty in opening the mouth. 6, tetanus: after being stabbed by contaminated rusty nails, especially into the deep tissue is very easy to lead to tetanus bacillus infection, the disease has an incubation period of about 20 days, the earliest symptom may be difficulty in opening the mouth, should be noted. 7, oral mucosa fibrous lesions: long-term betel nut chewing or excessive consumption of spicy food will stimulate the oral mucosa fibrous changes, oral mucosa loss of stretch and elasticity resulting in mouth opening difficulties.