Normal mouth opening degree of the above three fingers can be placed vertically into the upper and lower incisors between the incisal edge (equivalent to 4 – 4.5cm or so), according to the “Work Injury and Occupational Disease Disability Determination,” the degree of mouth opening is divided into the following four grades: 1, difficult to open the mouth I degree: large mouth opening, can only vertically into the index finger and the middle finger (equivalent to 3cm or so); 2, difficult to open the mouth Ⅱ: large mouth opening, can only be vertically into the Open mouth difficulty Ⅱ degree: when the mouth is wide open, can only be placed vertically into the index finger (equivalent to 1.7cm or so); 3, open mouth difficulty Ⅲ degree: when the mouth is wide open, the upper and lower incisors spacing is less than the transverse diameter of the index finger; 4, the teeth closed: completely unable to open the mouth. So why people’s mouth opening degree will be a problem, clinical analysis is as follows: 1, dental inflammation: wisdom teeth pericoronal inflammation and swelling, can spread to the edge of the biting muscle and pterygoid muscle, which leads to the muscle tension and spasm of the difficult to open the mouth. 2, gap infection: molar periapical inflammation further leads to gap infection, such as bite muscle gap, pterygomandibular gap, temporomandibular gap, etc., which can lead to the adjacent muscles are inflammatory invasion of spasm and difficult to open the mouth phenomenon. Malignant tumors: malignant tumors located in the inner and outer occlusal muscle and deep area of the facial side, as long as they infringe on the occlusal muscle, there will be difficulty in opening the mouth; in addition, malignant tumors located in the temporomandibular space and temporomandibular space infringe on the temporal muscle, the internal pterygoid and the external pterygoid muscles can lead to difficulty in opening the mouth. 4. Temporomandibular joint disorders: (1) Intra-articular ankylosis: it is mainly because of the joints being injured by traumas or inflammatory aggressions, especially when young people suffer from traumas and don’t receive proper treatments. (1)Intra-articular ankylosis: mainly due to trauma or inflammatory invasion of the joint, especially when the trauma is not properly treated at a young age, it can lead to intra-articular ankylosis or even bony ankylosis; (2)Intermandibular contracture: mainly due to the formation of inelastic scar tissue after the ulceration of intermandibular soft tissues, it can constrain the movement of the temporomandibular joints; (3)Displacement of the joint disk: the displacements of joint disk will hinder the sliding of condylar process, thus intermittent difficulties in opening the mouth can be seen. Trauma: (1) Zygomatic zygomatic arch fracture, especially the fracture piece or displacement of the broken end may press and stimulate the temporal muscle, resulting in difficulty in opening the mouth; (2) condylar fracture of the temporomandibular joint, resulting in inability of the condylar 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: contaminated rusty nail stab wound, especially into the deep tissue is very easy to lead to tetanus bacillus infection, the disease has about 20 days of incubation period, the earliest symptom may be difficult to open the mouth, should be noted. 7, oral mucosal fibrous lesions: long-term chewing betel nut or excessive consumption of spicy food will stimulate the oral mucosa fibrous lesions, the oral mucosa to lose stretch and elasticity resulting in difficulty in opening the mouth.