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Medical dictionaries describe lockjaw as a tightness or spasm of the jaw muscles due to a bacterial infection from Clostridium tetani, commonly called tetanus.  Tetanus has become very rare in the United States with only about 30 reported cases per year. Almost everyone who gets tetanus today has not had tetanus vaccinations or booster shots. 
Tetanus bacteria are common in soil and dust, especially where there is manure. That’s why you may need a tetanus booster shot if you step on a nail. Tetanus bacteria can only get into your body through an open wound. The infection is not spread from person to person. Other ways that people get tetanus include scrapes and cuts, surgical or dental procedures, open fractures, and IV drug use. 
Symptoms of tetanus usually start within 10 days of tetanus entering your body. The first sign may be spasm of the jaw muscles making it impossible to open your mouth, called lockjaw. Other symptoms include:
- Muscle spasms of the stomach
- Painful muscle stiffness all over the body
- Difficulty swallowing
- Fever 
Tetanus is a medical emergency and needs to be treated at a hospital. Although antibiotics and other medications are used for treatment, tetanus can be very serious and sometimes fatal. The good news is that as long as you get you tetanus vaccinations, you should not need to worry about this type of lockjaw. The tetanus vaccines are given to children and preteens. Adults should get a booster shot every 10 years. 
Trismus: Locked Jaw
Trismus is the medical term for several medical conditions that cause a locked jaw. These conditions can make it difficult to open your mouth, which can interfere with eating and swallowing. A locked jaw may also be painful. These are some of the conditions that can cause trismus:
- Infections in or around the jaw
- Fracture of the jaw bone or face
- Dental surgery or procedures
- Cancer of the jaw or jaw muscles
- X-ray treatment (radiation therapy) of the head and neck region
- Drug reactions, especially to tricyclic antidepressants and drugs called phenothiazines
- Nervous system diseases including multiple sclerosis, scleroderma, and lupus
- Temporomandibular joint and muscle disorders [3,4]
Treatment of trismus depends on the cause. To diagnose the cause, your doctor will do a complete history and physical exam and may do imaging studies of your jaw. Treatment may include antibiotics, physical therapy, muscle relaxants, pain medication, a soft diet, and surgery. [3,4]
Temporomandibular Joint and Muscle Disorders: TMJ
Your temporomandibular joint is the joint that attaches your lower jaw to the side of your skull. TMJ disorders affect over 10 million Americans. TMJ is a common cause of a stiff or locked jaw. Women have TMJ more often than men.  TMJ can affect one side of you jaw or both sides. 
The cause of TMJ is often unknown. Possible causes can be muscle pain, a displaced or dislocated joint, grinding your teeth (bruxism), or trauma to the joint. You may be at higher risk of you have arthritis in other joints. [5,6]
Symptoms of TMJ include having a painful or stiff jaw that makes it hard to open your mouth. In some cases, your jaw may lock due to a muscle spasm (trismus). Other symptoms include:
- Jaw pain that moves (radiates) to you face or neck
- A clicking, popping, or grinding noise when you move your jaw
- Pain that gets worse with chewing
- Pain in your ear
- Pain when pressing on the joint (tenderness) [5,6]
The cause of TMJ can be hard to find. Your doctor or dentist may examine your jaw and check to see how much you can open your mouth, if you have tenderness over your joint, and if your joint makes noise when you move it. You may need a dental x-ray or other imaging studies. [5,6]
TMJ usually goes away on its own, so treatment is aimed at giving you relief with a soft diet, over-the-counter pain relivers, and use of cold or hot packs to relieve swelling and muscle tension. You may be given a bite guard to put between your teeth to support and cushion your jaw. You may be asked to do some gentle stretching of your jaw muscles. For more severe symptoms, treatment may include a muscle relaxant, low-dose antidepressant, physical therapy, and in a very few cases, joint surgery. [5,6]
If you have TMJ symptoms:
- Switch to a soft diet.
- Place an ice pack or warm heat over the joint.
- Avoid movements that cause pain, like chewing gum and yawning.
- Avoid grinding your teeth, leaning on your jaw, and chewing your nails or the end of a pencil.
- If TMJ symptoms are persistent or cause a locked jaw, call your doctor or dentist. [5,6]
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