Trismus is the medical term for restricted jaw opening, sometimes called “lockjaw.” It’s defined as a maximum mouth opening of 35 mm or less, roughly the width of two fingers stacked on top of each other. Normal jaw opening allows you to fit three fingers between your upper and lower front teeth. When that range shrinks noticeably and your jaw feels tight, stiff, or locked, trismus is the likely explanation.
How Trismus Is Measured and Graded
The key measurement is the distance between your upper and lower front teeth when you open your mouth as wide as possible. This is called the maximal interincisal distance. Anything above 35 mm is considered normal (or 40 mm if you don’t have your natural teeth).
Below that threshold, severity breaks down into grades:
- Grade 1 (mild): 20 to 35 mm. You notice tightness and may struggle with larger bites of food.
- Grade 2 (moderate): 10 to 20 mm. Eating solid food becomes difficult, and dental exams are hard to perform.
- Grade 3 (severe): 5 to 10 mm. Only soft or liquid foods can pass through.
- Grade 4 (very severe): Less than 5 mm. The jaw is essentially locked shut.
You can estimate your own opening at home with a ruler or by stacking fingers between your front teeth. Each finger is roughly 17 to 18 mm wide, so if you can barely fit one finger, you’re in moderate-to-severe territory.
Common Causes
Trismus has a long list of possible causes, but most cases fall into a few main categories: dental problems, infections, trauma, cancer or cancer treatment, and medication side effects.
Dental and Surgical Causes
The most common trigger people encounter is a wisdom tooth extraction. Swelling and inflammation around the jaw muscles after surgery restrict opening. This post-surgical trismus typically starts on the second day after the procedure, peaks around day three or four, and then gradually improves. Impacted wisdom teeth can also cause trismus before they’re removed, especially when the surrounding gum tissue becomes inflamed (a condition called pericoronitis, which is the dental infection most closely linked to trismus).
Infections
Any infection near the jaw muscles can trigger trismus. Dental abscesses are the most common culprit, but tonsillitis, abscesses behind the tonsils, parotid gland infections, and bone infections of the jaw all qualify. Tetanus, the classic “lockjaw” disease, causes trismus through a completely different mechanism: the bacteria release a toxin that overstimulates nerves controlling the jaw muscles, causing them to spasm uncontrollably. Thanks to vaccination, tetanus-related trismus is rare in developed countries.
Trauma and Joint Problems
A fracture or dislocation of the jaw, a blow to the temporomandibular joint (TMJ), or direct injury to the chewing muscles can all produce trismus. Chronic TMJ disorders, where the joint and its surrounding muscles don’t function properly, are one of the more common causes of long-lasting jaw restriction. Arthritis affecting the TMJ, whether osteoarthritis or rheumatoid arthritis, can gradually reduce opening over time as the joint stiffens or becomes damaged.
Cancer and Radiation Therapy
Tumors in the throat, mouth, or salivary glands can physically obstruct jaw movement or invade the muscles responsible for chewing. But radiation therapy for head and neck cancers is an even more frequent cause. Radiation damages muscle fibers and causes scar tissue (fibrosis) to form in the jaw muscles over time. In one study of head and neck cancer patients treated with combined chemotherapy and radiation, about 27% had trismus 10 weeks after treatment, and that number climbed to 37% at six months. The worsening over time reflects the gradual buildup of fibrosis in irradiated tissue.
Medications
Certain drugs cause trismus as a side effect by disrupting the brain’s movement-control signals. Antipsychotic medications are the best-known offenders, but anti-nausea drugs like metoclopramide, some antidepressants (both SSRIs and tricyclics), mood stabilizers like lithium, and even some stimulant medications can trigger involuntary jaw clenching. These reactions happen because the drugs interfere with dopamine signaling, which helps coordinate muscle movements.
What Trismus Does to Daily Life
The practical impact depends on severity, but even mild trismus creates real problems. Eating becomes the first challenge. When you can’t open your mouth wide enough for normal bites, meals take longer, food choices narrow, and nutrition can suffer. At moderate-to-severe levels, you may be limited to soft foods, liquids, or foods cut into very small pieces.
Oral hygiene deteriorates because you can’t open wide enough for proper brushing, flossing, or dental cleanings. This creates a frustrating cycle: dental problems can cause trismus, and trismus makes it harder to prevent new dental problems. Speech can become muffled or strained. Even simple medical procedures like intubation for surgery become complicated when the jaw won’t open adequately.
Treatment and Jaw Rehabilitation
Treatment depends entirely on the underlying cause. If an infection is driving the trismus, treating the infection resolves the jaw restriction. If a medication is responsible, adjusting or switching the drug typically helps. Post-surgical trismus from wisdom tooth removal usually resolves on its own within a week or two as swelling subsides.
For persistent or chronic trismus, particularly after radiation therapy, structured stretching exercises are the main treatment. The goal is to gradually increase jaw opening by repeatedly stretching the muscles and scar tissue. Patients typically work through a program lasting at least three months.
Two approaches are common. The simplest uses stacked wooden tongue depressors (spatulas) placed between the teeth, adding one more depressor as opening improves. More specialized stretching devices, such as the TheraBite system and the Dynasplint Trismus System, use spring-loaded or lever mechanisms to apply controlled, sustained pressure. These devices have been reported to increase mouth opening by up to 14 mm, which can mean the difference between liquid-only meals and being able to eat solid food again.
Physical therapy for the jaw muscles, including massage, heat application, and guided range-of-motion exercises, often complements device-based stretching. In rare cases where fibrosis or joint fusion is severe, surgical intervention may be needed to release the restriction mechanically.
Recovery Expectations
How quickly trismus resolves varies widely. Post-surgical trismus after a tooth extraction follows a predictable arc: onset on day two, worst around days three to four, then steady improvement. Most people regain normal opening within one to two weeks. Trismus from a treatable infection typically clears within days of starting effective treatment.
Radiation-induced trismus is a different story. Because the underlying problem is progressive scarring of muscle tissue, it often worsens for months after treatment ends and can become permanent without active rehabilitation. Starting stretching exercises early, ideally during or shortly after radiation treatment, gives the best chance of preserving jaw mobility. Even with diligent therapy, some patients never fully recover their pre-treatment opening, but most can improve enough to eat and speak comfortably.
Drug-induced trismus tends to resolve relatively quickly once the offending medication is stopped or the dose is adjusted, often within hours to days depending on the drug.