Trismus: When a Jaw That Will Not Open Is an Emergency
Trismus means a restricted ability to open the mouth. It is easy to treat it as a side effect of swelling and to focus on the swelling instead. That is a mistake, because the restriction itself carries information that the swelling does not.
Why restricted opening is a depth marker
The muscles that close the jaw do not sit on the surface. Two of them — the medial and lateral pterygoids — lie deep, on the inner side of the jaw, in spaces adjacent to the throat and the base of the skull. The masseter lies over the angle of the jaw, and the temporalis runs up the side of the head.
When an infection spreads from a tooth, it does not travel randomly. It follows the fascial planes — the sheets of connective tissue that divide the head and neck into communicating compartments. As it enters a compartment containing a chewing muscle, that muscle becomes inflamed and goes into protective spasm, and jaw opening becomes restricted.
The consequence is straightforward and important: trismus generally means the infection has left the area immediately around the tooth and entered the deep tissue spaces. A swelling in the cheek with a normal opening is a more contained problem than a smaller-looking swelling accompanied by a jaw that will not open. The second has travelled further, and those deep spaces communicate with the spaces around the airway.
This is the reason trismus appears on essentially every clinical list of signs indicating that a dental infection requires urgent, often hospital-based, assessment.
Measuring it
Normal opening is roughly 35 to 55 millimetres between the biting edges of the upper and lower front teeth — around three of your own fingers stacked vertically.
A rough scale that is useful for tracking:
• Three fingers: normal
• Two fingers: moderate restriction, roughly 20 to 30 millimetres
• One finger: marked restriction, under about 20 millimetres
• Cannot separate the teeth: severe
What matters more than any single reading is the direction of travel. Opening that is reducing over hours is an escalating situation.
Go to A&E now if any of these are present
These indicate the airway or deeper structures may be involved:
• Difficulty breathing, noisy breathing, or a change in voice — a muffled "hot potato" voice is a recognised warning sign
• Difficulty swallowing, or being unable to swallow your own saliva — drooling because swallowing is too difficult is a serious sign
• Swelling under the tongue or in the floor of the mouth, particularly if the tongue is being pushed upwards or backwards. Bilateral swelling of the floor of the mouth with a raised tongue is the presentation of Ludwig's angina, which is a life-threatening emergency
• Swelling spreading down the neck, or neck stiffness
• Being unable to open more than a finger's width, with swelling
• Fever, shivering, feeling very unwell, rapid heartbeat or confusion
• Swelling closing the eye, or any change in vision
• Swelling that has advanced markedly over a few hours
If breathing or swallowing is affected, call 999. Do not wait for a dental appointment, and do not lie flat.
Our article on fever with facial swelling, our article on swelling spreading into the neck and our article on whether a tooth abscess can become life-threatening cover why these matter. Our article on swelling under the eye from an upper tooth covers the upper jaw equivalent.
Same-day dental care if the airway is clear
Where none of the above applies but you have restricted opening with swelling, pain, or a bad taste, this is a same-day dental problem. The source needs treating — drainage, root canal treatment or extraction — because antibiotics alone do not resolve a collection of pus, and the restriction will not settle until the cause is dealt with.
Our article on what counts as a true dental emergency covers the thresholds, and our article on dental abscess symptoms covers the presentation.
The common dental causes
Pericoronitis around a lower wisdom tooth. The single most frequent cause. The gum flap over a partly erupted wisdom tooth becomes inflamed and infected, and because those teeth sit immediately adjacent to the deep muscle attachments, trismus develops early. Our article on a swollen gum behind the last molar covers it, along with our article on a bad taste from an infected wisdom tooth and our article on pain radiating to the ear from a wisdom tooth.
A spreading abscess from a lower molar. Our article on jaw lock from an abscess covers this, and our article on dental causes of sudden facial swelling covers the wider picture.
After a lower wisdom tooth extraction. Common and expected, from surgical access, tissue swelling and muscle inflammation. It should improve steadily after the first two to three days. Worsening restriction after day three warrants a call. Our article on why the jaw feels tight after implant surgery covers the equivalent after implant placement.
After a local anaesthetic injection. The inferior dental block passes close to the medial pterygoid, and needle trauma, a small bleed within the muscle, or irritation can cause restriction that characteristically appears a day or more after the appointment and then peaks. It is uncomfortable but generally resolves over one to several weeks with gentle exercises and heat. The delayed onset is the diagnostic feature.
The causes that are not infective
These matter because the management is different and reassurance is often appropriate.
Jaw joint disc displacement without reduction. A sudden restriction, typically to around 25 to 30 millimetres, with the jaw deviating towards the affected side on opening, no swelling and no fever. Often there was a history of clicking that stopped abruptly at the moment the restriction began. This is a joint problem, not an infection, and is managed quite differently.
Muscle spasm from clenching and grinding, producing stiffness that is worse in the morning and eases through the day, with tender muscles and no swelling.
Fibrosis after head and neck radiotherapy, which develops gradually over months.
Oral submucous fibrosis, associated with areca nut or paan chewing, producing progressive restriction alongside a pale, stiff, banded appearance of the lining of the mouth. This is a condition requiring onward referral, as it carries an increased risk of malignant change.
A tumour in the region of the deep muscle spaces, which can present as progressive restriction without pain — one reason unexplained, gradually worsening trismus without infection should always be investigated.
Tetanus, rare in a vaccinated population but historically the origin of the term "lockjaw".
Medication-related muscle reactions, which can follow certain antipsychotic and antiemetic drugs.
What you can do while arranging care
If the airway is clear and this is a dental infection, contact a dentist the same day. In the meantime: analgesia per packet instructions; soft or liquid food, since chewing may be impossible; keep the mouth as clean as possible; warm salt water rinses if you can get water into the mouth; sleep with the head elevated; and do not apply heat to the outside of the face where infection is suspected, as it encourages spread.
Do not force the jaw open. Do not place a tablet against the gum. Do not attempt to drain a swelling yourself.
Where the cause is post-injection or muscular, gentle range-of-motion exercises and warmth are usually appropriate — but only once infection has been excluded by someone who has looked at it.
Key points
• Trismus indicates that a dental infection has entered the deep tissue spaces, not just the tissue around the tooth.
• Those deep spaces communicate with the spaces around the airway, which is why it is treated as a serious sign.
• Normal opening is about three of your own fingers; restriction reducing over hours is escalating.
• Difficulty breathing or swallowing, drooling, a raised tongue, voice change or neck swelling mean A&E immediately.
• Antibiotics alone do not resolve a collection of pus; the source needs treating.
• Delayed-onset restriction after a dental injection, and sudden restriction with no swelling, have non-infective explanations.
Frequently Asked Questions
Why does a dental infection stop me opening my mouth?
Because the infection has spread along tissue planes into compartments containing the chewing muscles, which go into protective spasm. It indicates the infection has travelled beyond the immediate area of the tooth.
When should I go to A&E rather than see a dentist?
Immediately if you have difficulty breathing or swallowing, are drooling, notice swelling under the tongue or spreading down the neck, have a changed voice, a high fever, or can barely separate your teeth.
How much should I normally be able to open?
Roughly 35 to 55 millimetres, or about three of your own fingers stacked vertically. Under about 20 millimetres with swelling is significant.
My jaw was stiff a day after an injection. Is that an infection?
Delayed onset after a lower jaw injection is characteristic of needle trauma or a small bleed within a deep muscle. It usually settles over one to several weeks, but it should be assessed to exclude infection.
Will antibiotics resolve it?
Antibiotics may help control spread, but they do not clear a collection of pus. The underlying source usually needs drainage, root canal treatment or extraction, and the restriction settles once it is addressed.
My jaw locked suddenly but there is no swelling. What is that?
Sudden limitation to around 25 to 30 millimetres with deviation to one side, no swelling and no fever suggests a jaw joint disc problem rather than infection. It is managed quite differently and should be assessed.
Next Steps
If your jaw will not open fully and there is swelling, seek care the same day. You can contact our team or use our emergency dentist service. Where a jaw joint problem is the cause, TMJ treatment may be appropriate.
Dental Disclaimer
This article is provided for general information only and does not constitute dental or medical advice. Restricted jaw opening accompanied by swelling may indicate a spreading infection requiring urgent hospital assessment. If you have difficulty breathing or swallowing, cannot swallow your saliva, or have swelling in the floor of the mouth or neck, call 999 or attend an accident and emergency department immediately.
Next review due: 13 August 2027
Written by Dr Elisabeth Lichtmannegger · reviewed by Dr Elisabeth Lichtmannegger, GDC 319325
This article is general information, not personal clinical advice. For a diagnosis and a plan tailored to you, book a consultation with a GDC-registered dentist.
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