I can't open my mouth: why the jaw locks, and when it is an emergency

Opening the mouth depends on two things: the jaw joint just in front of the ear being able to glide, and the muscles that close the jaw being able to relax. The muscles that close the jaw are powerful: the chewing muscle in the cheek (masseter), the temporalis muscle at the temple and the medial pterygoid on the inner side of the lower jaw. When one of these muscles goes into spasm because of inflammation, bleeding or overload, or when the disc inside the joint blocks the way, the mouth will not open. In other words, trismus is not a disease but a symptom; its cause has to be found.

Knowing what is normal lets you make the first distinction at home. MSD Manuals writes that an adult of average build can open their mouth at least 40 mm between the biting edges of the upper and lower front teeth. To allow for differences in body size, it also gives a practical measure: you should be able to fit your index, middle and ring fingers, stacked upright, into your mouth up to the first knuckle. If three fingers do not fit, the opening is restricted. A study of 89 patients treated for head and neck cancer set the point at which patients felt restricted at 35 mm or less; this threshold was developed for that group of patients and should not be applied to everyone.

The first route is muscle fatigue and jaw disorder. The NHS describes a disorder of the jaw joint and chewing muscles (temporomandibular disorder, TMD) as being unable to open the mouth fully, the jaw locking, and pain around the jaw, ear and temple; clenching the teeth, wear of the joint, a blow to the head or face, stress and an uneven bite are among the causes. In this picture the restriction usually gets worse in the morning or after a lot of chewing, and the pain gets worse with chewing.

The second route is the disc inside the joint slipping forward permanently (disc displacement without reduction, commonly called closed lock). MSD writes that in this condition the mouth can open only about 30 mm or less between the front teeth, the jaw drifts towards the affected side on opening, and the clicking often disappears. The typical story is this: a joint that has clicked for years goes quiet one morning and the mouth will not open. MSD writes that the symptoms sometimes settle on their own within 6 to 12 months, and that mouth splints can only help if the disc has not changed shape markedly. A review in the journal of the American Dental Association also stresses that early diagnosis is critical for stopping the restriction from becoming irreversible. The details of clicking and locking are on our jaw joint page.

The third route is an infection coming from a tooth. Inflammation of the flap of gum over a partly erupted lower wisdom tooth (pericoronitis), or a dental abscess, irritates the chewing muscles right next to the inflammation, and the mouth will not open. The NHS lists difficulty opening the mouth and chewing, swelling in the face or jaw, and a high temperature among the symptoms of a dental abscess. On the same page, it writes that having trouble opening your mouth with an abscess is one of the situations that call for 999 or A&E. Of 102 patients admitted to hospital in Australia because of a dental infection, 40.2% had restricted mouth opening and 27.4% had difficulty swallowing; 52.5% of the patients had taken antibiotics before admission. Complications developed in 8.8% of the patients, mostly because of narrowing of the airway.

The fourth route is the dental treatment itself. With the injection used to numb the lower teeth (an inferior alveolar nerve block), the needle passes close to the muscle on the inner side of the lower jaw. If it touches the muscle or the small blood vessels inside it, there can be irritation or a small bleed within the muscle, and a day or two later opening the mouth becomes difficult. Published case reports on this say that restriction of muscular origin like this usually resolves within a few weeks with early measures. The NHS also says that after wisdom tooth removal the jaw can be painful and stiff for up to 2 weeks. What sets it apart is the course: this restriction should ease as the days go by, not get worse.

The fifth route is deep infections of the throat and neck, and causes that have nothing to do with the teeth. According to MSD, pus collecting between the tonsil and the wall of the throat (a peritonsillar abscess, or quinsy) is the commonest of the deep infections of the head and neck; it comes with severe sore throat on one side, being unable to open the mouth, a muffled voice as if speaking with a hot potato in the mouth, and the uvula being pushed to one side. Among the conditions that mimic a jaw disorder, MSD also lists a depressed fracture of the cheekbone arch, a tumour on the bony projection of the lower jaw where a muscle attaches (coronoid osteochondroma) and tetanus. According to the NHS, the symptoms of tetanus begin 4 to 21 days after infection, usually around day 10, and one of the first symptoms is spasm of the jaw muscles.

How long the restriction has been there matters too. A review published in the journal of the American Dental Association writes that restriction that has only just started is mostly linked to pain, inflammation or the muscle's protective reflex, whereas long-standing restriction is associated with changes that are hard to reverse, such as the tissue stiffening (fibrosis). The authors' emphasis is on early diagnosis: waiting for weeks can make a reversible condition permanent. This is also why mouth opening is measured and monitored regularly in people who have had radiotherapy to the head and neck.

The situations in the first list usually loosen up within a few days to a few weeks. If even a single point from the second list applies, do not wait for an appointment.

The sequence deliberately starts with danger: first the situations that threaten the airway or involve a spreading infection are ruled out, and only then are the muscles and joint looked at.

1. 1

   First, the airway and signs of spread

   You are asked about difficulty breathing, speaking or swallowing, being unable to swallow your saliva and drooling, swelling spreading to the neck or under the eye, fever and feeling unwell. If any of these is present, the assessment is done at A&E, not in the dental chair. If difficulty opening the mouth comes with double vision or loss of vision in one eye, tenderness at the temple or on the scalp, or frequent and severe headaches, the NHS also wants an urgent same-day assessment by a doctor; if there is a symptom affecting your vision, go to A&E without waiting.

2. 2

   Measure the opening at home and write it down

   In front of a mirror, open your mouth as wide as you can without forcing it. Check whether your index, middle and ring fingers fit upright between the upper and lower front teeth, or measure it in millimetres with a ruler. Repeating the same measurement every morning shows whether things are getting better or worse, and gives your clinician something concrete to go on.

3. 3

   Look at how it started

   Did it start suddenly one morning, a day or two after an injection or an extraction, over a few days together with swelling and pain, or gradually over months? A sudden start with the clicking gone points to the joint, a start after an injection points to the muscle, a start with pain and swelling points to infection, and a slow build-up points to the tissue stiffening or to rarer causes.

4. 4

   Where does the jaw drift as it opens

   MSD writes that when the disc inside the joint has slipped permanently, the jaw deviates towards the affected side on opening, and that with muscle pain drifting towards the painful side is common as well. Looking in the mirror, you may see the centre line of the lower front teeth move off to one side.

5. 5

   Examination and imaging if needed

   The clinician presses on the muscles and the joint, examines the teeth and the wisdom tooth area and looks at the throat. An X-ray is requested if a dental infection is suspected; if the disc inside the joint is suspected, as MSD notes, a magnetic resonance scan (MRI) taken with the mouth open and closed; and for a deep neck infection, a CT scan.

6. 6

   Referral according to the source

   A dental abscess or an inflamed wisdom tooth is for a dentist, a peritonsillar abscess for an ear, nose and throat (ENT) specialist, a fracture for oral and maxillofacial surgery, joint and muscle problems for a dentist or a jaw joint specialist, and tetanus for A&E.

There is no single treatment for being unable to open the mouth; what is done depends on where the restriction comes from.

The following are decisions often seen with this complaint that drag the problem out or let a dangerous situation slip by unnoticed.

The timings vary a great deal depending on the cause. The sequence below is for the commonest conditions; the plan of the clinician who examines you comes first.

1. The first few days

   With muscle fatigue and jaw disorder, the restriction starts to loosen within a few days with soft food and heat or cold. The NHS writes that jaw disorder usually gets better even without treatment.

2. The 2 weeks after wisdom tooth removal

   The NHS writes that there can be pain, swelling and jaw stiffness during this time, and that the pain and swelling should start to improve after 1 to 2 days. Swelling that gets worse, a bad taste or a fever is not the usual course.

3. Once a dental infection has been treated

   Once the pus has been drained and the source cleared, the irritation of the muscles eases and the mouth opening gradually comes back.

4. Months with a permanently slipped disc

   MSD writes that the symptoms sometimes settle on their own within 6 to 12 months. Rather than waiting out this time without treatment, an early assessment is recommended.

Behind being unable to open your mouth there may be a few days of muscle fatigue, or an abscess that needs an operation. These are the items that determine the scope:

The cause of the restriction

Muscle, joint, dental infection, throat infection or a fracture fall within the remit of entirely different clinicians and involve different procedures.

The need for imaging

An X-ray may be needed for dental problems, an MRI scan for the disc inside the joint, and a CT scan for a deep neck infection.

How far the infection has spread

There is a big difference between drainage done at the dentist and a spreading infection that needs a hospital stay and surgical drainage.

The type of joint treatment

A condition managed with medicine and exercises and one that needs a mouth splint or a procedure inside the joint are different scopes.