Locked Jaw (Lockjaw and Slack Jaw) Meaning and Causes

What is a locked jaw?

Locked jaw is a broad term to describe any dysfunction with the nerves, muscles and jaw joints that hampers opening or closing of the mouth. The term lockjaw refers to tetanus of the jaw muscles which means that the muscles are in spasm (tightly contracted). It therefore prevents opening of the mouth meaning that the mouth is stuck in the closed position. The terms are often confused. However, lockjaw is a specific condition associated with the toxin secreted by the Clostridium tetani bacteria that affects the nerves thereby leading to muscle spasm, most notably of the jaw muscles. Another term – slack jaw – indicates a difficulty in closing the mouth due to any number of causes but is also sometimes referred to as a locked jaw meaning that the mouth is stuck in the open position. Locked jaw is therefore discussed here as a commonly used term for problems with lower jaw movement.

Opening and closing of the Jaw

The human jaw is made up of two bones – one that is fixed to skull (upper jaw) and the other that is mobile (lower jaw). The proper name for the upper jaw is the maxilla and the lower jaw is the mandible. The joint where the jaw moves is known as the temporomandibular joint (TMJ). It lies just in front of the ears and is a both a hinge and gliding joint. Several muscles facilitate movement of the jaw, causing it to protrude forwards, recede backwards, drop and rise to open and close the mouth, and slide side-to-side. The main muscles that move the jaw are the :

  • masseter
  • medial pterygoid
  • lateral pterygoid
  • temporalis

The muscles are also known as the muscles of mastication (chewing). The masseter, medial pterygoid and temporalis muscles close the mouth while the lateral pterygoid aids with opening the jaw. Several other accessory muscles help these primary muscles of mastication with opening and closing of the jaw. The primary muscles of mastication are innervated by the mandibular branch of the trigeminal nerve (CN V) which means that it controls the action of these muscles. Strong bands of tissue known as ligaments further support and strengthen the joint, position and movement of the joints. There are three ligaments of the jaw – temporomandibular ligament, stylomandibular ligament and sphenomandibular ligament.

Meaning of Locked Jaw

A problem that affects the jaw bones, jaw joints, muscles or nerves supplying it can lead to difficulty with opening and closing the mouth. In minor cases this is perceived as a difficulty chewing. In severe cases, however, the ability to open or close the jaw may be hampered to such a degree that it is also impossible to open or close the jaw at all. The term trismus describes a state where there is difficulty or inability to open the mouth (lockjaw) whereas the term slack jaw indicates the inability to close the mouth. Both these terms usually refer to a problem with the muscles of mastication or its nerves but may also extend to jaw joint dysfunction which is more correctly known as temporomandibular joint dysfunction (TMJ).

Causes of Locked Jaw


  • Myositis is inflammation of the muscles and when the muscles of mastication are affected, it may lead to a locked jaw in severe cases. Inflammation tends to follow extensive dental surgery like a wisdom tooth extraction.
  • Jaw fracture which may occur after significant head trauma associated with assault, car accident injuries or falls.
  • Temporal arteritis which is inflammation of the temporal arteries also known as giant cell arteritis.


  • Osteomyelitis of the jaw is an infection of the boneĀ  (mandible or maxilla).
  • Tetany which is the painful spasm of muscles due to the action of the Clostridium tetani bacterial toxin and not an infection of the tissue itself.
  • Pericoronitis is an infection of the tissue around a partially erupted tooth, often the wisdom tooth.
  • Abscess which is an accumulation of pus associated with an infection. In locked jaw it may be seen with a dentoalveolar, peritonsillar (quinsy), retropharyngeal or parapharyngeal abscess.
  • Acute tonsillitis which is inflammation of the tonsils often associated with bacterial infections.

Medication and Substances

  • Succinylcholine
  • Anesthetics
  • Narcotics – more commonly associated with teeth grinding and clenching (bruxism) than with trismus.
  • Strychnine poisoning.
  • Toxin (bacterial).

Joint Problems

  • Temporomandibular joint dysfunction (TMD) or TMJ syndrome is a broad term for a number of conditions that affect the joint between the mandible and skull. Read more on clicking jaw.
  • Jaw dislocation when the mandible (lower jaw) portion of the joint slips out of position from its normal orientation.
  • TMJ ankylosis is a difficulty with movement at the jaw joint associated stiffness as a result of a number of different conditions including arthritis, injury or congenital (present from birth) deformities.

Bone Problems

  • Jaw fracture – discussed above.
  • Osteomyelitis – discussed above.
  • Osteonecrosis (jaw) where the blood supply to the bone is compromised leading to death of the bone tissue.
  • Osteoarthritis where the articular cartilage is eroded leading to damage of the jaw bones.

Muscle and Nerve Problems

The muscles of mastication are primarily responsible for opening and closing of the mouth. These muscles are controlled by the brain through signal relayed via nerves. Therefore nerve problems will lead to muscle dysfunction even though the muscles are not diseased in any way.

  • Myasthenia gravis is a chronic disease marked by weakness of the voluntary muscles in the body.
  • Amyotrophic lateral sclerosis (ALS or Lou Gehrig’s disease) is a degenerative condition marked by loss of the motor neurons which are the nerves that control muscle movements.
  • Myositis – discussed above.
  • Tetany – discussed above.

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