Overview 

The temporomandibular joint (TMJ) is a synovial joint that allows complex movement for the mouth. The TMJ, teeth, and surrounding soft tissue form a complex system that facilitates breathing, eating, and speech.¹ Trismus, often referred to as "lockjaw," is a condition characterised by the reduced ability to open the mouth fully due to muscle spasms, pain, or dysfunction. This disease primarily affects the TMJ, which is primarily responsible for moving the lower jaw.^1,2 

Anatomy of the temporomandibular joint (TMJ) 

TMJ is a hinge synovial joint (it opens and closes in one direction) that allows different kinds of movement to happen. It has two cavities for different movements of the temporal bone. 

Part of the superior compartment is responsible for the convenience of the translational movements at the joint. Translation (or sliding) is a movement in which each point within the moving object is similar in its direction and velocity. One example of translation at the TMJ is when the mandible translates forward and both the teeth and the condyles translate forward, and an equal distance simultaneously has the same direction and velocity. These movements include the:^1,3,4 

• Protrusion: the act of moving the mandible forward 
• Retraction: the posterior displacement in the jaw
• Convergent movements: these are left and right lateral deviations of the mandible, usually referring to the acts of grinding and chewing

The inferior compartment is designed to receive and enable rotational movement. More specifically, the movements can be between the superior surface of the condyle and the inferior surface of the articular disk. Rotational movement of the mandible can occur in all planes: sagittal, frontal and transverse. These movements include the:^1,3,4 

• Depression: a movement of opening the mouth. While the majority of this movement is a result of gravity, muscles help out where there is resistance
• Closing: a movement to close the jaw 

In most cases, not only rotation but also translation takes place at the same time. This leads to complicated motions that enable people to chew and speak.^1,3,4 

Muscles acting on the temporomandibular joint (TMJ)

The muscles involved in the functions of the TMJ are the temporalis, masseter, lateral pterygoid, and medial pterygoid muscles, though the latter two are deep muscles in the jaw.  Also of more importance are indeed the infrahyoid and suprahyoid muscles because these aid in mastication. Every movement of the TMJ occurs when specific muscles are contracted.

The role of TMJ in trismus

Trismus, also named lockjaw, is characterised by the reduction in the ability to open the mouth fully due to pain, muscle spasms, or dysfunction. Multiple conditions can cause it, including disorders related to the TMJ.⁵

TMJ Dysfunction (TMD) and trismus

TMDs may cause pain and dysfunction of related muscles and joints, such as the masseter, temporalis muscles, and pterygoids. When the muscles are swollen, tired, or contracting, this causes trismus or the inability to open the mouth wide.^3,6

Joint dysfunction

Internal derangement affects the normal movement and function of the TMJ; for example, the disc is displaced within the joint, or there are degenerative changes in the joint. This can lead to an impairment of the ability to open the mouth all the way or trismus.^3,5,6

Muscle spasm and tightness

The muscles of mastication consist of the masseter, temporalis, medial, and lateral pterygoid muscles, and they are the muscles for chewing and jaw movement. When these muscles are affected by TMJ disorders, they become inflamed, and then the muscles go into spasm, leading to limited mouth opening and hence trismus.^3,5,6

Pain and inflammation

This is because when the TMJ or the associated muscles become sore, then movement initiates the contraction of pain-protecting muscles. This muscle guarding can substantially decrease the range of jaw motility and lead to trismus. Severe pain that emanates from the TMJ will restrict movement of the jaw because of the pain felt when opening the mouth.^3,5,6

Psychosomatic factors

Both TMJ dysfunction and trismus are prone to worsening when a person is stressed and anxious. Mental conditions may also cause spasms, and the patient may be totally unaware that they are grinding their teeth and could be pulling the jaw tighter, thereby worsening the situation of the patient being unable to open the mouth.^5,6

Trauma or surgery

Any form of trauma to the TMJ, such as from an injury, leads to pain and concomitant muscle spasms, which cause trismus. Likewise, after oral/dental surgery, particularly the hemi or full mandibular or maxillary surgery, which involves the TMJ area, might cause that the patient develop temporary or permanent trismus as a result of muscle spasm or post-operative swelling.^3,5,6

Chronic TMJ issues

However, if the TMJ dysfunction is chronic, the constant stress in the muscles of the jaw may cause a condition where the mouth does not open wide. Therefore, this long-term interruption of the musculature, neural, and occlusal relationships may have something to do with the chronicity of trismus.^3,5,6

Management and treatment

1. TMJ-focused interventions: Treatment for TMJ dysfunction and associated trismus includes the prescription of oral appliances (splints), medicines such as muscle relaxers or anti-inflammatory drugs, and even Botox injections^5,6,7
2. Physical therapy: A physical therapist who is experienced should be able to perform exercises that can stretch and relax jaw muscles and increase joint fluidity^5,6
3. Stress management: Stress relief measures such as taking deep breaths could go a long way in preventing the clenching of the jaws, an action that worsens both TMJ disorders and trismus^5,6

Summary

TMJ is a synovial joint that permits multipositional activities for feeding, breathing, and speaking. The joint is formed by the mandible, the temporal bone, and the articular disc. It permits linear transverse and rotational movements, and it is reinforced by ligaments and muscles. The TMJ is involved in the causes of trismus, which is the inability to open the mouth fully. TMJ disorders can cause muscle spasms, pain and inflammation, psychosomatic disorders, trauma, and chronic trismus. The available treatment measures include TMJ-oriented management, medication, physical therapy and stress control.