Introduction

Trismus is when your jaw muscles become so tight that you cannot open your mouth. It involves spasms in your temporomandibular joint (your jaw joint located next to your ear). It commonly occurs after head and neck cancer treatment. It is usually temporary and typically resolves in less than 2 weeks. However, permanent trismus can still happen and interfere with daily activities such as speaking, eating, swallowing, and complicate dental procedures.¹

Symptoms of trismus 

Trismus is mainly caused by head and neck cancer treatment, temporomandibular joint disorder (TMD) problems, or wisdom teeth removal. Other causes can include:

• Inflammatory conditions
• Infections
• Trauma

Symptoms of trismus include: 

• Not being able to open your mouth as wide as usual 
• Your jaw hurts when you are not talking
• It hurts to chew solid food 
• It hurts when you yawn 

Causes of trismus 

Multiple conditions can result in trismus:

Trismus due to local anaesthesia 

The administration of anaesthesia can cause trismus. The inferior alveolar nerve block is a dental procedure that numbs the lower teeth, gums, and lip. It is the most common nerve block used by dentists. During the administration of this block, the masticatory muscles - the muscles used for chewing food - can accidentally be penetrated, leading to pain-related trismus. When the affected muscle stretches, it triggers pain, causing a reflex contraction and limitation of the mandibular opening.² The mandibular opening is the distance between the upper and lower front teeth when the mouth is open.  Another form of trismus caused by anaesthesia is caused by a needle tract infection, which can lead to trismus 2-3 days after infection.

Trismus as a complication of dental extractions 

Trismus is a common complication that occurs during the surgical extraction of the mandibular molars.³ This is because an inflammatory response to surgical trauma occurs. This type of trismus is usually short-term.

Trismus as a sign of temporomandibular disorders (TMD) 

Temporomandibular disorders are classified by the dysfunction and pain of the temporomandibular joint (TMJ) and muscles. The TMJ is the joint in front of your ear where your lower jaw meets your skull, allowing you to open and close your mouth. Issues with your TMJ can be caused by muscle tension or misalignment from clenching your teeth. This leads to facial pain, functional noises in the TMJ, and limitations in mouth opening. TMD can cause trismus due to muscular pain.⁵

Trismus as a complication of dental infection 

Severe dental infection in the masticatory muscles (chewing muscles) can cause trismus.⁶ Pericoronitis is the most common dental infection associated with trismus -  an infection of the gum tissue surrounding the wisdom tooth.¹ Other infections that do not involve the formation of teeth that can cause trismus include tonsillitis and meningitis. 

Trismus as a sign of malignancy 

Trismus may be caused by malignancy in rare cases. There have been cases where the cause of trismus has been attributed to TMD, leading to a delayed diagnosis of malignancy in the temporal region.⁷ Specialist TMD clinics have therefore developed a checklist to identify trismus cases with atypical features. If a patient presents with the following 5 signs and symptoms, radiographs and referral to a senior clinician must be considered. 

1. Opening less than 15mm
2. No history of clicking 
3. Lymphadenopathy: the enlargement of one or more lymph nodes 
4. Nerve pain caused by damage or irritation to nerves 
5. An abnormal spot in the mouth that could be cancerous

Trismus as a side effect of head and neck cancer treatment 

Trismus is a common side effect of head and neck cancer treatment.⁸ Studies have shown that trismus develops in approximately 40% of head and neck cancer patients.⁹ When the masticatory muscles and TMJ are exposed to radiotherapy, trismus can occur.⁸ Additionally, surgical procedures used to treat head and neck cancer can lead to scarring of the mastication muscles, therefore restricting the mandibular opening.¹⁰

Conclusion

Trismus is a condition that restricts jaw movement due to muscle spasms, often resulting from head and neck cancer treatment, dental procedures, or infections. While it is commonly temporary, persistent cases may indicate underlying disorders like TMD or, in rare instances, malignancy. Early diagnosis and treatment are crucial in managing symptoms and preventing complications.