Introduction

What is trismus?

Trismus, also known as "lockjaw," is a condition where you have difficulty opening your mouth. It is due to the muscles of the lower jaw becoming restricted for various reasons, making normal movements like eating, speaking, or even brushing your teeth more challenging. 

Several factors can cause trismus. One of the most common reasons is dental treatment or surgery. It can also occur after dental extractions, untreated dental infections, trauma to the jaw, TMJ disorders, and as a side effect of head and neck cancer treatment.

Anatomy and physiology of jaw movement

Anatomy of jaw muscles

The lower jaw moves around the temporomandibular joint. These are 2 joints on either side of your face, which help in the function of opening and closing your mouth. There are four important muscles attached to these joints, and they are:

1. Masseter
2. Temporalis
3. Medial pterygoid
4. Lateral pterygoid

The jaw closure is the most active process involving three muscles- masseter, temporalis and medial pterygoid, whereas jaw opening involves just one muscle- lateral pterygoid. Jaw closure seems to be more powerful than the jaw opening movement. If for any reason, these muscles are affected, it might lead to trismus.

 Causes of trismus

• Trauma and post-surgical complications
• Infections (e.g., pericoronitis, tetanus)
• Radiation therapy and fibrosis
• Temporomandibular joint (TMJ) disorders
• Neurological and systemic conditions

What is interincisal distance?

Interincisal distance is the distance measured between the upper and lower incisors on maximal mouth opening. People affected by this lockjaw will typically have a much smaller interincisal distance.

Normal range of interincisal distance

Normally, a healthy person can open their mouth about 35- 55 millimetres wide, roughly the width of three fingers. With trismus, this range is often reduced significantly, which impacts daily functions like speaking, eating and maintaining good oral hygiene.

Methods for measuring interincisal distance

The following are the different ways to measure the interincisal distance:

• Direct measurement using a calliper or ruler- vernier scale

People affected by this condition will be asked to sit upright with their head placed in a neutral position. After about a minute, he/she will be asked to open their mouth as wide as possible without feeling the pain. The vernier ruler will then be placed between the upper and lower central incisors, and the measured value will be recorded. The measurements will be taken periodically during and after treatment to check the progress of the treatment.

• Photographic and imaging techniques

A study shows that in a clinical research application, a photograph of the patient’s mouth opening can be taken by an investigator with the reference frame, so the landmarks can be used for calibration of the image. Red, green and blue are the reference points in the frame for comparison and assessment.

• Digital and intraoral scanning methods.

Intraoral scanning could be used for people with mild to moderate trismus. Dental treatments might be uncomfortable for patients who have recently undergone surgeries or radiotherapy. Using intraoral scanners instead of conventional impressions could help make dental appliances for them more comfortable.

Normal vs. restricted interincisal distance

Normal reference values (age, sex, and ethnicity considerations)

Normally, a healthy person can open their mouth about 35-45 millimetres wide, roughly the width of three fingers. 45-55mm is the average mouth opening for females, and for males it's slightly higher, 50-60mm.

Classification of trismus severity based on interincisal distance

If the mouth opening is 35-45 mm interincisally, it is categorised as mild trismus. Moderate trismus, if the mouth opening is 15mm to 29mm and severe trismus if the mouth opening is 15mm or less than that.

Clinical assessment and diagnostic criteria

Patient history and symptom evaluation are the initial assessment for diagnosis. Physicians might perform a manual examination of jaw muscles to assess areas of tenderness, and the mouth opening will be recorded. People with trismus might also complain of pain in the jaw muscles, with clicking sounds on opening and closing. Diagnosis will be based on trismus of dental origin or non-dental origin, and treatment will be planned accordingly.

Implications for treatment and management

If trismus is of dental origin or post-surgical dental extractions, conservative measures such as heat therapy, warm saline rinses, analgesics, and muscle relaxants will be recommended to manage the initial stage of muscle spasm.

If trismus is due to fibrosis, a trismus appliance could be of use. There are different types of trismus appliances, and the most commonly used are the Dynamic bite opener, the threaded tapered screw, and screw type mouth gag. However, if trismus is due to TMJ disorder or fibrotic bands, surgical intervention will be the choice of treatment to lyse those fibrotic bands.

The speech-language pathologist (SLP) also plays a vital role in people affected with trismus by engaging them in proactive/preventive care and assisting them to improve upon speech, social communication and swallowing. 

Future directions and research

With the advancement in AI, measurement of interincisal distance will be much easier and comfortable in future. There will also be potential development of standardised assessment tools for remote monitoring and telemedicine applications.

Summary

Trismus refers to restricted mouth opening. Healthy individuals with normal mouth opening have an interincisal distance of 35mm- 55mm. People affected with this condition will have <35mm mouth opening. Infants might be born with trismus as a congenital disorder, or trismus could be acquired due to post-surgery dental extractions, as a complication of trauma to the face and lower jaw or as a sign of TMJ disorder, malignancy or infection. Trismus affects patients' quality of life. Sometimes trismus is self-limiting and the condition will improve in a couple of weeks with conservative measures, physiotherapy and muscle relaxants. However, in very rare cases, surgical intervention might be required.