Trismus (Lockjaw): How to Measure Jaw Opening, Causes, and When to Seek Medical Advice

How to check trismus?

Trismus, commonly known as lockjaw, refers to a reduced ability to open the mouth, primarily due to spasm or restricted movement of the muscles of mastication. Checking for trismus involves a simple, objective measurement of the maximum interincisal opening (MIO), typically using a ruler or specialized device, to quantify the range of jaw motion.

Understanding Trismus: An Overview

Trismus is a condition characterized by the inability to fully open the mouth. While often associated with tetanus (hence "lockjaw"), it can arise from a variety of causes, ranging from temporary muscle spasms to more serious underlying conditions.

Common Causes of Trismus:

• Dental Procedures: Most commonly after wisdom tooth extraction or other invasive oral surgeries, due to inflammation or muscle trauma.
• Infection: Peritonsillar abscess, dental infections, or other oral/facial infections.
• Trauma: Fractures of the jaw or surrounding structures.
• Temporomandibular Joint (TMJ) Disorders: Dysfunction or inflammation within the jaw joint.
• Radiation Therapy: Particularly for head and neck cancers, leading to fibrosis of masticatory muscles.
• Neurological Conditions: Tetanus, certain dystonias, or stroke.
• Medication Side Effects: Certain antipsychotics or antiemetics.

Why is it Important to Check? Assessing trismus is crucial for several reasons:

• Diagnosis: To identify the presence and severity of jaw restriction.
• Monitoring Progress: To track improvement or deterioration over time, especially during rehabilitation or treatment.
• Functional Impact: Trismus significantly impairs essential functions such as eating, speaking, oral hygiene, and even breathing in severe cases.
• Guiding Treatment: The degree of trismus helps clinicians determine appropriate interventions, from medication and physical therapy to more advanced medical procedures.

Anatomical Basis: Muscles of Mastication

The ability to open and close the mouth relies on a complex interplay of several muscles. The primary muscles involved in mastication (chewing) and jaw movement include:

• Masseter: A powerful muscle on the side of the jaw, primarily for closing the mouth.
• Temporalis: A fan-shaped muscle on the side of the head, also for closing and retracting the jaw.
• Medial Pterygoid: Located internally, assists in closing the jaw and lateral movements.
• Lateral Pterygoid: Crucial for opening the mouth (depression), protrusion, and lateral movements.

Trismus often involves spasm, inflammation, or fibrosis of one or more of these muscles, particularly the jaw-closing muscles, leading to restricted opening.

The Standard Method: Measuring Maximum Incisal Opening (MIO)

The most common and objective method to check for trismus is by measuring the Maximum Incisal Opening (MIO). This quantifies the distance between the upper and lower front teeth when the mouth is opened as wide as possible.

What You'll Need:

• A clean, rigid ruler (preferably a small, flexible plastic or metal ruler designed for medical use) or a specialized TheraBite® device/TheraMeasure®.
• Good lighting.

Preparation:

• Ensure the individual is relaxed and comfortable.
• Explain the procedure clearly to gain their cooperation.
• Have the individual remove any dentures or removable orthodontic appliances.

Step-by-Step Measurement:

1. Positioning: Have the individual sit upright with their head in a neutral, relaxed position.
2. Maximal Opening: Instruct the individual to open their mouth as wide as they possibly can, without causing excessive pain or strain. Encourage them to push past any initial discomfort but to stop if sharp pain occurs.
3. Ruler Placement:
   • Place the end of the ruler (zero mark) on the incisal edge (the biting edge) of the upper central incisor (front tooth).
   • Measure the vertical distance straight down to the incisal edge of the corresponding lower central incisor.
4. Accounting for Overlap:
   • Overbite: If there is a normal overbite (upper teeth overlap lower teeth vertically when closed), this overlap distance should be added to the measured MIO. To measure overbite, have the individual gently close their mouth and measure the vertical overlap of the upper incisor over the lower incisor.
   • Overjet: If there is an overjet (upper teeth protrude horizontally beyond lower teeth), this does not typically affect the MIO measurement.
5. Repeat and Record: Take the measurement two or three times to ensure accuracy and consistency. Record the largest comfortable measurement in millimeters (mm).
6. Assess Pain: Note any pain experienced during the measurement and its location (e.g., jaw joint, muscle, ear).

Interpreting Your Measurement

Understanding what constitutes a normal or restricted MIO is key to interpreting your measurement.

Normal Ranges:

• Adults: A typical healthy MIO for adults generally ranges from 40 mm to 60 mm.
• Functional Opening: An opening of at least 35-40 mm is usually considered adequate for most daily activities like eating and speaking.
• Children: Normal ranges are typically smaller for children and increase with age.

What Constitutes Trismus: While definitions can vary slightly, trismus is generally diagnosed when the MIO falls below certain thresholds:

• Mild Trismus: 30-39 mm
• Moderate Trismus: 10-29 mm
• Severe Trismus: Less than 10 mm

Subjective Assessment: Beyond the objective MIO measurement, also consider:

• Pain: Is there pain with opening? Where is it located? Is it sharp, dull, or aching?
• Difficulty with Function: Does the restricted opening interfere with chewing, swallowing, speaking, or maintaining oral hygiene?
• Associated Symptoms: Swelling, redness, fever, limited jaw movements in other directions (side-to-side).

When to Seek Professional Medical Advice

While self-assessment of MIO can provide an initial indication, it is crucial to consult a healthcare professional if you suspect trismus or experience persistent jaw restriction.

Consult a professional if:

• Your MIO is consistently below 35-40 mm.
• You experience significant pain with jaw movement.
• The trismus developed suddenly or is worsening.
• You have associated symptoms such as fever, swelling, difficulty swallowing, or breathing problems.
• The condition is impacting your ability to eat, speak, or maintain oral hygiene.
• The trismus persists for more than a few days following a dental procedure or injury.

A dentist, oral surgeon, or physician can accurately diagnose the cause of trismus and recommend appropriate treatment, which may include medication, physical therapy, jaw exercises, or other interventions.

Management and Rehabilitation Considerations

Management of trismus is highly dependent on its underlying cause. While this article focuses on how to check for trismus, it's important to know that treatment often involves:

• Pain Management: NSAIDs, muscle relaxants.
• Physical Therapy: Jaw exercises, stretching, and mobilization techniques to gradually increase range of motion.
• Therapeutic Devices: Devices like the TheraBite® or dynamic jaw opening devices.
• Addressing the Cause: Treating infections, managing TMJ disorders, or addressing post-surgical complications.

Important Considerations and Disclaimer

This guide provides information on how to objectively measure maximum incisal opening to check for trismus. It is intended for informational purposes and for knowledgeable individuals such as fitness enthusiasts, personal trainers, and student kinesiologists to better understand the assessment process. It is not a substitute for professional medical diagnosis or treatment. Always consult with a qualified healthcare provider for any health concerns or before making any decisions related to your health or treatment.