Grading Trismus
Trismus is graded by measuring maximal interincisal distance (mouth opening), with ≤35 mm defining clinically significant trismus that warrants intervention. 1, 2
Measurement Technique
- Measure the maximal interincisal distance in millimeters using a ruler or caliper between the incisal edges of the upper and lower central incisors when the patient opens their mouth as wide as possible 3, 1
- Normal mouth opening ranges from approximately 40-60 mm, with a median of 32-38 mm in healthy populations 1, 4
Classification System
Clinically Significant Trismus
- Mouth opening ≤35 mm is the validated cut-off point for defining trismus, confirmed by receiver operating characteristic curve analysis with a sensitivity of 0.71 and specificity of 0.86 2
- This threshold correlates strongly with patient-reported chewing deficits, dietary restrictions, and reduced quality of life 1
Treatment-Specific Considerations
The 35 mm cut-off may be adjusted based on treatment modality:
- Surgery alone: 37 mm or less indicates trismus 2
- Radiotherapy alone: 33 mm or less indicates trismus 2
- These variations reflect different mechanisms of restriction (surgical fibrosis versus radiation-induced muscle changes) 2
Severity Stratification
While not formally standardized, clinical severity can be conceptualized as:
- Mild trismus: 30-35 mm - functional limitations begin to appear 1
- Moderate trismus: 20-29 mm - significant functional impairment 4
- Severe trismus: <20 mm - marked disability affecting nutrition and oral hygiene 3, 4
Clinical Context Assessment
Beyond the numerical measurement, assess:
- Tumor invasion into masticatory muscles (particularly medial pterygoid), which may manifest as trismus and represents a contraindication to certain surgical approaches 5
- Temporomandibular joint disease or fibrosis from previous radiation, which requires differentiation from tumor-related trismus 5, 6
- Time course: Pre-treatment trismus (42% incidence in oral malignancies) versus post-treatment trismus (54% at 6 months post-surgery) 4, 7
Functional Impact Documentation
Complement the objective measurement with:
- Patient-reported change in mouth opening since treatment or diagnosis, as this subjective element combined with millimeter measurement provides superior outcome assessment 1
- Impact on chewing, diet consistency, dental hygiene, and quality of life, which correlate strongly with the 35 mm threshold 1
Common Pitfalls
- Avoid using variable cut-offs (15-40 mm range exists in older literature), as this prevents meaningful comparison and standardized care 3
- Do not rely solely on visual estimation - objective measurement is essential for tracking progression and treatment response 4
- Recognize that edentulous patients are eight times more likely to have trismus, which may confound measurement if not using consistent landmarks 7