Trismus is a common side effect in head and neck cancer patients after radiation therapy. This project aims to explore the therapeutic effect of Botulinum toxin (Botox) injection to the masticatory muscles in the above-mentioned patients.

In this project, Botulinum toxin will be injected into the masticatory muscles (medial pterygoid and masticatory muscles), combined with oral rehabilitation exercises, to reduce the patient's masticatory muscle tension and spasm. Patient's mouth opening range, quality of life, trismus condition and pain will be evaluated to understand its efficacy.

Inclusion Criteria:

1. Aged over 18 years old;
2. Head and neck cancer patients must have completed radiotherapy for at least 3 months;
3. Those whose mouth-opening distance is less than or equal to 3.5 cm (the functional cut-off point of trismus) and who have tightness or pain in the masticatory muscles;
4. Medical condition is stable;

Exclusion Criteria:

1. Those who have undergone surgery on their masticatory muscles;
2. Patients with obvious muscle atrophy or complete fibrosis of masticatory muscles ;
3. Those with tumor recurrence at the time of admission;
4. Those whose vital signs are unstable due to acute illness.