[Noninvasive monitoring of changes in the structure and function of masseter muscles after the injection of botulinum toxin type A].

[OBJECTIVE] To evaluate the effect and maintaining period of botulinum toxin A on the masseter muscles through noninvasive monitoring of changes in the structure and function of masseter muscle.

[METHODS] We randomly selected 20 female patients with benign masseter hypertrophy,22-43 years old, who were treated with injection of botulinum toxin A. High-frequency ultrasound was adopted to measure thickness and hardness of muscles.Surface electromyography (sEMG) was used to observe the changes of muscle force. All patients were followed up before and 1,4,12 and 24 weeks after the injection of botulinum toxin type A. Subjective evaluation of the efficacy was performed by questionnaire survey.

[RESULTS] The thickness and hardness of muscle decreased 1 week after injection which attained the lowest level at 12 weeks. The thickness and hardness reduced by 18％-39％ and 20％-32％ respectively at relaxed state of masseter muscles, the muscle force reduced by 78％ during muscle contraction. The muscle recovered to previous state 24 weeks after injection. The subjective evaluation by doctor and patients was consistent with the results by the high frequency ultrasound and EMG detection results.

[CONCLUSIONS] High frequency ultrasound and surface electromyography are effective and noninvasive means for objective evaluation of masseter muscles after the injection of botulinum toxin type A.