Muscle Weakness After Repeated Injection of Botulinum Toxin Type A Evaluated According to Bite Force Measurement of Human Masseter Muscle

BACKGROUND: Botulinum toxin type A (BTX-A) has been applied successfully to treat masseteric hypertrophy, but it can cause muscle weakness. OBJECTIVE: To measure the change in maximum bite force (MBF) after BTX-A injection into the human masseter muscle and to evaluate the influence of a booster (repeated) injection. METHODS: Thirty volunteers completed 18-week follow-up, and MBF was measured. At 18 weeks after the first injection, a booster injection was given to 14 patients, and they were followed up until 18 weeks from the booster injection. RESULTS: Mean MBF was approximately 20% lower at 2 weeks than before the injection, and it recovered gradually after 4 weeks to return to the preinjection level at 12 weeks. MBF differed significantly between before the injection and at 2, 4, and 8 weeks after the injection (p<.05). In the booster injection group (n=14), MBF was markedly lower at 6 weeks (p<.05), and it recovered gradually in 12 weeks. CONCLUSION: MBF was significantly lower after booster injection of BTX-A into the human masseter muscle, but it gradually recovered in a predictable pattern, and the degree of discomfort experienced by the subjects had little effect on normal mastication.