The change of maximum bite force after botulinum toxin type A injection for treating masseteric hypertrophy

Background: A botulinum toxin type A injection into the masseter muscle has been used as a noninvasive treatment for masseteric hypertrophy. However, muscle atrophy inevitably causes a change of bite force. The aim of this study was to evaluate the change in the maximum bite force after a botulinum toxin type injection for the treatment of masseteric hypertrophy. Methods: Seven patients who had presented for treatment of masseteric hypertrophy participated in this study. Twenty-five units of botulinum toxin type was injected into each masseter muscle, 50 units in total, at two to five points at the prominent portions of the mandibular angle. The bite-force measurement apparatus included a digital multimeter and a bite-force transducer. The maximum bite force between the maxillary and mandibular first molars was measured before injection and at 2, 4, 8, and 12 weeks after injection. Results: The difference in maximum bite force between the preinjection and 2-, 4-, and 8-week postinjection time points was statistically significant. However, there was no such difference between the preinjection and 12-week postinjection values (p < 0.05). Conclusions: The maximum bite force was significantly reduced after injection of botulinum toxin type A for the treatment of masseteric hypertrophy. However, it gradually recovered by 12 weeks.