Botulinum Toxin Type A for Treatment of Masseter Hypertrophy: Volumetric Analysis of Masseter Muscle Reduction over Time

ABSTRACT

BACKGROUND: There has been increasing interest in facial contouring procedures throughout Asian countries. As such, botulinum toxin A injections for masseteric hypertrophy have become a common procedure provided to patients who desire non-surgical correction of a square-angled mandible. We published a retrospective review of our initial results and our technique and treatment protocol in 2005. We also completed a long-term follow-up of the results (average follow-up period of 4.28 years) and the efficacy of repeated injections in 2010. The purpose of the current study is to systematically evaluate the changes to the masseter muscle at weekly intervals to determine the physiologic effects of botulinum toxin A injection. METHODS: Eight patients were prospectively followed on a weekly basis after botulinum toxin A injection for masseteric hypertrophy. Eight patients were followed for 15 weeks and four patients were followed for 25 weeks. Changes in the thickness of the muscle were recorded and analyzed. RESULTS: A reduction in the muscle thickness was found during the clenching phase of the muscle in the first week followed by a reduction in thickness during the resting phase in the second week. The reduction in muscle thickness continued until the eleventh week after which there was a gradual, but incomplete, return of muscle thickness over the study period. CONCLUSIONS: There is a predictable, phasic reduction in muscle thickness after botulinum toxin A injection for masseteric hypertrophy. This reduction first occurs during the clenching phase followed by a concomitant reduction during the resting phase. Maximal size reduction occurs at 11 weeks followed by gradual muscle size recovery.