Temporal Muscles Thicken Compensatorily After Injection of Botulinum Neurotoxin A Into Masseter Muscles

The masseter and temporal muscles are primary jaw muscles vital for chewing and speaking, which work in tandem for effective mastication. On the basis of the reported studies that masseter botulinum toxin A injection causes masseter muscle atrophy without impairing chewing function in most patients, this study explores whether the temporal muscle thickens compensatively after injection of botulinum toxin A into masseter muscle. In this prospective study, the authors analyzed data from 10 patients who were treated with botulinum toxin A injection into the masseter. Ultrasonographic images of the masseter and temporal muscles were obtained to measure the muscle thickness at rest and in contraction before and 1, 2, 4, 8, and 12 weeks after the injection. Repeated measures one-way ANOVA was used to compare the statistical significance of the data and simpler linear regression to find the relationship of thickness changes between the masseter and temporal muscles. During the 3-month follow-up, there was a statistically significant decrease in masseter thickness and a corresponding increase in temporalis thickness. Masseter thickness decreased from 14.21±2.95 to 10.42±1.59 mm at rest, and from 15.08±3.11 to 11.30±1.8 mm in contraction ( P <0.0001). Temporalis thickness increased from 9.27±2.18 to 13.05±1.49 mm at rest and from 10.12±2.44 to 13.74±1.59 mm in contraction ( P <0.0001). There was a negative linear relationship between the thickness of masseter and temporal muscles. No serious complications occurred. Temporal muscle thickness increased after the injection of botulinum toxin A into the masseter.