Repeated Injections of Botox in the Masseter Muscle: A Longitudinal Study

The authors of this study examined mandibular bone before and after subjects received Botox injections into each masseter muscle. These volunteers were healthy adults (22-48 years old), both male and female, who wanted injections to slim their faces. (This happens because the Botox causes temporary muscle paralysis and the masseters undergo atrophy when they are not used.) The 20 subjects were randomized into two groups. One group received a single dose of Botox in each muscle while the second group got two doses. The second dose was given 4 months after the first. The bone scans were cone-beam computed tomography (CBCT), a 3-D technique with relatively low radiation. The after-Botox CBCT scan was performed 6 months after the initial injection. The scans were directed at the mandibular angle, where the masseter attaches. The TMJ region was not examined.

Even 6 months after a single injection, the masseter muscles were much smaller than before injection (loss of about 70 mm²). Predictably, the shrinkage was much greater for the two-injection group (loss of about 140 mm²).

The very limited amount of mandibular bone examined is only part of the masseter’s attachment, and an unaffected muscle, the medial pterygoid, attaches to it as well. Therefore, it is surprising that the authors were able to document a loss of bone volume which was statistically significant in the 2-injection group. The loss was due to thinning of the bone, because the mandible as a whole did not become smaller. The authors refer to this finding as an “unwanted side effect” of multiple Botox injections of the masseter muscles.

This study was not ideal and leaves many questions unanswered. Sample size was small, there were no uninjected control subjects, differences between the groups at the start were not reported, the bone examination was very restricted, and it is unknown whether full restoration of muscle and bone would eventually have occurred.  

Nevertheless, it is the first prospective, randomized, longitudinal study of the effects of Botox on the human jaw. The finding of bone loss is consistent with the only other human Botox study on jaws, a cross-sectional examination of the condylar bone of TMJ disorder patients who did or did not receive masseter injections (Raphael et al. 2014). The paper by Lee et al. reinforces the idea that thinned and weakened jaw bones are likely side effects of Botox treatment.