Early Molecular Response and Microanatomical Changes in the Masseter Muscle and Mandibular Head After Botulinum Toxin Intervention in Adult Mice

In this small study, young adult mice received an injection of Botox in the right masseter muscle, while the left masseter received the same volume of saline as a control. The authors examined gene expression in the mandibular condyles 2 days after the injections and in the muscles 7 days after the injections. Two weeks after the injections, they looked for structural changes in the condyles and masseters.

The results showed dramatic early changes in gene expression. Only 2 days after treatment, the Botox side condyles showed four times as much activity in a gene that promotes bone resorption (RankL) than the saline side condyles. The Botox-injected masseters had greatly increased expression of genes involved in muscle atrophy at the 1 week time point compared to the control side muscles. At the end of the study, 2 weeks after injection, the Botox-injected masseters were about 20% smaller than the control side masseters, and the Botox-side condyles had lost about 40% of relative bone area compared to the control side condyles.

This study is preliminary in many ways. Sample size was very small and only included male mice of one inbred strain. Each outcome was measured at just one time point. The findings that Botox treatment of the masseter cause atrophy of the paralyzed muscle and severe loss of bone in the TMJ region are not new. Nevertheless, the study illustrates how rapidly the tissues react to treatment with this powerful toxin. It is striking that bone loss in the condyles was already triggered by the second day, because this bone loss is likely not a direct effect of the toxin, but rather a response to the absence of muscle loading.

From the point of view of The TMJ Association, important questions that remain include (1) does the bone of the condyle eventually recover, and if so, how long does it take? and (2) does the loss of condylar bone threaten the integrity of the TMJ and make it more vulnerable to injury? To date, there are no studies that address these questions in either animals or humans.