Botulinum toxin type A (BoNT-A) is increasingly used in treating masticatory muscle pain disorder; however, safe doses and reinjection intervals still need to be established. The purpose of this randomized clinical trial was to evaluate the degree and duration of the impairment of masticatory muscle performance. Fifty-seven subjects were randomly divided into two groups: one of which received BoNT-A first (n = 28) while the other received saline first (n = 29), with the cross-over being in week 16, and a total follow-up period of 32 weeks. A total dose of 50 U of BoNT-A was injected in the masseter and temporal muscles bilaterally. Electromyographic (EMG) activity and bite forces were assessed. A significant reduction in EMG activity was observed up to week 18 (p ≤ 001), with total recovery at week 33. A significant reduction in maximum bite force was observed up to week 11 (p ≤ 005), with total recovery at week 25. In conclusion, when treating masticatory muscle pain disorder with 50 U of BoNT-A, a reinjection interval of 33 weeks can be considered safe since the recovery of muscle function occurs by that time.

Background: Injections of botulinum toxin type A (BoNT‐A) have been proposed as an additional treatment modality for patients suffering chronic temporomandibular disorder (TMD)‐related myofascial pain (MFP). BoNT‐A impairs muscle function, along with its analgesic effect, and a minimal effective dose should be used. The objective of this randomized placebo‐controlled crossover study was to evaluate the clinical benefit of a moderate dose (50 U) of BoNT‐A.Methods: Sixty‐six subjects were randomized into two groups, one which received BoNT‐A first and a second which received a saline solution (SS) first. Follow‐ups were performed 2, 11, and 16 weeks after the injections. Diagnostic criteria for temporomandibular disorders (DC/TMD) diagnostic algorithms were used to evaluate characteristic pain intensity (CPI) and pain‐related disability based on the Graded Chronic Pain Scale (GCPS). Electromyographic and bite force were also evaluated.Results: The within‐group analysis showed a significant improvement in pain intensity and pain‐related disability after BoNT‐A (p < 0.001, p = 0.005, p = 0.011) and SS (p = 0.003, p = 0.005, p = 0.046) injections up to week 16. The between‐group analysis of pain‐related variables revealed no differences between groups at any time. Nonetheless, BoNT‐A, but not SS, caused a significant decline in muscle performance. The number needed to treat (NNT) regarding a clinically significant pain reduction (≥30%) was 6.3, 57.0, and 19.0 at 2, 11, and 16‐week follow‐ups favoring BoNT‐A.Conclusions: Injections of 50 U of BoNT‐A might improve MFP symptoms, but the specific effect of the drug on pain compared to the placebo is not obvious.

The outcome evaluation method presented in this case study, including Axes I and II findings combined with the results of quantitative bite force and EMG measurements, provides a good tool for proper evaluation of the effect of BoNT‐A on patients with myofascial orofacial pain and changes in jaw muscle function.

Botulinum toxin is widely used in the treatment of temporomandibular disorder symptoms even though consensus on this treatment modality has not yet been reached. Outcome assessment methods described herein could serve as a novel scientific method for the reliable evaluation of the effectiveness of botulinum toxin in further investigations.