Probiotics for temporomandibular disorders: a critical appraisal of a therapeutic inference

Background and Objective: The gut-brain-jaw axis has been proposed as a systemic route contributing to trigeminal sensitisation in temporomandibular disorders. A therapeutic inference increasingly follows: that dysbiosis reduces short-chain fatty acid availability, and that probiotic supplementation corrects this deficit. This review appraises the evidence supporting that inference, link by link. Databases and Data Treatment: PubMed, Embase, Scopus and the Cochrane Central Register of Controlled Trials were searched without restrictions on language, publication date or study design. Of 180 records identified, 103 were screened after deduplication and 42 were included. Randomised trials carried into the appraisal were assessed with RoB 2. Each claim is labelled by the level of evidence supporting it, and points of extrapolation are stated. Results: No published randomised trial of a probiotic or synbiotic in a temporomandibular disorder population was identified; two are registered and unpublished. Across seven trials in adjacent chronic pain conditions, methodological quality runs opposite to the direction of the result: the single trial at low risk of bias in every domain is null, while the two clearest positive signals come from trials without a registered protocol or without blinding. Faecal short-chain fatty acid concentrations index an unabsorbed escape fraction rather than tissue delivery. Human genetic analyses do not show the protective direction the deficiency reading predicts for taxa defined by butyrate production. Conclusions: Current evidence does not establish a clinically meaningful short-chain fatty acid deficiency in temporomandibular disorders, nor demonstrate that supplementation relieves pain. Supplementation cannot be recommended outside research.