The primary aims of this randomized controlled trial are to evaluate the short-term efficacy of duloxetine on pain intensity reduction in patients with chronic temporomandibular disorders (TMD) and to investigate the predictive value of the inhibitory pain modulation phenotype on this response. Eighty patients with chronic TMD will be randomized and concealed allocated into two treatment groups (duloxetine and placebo). The following phenotypic domains (outcomes) will be investigated: pain clinical characteristics, psychosocial factors, sleep quality, mechanical somatosensory profile and endogenous pain inhibition. The treatment duration will totalize 3 months and the outcome variables will be measured before (T0) and after treatment (T1). In addition, the primary outcome will be the pain reduction magnitude, which will be assessed by the difference between the mean pain intensity during the last week measured at T0 and T1. Furthermore, the conditioned pain modulation (CPM) test at T0 will be regarded as the primary predictor for the treatment response variability. Analysis of covariance (ANCOVA) will be computed with the between-subject factor treatment (2 levels) and the quantitative covariate CPM at T0 in order to establish the main effects of treatment group on pain reduction while controlling the possible interaction with the inhibitory pain modulation capacity (CPM in T0) as a possible predictor of this reduction. The significance level will be set at 5% (p <0.050).
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