A randomized controlled clinical trial was designed to evaluate a physiotherapy treatment for chronic myofascial TMD pain. The intervention group was treated with conventional treatment (occlusal splint and self-care management) and PRT and the control group with conventional treatment and sham PRT. The primary endpoint was self-reported pain at post-treatment (T1) and 3 months follow-up (T2) compared with baseline (T0). Secondary outcomes included Pressure Pain Thresholds (PPTs), Range of Opening of the Mouth (ROM), Neck Disability Index (NDI), and measures of biopsychosocial disabilities like pain catastrophizing (PCS), kinesiophobia (TSK-11), anxiety (STAI) and depression (ST-DEP). The current report follows the CONSORT (Consolidated Standards of Reporting Trials) guidelines for clinical trials . The study was approved by the Ethic and Clinical Research Committee (Institutional Review Board) of the Hospital Clínico San Carlos with protocol number C.P.-C.I. 15/105-E, on the 16th of march, 2015, and the clinical trial was registered (ClinicalTrials.gov: NCT03619889).
Ninety one patients tentatively diagnosed with chronic myofascial TMD pain from the clinic at Faculty of Dentistry (Universidad Complutense de Madrid, Spain) were screened for eligibility criteria. Participants were invited to participate in the study during the routine medical visit. Inclusion criteria were: 1, over 18 years of age; 2, diagnosed with myofascial TMD according to the DC/TMD with or without referred pain and with or without associated TMJ pathology; 3, the primary reason for consultation was the pain of more than 6 months duration; 4, if already treated with an occlusal splint that this had not been modified in the past 6 months; and 5, self-care therapy for at least the last 6 months. Patients were excluded if they exhibited: 1, systemic, neurological or muscle pathology; 2, psychiatric or psychological pathology; and 3, cervical pathology. All participants signed an informed consent prior to their inclusion in the study in accordance with the Helsinki declaration.
Sample size determination
The sample size calculations were based on detection of between-group differences of 1.2 cm of self-reported pain (0-10 cm VAS) as the main outcome measure, assuming a standard deviation of 1.5, a 2-tailed test, an alpha level (α) of 0.05 and desired power (β) of 98.4% for dependent samples and 81.1% for independent samples. The estimated desired sample size was calculated to be at least 35 patients per group. A 10% dropout rate was expected.
Randomization and masking
Patients were randomly assigned to receive PRT or sham PRT. Concealed allocation was done using a computer-generated randomized table of numbers created by a statistician who did not participate in the main trial. Individual and sequentially numbered index cards with the random assignment were prepared, folded, and placed in sealed opaque envelopes. A second external researcher opened the envelope and proceeded with allocation. Another independent statistician carried out the final statistical analysis without knowing which patients had received PRT or sham PRT (Fig. 1).
The two groups received the same conventional treatment (all patients had an occlusal splint and used self-care for at least the past 6 months) by trained and experienced dentists.
Patients allocated to the PRT group received PRT applied to the TrPs of both sides of the masticatory and cervical muscles systematically independently if TrPs reproduced referred sensations or pain, during 45 minutes, once per week, for 5 weeks. The muscles included in the experimental treatment were: upper trapezius, sternocleidomastoid sternal and clavicular portions, superficial and deep masseters, and anterior, medium and posterior temporalis, both sides. The selection of these muscles was partly based on the Diagnostic Criteria for TMD (DC/TMD) . Since some muscles can exhibit multiple TrPs  a clinically pragmatic approach was applied. Therefore, if multiple active TrPs were found, the clinician selected the most painful TrP for receiving PRT. Participants received PRT on all the selected muscles, on both sides. In this study, the PRT described by Lewit  and extended by Simons  was applied. Once the TrPs were located, pressure was applied with the finger for 90 seconds until the resistance of the muscle tissue was felt, the operation was repeated increasing the pressure if the tissue resistance was not reduced (Fig. 2). By convention a decrease in tissue resistance and painful sensitivity suggest an adequate and successful technique. PRT was applied by a physical therapist with 20 years of clinical experience in this therapeutic approach. Patients allocated to the sham group received a similar treatment but applying only a superficial and non-specific pressure. The application of this pressure was trained using a scale and a target below 2 N / cm2 during 90 sec in accordance with Fryer et al.  on the same muscles and in the same sequence as in the PRT group.
Clinical records of all patients included questions regarding the intensity of the symptoms, range of motion and questionnaires. Self-reported pain, pressure pain thresholds, range of the opening of the mouth, neck disability, pain catastrophizing, kinesiophobia, anxiety and depression were assessed at baseline (T0), immediately after the treatment (T1) and at 3 months follow-up (T2).
It has been found that the self-reported orofacial pain is the most frequent symptom for TMD  and the principal motive for consultation . Therefore, we decided self-reported pain as the primary outcome. Self-reported pain was assessed with a 10 cm Visual Analogue Scale (VAS) and 0 defined as “no pain at all” and 10 as “the worst imaginable pain”.
The secondary outcomes were the PPTs at the TrPs  of upper trapezius, clavicular and sternal sternocleidomastoid, deep and superficial masseter, and anterior, medium and posterior temporalis; the ROM (mm) , that is an important parameter for assessment and evaluation of the TMD at follow-up; NDI , a 50-points NDI (0: no disability; 50: maximum disability) was used. It is described that neck disability is frequently associated to TMD ; the TSK-11 , an 11-44 points TSK-11 (11: low fear to movement and pain, 44: high fear to movement and pain); PCS , 0-52 points PCS (0: no catastrophizing level, 52: highest catastrophizing level); State Anxiety (STAI) , an 0-60 points STAI subscale (0: no State Anxiety and 60: highest state anxiety level), and State Depression (ST-DEP) , a 20-80 points ST-DEP (20: lowest state depression and 80: highest state depression).
We also defined a successful outcome when patients reported, at least, 1.2 cm  improvement from baseline (T0) on the VAS at the end of the treatment (T1) and 3 months follow-up period (T2).
Statistical analysis was performed using SPSS software, version 21.0 (Chicago, IL, USA) and SAS® 9.4 (TS1M4). It was conducted by an independent statistical technician as described above. Q-Q plots indicated normal distribution of the primary and secondary outcome parameters. Our primary and secondary evaluations included ANOVA of repeated measurements for three related samples (T0, T1, T2), and adjusted for baseline outcomes for evaluating between-group differences in all the outcomes. To avoid limitations inherent to the test of significance of the null hypothesis and the interpretation of the results, the presence of the phenomenon analyzed was quantified by calculating the effect size. The choice of the magnitude of the effect was made based on the study conducted by Dominguez-Lara . P < 0.05 was considered statistically significant.