Short-term transcutaneous electrical nerve stimulation reduces pain and improves the masticatory muscle activity in temporomandibular disorder patients: a randomized controlled trial

Abstract Studies to assess the effects of therapies on pain and masticatory muscle function are scarce. Objective To investigate the short-term effect of transcutaneous electrical nerve stimulation (TENS) by examining pain intensity, pressure pain threshold (PPT) and electromyography (EMG) activity in patients with temporomandibular disorder (TMD). Material and Methods Forty patients with myofascial TMD were enrolled in this randomized placebo-controlled trial and were divided into two groups: active (n=20) and placebo (n=20) TENS. Outcome variables assessed at baseline (T0), immediately after (T2) and 48 hours after treatment (T1) were: pain intensity with the aid of a visual analogue scale (VAS); PPT of masticatory and cervical structures; EMG activity during mandibular rest position (MR), maximal voluntary contraction (MVC) and habitual chewing (HC). Two-way ANOVA for repeated measures was applied to the data and the significance level was set at 5%. Results There was a decrease in the VAS values at T1 and T2 when compared with T0 values in the active TENS group (p<0.050). The PPT between-group differences were significant at T1 assessment of the anterior temporalis and sternocleidomastoid (SCM) and T2 for the masseter and the SCM (p<0.050). A significant EMG activity reduction of the masseter and anterior temporalis was presented in the active TENS during MR at T1 assessment when compared with T0 (p<0.050). The EMG activity of the anterior temporalis was significantly higher in the active TENS during MVC at T1 and T2 when compared with placebo (p<0.050). The EMG activity of the masseter and anterior temporalis muscle was significantly higher in the active TENS during HC at T1 when compared with placebo (p<0.050). Conclusions The short-term therapeutic effects of TENS are superior to those of the placebo, because of reported facial pain, deep pain sensitivity and masticatory muscle EMG activity improvement.


Introduction
Transcutaneous Electrical Nerve Stimulation (TENS) has been used to control pain in patients with chronic temporomandibular disorders (TMD). However, the muscles should be further investigated, considering that the evidence regarding the improvement of clinical parameters, e.g., reported pain, jaw movement and electromyography (EMG) activity, are divergent and controversial 4,6, 18,22 .
It is well established that TENS can reduce patientreported pain intensity in acute pain conditions 13 , 2,15 . On the other hand, to the best of our knowledge, there is no published study on the effects of TENS on the pressure pain threshold (PPT) of masticatory muscles.
shown that TENS did not affect the PPT of upper trapezius trigger points 11 .
One study showed that the main effects of TENS on the muscular tonus of patients with TMD were: 1) reduction in muscle activity of the anterior portion of the anterior temporalis muscle, during resting posture of the jaw, and 2) increased muscle activity of the masseter muscles, during maximal voluntary contraction (clenching) 22 TENS as effective in reducing the EMG activity of the anterior temporalis and masseter muscles during resting posture of the jaw 19 .
Mandibular kinematics may be subjected to changes that may compromise the muscle and articular functions in TMD patients 16 . Therefore, in addition to pain assessment, identifying dysfunctional EMG behaviors will also be useful in providing therapeutic management and preventing the progression of signs and symptoms 16 . Furthermore, pain reduction and improved function are commonly the proposed goals for treating chronic musculoskeletal pain 12 .
Nevertheless, pain and muscle function are often evaluated separately, and studies to assess the effects of therapies on pain and masticatory muscle function are scant in the literature.
Based on the above, the aim of this study was to investigate the short-term effect of TENS by examining pain intensity, PPT and EMG parameters in subjects with myofascial TMD. Our initial hypothesis was that TENS is as effective in reducing facial pain as it is in improving masticatory muscle EMG activity of TMD patients.

Material and Methods
Participants and design Academic staff and undergraduate students of both genders from the Federal University of Sergipe were eligible. They underwent clinical evaluation for examination of signs and symptoms of TMD and, after the inclusion and exclusion criteria assessment, participants were divided into two groups: (1) TENS placebo (n=20) and (2) TENS active (n=20). The study design was a randomized placebo-controlled trial.
One investigator (R1) who did not participate in data collection performed the randomization with the aid of computed-generated combinatorial analysis, which was used to generate the random sequence. A second investigator (R2) performed the eligibility assessment and the group allocation was made by sealed and opaque envelopes (concealed allocation). Operational issues precluded masking of the researcher who applied the treatment. Then, the same investigator (R2) who performed the group allocation also performed the treatment. A third investigator (R3), blinded for group allocation, conducted the outcome assessments. Finally, a fourth investigator (R4) carried out the interpretation and analysis of results. It is important to note that the participants of this study were also blinded regarding the type of treatment.
Sample size of at least 20 subjects per group was determined based on pilot evaluations, which would be suitable to detect a pressure pain threshold (PPT) difference of 1.02 kgf/cm 2 , standard deviation (SD)        The strengths of this study are mainly related with the systematic assessment of myofascial TMD pain and muscle function using valid and reliable methods.
On the other hand, some limitations that can be highlighted in this study were: a) lack of a long-term assessment; b) lack of a control group without any and pain remission in TMD patients must be considered before any final judgment is made regarding such procedure should be considered; c) lack of a control group without TMD, which could also elucidate the effects of TENS on asymptomatic muscles; d) risk of treatment bias, because the researcher who applied the treatment was aware about the group allocations.

Conclusions
Short-term therapeutic effects of TENS are superior to those of the placebo, because of the reported facial pain, deep pain sensitivity and masticatory muscle EMG activity improvement. Accordingly, we recommend the use of TENS as an effective option for short-term management of masticatory myofascial pain. However, further investigations are required to term effects.