JOMP. Basic Understanding of Transcutaneous Electrical Nerve Stimulation INTRODUCTION. Jae-Kwang Jung 1,2, Jin-Seok Byun 2, Jae-Kap Choi 2

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1 JOMP Journal of Oral Medicine and Pain Review Article pissn eissn J Oral Med Pain 2016;41(4): Basic Understanding of Transcutaneous Electrical Nerve Stimulation Jae-Kwang Jung 1,2, Jin-Seok Byun 2, Jae-Kap Choi 2 1 Institute for Hard Tissue and Bio-tooth Regeneration (IHBR), Kyungpook National University, Daegu, Korea 2 Department of Oral Medicine, School of Dentistry, Kyungpook National University, Daegu, Korea Received November 28, 2016 Revised December 13, 2016 Accepted December 13, 2016 Correspondence to: Jae-Kap Choi Department of Oral Medicine, School of Dentistry, Kyungpook National University, 2177 Dalgubeol-daero, Jung-gu, Daegu 41940, Korea Tel: Fax: jhchoi@knu.ac.kr This research was supported by Kyungpook National University Research Fund, Transcutaneous electrical nerve stimulation (TENS) is one of the representative physiotherapical modalities used for the treatment of various musculoskeletal disorders by the application of electrical stimuli. In dental practice, it has long been used in the treatment of acute and chronic orofacial pain conditions including temporomandibular disorders. TENS is the delivery of therapeutic electrical stimuli with a variety of electrical intensity, frequency and duration to stimulate peripheral nerve through surface electrodes with various form and placement. While controversy still remains over the clinical effectiveness and application of TENS, basic understanding of its electrical properties and the expected biological reactions is important to increase the therapeutic effect and decrease the risk of possible side effects. This review, therefore, focuses on basic understanding of TENS including its underlying mechanisms and stimulation parameters. Key Words: Orofacial pain; Stimulation parameter; Transcutaneous electrical nerve stimulation; Underlying mechanism INTRODUCTION Temporomandibular disorders (TMDs) are one of the most common orofacial pain conditions, encompassing a wide variety of clinical problems involving the temporomandibular joint (TMJ) and the masticatory muscles. 1) They are considered to be a subclassification of musculoskeletal disorders. 2) As in other musculoskeletal disorders, non-invasive therapeutic modalities are generally recommended as the primary treatment options for TMDs. 1) Among them, physical therapy is applied to restore the normal function of the TMJ, masticatory muscles, and cervical muscles, by reducing inflammation and pain as well as promoting repair. 3) Transcutaneous electrical nerve stimulation (TENS) is a therapeutic stimuli using electrical current, applied through skin surface electrodes to stimulate peripheral nerves for the relief of pain. 4) TENS has long been widely used to relieve a wide range of painful conditions, especially musculoskeletal pain disorders including TMDs. In 1972, U.S. Food and Drug Administration has already approved TENS as a useful method of pain alleviation and classified as class II medical device. 5) TENS is considered to be non-invasive, inexpensive, safe and easy to administer. It can be even selfadministered by the patients after a short instruction and individually titrated by themselves according to the analgesic response to the given electrical stimuli. 6) While previous meta-analyses and reviews supported positive outcomes for the relief of various musculoskeletal and post-operative pains, many systematic reviews regarded them to remain inconclusive. 4,7-10) However, it was indicated that while some studies included in previous reviews were the randomized controlled clinical trials on TENS, but most studies had the methodological weakness like small sample sizes and possibility of underdosing stimuli. 6,11) A number of basic scientific studies proved that TENS could inhibit the central transmission of nociceptive impulses through the involvement Copyright C 2016 Korean Academy of Orofacial Pain and Oral Medicine. All rights reserved. CC This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial License ( which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

2 146 J Oral Med Pain Vol. 41 No. 4, December 2016 of endogenous opioids, gamma-amino butyric acid (GABA), acetylcholine, 5-HT, noradrenaline and adenosine at peripheral, spinal or supraspinal (brainstem) sites levels ) Moreover, clinical experience suggests that TENS is still beneficial for various pains with neuropathic, nociceptive or musculoskeletal origin and popular with patients as well as clinician. 11) Taken together, it is still supported that clinicians continue to offer patients TENS until enough evidences with better quality will be provided. 11) Each parameter of TENS defined in terms of electrical intensity, frequency, wave shape, the electrode and its placement may have different neuroelectric effect on peripheral and central nervous structures, possibly leading different analgesic profiles, while the efficacies of electrical parameters remain to be determined. 16) Meanwhile, a variety of TENS-like devices with various electrical properties were introduced along with the developments in electronic technology. However, their clinical effectiveness is not well clarified owing to a lack of randomized controlled clinical trials. 4) The increasing options of electrical parameters adopted in various TENS and TENS like devices appear to make patients and clinicians confused about the accurate applications of each parameters. Therefore, it is essential for clinicians to have the basic understanding of TENS and its electrical parameters. DEFINITION OF TENS TENS is defined as the electrical stimulation transcutaneously applied by a battery-powered TENS unit for relieving pain by interfering with the neural transmission of nociceptive signals from pain origin. 17) TENS unit is comprehensively defined as all types of therapeutic device which delivers electrical stimulations across the intact surface of the skin, though a variety of TENS-like devices are introduced to the medical device market under the different brand names and their discriminations from TENS devices are emphasized by manufacturers. 4) CLASSIFICATION OF TENS While TENS can be clinically applied at varying frequencies, intensities, and pulse durations of stimulation, it is broadly classified into 3 categories according to the amplitude and frequency of the applied electrical current; 1) conventional TENS, 2) acupuncture-like TENS, and 3) intense TENS. 4,18) Most of modern TENS units usually provide the adjust ment in electrical parameters, subsequently making them possible to function in four TENS modes. According to previous literature, conventional TENS is defined when low intensity current is applied at high frequency ( pps) and small pulse width ( μs), while acupuncture-like TENS is defined when high intensity current (<10 pps, usually 2-4 pps) is applied at low frequency and long pulse width ( μs). Intense TENS is defined when high frequency current ( pps) is applied at highest intensity tolerable with minimal muscle contraction. 15,19) Besides, some literatures have also introduced a few variation of TENS including burst TENS and Acu-TENS with modified electrical parameters and clinical applications (Fig. 1). 20,21) Burst TENS is defined when the current with relatively high carrier frequencies ( pps), modulated burst frequencies (2-5 bps) is applied above or below the motor intensity. 21) Acu-TENS is occasionally described as the distinct TENS technique in some literatures. 19,20) It is termed to describe the application of transcutaneous electrical stimulation on acupuncture points, independent of any particular stimulating parameters. 20,22) However, despite of the development of various TENS-like devices, conventional TENS is still regarded as the most frequently used modality to apply the electrical current in clinical practice. 4) Despite of the application of TENS with different profiles, the basic mechanism of action for TENS is based on a electrophysiology principle that electrical stimuli can depolarize Current (intensity) A B C Motor level Sensory level Microcurrent level High frequency Low frequency High frequency Time Fig. 1. Three main types of transcutaneous electrical nerve stimulation (TENS) and their electrical parameters. (A) Conventional TENS, (B) acupuncture-like TENS, (C) intense TENS. Response

3 Jae-Kwang Jung et al. Basic Understanding of TENS 147 the peripheral nerve membrane and then to generate the induced nerve impulse, subsequently modulating the flow of nociceptive impulse in central and/or peripheral nervous systems and, intimately reducing the pain. 19) However, it was also suggested that the different nerve structures were predominantly affected by each type of TENS with the different electrical properties, thereby indicating the different analgesic mechanisms and treatment outcomes, though these suggestions have been challenged due to debatable and inconsistent findings. 4,23,24) UNDERLYING MECHANISMS OF ANALGESIC EFFECT The effects of TENS is broadly divided into analgesic and non-analgesic effects. Although there are the increasing clinical applications in ischemic or wound tissue, TENS in clinical practice is still predominantly applied for the symptomatic relief of pain. 6) Analgesic effect of TENS has long been explained by two main theories; Gate control theory and endogenous opioid theory. 1. Gate Control Theory The gate control theory of pain perception, proposed by Melzack and Wall 25) in 1965, is the main theory to explain the mechanism of action of conventional TENS. Briefly, it suggested that there is a gate through which nerves delivered pain impulse to the brain. They also suggested that the nerve fibers including small fibers (A-delta and C fibers) and large fibers (A-beta) project to the substantia gelatinosa (SG) of the dorsal horn and the first central transmission (T) cells in the dorsal horn of spinal cord. SG consists of inhibitory interneurons that functions as a gate by determining which type of nerve would deliver the certain signal to T cells. Small C fibers transmit pain impulse and their activation keeps the gate in open condition. The activation of large myelinated A-beta fibers exhibit presynaptic inhibition on pain impulse from C fibers and is responsible for making the gate in closed condition, thus inhibiting pain impulse from transmitting to T cells. Theoretically, the electrical stimuli of conventional TENS with high frequency and low intensity would activate selectively large myelinated nerve fibers and block subsequently the transmission of pain input from small nerve fibers as described in gate control theory, because large diameter nerve fibers (A alpha and beta) have low activation thresholds to electrical stimuli when compared with small diameter fibers (A delta and C). More interestingly, the greatest separation and sensitivity of pulse amplitudes happens with pulse durations of 10-1,000 µs, probably making conventional TENS with pulse duration ( µs) possible to activate more selectively recruit large diameter afferents than small diameter afferents and motor efferents. 26) In addition, it is known that the amplitude of electrical current necessary to excite nerve fiber tends to diminish as its pulse duration and frequency increase. Some findings from animal and human studies reported that conventional TENS produces segmental analgesia with a rapid onset and offset which was predominantly localized to the dermatome. 27,28) These findings suggested that the analgesic effect by conventional TENS was consistent with analgesic patterns suggested by the gate control theory. 2. Endogenous Opioid Theory In 1969, Reynolds 29) found that the electrical stimulation of periaqueductal gray region in midbrain exhibited the analgesic effect equivalent to that induced by morphine. After then, this finding was followed by the discovery of several morphine-like substances like beta endorphins, existing at various levels of central nerve system. It was considered as main explanation for the mechanism of action of acupuncture-like and, sometimes, intense TENS of which the electrical stimuli with high intensity and low frequency evoked the release of endogenous opioids in central nerve system though the activation of local circuits within the spinal cord or from the activation of descending pain-inhibitory pathways. 30) Previous studies showed that acupuncture-like TENS elevated the concentration of endorphin in cerebrospinal fluid of nine patients with chronic pain and the analgesia produced by acupuncture-like TENS was reversed by the application of naloxone. 31,32) The released beta endorphins following intense TENS was known to induce analgesic effect for up to 2 hours. 4) Hansson et al. 33) reported that the application of naloxone failed to reverse the analgesia induced by conventional TENS in orofacial patients with

4 148 J Oral Med Pain Vol. 41 No. 4, December 2016 post-operative pain. These results suggested that not conventional, but acupuncture-like TENS was more likely to provide the analgesic effect through the release endogenous opiates in patients with chronic pain. In recent, several studies reported that high and low frequency TENS activate different opioid receptors, while other study showed that TENS with low frequency is less effective than TENS with high frequency at reducing inflammatory hyperalgesia in morphine-tolerant rats. 11,34,35) It was also known that both TENS with high and low frequency have analgesic effects at the peripheral site by altering the excitability of peripheral nociceptors, thereby reducing pain input to the central nervous system. 35) However, others studies reported that blockade of peripheral opioid receptors prevented the analgesic effect produced by low, but not high frequency TENS ) In addition, it was known that a variety of neurotransmitters including endogenous ligand norepinephrine, GABA and serotonin were involved in analgesic actions of TENS. 14,38-40) THE ELECTROPHYSIOLOGY OF TENS Electric current of TENS is produced by the flow of negatively charged electrons. The cathode electrode of TENS attracts the negatively charged electrons outward nerve membrane, causing the depolarization on the axonal membrane and the subsequent action potential, while the anode electrode causes the hyperpolarization, blocking the nerve transmission (Fig. 2). That might explain why cathode is called as active electrode in TENS unit. Patient also tends to feel stronger TENS sensation on the cathode than on anode. 19) Therefore, it is recommended that in most case, both electrodes should be aligned along a peripheral nerve with the cathode positioned proximal to the anode when using monophasic or asymmetric biphasic waveform. 19) THE ELECTRICAL PARAMETERS The adjustable electrical parameters in TENS include pulse frequency, amplitude, duration and pattern. 1. Pulse Intensity Pulse intensity is broadly classified as three levels depending on the response of the patient; sensory, motor, microcurrent level. 41) Sensory level intensity is defined as the amplitude making the patient feels a comfortable paresthesia like tingling or tapping sensation without any motor contraction. This amplitude is also referred to low intensity, which is usually used for conventional TENS. Motor level intensity is defined as the amplitude producing a motor contraction with paresthesia. It is also called to high intensity, which is used for acupuncture-like TENS. Sometime the intensity can be increased to the maximal level just before becoming noxious. Such a intensity is usually applied in intense TENS. 4) Previous studies showed that there was a dose-response TENS electrode Skin Anode ( ) Cathode (+) Nerve (polarity) (Hyperpolarity) (Depolarity) Impulse conduction Blocking by the formation of hyperpolarity under the anode Antidromic impulse by TENS Action potential Prodromic impulse by TENS Fig. 2. Transcutaneous electrical nerve stimulation (TENS) induced the polarity and impulse on nerve. Nerve underneath cathode is depolarized to induce action potential and bi-directional conduction of impulse and one underneath anode is hyperpolarized to blocks the impulse, subsequently allowing prodromic conduction of impulse.

5 Jae-Kwang Jung et al. Basic Understanding of TENS 149 effect of TENS with the greatest analgesic effect at the highest tolerable current amplitude. It was suggested that the intensity of electrical stimuli should be titrated to enough extent to achieve maximum analgesic effect. 42,43) A recent systemic review reported that while moderate evidence of efficacy was reported for intense TENS, conventional TENS had overall conflicting evidence of efficacy and TENS with low-intensity and low-frequency had the lack of efficacy, which may be linked with an insufficient stimulus to active nerve fibers. 16) 2. Pulse Frequency Pulse frequency of electrical current is usually classified as high frequency (>50 Hz), low frequency (<10 Hz) and burst (bursts of high frequency current applied at a much lower frequency). 41) In last decade, it was revealed that highfrequency TENS could also cause analgesia through the release of endogenous opioids in the central nervous system, but through different target receptor. While low-frequency TENS acted via mu opioid receptors, high-frequency TENS acted via activation of delta opioid receptors, subsequently reducing the increased release of glutamate and aspartate in the spinal cord receptors ) However, a recent systemic review concluded based on an analysis of previous twenty studies that the pulse frequency of TENS didn t influence clinical analgesic effect as long as its pulse intensity, pulse pattern, and pulse duration were constant. 23) A previous study showed that both frequency-modulated TENS and constant-frequency TENS were superior in analgesic effect to placebo TENS but the modulation of pulse frequency did not increase hypoalgesia. 47) Therefore, when compared with previous findings on pulse intensity, it can be carefully postulated that pulse frequency of TENS does not influence hypoalgesia as much as intensity. varying pulse duration had no effect on the degree of antihyperalgesia produced by TENS with high frequency. 48) 4. Waveform Pulse waveform is classified into monophasic and biphasic waveform depending on the switch of electrode polarity, which means the alternation of cathode and anode between the two electrodes (Fig. 3). Biphasic waveform is subclassified into symmetrical and asymmetrical waveform. 4) Symmetrical biphasic waveform defined as symmetrical waveform of which the first phase of the wave has mirror image with opposite polarity to the second phase the wave, subsequently being equivalent in the quantities of electricity between electrical flows under cathode and anode. 19) Net current flow will be theoretically zero beneath two electrodes of TENS unit with symmetrical biphasic waveform, while the build-up of ion concentrations with a certain charge will occur beneath electrodes of TENS unit with monophasic waveform. 49) As excess build-up of ion concentrations is more likely to cause the adverse skin reactions, almost current TENS units adopt the biphasic waveform for the safety. 19) Sometimes, asymmetrical biphasic waveform was adopted for the mild accumulation of ions, resulting in slight concentration preferential polarity beneath the electrodes. A previous study reported that subjects strongly preferred the symmetrical biphasic waveform to asymmetric biphasic waveform for neuromuscular electrical stimulation. 50) These perceived discomfort with the asymmetric waveform could be attributed to the accumulation of ions under the electrodes. 51) However, other study reported that the differences in waveform characteristics had a negligible effect on analgesic efficacy produced by TENS when A B C D E (+) 3. Pulse Duration Pulse duration is usually classified as longer and shorter duration (<200 μs). It was stated that longer pulse duration evoked more intense sensation and deeper effect. 19) A previous animal study reported that prolonged pulse duration resulted in the increased inhibition of neuronal activity in dorsal horn. 27) However, other study showed that the Current ( ) Time Fig. 3. Waveforms commonly used in transcutaneous electrical nerve stimulation. (A) Monophasic pulsed square wave, (B) biphasic symmetric square wave, (C) biphasic asymmetric spike-like wave, (D) biphasic symmetric sine wave, (E) biphasic symmetric triangular wave.

6 150 J Oral Med Pain Vol. 41 No. 4, December 2016 compared between symmetrical and asymmetrical biphasic waveform. 51) In addition, there was no significant difference in pain threshold and tolerance between monophasic and asymmetric biphasic waveform. 52) Regarding the shape of current waveform, there are various shapes of waveforms determined by manufacturers, including square, rectangular, sine wave or triangular/spiked waveform. However, previous study showed that there were individual differences in preferences for three different waveforms of sinusoidal, sawtooth, and square symmetric biphasic waveforms and there was no particular waveform that was either least or most comfortable to the patient during neuromuscular electrical stimulation. 53) THE PLACEMENT OF ELECTRODE For conventional TENS, the electrodes would be placed on healthy innervated skin with straddling the pain origin. However, when it is not allowed to position the electrode around the pain origin due to the absence of sound skin tissue or abnormal hypersensitivity as in trigeminal neuralgia or postherpetic neuralgia, electrodes can be alternatively applied at the proper vertebral area over the corresponding spinal nerve arising from the pain origin. 4) However, a previous study suggested that TENS might have ineffective hypoalgesia if electrodes are placed contralaterally or distant to the pain site. 42) The size of the electrodes should be also considered because of possible effect on the strength of the electrical field. On using the electrodes with different size, the electrical field usually accumulated on the side of the smaller electrode. The current density defined as the amount of current per unit of conduction area, is likely to increase beneath smaller electrode. 4) The increased current density can cause the perceived intensity of stimuli in patients. In addition, it is also more likely to cause excessive heat at the superficial layers as well as the reduction in penetration depth beneath smaller electrode. 19,54,55) However, the intensity beneath smaller electrode of the field tends to be more likely to decrease as the distance between the electrodes increase. 54) Therefore, it should be considered to prevent the possible side effects by increasing the distance between the electrodes. According to the number of polarities delivered on each electrode, electrode can be classified into monopolar, bipolar and quadripolar electrode. 56) When the current with monopolarity was applied for TENS, one electrode is placed over the target tissue where the therapeutic effect is needed and the other electrode is placed on the sound tissue away from the target tissue for electrical circuit. The electrode placed in the targe tissue is termed as the active or stimulating electrode, usually corresponding to cathode, while the electrode placed away from the target tissue is termed as the dispersive, sometimes reference electrode. For convenience, cathode is commonly distinguished into black lead wire, while anode is distinguished into red lead wire, whereas most modern TENS devices adopt biphasic waveforms with zero net current flow, subsequently making the delimited placement of red and black lead wires in practice less important. 11) Smaller electrode is usually used as the active electrode for exerting more stimuli, while larger electrode is used as the dispersive electrode. Comparatively, the paired electrodes with equal size was used for TENS using bipolarity, but the asymmetric electrode pair with unequal size is sometimes used to deliver the intensified electrical stimuli on the pain origin by the application of smaller electrode. Quadripolar electrode is defined when two electrode pairs are positioned on the target tissue with two currents separately circuit. Such a electrode positioning can produce more complex pattern in electrical current by intersection, interaction or interference between each electrical current. 19) Too close placement of electrodes can produce the electrical field more restricted in superficial tissue, while the penetrating depth of electrical field can be increased by increasing the distance between the two electrodes (Fig. 4). 57) Accordingly, the distance between electrodes should be determined in light of the depth of targeting tissues for electric flow to reach. On the other hand, closer placement of two electrodes can cause the more potent electrical current localized in superficial epidermal area, possibly increasing the risk of temporary skin irritation or adverse skin reactions. 54,56) Recent study demonstrated using a finite element method that the volume of electrical activation tended to increase either by using larger electrode or positioning electrodes with wider inter-electrode distance, whereas the selectivity of electrical activation was improved by using

7 Jae-Kwang Jung et al. Basic Understanding of TENS 151 A B Skin Skin Fig. 4. Spatial pattern of electrical flow depending on the distance between the paired electrode. (A) Superficial distribution and increased density of electrical flow between the closely positioned pair of electrodes. (B) Deepened distribution and decreased density of electrical flow between the distantly positioned pair of electrodes. smaller electrodes. 54) In addition, it was also found that the shape of electrodes made no significant difference in the distribution of electrical current. 54) However, as skin tissue have very complex structure with heterogeneous components, its current impedance can t be homogeneous just as in materials used for in vitro experiment. 58) In addition, skin is known to have the increased impedance in the range of pulse frequencies commonly used by TENS. These implied that the experimental findings in in vitro experiments couldn t be directly adapted in practice and, therefore, it was not easy to predict exactly the distribution of the given currents. 4,59) However, it is still important to take these scientific findings into consideration when determining appropriate size and position of electrode. 4,57) The possibility of individualized adjustments should be also considered for better safety and effectiveness based on patient s response during the application of TENS. THE CLINICAL EFFECTS OF TENS The clinical benefits of TENS for the treatment of TMDs are widely reported both in literature and in most of the textbooks ) In contrast, systematic reviews indicated insufficient evidence for clinical effectiveness of electrophysical modalities including TENS. 9,10) To date, research data regarding clinical efficacies remains debatable and inconsistent. A previous study found that the functional pain and jaw function were improved significantly more after the application of high frequency TENS than placebo TENS in 19 patients with orofacial pain involving TMJ. 65) Other study revealed in 21 females with chronic orofacial pain that the application of TENS yielded the significant improvement in a clinical examination, self-monitoring of pain, and an assessment of EMG activity during resting and task conditions. 66) Konstantinović and Lazić 67) also reported that the signs and symptoms of craniomandibular disorders significantly decreased in 83% of patients after the application of occlusal splints, physical therapy and TENS for six weeks. In contrast, Linde et al. 68) pointed out that TENS in several respects are less than occlusal splints in the treatment of symptoms associated with disk displacement without reduction in TMJ. However, most of these clinical studies were designed to determine the effectiveness of TENS in management of TMDs and rehabilitation of stomatognathic system after the application of TENS and other therapies in combination rather than TENS alone. 9,10) There have also been fewer clinical studies with randomized controlled study design as well as the sufficient sample size and the standardized application of TENS. 9,10) Therefore, previous clinical findings should be interpreted with caution and its role in the functional rehabilitation of the masticatory system is still significant, while the value of TENS to manage chronic pain in TMD patients is controversial. 69) Similarly, the controversy over the effectiveness of TENS on other musculoskeletal pains has also been around up to recently. 7,8) Especially, a current review had a great resonance by stating that TENS is ineffective for chronic low back pain. 7) However, it was being challenged by other researchers indicating that its previous conclusion was based on an analysis of only two Class I randomized controlled trials with 114 subjects. 8) In addition, clinical experience have suggested that TENS was beneficial for acute and chronic pains with nociceptive, neuropathic or musculoskeletal origin. 6,11) Large meta-analyses also suggested the positive outcomes for the relief of musculoskeletal and post-operative pain ) Moreover, a number of basic scientific researches so far demonstrated the neurophysiological

8 152 J Oral Med Pain Vol. 41 No. 4, December 2016 mechanism that TENS inhibits the synaptic transmission of nociceptive impulses in the peripheral and central nervous system. 73,74) Although a number of clinical studies showed the effectiveness of TENS for pain, there are still much scientific controversies over clinical application of TENS. Therefore, further studies with sufficient sample size and well established experimental design are required to establish the clinical guideline with scientific evidence for the application of TENS. For now, basic understanding of TENS and it related findings is important in relieving chronic orofacial pain more effectively and safely in clinical practice. CONFLICT OF INTEREST No potential conflict of interest relevant to this article was reported. ACKNOWLEDGEMENTS Part of this paper was presented at The Korean Academy of Orofacial Pain and Oral Medicine, Annual Continuous Education held on December 6th, 2015, at 1st lecture room of Seoul National University Dental Hospital in Seoul, Korea. REFERENCES 1. Wright EF, North SL. Management and treatment of temporomandibular disorders: a clinical perspective. J Man Manip Ther 2009;17: Okeson JP. Bell s orofacial pains: the clinical management of orofacial pain. Chicago: Quintessence Pub.; Romero-Reyes M, Uyanik JM. Orofacial pain management: current perspectives. J Pain Res 2014;7: Kitchen S, Bazin S, Clayton BE. Electrotherapy: evidence-based practice. Edinburgh: Churchill Livingstone; Katch EM. Application of transcutaneous electrical nerve stimulation in dentistry. Anesth Prog 1986;33: Naka A, Keilani M, Loefler S, Crevenna R. Does transcutaneous electrical nerve stimulation (TENS) have a clinically relevant analgesic effect on different pain conditions? A literature review. European Translat Myol 2013;23: Dubinsky RM, Miyasaki J. Assessment: efficacy of transcutaneous electric nerve stimulation in the treatment of pain in neurologic disorders (an evidence-based review): report of the Therapeutics and Technology Assessment Subcommittee of the American Academy of Neurology. Neurology 2010;74: Johnson MI, Walsh DM. Pain: continued uncertainty of TENS effectiveness for pain relief. Nat Rev Rheumatol 2010;6: List T, Axelsson S. Management of TMD: evidence from systematic reviews and meta-analyses. J Oral Rehabil 2010;37: Medlicott MS, Harris SR. A systematic review of the effectiveness of exercise, manual therapy, electrotherapy, relaxation training, and biofeedback in the management of temporomandibular disorder. Phys Ther 2006;86: Johnson MI. Transcutaneous electrical nerve stimulation (TENS). Chichester: John Wiley & Sons Ltd; doi: / a Duggan AW, Foong FW. Bicuculline and spinal inhibition produced by dorsal column stimulation in the cat. Pain 1985;22: Ma YT, Sluka KA. Reduction in inflammation-induced sensitization of dorsal horn neurons by transcutaneous electrical nerve stimulation in anesthetized rats. Exp Brain Res 2001;137: Maeda Y, Lisi TL, Vance CG, Sluka KA. Release of GABA and activation of GABA(A) in the spinal cord mediates the effects of TENS in rats. Brain Res 2007;1136: Tashani O, Johnson M. Transcutaneous electrical nerve stimulation (TENS) A possible aid for pain relief in developing countries? Libyan J Med 2009;4: Claydon LS, Chesterton LS, Barlas P, Sim J. Dose-specific effects of transcutaneous electrical nerve stimulation (TENS) on experimental pain: a systematic review. Clin J Pain 2011;27: Pease Jr RW. Merriam-webster s medical dictionary. Springfield: Merriam-Webster; Kasat V, Gupta A, Ladda R, Kathariya M, Saluja H, Farooqui AA. Transcutaneous electric nerve stimulation (TENS) in dentistry: a review. J Clin Exp Dent 2014;6:e562-e Johnson MI. Transcutaneous electrical nerve stimulation (TENS): research to support clinical practice. Oxford: OUP Oxford; Brown L, Holmes M, Jones A. The application of transcutaneous electrical nerve stimulation to acupuncture points (acu-tens) for pain relief: a discussion of efficacy and potential mechanisms. Phys Ther Rev 2013;14: Sherry JE, Oehrlein KM, Hegge KS, Morgan BJ. Effect of burstmode transcutaneous electrical nerve stimulation on peripheral vascular resistance. Phys Ther 2001;81: Yip YB, Tse HM, Wu KK. An experimental study comparing the effects of combined transcutaneous acupoint electrical stimulation and electromagnetic millimeter waves for spinal pain in Hong Kong. Complement Ther Clin Pract 2007;13: Chen C, Tabasam G, Johnson MI. Does the pulse frequency of transcutaneous electrical nerve stimulation (TENS) influence hypoalgesia?: a systematic review of studies using experimental pain and healthy human participants. Physiotherapy 2008;94: Chesterton LS, Barlas P, Foster NE, Lundeberg T, Wright CC, Baxter GD. Sensory stimulation (TENS): effects of parameter manipulation on mechanical pain thresholds in healthy human subjects. Pain 2002;99: Melzack R, Wall PD. Pain mechanisms: a new theory. Science

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