Massiter Muscle Relaxation with Tens
More than 50 years ago, Bernard Jankelson introduced a new device to relax chewing muscles, the Myomonitor. The 1st Tens Unit. At the peak of the gnathologic era, colleagues in search of the terminal hinge axis could not understand the use of such a device for the most part and this marked the beginning of a different understanding of how the mandibular movement system functions. Gnathologists viewing the TMJ’s as mechanical guides, postulated that the mandible follows a circular path upon closure with the hinge axis at its center. Neuromuscular dentists on the other hand were (and are) convinced that the intercuspation of the teeth can be exactly hit by a mandibular movement guided by afferent and efferent neuromuscular reflexes, regardless where it is located with regard to any hinge axes.
This opened the door to the concept of „accommodation", a process by which mandibular closure accurately meets the intercuspal position even if the physiological path of the natural muscle vectors does not lead there. Actually, more often than not the path of closure does follow physiologic vectors, but the starting point of the closure (with the mandible at rest) is shifted in order to accomplish this. Nice as this sounds, it necessitates holding the mandible at rest somewhere else than where it would relax and brings with it chronic muscle tension which remains active, even when the patient does not bite on his teeth, i. e. all the time!
Jankelson had realized this and tried to find a way to restore true, so called „physiologic“ rest by relaxing the muscles positioning the mandible at accommodated rest.
Low-frequency TENS with single impulses of relatively long duration (500 µsec) are effective means to this end is a well-established fact today, with more than 30 published studies citing over 200 secondary references having been collected by the manufacturer of the original Tens Unit Myomonitor, Myo-Trinics, Inc.
Yet, it has never been quite clear what exactly the mechanisms are. Blood flow is increased so that more oxygen reaches muscle cells and more waste products, mostly of an acidic nature, are removed. Next to such histochemical changes in the tissues the release of endorphins may play a role. However, in reference to TENS, one usually finds a 1965 publication by Melzack and Wall quoted, „Pain Mechanisms: A new Theory“. Here the authors describe their „Gate Control Theory“: Non painful stimuli, conducted via fast, myelinated nerve fibers, reach the synapse before pain signals arrive along slow unmyelinated fibers and block it. They „close the gate“ to pain.
Yet we are not talking about pain here and a stimulation frequency of typically <1 Hz is unlikely to hold any gates closed. In a German publication in the ICCMO-Kompendium I therefore proposed a different theory: The slow individual impulses (in the case of the Myomonitor 0,75 Hz) actually cause movement in the stimulated muscles.
Why Tens Relaxes
These movements are of a ballistic nature, meaning that they are characterized by an initial sudden acceleration and a continuation of movement even after force is no longer applied. This is the exact opposite to isometric contractions known for their stimulation of muscle cell hypertrophy due to muscle exhaustion. In turn, the opposite of exhaustion is relaxation which is here accomplished by providing the minimal movement necessary to improve mircocirculation in the muscle while stressing muscle cells as little as possible.
The quintessence of this theory is what if movement is provided in some other way?
TENS most often is able to restore almost perfect symmetry in the masseter muscles. However, to achieve this, the therapy can take up to 40 minutes.
In summary, TENS-therapy, if given enough time, provides additional benefits and is standard treatment protocol in many offices for muscle relaxation.
Patients who do not tolerate TENS well, or where TENS is contraindicated, f. i. due to a pacemaker, or when 40-60 minutes of therapy time are not available, they’re are alternative way of relaxing chewing muscles.