It is only within the last two decades that electromyography has become recognized as a separate discipline widely used in clinical diagnosis. Electromyography originally was developed out of neurophysiology; a method employed to record action potentials from human muscle fibers in healthy and diseased patients. It is the technique by which the action potentials of contracting muscle fibers and motor units are recorded and displayed. In recent years, the expression has had a wider use and now frequently EMGs have a whole range of electrodiagnostic techniques that are now being applied to peripheral nerves and muscles.
Regardless of occlusal philosophy or technique, the ability to monitor muscle relaxation and function provides new capabilities to insure a predictable and physiologically acceptable result.
The history of occlusal theories in dentistry is long and varied. Basic physiologic facts, however, transcends these differences of “opinions” in occlusal philosophies. The most obvious of these facts is that antagonistic muscles must function synergistically and must periodically achieve relaxation. Synergy and relaxation allow for muscle recovery. Many of the so called TMJ symptom so prevalent in patients are in fact, a result of spasm of the cervical, facial, or the masticatory muscles. (Duchene, Selzer, Shaber, Basmajean and many others in the scientific EMG arena have shown EMGs as relevant tool to muscle hyperactivity). This spasms is created when craniomandibular position requires muscles to repeatedly over accommodate to reach the intercuspal position during occlusal function.
This condition of muscle hyerptonicity results in elevated electrical activity in the affected muscles when at rest. To restore an occlusion when muscles are in this state of hypertonicity spasticity encourages the perpetuation of existing pathologies.
Monitoring levels of muscle resting activity with EMG is an invaluable technique for verifying the physiologic status of the stomatognathic system. It readily verifies the efficacy of the Myomonitor TENS in achieving muscle relaxation. With EMG we can confirm quantitatively that the mucscles are relaxed, then subsequently take our occlusal registration in this physiologically optimal muscle environment.
Just as radiography has become the definitive diagnostic tool for evaluation of tooth and skeletal pathology, electromyography is now becoming the definitive tool in dentistry for the diagnosis of neuro-muscular pathology.
The preference of bipolar electrodes to needle electrodes is obvious when simultaneous activity or interplay of activity is being studied in a fairly large group of muscles. Surface electrode (EMG) do not interfere with natural function, and the mean amplitude recorded varies almost linearly with the force generated at constant length, or during contractions with constant velocity.
The disadvantage of surface electrodes is the inability to specifically monitoring deep muscles. Surface electrodes is not intended to study single motor unit activity like the needle electrodes which are necessary for analyzing neuropathies. Needle electrodes is the modality of choice for conduction velocity and conduction latency testing for neurapathy.
In general, it is desirable to place the surface electrode in the long axis of the muscle fibers, parallel and over the body of the muscles to get the optimal signal collection.
Three variables which might affect the electronic signal picked up by the surface electrodes are:
1) Depth of individual muscle from the skins surface.
2) Patient morphotype
3) Tissue tone.