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Interferential therapy, during the past ten years, has increased in popularity to the point that it is now perhaps the most widely used form of electrotherapy in the United States.
First developed in Europe, where this unique form of stimulation has been utilized for numerous indications, interferential units have been marketed since the early 1950's.
It seems, however, that a great deal of confusion, mystery and perhaps even misinformation still exists concerning this therapy.
The purpose of this article is to shed some light on areas that may be confusing to the clinician, share information on proper treatment protocols and offer a few insights into treating patients with interferential therapy effectively and safely."TRUE INTERFERENTIAL" VS. "PRE-MODULATE INTERFERENTIAL"
The original concept of interferential therapy was developed by Austrian physician, Dr. Hans Nemec, approximately forty years ago. Dr. Nemec proposed that by crossing two slightly different medium frequency alternating currents within the tissue; a third frequency current of greater intensity is created in the deeper tissue.
As an example, a frequency of 4000Hz interfering with another frequency of 4080Hz creates a third current of 80Hz. This is caused by the in phase and out of phase relationship of the two original currents as they alternate from positive to negative polarity.
The third current, referred to as the "beat frequency" becomes the actual therapeutic frequency. One output of the unit is a constant 4000Hz while the second output frequency is adjustable from perhaps 4001Hz to as much as 4250Hz. This form of interferential therapy has become known as "true interferential frequency difference interferential".
A second method of creating the interference effect has been developed in recent years and has become known as "pre-modulated interferential". With this method, both outputs of the unit provide a carrier frequency of 4000Hz, however, each output has the ability to pre-modulate or burst the frequency within the unit. It is important that this unit has the capability of perfectly synchronizing these bursts in the same polarity, at the same time in order to create "pre-modulated interferential" Units capable of pre-modulation are not necessarily pre-modulated interferential and may only provide pre-modulation for the purpose of bi-polar (two electrodes) stimulation.
When considering the relative merits of these two methods, many clinicians have noted that while both create the interference effect, there may be a distinct advantage to the pre-modulated technique. Since the "true interferential" provides an uninterrupted, constant 4000Hz frequency to the tissue, a condition known as Widensky inhibition (depolarization of the nerve fibers) will occur beneath the electrodes.
This will create numbness and what will be perceived by the patient as a reduction in the intensity of current. With pre-modulated interferential, however, since the current is being burst inside the unit itself, Widensky inhibition will not occur and a larger treatment area is established with the actual therapeutic frequency.ELECTRODES, CONTACT AND SAFETY
Virtually all interferential clinical units are supplied with carbon rubber electrodes. The clinician should be aware that either water soaked sponges or a conducting gel should always be used between the electrode and the tissue. This will insure a uniform contact and provide for even disbursement of the current over the entire surface area of the electrode. If water only is used as a conductive agent, pooling may occur with resulting dry spots under the electrode. The current will then become intensified at the site of best conduction, the water pools, with little or no current flow elsewhere. With "true interferential" units this could result in over stimulation of tissue under the water pools and even possible tissue burns as depolarized tissue will not be able to sense the over stimulation.
While some interferential units still offer the vacuum electrode system, many clinicians have discontinued their use. Extra maintenance, tissue bruising and uneven current flow have been cited as reasons for a reduction in the popularity of vacuum systems.
Self adhesive electrodes are rapidly becoming the favorite of clinicians due to the ease of use, patient acceptance and elimination of possible cross-contamination. Difficult to apply areas such as shoulders, hips and the cervical spine are easily treated with the self-adhesive electrodes. Also, recent improvements in adhesive agents have made longer use possible and prices have been reduced substantially.
If carbon rubber electrodes are used, care should be taken to insure proper current flow. When conductive gels are used, the gel will create a glaze over the surface of the electrodes with long-term use. The glaze may prevent the flow of current over the entire electrode surface. Cleaning the electrode periodically with a mild soap and water and soft brush is recommended. It is not a good practice to use conducting mist sprays in lieu of other conducting agents. This is due to the saline content of the sprays which has been shown to destroy the carbon content of the electrode, thus rendering the electrode useless.
The best selection for this device is re-usable Silver Connector Electrodes.TREATMENT FREQUENCIES
While frequency ranges vary from manufacturer to manufacturer, basic therapy ranges are fairly consistent. Frequencies which vary from approximately 80Hz to 120Hz are considered most effective for acute pain while lower frequencies of perhaps 3Hz to 5Hz or 2Hz to 10Hz are preferred for the treatment of chronic pain. Some units feature a nerve block setting where both channels produce an output of 4000Hz to create an interferential nerve block to quickly block out acute pain. Most clinicians prefer a setting of 1 Hz to 15Hz for treating acute edema.TREATMENT TIME
When treating acute pain with the 80Hz to 120Hz setting, interferential therapy will provide a release of enkephalin with a treatment time of 10 to 12 minutes. Chronic pain, however, requires 15 to 20 minutes of the 3Hz to 15Hz setting to provide release of beta-endorphins. Nerve block techniques, 4000Hz, normally require 10 minutes or more depending upon the size of the area being treated.INTENSITY OF CURRENT
Interferential therapy provides a comfortable, soothing stimulation and should never be strong enough to cause any discomfort to the patient. Higher intensities should not be considered "better" as far as obtaining results. It is important to note that once the patients comfort level is established at the onset of therapy, the intensity should not be increased during the treatment. This could cause over stimulation of the tissue and even minor burns, particularly when treating with a unit that produces "true interferential" due to the Widensky inhibition effect.RUSSIAN STIMULATION
This procedure is utilized for muscle strength and rehabilitation and is an added feature of interferential units. Space does not permit adequate explanation of this technique at this time; however, Russian Stimulation may be the topic of a future articleCONTRAINDICATIONS AND PRECAUTIONS
Interferential therapy is considered a very safe modality when used properly for appropriate conditions. Most manufacturers list similar contraindications and precautions, most of which are the same as other electrotherapy devices. It is always recommended that the clinician review each manufacturer’s warnings prior to treatment with any device.Interferential devices are non-addictive.
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