Definition:
Interferential therapy utilizes two medium frequency currents which pass through the tissues simultaneously. They are set up so that their paths cross; and in simple terms they interfere with each other. This interference gives rise to an interference or beat frequency, which has the characteristics of low-frequency stimulation (Fig. 1) .
Fig. (1): Interferential current
The exact frequency of the resultant beat frequency can be controlled by the input frequencies. For example, if one current was at 4000 Hz and the other current at 3900 Hz, the resultant beat frequency would be at 100 Hz.
Principle of interferential therapy:
The basic principle of the interferential therapy (IF) is to utilize the strong physiological effects of the low frequency electrical stimulation of muscle and nerve tissues at sufficient depth, without the associated painful and somewhat unpleasant side effects of such stimulation. The medium frequency currents penetrate the tissues with very little resistance, whereas the resulting interference current (low frequency) is in the range that allows effective stimulation of the biological tissues. The resistance (impedance) of the skin is inversely proportional to the frequency of the stimulating current.
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In other words, the lower the stimulation frequency, the greater the resistance to the passage of the current, so more discomfort is experienced.
The skin impedance at 50 Hz is approximately 3200 ohms, whilst at 4000 Hz it is reduced to approximately 40 ohms. The result of applying this latter frequency is that it will pass more easily through the skin, requiring less electrical energy input to the deeper tissues, giving rise to less discomfort.
Uses of interferential therapy:
1. Relief and management of acute and chronic pain:
The interferential current is used in management of acute and chronic pain of different origins, especially post traumatic pain and sympatheticallymaintained pain as in shoulder hand syndrome and Raynaud’s disease (spasm of the digital arteries producing pallor or cyanosis of fingers or toes).
Mechanisms of pain control by interferential therapy:
- Stimulation with the higher frequencies of about 100 Hz at the sensory level intensities stimulates pain gate mechanisms, thereby mask the pain symptoms.
- Stimulation with lower frequencies up to 10 Hz at motor level intensities can be used to activate the opoid mechanism, providing a degree of relief.
- Decreased activity of the sympathetic ganglion and sympathetic nerves in cases of sympathetically-maintained pain.
2. Muscle stimulation:
Stimulation of the motor nerves to induce contraction of muscle can be achieved with a wide range of frequencies (10 – 50 Hz). Clearly, stimulation at low frequency (1 – 10 Hz) will result in a series of twitches, whilst stimulation at 50 Hz will result in a tetanic contraction. The choice of treatment parameters will depend on the desired effect.
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If IF is used as neuromuscular stimulation, it can be used for:
- Relaxation of muscle spasm.
- Prevention and retardation of disuse atrophy.
- Muscle re-education.
- Maintenance of range of motion.
3. Increase blood flow:
Increased vasodilatation is noticed following interferential stimulation due to its effect on the parasympathetic nerve fibers for increased blood flow.
So, interferential stimulation is used in treatment of circulatory disorders.
Moreover, depressing the activity of certain cervical and lumbo-sacral sympathetic ganglia in patients with increased arterial constrictor tone, increases circulation as in Raynaud's disease.
4. Reduction of edema:
Chronic post-traumatic edema can be reduced by the use of interferential stimulation. This effect may be attributed to milking of the venous and lymphatic return through electrically-evoked contraction. The beat frequency is of approximately 15 Hz or a sweep of 10-25 Hz is often used.
5. Stimulation of soft tissue healing and repair:
The interferential current accelerates ossification, so may be used to enhance fracture healing. Furthermore, it may be used also to accelerate wound healing by improving the cellular function and increasing cell proliferation.
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Clinical applications:
By manipulation of the input currents, it is possible to achieve any beat frequency desired to be used clinically. Modern machines usually offer frequencies of 1-150 Hz, though some offer a choice of up to 250 Hz or more.
Electrodes placement:
- Electrode positioning should ensure adequate coverage of the stimulated area.
- Placement of the electrodes should be such that a crossover effect is achieved in the desired area. When the electrodes are properly positioned, the stimulation should be felt only between the electrodes, not under them.
- If the electrodes are not placed so that a crossover is achieved, the physiological effects of interferential stimulation can not be attained (Fig. 2) . a) Quadripolar technique:
Fig. (2): Quadripolar technique
The four electrodes are positioned around the target area so that each channel runs perpendicular to the other and the current crosses at a midpoint.
The interference effect branches off at 45° from the center of the treated area, in the shape of a four-leaf clover. Tissues within this area receive the maximal treatment effect (Fig. 3) . The electrodes are positioned in a coplanar arrangement to treat a flat surface (Fig. 4) .
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Fig. (4): Coplanar application for the back b) Bipolar technique:
When the IF current is applied using a bipolar technique, the mixing of the two channels occurs within the generator rather than in the tissue. This technique does not penetrate the tissues as deeply as quadripolar application, therefore it tends to be of more sensory sensation than with four pole technique, although still less than with low frequency stimulation. In some circumstances, a bipolar technique is preferable if a longitudinal zone requires stimulation rather than an isolated tissue area. When muscle contraction is the goal of the treatment, bipolar electrode placement is used (Fig. 5).
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Types of electrodes:
- Metal plate and pads:
The interferential current is applied by metal electrode with watersoaked sponges.
- Carbon rubber electrodes:
The interferential current is applied by carbon rubber electrodes with conducting gel.
- Suction cup electrodes:
Suction unit can be connected to the interferential machine. The electrodes are applied to the patient through flexible rubber cups, which have a sponge pad and electrode incorporated inside the cup to carry interferential current. This cup provides rhythmic negative pressure. This pressure produces mild massaging effect on the skin, stimulating cutaneous sensory nerves and causing slight vasodilatation.
Treatment Parameters:
* Intensity:
The intensity of the current is dependent on the desired physiological effect and the tissue to be stimulated. In general, the current could be administered at sensory, motor or noxious levels.
* Frequency:
The excitable tissues can be stimulated by low frequency alternating currents. Although to some extent all tissues in this category will be affected by a broad range of stimulations, it is thought that different tissues will have an optimal stimulation band. These bands are detailed as following (Table 1).
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Table (1): Different frequencies used in IF therapy.
Tissue
Sympathetic Nerve
Parasympathetic Nerve
Motor Nerve
Sensory Nerve
Nociceptive fibers
Smooth Muscle
Frequency
1-5 Hz
10-150 Hz
10-50 Hz
90-100 Hz
90-150 Hz
1-10 Hz
Sweep frequency:
Nerves are known to accommodate to a constant signal, so a sweep
(gradually changing) frequency is often used to overcome this problem. The sweep (range) should be appropriate to the desired physiological effects. The mode of delivery of the selected sweep varies with machines. The most common application is the 6-second rise and fall between the pre-set frequencies. For example, if a 10-25 Hz range has been selected, the machine will deliver a changing frequency, starting at 10 Hz, rising to 25 Hz over a 6second period. Once this upper limit has been achieved, the frequency will once again falls, over a 6-second period to its starting point at 10 Hz. This pattern is repeated throughout the treatment session (Fig. 6) .
Fig. (6): Sweep frequency
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Other patterns of sweep can be produced on many machines as illustrated below (Fig. 7).
Fig. (7): Other patterns of sweep frequency
Treatment duration:
It varies widely according to the usual clinical parameters of conditions
(acute / chronic) and the type of physiological effect desired.
- Acute conditions: 5-10 minutes.
- Chronic conditions: 20-30 minutes.
Setup and application:
The usual steps for preparation of patient and the apparatus during electrical stimulation are followed:
- Apply the electrodes to the treated area.
- Turn on the apparatus by activating the power switch.
- Select the appropriate beat frequency, based on the goal of treatment.
- Use the appropriate sweep frequency for this treatment protocol.
- Set the duration of the treatment by adjusting the timer.
- Start the treatment by pressing the start button.
- Slowly increase the intensity until the appropriate current level is obtained, guided by the patient's feeling.
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Precautions:
* Care should be taken to maintain the suction at a level below that which causes damage or discomfort to the patient.
* For patients who have febrile conditions, the outcome of the first treatment session should be monitored.
* Patients who have epilepsy, advanced cardiovascular conditions or cardiac arrhythmias should be treated in consultation with the appropriate medical practitioner.
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