Interferential Therapy

The basic principle of Interferential Therapy (I/F) is to utilise the strong physiological effects of low frequency (@ <250pps) electrical stimulation of muscle and nerve tissues without the associated painful and somewhat unpleasant side effects of such stimulation.

To produce low frequency effects at sufficient intensity at depth, most patients experience considerable discomfort in the superficial tissues (i.e. the skin). This is due to the resistance (impedance) of the skin being inversely proportional to the frequency of the stimulation. 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 50Hz is approximately 3200W whilst at 4000Hz it is reduced to approximately 40W . The result of applying this latter frequency is that it will pass more easily through the skin, requiring less electrical energy input to reach the deeper tissues & giving rise to less discomfort.

The effects of tissue stimulation with these 'medium frequency' currents (Medium frequency in electromedical terms is usually considered to be 1KHz-100KHz) is unknown. It is unlikely to be innocuous, but in terms of current practice, little is known of its physiological effects. It is not capable of direct stimulation of nerve in the common context of such stimulation.

Interferential therapy utilises two of these medium frequency currents, passed through the tissues simultaneously, where they are set up so that their paths cross & in simple terms they interfere with each other. This interference gives rise to an interference or beat frequency which has the characteristics of a low frequency stimulation.

The exact frequency of the resultant beat frequency can be controlled by the input frequencies. If for example, one current was at 4000Hz and its companion current at 3900Hz, the resultant beat frequency would be at 100Hz, carried on a medium frequency 3950Hz amplitude modulated current.

By careful manipulation of the input currents it is possible to achieve any beat frequency that you might wish to use clinically. Modern machines usually offer frequencies of 1-150Hz, though some offer a choice of up to 250Hz or more. To a greater extent, the therapist does not have to concern themselves with the input frequencies, but simply with the appropriate beat frequency which is selected directly from the machine.

The use of 2 pole I/F stimulation, where there is clearly no interference within the body, is made possible by electronic manipulation of the currents - the interference occurs within the machine.

The magnitude of the low frequency interference current is approximately equivalent to the sum of the input amplitudes. In other words, the result of the interaction between the two input currents is a low frequency current, which has an amplitude greater than either of the individual input currents.

Physiological Effects:
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, which can be estimated by the conduction velocity of the tissue, its latency and refractory period. These are detailed below:

Sympathetic Nerve 1-5Hz
Parasympathetic Nerve 10-150Hz
Motor Nerve 10-50Hz
Sensory Nerve 90-100Hz
Nociceptive fibres 90-150Hz (?130Hz specific)
Smooth Muscle 0-10Hz

The clinical application of I/F therapy can be based logically on this data together with a knowledge of physiological behavior of stimulated tissue. Selection of a wide treatment band can be considered less efficient than a smaller selective band in that by treating with a frequency range of say 1-100Hz, the appropriate treatment frequencies can be covered, but only for a relatively small percentage of the total treatment time. Additionally, some parts of the range might be counterproductive for the primary aims of the treatment.

The are 4 main clinical applications for which I/F appears to be used:
Pain relief
Muscle stimulation
Increased blood flow
Reduction of edema

In addition, claims are made for its role in stimulating healing and repair.

As I/F acts primarily on the excitable tissues, the strongest effects are likely to be those which are a direct result of such stimulation (i.e. pain relief and muscle stimulation). The other effects are more likely to be secondary consequences of these.

Pain Relief:
Electrical stimulation for pain relief has widespread clinical use, thought the direct research evidence for the use of I/F in this role is limited. Logically one could use the higher frequencies (90-150Hz) to stimulate the pain gate mechanisms & thereby mask the pain symptoms. Alternatively, stimulation with lower frequencies (1-5Hz) can be used to activate the opoid mechanisms, again providing a degree of relief. These two different modes of action can be explained physiologically & will have different latent periods & varying duration of effect. It remains possible that relief of pain may be achieved by stimulation of the reticular formation at frequencies of 10-25Hz or by blocking C fibre transmission at >50Hz.

Muscle Stimulation:
Stimulation of the motor nerves can be achieved with a wide range of frequencies. Clearly, stimulation at low frequency (e.g. 1Hz) will result in a series of twitches, whist stimulation at 50Hz will result in a tetanic contraction. The choice of treatment parameters will depend on the desired effect, but to combine muscle stimulation with an increase in blood flow and a possible reduction in oedema, there is some logic in selecting a range which does not involve strong sustained tetanic contraction & a sweep of 10-25Hz is often used.

There is no primary nervous control of oedema reabsorption & the direct electrical stimulation of blood flow is limited in its effectiveness. It is suggested therefore that in order to achieve these effects, suitable combinations of muscle stimulation can be made.

Treatment Parameters:
Electrode positioning should ensure adequate coverage of the area for stimulation. In some circumstances, a bipolar method is preferable if a longitudinal zone requires stimulation rather than an isolated tissue area. Placement of the electrodes should be such that a crossover effect is achieved in the desired area. If the electrodes are not placed so that a crossover is achieved, the physiological effects of I/F can not be achieved.

Nerves will accommodate to a constant signal & a sweep (or gradually changing frequency) is often used to overcome this problem (as well as generating a range of effects). The sweep (range) should be appropriate to the desired physiological effects, though again it is suggested that an excessive range may minimise the clinical effect.

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 - 25Hz range has been selected, the machine will deliver a changing frequency, starting at 10Hz, rising to 25Hz over a 6 second period. Once this upper limit has been achieved, the frequency will once again fall, over a 6 second period to its starting point at 10Hz. This pattern is repeated throughout the treatment session

Treatment times vary widely according to the usual clinical parameters of acute/chronic conditions & the type of physiological effect desired. In acute conditions, shorter treatment times of 5-10 minutes may be sufficient to achieve the effect. In other circumstances, it may be necessary to stimulate the tissues for 20-30 minutes. It is suggested that short treatment times are initially adopted especially with the acute case in case of symptom exacerbation. These can be progressed if the aim has not been achieved and no untoward side effects have been produced. There is no research evidence to support the continuous progression of a treatment dose in order to increase or maintain its effect.

 

Ultrasound:
Ultrasound is a form of MECHANICAL energy, not electrical energy and therefore the waveform is different from those previously considered. Mechanical vibration at increasing frequencies is known as sound energy. Below about 20Hz, these vibrations are not recognisable as sound, and the normal human sound range is from 20Hz to something approaching 15-20,000 Hz (in children and young adults). Beyond this upper limit, the mechanical vibration is known as ULTRASOUND. The frequencies used in therapy are typically between 0.75 and 3.0 MHz (1MHz = 1 million cycles per second).

Sound waves are LONGITUDINAL waves consisting of areas of COMPRESSION and REFACTION. Particles of a material, when exposed to a sound wave will oscillate about a fixed point rather than move with the wave itself. As the energy within the sound wave is passed to the material, it will cause oscillation of the particles of that material. Clearly any increase in the molecular vibration in the tissue will result in heat generation, and ultrasound (U/S) can be used to produce thermal changes in the tissues, though current usage in therapy does not focus on this phenomenon.

In addition to thermal changes, the vibration of the tissues appears to have effects which are generally considered to be non thermal in nature, though, as with other modalities (e.g. Pulsed Shortwave) there must be a thermal component however small. As the U/S wave passes through a material (the tissues), the energy levels within the wave will diminish as energy is transferred to the material. The energy absorption and attenuation characteristics of U/S waves have been documented for several types of tissue.

Ultrasound Waves :
FREQUENCY - the number of times a particle experiences a complete compression/rarefaction cycle in 1 second. Typically 1 or 3 MHz.

WAVELENGTH - the distance between two equivalent points on the waveform in the particular medium. In an ‘average tissue’ the wavelength @ 1MHz would be 1.5mm and @ 3 MHz would be 0.5 mm.

VELOCITY - the velocity at which the wave (disturbance) travels through the medium. In a saline solution, the velocity of U/S is approximately 1500 m sec-1 compared with approximately 350 m sec-1 in air (sound waves can travel more rapidly in a more dense medium). The velocity of U/S in most tissues is thought to be similar to that in saline.

These three factors are related, but are not constant for all types of tissue. Average figures are most commonly used to represent the passage of U/S in the tissues. Typical U/S frequencies from therapeutic equipment are 1 and 3 MHz though some machines produce additional frequencies (e.g. 0.75 and 1.5 MHz).

Ultrasound Transmission through the Tissues:
All materials (tissues) will present an impedance to the passage of sound waves. The specific impedance of a tissue will be determined by its density and elasticity. In order for the maximal transmission of energy from one medium to another, the impedance of the two media needs to be the same. Clearly in the case of U/S passing from the generator to the tissues and then through the different tissue types, this can not actually be achieved. The greater the difference in impedance at a boundary, the greater the reflection that will occur, and therefore, the smaller the amount of energy that will be transferred.

The coupling media used in this context include water, various creams and gels. Ideally, the coupling medium should be fluid so as to fill all available spaces, relatively viscous so that it stays in place (!!), have an impedance appropriate to the media it connects, and should allow transmission of U/S with minimal absorption, attenuation or disturbance.  At the present time the gel based media appear to be preferable to the creams. In fact many gels now contain ketoprofen or Ibuprofen as a pain releiver as well.  Water is a good media and can be used as an alternative but clearly it fails to meet the above criteria in terms of its viscosity.  Water is often used in a tank (or bucket) for extremity applications (ie. sprained ankle or wrist).

In addition to the reflection that occurs at a boundary due to differences in impedance, there will also be some refraction if the wave does not strike the boundary surface at 90° . Essentially, the direction of the US beam through the second medium will not be the same as its path through the original medium - its pathway is angled. The critical angle for U/S at the skin interface appears to be about 15° . If the treatment head is at an angle of 15° or more to the plane of the skin surface, the majority of the U/S beam will travel through the dermal tissues (i.e. parallel to the skin surface) rather than penetrate the tissues as would be expected.

Absorption and attenuation:
The absorption of U/S energy follows an exponential pattern - i.e. more energy is absorbed in the superficial tissues than in the deep tissues. In order for energy to have an effect it must be absorbed, and at some point this must be considered in relation to the U/S dosages applied to achieve certain effects.

Because the absorption (penetration) is exponential, there is (in theory) no point at which all the energy has been absorbed, but there is certainly a point at which the U/S energy levels are not sufficient to produce a therapeutic effect.

As the U/S beam penetrates further into the tissues, a greater proportion of the energy will have been absorbed and therefore there is less energy available to achieve therapeutic effects. The half value depth is often quoted in relation to U/S and it represents the depth in the tissues at which half the surface energy is available. The will be different for each tissue and also for different U/S frequencies.

As the penetration (or transmission) of U/S is not the same in each tissue type, it is clear that some tissues are capable of greater absorption of US than others. Generally, the tissues with the higher protein content will absorb U/S to a greater extent, thus tissues with high water content and low protein content absorb little of the US energy (e.g. blood and fat) whilst those with a lower water content and a higher protein content will absorb U/S far more efficiently. It has been suggested that tissues can therefore be ranked according to their tissue absorption.

Pulsed Ultrasound:
Most machines currently used in physiotherapy departments offer the facility for pulsed U/S output, and for many clinicians, this is a preferable mode of treatment. Until recently, the pulse duration (the time during which the machine is on) was almost exclusively 2ms (2 thousandths of a second) with a variable off period. Some machines now offer a variable on time. Typical pulse formats are 1:1 and 1:4 though others are available. In 1:1 mode, the machine offers an output for 2ms followed by 2ms rest. In 1:4 mode, the 2ms output is followed by an 8ms rest period. The effects of pulsed U/S are well documented and this type of output is preferable especially in the treatment of acute lesions.

Therapeutic Ultrasound & Tissue Healing
One of the therapeutic effects for which ultrasound has been used is in relation to tissue healing. It is suggested that the application of U/S to injured tissues will, amongst other things, speed the rate of healing & enhance the quality of the repair. The following information is intended to provide a summary of some of the essential research in this field together with some possible mechanisms through which US treatments may achieve these changes. It is not intended to be a complete explanation of these phenomena or a comprehensive review of the current literature. It may, none the less, provide some useful basic information for clinical application.

The therapeutic effects of U/S are generally divided into two main catagories: THERMAL & NON-THERMAL.

THERMAL:
The desirable effects of therapeutic heat can be produced by U/S. It can be used to selectively raise the temperature of particular tissues due to its mode of action. Among the more effectively heated tissues are periosteum, superficial cortical bone, collagenous tissues (ligament, tendon & fascia) & fibrotic muscle. If the temperature of the damaged tissues is raised to 40-45°C, then a hyperaemia will result, the effect of which will be therapeutic. In addition, temperatures in this range are also thought to help in initiating the resolution of chronic inflammatory states. Most authorities currently attribute a greater importance to the non-thermal effects of U/S as a result of several investigative trials in the last 15 years or so.

NON-THERMAL:
The non-thermal effects of U/S fall into 3 main catagories - CAVITATION, ACOUSTIC STREAMING & MICRO-MASSAGE.

CAVITATION in its simplest sense relates to the formation of gas filled voids within the tissues & body fluids. There are 2 types of cavitation - STABLE & UNSTABLE which have very different effects. STABLE CAVITATION does seem to occur at therapeutic doses of U/S. This is the formation & growth of gas bubbles by accumulation of dissolved gas in the medium. They take apx. 1000 cycles to reach their maximum size. The `cavity' acts to enhance the acoustic streaming phenomena & as such would appear to be beneficial. UNSTABLE (TRANSIENT) CAVITATION is the formation of bubbles at the low pressure part of the US cycle. These bubbles then collapse very quickly releasing a large amount of energy which is detrimental to tissue viability. There is no evidence at present to suggest that this phenomenon occurs at therapeutic levels if a good technique is used.

ACOUSTIC STREAMING is described as a small scale eddying of fliuds near a vibrating structure such as cell membranes & the surface of stable cavitation gas bubble. This phenomenon is known to affect diffusion rates & membrane permeability, with a result that protein synthesis is enhanced. Sodium ion permeability is altered resulting in changes in the cell membrane potential. Calcium ion transport is modified which in turn leads to an alteration in the enzyme control mechanisms of various metabolic processes, especially concerning protein synthesis & cellular sectretions.

The result of the combined effects of stable cavitation and acoustic streaming is that the cell membrane becomes ‘excited’, this increasing the activity levels of the whole cell. The U/S energy acts as a trigger for this process, but it is the increased cellular activity which is in effect responsible for the therapeutic benefits of the modality.

MICROMASSAGE is a mechanical effect which appears to have been attributed less importance in recent years. In essence, the sound wave travelling through the medium will cause the molecules to vibrate, possibly enhancing tissue fluid interchange & affecting tissue mobility.

The healing process is usually divided into 3 phases - those of INFLAMMATION, PROLIFERATION & REMODELLING. There is good evidence to suggest that U/S can be used effectively at all stages, though for different reasons.

INFLAMMATION:
There are 2 essential responses at this stage, both of which are related to chemical release from MAST CELLS, PLATELETS & LEUCOCYTES. Firstly, vasopermeability factors (e.g. HISTAMINE) when released have a direct effect on the local capillary membrane, resulting in an increased permeability. Secondly, chemotactic factors are released which attract neutrophils & other inflammatory cells to the area. These cells in turn release various factors resulting in the stimulation of the proliferative phase. In addition to the release of these factors, neutrophils phagocytose cell debris & toxins. U/S can stimulate MAST CELLS to degranulate & MACROPHAGES to release several chemical factors which in turn activate local FIBROBLASTS & stimulate the PROLIFERATION phase.  U/S in the early stage would therefore appear to be a PRO-INFLAMMATORY treatment which results in the stimulation of the proliferative repair stage.

PROLIFERATION:
During the proliferative stage, the FIBROBLASTS & ENDOTHELIAL CELLS which have been stimulated by mediators released during the inflammatory stage, migrate into the area from adjacent tissues. Once there, they proliferate & form the early scar tissue. The fibroblasts produce COLLAGEN, the basic building block for repair, a process which is enhanced the raised intracellular calcium ion levels. In addition during this phase, U/S serves to improve the local blood supply & stimulates the MYOFIBROBLASTS to contract, giving rapid initial wound strength.

REMODELLING:
During this phase, the immature scar tissue gradually matures (a collagenous, low cell content tissue with a poor blood supply). This tissue is progressively reshaped in response to local stress (c/f bone) so that it is optimally structured to meet the requirements of the parent tissue.