Medical ultrasonic energy (MUE) is a form of acoustical vibration for clinical application with a frequency commonly in the range of 0.8 to 1.0 megacycles (800,000 to 1,000,000 cps).It is a conversive energy form which is converted into vibrational energy from electrical energy by way of a piezoelectric crystal. This is not a form of diathermy.
When applied to human tissue, this acoustical vibration results in a radiation pressure change within the respective body fluids. This radiation pressure change provides for a particle acceleration of cellular components which approximates 100,000 X G. This change creates a secondary tissue reaction referred to as micromassage. It is the effect of such forces which results in alteration of the concentration of electrolytes, anions, and cations on both sides of the cellular membranes. It is this alteration in dissolved constituents and the respective effect on cellular function, which is consistent with the achievement of the desired therapeutic goals.
Because human tissue fluid is relatively homogeneous with a density gradient of 1, it tends to transmit MUE. Also, being relatively heterogeneous, adipose tissue absorbs MUE poorly and transmits it well. Since muscle tissue is relatively heterogeneous, it tends to absorb MUE well and transmits poorly. For these reasons, MUE tends to achieve therapeutic effects in muscle tissue without significant danger of thermal injury to adipose tissue even when applying MUE in the continuous form.
Water is used as the standard for determining coupling agent MUE velocity since human tissue fluid is largely water. MUE velocity in water is about 1500 m/sec, which is the standard. The velocity of MUE in cortical bone is approximately 3500 m/s. Because of the geometric configuration of protein molecules, as well as their large molecular weight, MUE is selectively absorbed by them. Approximately 80% of MUE is absorbed by protein molecules in the various tissues with muscle being the principle tissue of absorption.
Approximately one half of the energy contained in the MUE wavelength is consumed in achieving tissue depth while the balance of its energy is absorbed, or reflected and absorbed.
Although pulsed MUE is nonthermal in its clinical effect, it does provide therapeutic benefit, which is probably due to radiation pressure changes; albeit related to membrane ion transfer effects.
Clinically, if it is desired to achieve the effects of enhanced cellular delivery without an actual increase in peripheral blood flow, it may be possible to achieve this result by applying pulsed MUE. Pulsed MUE tends to have the following physiological effects:
- increased permeability of biologic membranes;
- change in membrane potentials;
- elevation of peripheral nerve pain thresholds;
- increase in lymph flow;
- without necessarily resulting in a significant increase in peripheral blood flow to the part.
- increased peripheral arterial blood flow;
- increased cellular metabolic rate;
- increased enzymatic activity (probably due to an increase in cellular thermodynamics).
Although MUE may be administered by direct contact using a coupling agent, by immersion under water, or by a fluid filled bag method, the immersion technique would tend to be the most efficient method.
Phonophoresis is an excellent method of delivering large, complex, whole molecules away from the skin surface into the tissue interstices under the influence of MUE. This delivery is probably due to resultant radiation pressure changes. In this author's opinion, phonophoresis is far superior to iontophoresis when the agents being delivered are large in molecular weight. However, phonophoresis appears to be inferior in comparison when transferring agents of small molecular weight, such as salts (crystals).
In the event that the agents to be transferred are oil soluble, an oil coupling agent must be used. The same condition applies to water soluble agents, in which case, water must be used as the coupling agent.
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R. Vincent Davis, DC, PT, DNBPM