April/May 2000

Penetrating the Problem

By Betty Tabakovic Oakley, MSPT

Therapeutic ultrasound is one of the most commonly used physical agents for the treatment of musculoskeletal injuries. It is used as part of an overall treatment program for diagnoses such as temporomandibular joint pain, myofascial pain, lateral epicondylitis, soft-tissue shoulder and ankle disorders, and various other conditions.  

Clinical Applications of Ultrasound

Ultrasound is most effectively used to help patients move through and resolve various symptoms associated with the phases of the healing process. Trauma, surgical procedures, and open wounds go through the process of inflammation and repair that consists of three phases: inflammation, proliferation, and maturation. The inflammatory phase begins when injury or disease causes a disruption in the normal physiological function of the tissue, which can last up to 10 days. It is characterized by swelling, heat, redness, pain, and loss of function.2 During this stage, ultrasound is used primarily for its nonthermal properties to facilitate the process of inflammation and therefore healing. This is attributed to the acoustic streaming of the ultrasound beam in the tissue, which has been shown to increase intracellular calcium, macrophage responsiveness, skin and cell permeability, mast cell degranulation and histamine release. These effects have been attributed to pulsed ultrasound with a duty cycle of 20% at an intensity of .5 W/cm2 or continuous mode at a very low intensity of .1 W/cm2.3

The proliferative phase begins around day three and lasts up to 20. This stage is characterized by migration of epithelial cells and fibroblasts to cover and impart strength to the injured area.2 During this stage, ultrasound has been shown to stimulate fibroblast to produce more collagen and accelerate the development of new blood vessels.

The maturation phase begins around day 9 and may last for years. It is characterized by the gradual disappearance of fibroblasts. Collagen is initially laid down in a random fashion and it is during this phase that the fibers become more organized and mature. Since collagen fibers will orient themselves parallel to lines of stress, the type of tension applied to the scar will affect how it remodels. Scars are inelastic and during this stage clinical techniques are incorporated to avoid the development of short, dense adhesions that would restrict motion. Soft- tissue shortening or adhesion formation can also result from immobilization or chronic inflammation in which healthy tissue is often replaced with scar tissue.2

The thermal properties of ultrasound have been shown to increase elasticity and decrease the viscosity of collagen fibers and soft tissue—allowing for greater residual length gains while reducing the risk of damage through the applied stretching force. The change in viscoelastic properties is transient and in order to get optimal effects, the stretching should be applied during heating or immediately following.3,4

Both thermal and nonthermal properties of ultrasound have been shown to decrease pain. The mechanism of pain reduction is still unclear, but it is thought that ultrasound may control pain by altering its transmission or perception due to the stimulation of the cutaneous thermal receptors, altering nerve conduction velocity, and increasing tissue temperature or modulation of the inflammatory process.5

Two natural laws need to be taken into consideration when applying ultrasound: ultrasonic energy must be absorbed by the target tissue in order to have an effect, and the right amount of energy needs to be delivered and absorbed by the tissue in order to produce the desired physiological responses. If the energy absorbed is insufficient to stimulate the tissue, no reactions or changes will occur.6 With these laws in mind, it is logical to conclude that one favorite or standard intensity and duration for ultrasound cannot and will not be the correct dosage for every diagnosis.

Frequency—1-MHz vs 3-MHz

Ultrasound can have an effect on the target tissue only if the energy delivered reaches the tissue and is absorbed. The depth of tissue penetration is not intensity dependent, but frequency dependent. Therefore, in order to determine the appropriate frequency, the depth of the target tissue must be ascertained. The 1-MHz frequency can heat tissue up to 3-5 cm deep, while the 3-MHz can penetrate up to 2 cm. The higher the frequency, the higher the rate of absorption and attenuation. Therefore, most of the ultrasonic energy with a 3-MHz frequency will be absorbed in the superficial tissue. In contrast, the slower, 1-MHz frequency will have less energy absorbed superficially, allowing for deeper penetration.

Can you compensate for a 1-MHz frequency in a superficial structure by lowering the intensity used? Since depth of penetration is not intensity dependent, lowering the intensity may delay periosteal overheating but will not facilitate absorption of energy in the superficial target tissue. Draper et al experimented with the two frequencies applying both to the patellar tendon at 1 W/cm2. They found that at 4 minutes, the 3-MHz ultrasound started to produce heat that was uncomfortable, requiring them to decrease the intensity, while the 1-MHz ultrasound caused periosteal aching within 1 minute, resulting in the termination of the treatment. The energy delivered by the 1-MHz frequency was not absorbed in the superficial tissue and therefore was able to penetrate and overheat the periosteum, while the energy with the 3-MHz was quickly absorbed in the tendon, producing heat.7 In light of this information, substitution of frequencies is not recommended due to the attenuation (decrease in energy of ultrasound as distance traveled increases) and absorption characteristics of ultrasound.

Intensity/Duration

The intensity chosen will be dependent on the treatment goal. The nonthermal effects of ultrasound are desired for the treatment of acute injury, edema, and wound healing. Low-intensity ultrasound at .5 W/cm2, pulsed mode with a 20% duty cycle are the parameters most frequently cited in related research. The physiological effects associated with thermal applications of ultrasound occur at specific tissue temperature increases. Studies done by Lehman indicate that an increase of 1°C accelerates the metabolic rate of tissue. An increase of 2-3°C reduces muscle spasm and pain, increases blood flow, and reduces chronic inflammation, and greater than 3-4°C tissue temperature rise decreases the viscoelastic properties of collagen. Table 1 organizes these physiological effects and links them to the stages of healing.7 Once the treatment goal has been established, the degree of tissue temperature is chosen and the appropriate treatment at a given intensity is determined. A study done by Draper et al looked at the rates of temperature increase in human muscle during 1-MHz and 3-MHz continuous ultrasound. From this study, the researchers were able to predict tissue temperature with both frequencies at doses of .5, 1.0, 1.5. and 2.0 W/cm2. Table 2 summarizes the these findings and assists in determining an appropriate treatment duration at a given intensity level. Too often ultrasound treatments are given at a standard duration of 5 minutes. On the basis of the data from this study, the intensity and the frequency used should determine the appropriate duration. In general, the higher the intensity and frequency, the faster the rate of heating. With a 3-MHz frequency, we can expect the rate of heating to be 3-4 times faster than at a 1- MHz frequency.8

Suppose I have a patient who is in the subacute stage of healing. My treatment goal is to accelerate the metabolic rate of the tissue and therefore accelerate the rate of healing. In order to obtain this physiologic effect, I need to create a 1°C tissue temperature rise. Using a 1-MHz frequency at an intensity of 1.0 W/cm2, the rate of heating would be .2°C per minute for a total treatment time of 5 minutes to reach a 1°C tissue temperature rise. Using the same frequency at an intensity of .5 W/cm2, the rate of heating would be .04°C per minute for a total treatment time of 25 minutes.

It is evident that any adjustments in the intensity should be followed by changes in treatment time. It is important to recognize that even though a continuous mode is used for thermal effects, at a .5 W/cm2 intensity with a 1-MHz frequency, the rate of heating is very slow and the typical 5- minute treatment time would result in very minimal tissue heating, thus, treatment would be ineffective. With this information in mind, it is important to remember that intensity is ultimately determined by patient tolerance.

Moving the transducer

During the application of ultrasound, it is very easy to inadvertently increase the speed of the movement of the ultrasound head. When this happens, the tissue being treated does not have enough time to absorb the energy.

A frequently recommended rate for the movement of the transducer is 4 cm/second.1-3 Castel recommends moving the soundhead as slowly as possible without causing pain, while Draper recommends 2 inches per second, lowering the intensity and making appropriate adjustments in treatment time if the patient complains of discomfort.9 However, the speed at which the transducer should be moved over the treatment area depends on the beam nonuniformity ratio (BNR) identified by the manufacturer on the ultrasound unit.

The ultrasound beam is nonuniform in nature and the higher the BNR ratio, the greater the nonuniformity of the beam and potential hot spots. For most units, the BNR ratio is usually 5:1 or 6:1, creating peak intensities five to six times greater than that set by the clinician. This means that at a therapeutic intensity of 1.5 W/cm2, a machine with a BNR of 6:1 would produce a peak intensity of 9 W/cm2, exceeding the 8 W/cm2 safety imit. These high peak intensities are what often cause pain or discomfort associated with ultrasound applications. A unit with a low BNR will allow for slower movements of the soundhead and less discomfort for the patient.

The keys to maximizing treatment outcomes with ultrasound in musculoskeletal conditions are: determining the stage of healing, determining the treatment goal, and choosing parameters that will accomplish the desired treatment outcome. N

Betty Tabakovic Oakley, MSPT, is an associate professor and clinical science director of the Department of Physical Therapy at Andrews University, Berrien Springs, Mich.

References

1. Cameron MH. Physical Agents in Rehabilitation: From Research to Practice. Philadelphia: WB Saunders Company; 1999:280-284, 293.
2. Michlovitz SL. Thermal Agents in Rehabilitation. 3rd ed. Philadelphia: FA Davis Company; 1996:3-17, 200.
3. Prentice WE. Therapeutic Modalities for Allied Health Professionals. New York: McGraw-Hill Health Professions Division: 1998:271, 279, 280, 289.
4. Hecox B, Mehreteab TA, Weisberg J. Physical Agents: A Comprehensive Text for Physical Therapists. Norwalk, Conn: Appleton & Lange; 1994:116.
5. Fedorczyk J. The role of physical agents in modulating pain. J Hand Ther. April-June 1997:110-121.
6. Griffin JE, Darselis TC. Phsical Agent for Physical Therapists. 2nd ed.. Springfield, IL: Charles C. Thomas; 1982:32-34.
7. Draper DO. Ten mistakes commonly made with ultrasound use; current research sheds light on myths. Journal of Athletic Training. 1996;2(2):95-107
8. Draper DO, Castel JC, Castel D. Rate of temperature increase in human muscle during 1MHz continuous ultrsound. J Othop Sports Phy Ther. 1995;22:142-149.
9. Enwemeka CS. The effects of therapeutic ultrasound on tendon healing. Am J Phys Med Rehabil. 1998;68:283-286.

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