Therapeutic ultrasound is one of the most commonly used physical agents for the treatment of musculoskeletal injuries. The therapy has been used in the treatment of musculoskeletal injuries and conditions for years, primarily because it effectively promotes healing in the injured tissue and helps reduce pain. Its ability to penetrate deeper than other heat-based modalities facilitates a beneficial effect on deep muscle tissue.

Unique capabilities that allow either thermal or nonthermal applications make ultrasound a valuable option as a therapeutic treatment for a wide range of acute and chronic conditions, such as tendinitis, muscle spasms, bursitis, fasciitis, edema, sprains, contractures, and adhesions. It also 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.

HELPING THE HEALING PROCESS

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 inflammation 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. During this stage, ultrasound is used primarily for its nonthermal properties to facilitate the process of inflammation and, therefore, healing.

The proliferation phase begins around day 3 and lasts up to day 20. This stage is characterized by migration of epithelial cells and fibroblasts to cover and impart strength to the injured area. During this stage, ultrasound has been shown to stimulate fibroblasts 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 also can result from immobilization or chronic inflammation in which healthy tissue is often replaced with scar tissue.

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 afterward.

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.

CLINICAL FUNDAMENTALS

Ultrasound delivers sound waves to tissue at a frequency above the range of human hearing, providing a "micromassage" via mechanical vibrations. Clinical ultrasound is delivered as either a thermal (continuous) or nonthermal (pulsed) application, generally at frequencies of 0.75 MHz to 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 to 5 cm deep, while the 3-MHz frequency 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.

A topical medium, such as water, gel, cream, lotion, or mineral oil, must be applied to the skin surface before treatment. Determination of frequency, duration, and whether to use pulsed or continuous application depends on the equipment being used, as well as the type of injury (such as adhesion, muscle spasm, tendinitis, contracture, or edema).

To ensure correct application and the desired outcome, it is important for PTs to understand how ultrasound works. Combining knowledge of the physical agent, the pathology of the injury, and the correct technique for delivering it significantly increases the potential for success.

Ultrasound works by delivering acoustic vibrations via a transducer that utilizes a quartz crystal to convert electrical energy to ultrasonic waves, which can then be applied to human tissues for therapeutic purposes. Absorption of this energy takes place at the molecular level with the protein in the tissue acting as the absorbing agent. Muscle, fat, and hemoglobin all absorb ultrasound. Since numerous factors affect absorption and the benefits received, an understanding of the physiology, the delivery process, and any contraindications are key for ensuring that patients' results are maximized.

Once the initial diagnostic exam has been performed, and ultrasound has been named the modality of choice, PTs must choose the appropriate dosages, treatment times, and frequencies. It all begins by examining the affected tissue itself. Is it superficial, or is it deep? Do you want to heat the tissue, or do you want to just do the micromassaging that ultrasound does in a pulsed wave?

For deep structures that require heating, PTs might apply a 1-MHz frequency, which penetrates and heats deeper tissue. In this scenario, dosage depends on how quickly the therapist wants to heat the tissue. To heat quickly, a higher dosage—such as 1.5 watts per square centimeter—is used.

It also is important to keep the size of the affected area in mind when choosing the soundhead to be used during the treatment. Based on the size of the soundhead, treatment time can be determined—for example, a larger area might require a 10-minute treatment. Generally, treatment times run between 5 and 15 minutes.

Once frequency and the size of the affected area have been determined, transmission gel is applied to the affected area and the PT moves the soundhead slowly over the area to be treated until the prescribed time is up. Continuously moving the soundhead is important, as resting it on one area for too long could cause pain or damage, especially if the ultrasound is concentrated on the bone.

The basic benefits of ultrasound treatment include:

• Increased elasticity of collagen in tendons, joint capsules, and scar tissue.
• Increased motor and sensory nerve-conduction velocities, which assists in reducing pain.
• Altered contractile activity to skeletal muscle, which reduces muscle spasm.
• Diminished muscle spindle activity, another factor in muscle spasm reduction.
• Increased blood flow.

These factors make ultrasound the modality of choice for conditions involving muscle spasm, pain, scar tissue, and acute and chronic soft-tissue inflammation. It can be delivered over smooth, even surfaces such as the lower back, as well as uneven joint surfaces such as the ankle and wrist. For example, for an elderly patient whose muscles are tightening up due to aging, ultrasound's unique characteristics allow the tissue to be stretched more easily and with less trauma, resulting in better gains. Another example would be a case of edema, in which it can be challenging to initiate movement, but the ultrasound's mechanical process helps increase the mobility of the fluid, thus reducing the chances of increasing trauma or injuries to that area.

While all modalities have some overlapping qualities, it is important to know the distinctive qualities and contraindications of each option to determine the most appropriate choice. For instance, hot packs provide heat but do not penetrate as deep as ultrasound. With electromagnetic radiation, the depth of penetration is slightly less compared to ultrasound, in addition to a loss of energy through the surface of the skin during sweating. Ultrasound, which is very deep, produces heat proportional to the heat absorbed in the tissues so there is no loss of energy from the skin, thus having maximal benefit of the diathermy modality.

The only real limitation for ultrasound is in the size of the area that can be treated, which is determined by the size of the transducer head (that is, an acute muscle spasm over a large part of the lower back would be difficult to treat and would tend to make electromagnetic radiation the better choice of diathermy for such cases). The treatment selected also depends on whether the injury being dealt with is acute or chronic. While ultrasound can work effectively for either one because of its thermal and nonthermal qualities, some of the other options do not possess that capability.

TECHNIQUES FOR OPTIMAL RESULTS

Once again, the key to determining the most effective method to use is in knowing as much as possible about the treatment options and the injury.

To ensure the best possible outcome, ultrasound is typically prescribed in combination with other modalities. For example, in acute injuries (such as a back or neck injury, or a knee or ankle sprain), the therapist may use ice, compression, and gentle exercise, depending on the severity and location of the injury and the specific symptoms. For chronic injury, treatment might include aggressive stretching and manual therapies in addition to ultrasound, which is useful for allowing muscle spasms to relax, reducing edema, and mobilizing adhered tissues. A combination of ultrasound and electrical stimulation is very effective in treating muscle spasms, controlling pain, and increasing range of motion.

Various pharmaceuticals, such as hydrocortisone creams, vitamin E, or a local anesthetic, also can be delivered to an injury site more effectively via ultrasound because it increases cell membrane permeability through the positive and negative forces, allowing the entire molecule to penetrate into the tissue.

When combining ultrasound with certain other forms of treatment such as stretching, it is important to provide the stretching component of the therapy immediately afterward to take advantage of the increased elasticity and reduced pain achieved in the therapy session.

TODAY'S TECHNOLOGY

Ultrasound models are sleeker than their ancestors, thanks in large part to the development of more compact computer technology. You can get ultrasound units that you can combine with electrical stimulation and portable units that you can carry with you—the result is a unit that will not take up too much space in the practice.

For the most part, the ultrasound units are ready to apply when clients come into the practice. Cleaning the soundheads between clients is generally as easy as wiping off the gel from the soundhead, then using an alcohol-based spray cleaner for sterilization. The soundheads come in a variety of sizes so that PTs can choose the one that best fits the size of the treatment area.

Each year, the units should have a routine maintenance check to make sure there is no line leakage and the power level is where it should be. For the most part, maintenance is easy, but proper diligence is still required.

Essentially, the key to increased use of ultrasound—and the opportunity for more patients to take advantage of its many benefits—lies in rehabilitation professionals updating their knowledge and training. Having an understanding of the procedure, its benefits, effects, proper application, and potential outcomes provides a foundation for more effective application of the process. That is important in the health care industry now, because the number of therapy visits a patient is allowed is usually limited by insurance coverage.

When determining a therapy plan, it is vital to know as much as possible at the outset about the patient's general condition, the injury to be treated, the treatment goals, as well as any potential modalities that can offer an appropriate solution. Ultrasound may not be the answer for every patient; however, it does offer unique benefits that should be taken into consideration.

Susan Keller, PT, is a pain management specialist at Peak Physical Therapy in Seattle. For more information, contact .