The use of ultrasonic sound for diagnosis and treatment in human and equine medicine is not new, and in fact is becoming commonplace. Most horse breeders, for example, are familiar with the use of diagnostic ultrasound to detect and monitor reproductive problems and pregnancy. Sport horse owners in large numbers have seen ultrasound employed in diagnosing ligament and tendon injuries. But perhaps not quite so common is the use of ultrasound to facilitate recovery from injury. It is an intriguing and beneficial modality for treating a variety of problems in equines, particularly in the limbs.

Therapeutic ultrasound is capable of significantly raising the temperature of deep tissues. This provides for much greater therapeutic potential than, for example, merely applying external heat to a limb. When heat is applied, the skin becomes very warm, but often the deep tissues–where heat is needed–are not affected by surface warming.

Ultrasound is capable of heating these deep tissues without elevating the temperature at the skin surface, says Mimi Porter, an equine physical therapist from Lexington, Ky., who is the author of Equine Sports
, a book that delves into a variety of therapeutic modalities.

Because of its penetrating nature, ultrasound can be used to selectively heat certain tissues, such as muscles and ligaments, while avoiding excessive heating of the overlying skin.

As pointed out in an earlier article on basic physical therapy (The Horse of May 1997, page 59), heat can be beneficial in the healing process.

Porter explains it this way: “Heating, being a form of energy, increases metabolic activity in the cells. This increased activity causes an increase in oxygen demand locally. As a result, vasodilation occurs to increase the amount of blood bringing oxygen and nutrients to the area. Membrane diffusion and enzymatic activity also increase, enabling oxygen consumption and waste removal. The waste products of injury include prostaglandins, bradykinin, and histamine, all implicated in nerve fiber sensitization and pain.

“Methods of heat application include heating pads, hot water bottles, hydrocollator packs, heating lamps, hot water whirlpools, hot towels, counter-irritating liniments, and ultrasound. Of all the heating devices listed, only ultrasound has the ability to penetrate through the skin to the deeper structures, such as joint structures, tendons, and muscles. All the other sources heat only the skin and perhaps the underlying connective tissue. These two structures are not involved in most sports injuries. The temperature of the injured tissue must increase to have a significant effect.”

While ultrasound can produce a highly beneficial effect in treating certain types of injury, it also has the capability of causing harm if not applied properly.

The window of opportunity between therapeutic healing and ultrasound damage is narrow–only about 5° Centigrade. At 40° Centigrade, the potential for healing is at or near its maximum. However, temperatures beyond 45° Centigrade can cause serious tissue damage.

With ultrasound, Porter repeats with frequency, more is not better.

It is for this reason that therapeutic ultrasound should be used only by persons properly trained in administering it, and only after an evaluation is made by a veterinarian.

“A complete veterinary evaluation is necessary to determine the tissues involved in an injury,” says Porter, “and the status of the healing process before ultrasound therapy is indicated.”

How Ultrasound Heats

Sound is made up of vibrations that are audible to the human ear at cyclic frequencies between 16,000 to 18,000 cycles per second. Frequencies above that range that are inaudible to the human ear are termed ultrasonic.

One of man’s first clues that certain creatures made use of these ultrasonic sounds came from observing bats in flight. It was found that bats could avoid flying into objects by bouncing ultrasonic sound waves off of solid surfaces and listening for the echo.

It took a major disaster to spur on research efforts. In the early 1900s, the luxury liner Titanic crashed into an iceberg and sank to the ocean floor.

As the research effort into the use of ultrasonic sound cranked up, it was found that pulses of high frequency sound could be bounced off objects such as icebergs. However, researchers noticed a negative effect that has since been turned into a strong positive.

“It was noted,” says Porter, “that the high frequency sound emissions were injuring marine life when the use of echo-location increased. Scientists began to look for medical uses of ultrasound once it was realized that high frequency sound had an effect on living tissues. The first recorded use of ultrasound was in 1938, when it was used to treat a case of sciatica, or inflammation of the sciatic nerve–a bothersome problem for both humans and horses. Therapeutic ultrasound has been in use in hospitals for over 50 years and remains the treatment of choice for many pathologies.”

There were a variety of offshoots from the medical research. For example, some burglar alarms and fish locators make use of ultrasound.

Medical uses other than for therapeutic purposes, says Porter, include pulse-echo diagnostic imaging, doppler shift detection of blood flow velocity, and surgical techniques such as cataract removal and tumor irradiation.

There is a basic difference between sound waves employed in diagnostic ultrasound and those involved in therapeutic ultrasound. The fundamental difference, Porter explains, is that diagnostic equipment is designed to obtain information without intentionally producing a biological effect. Therapeutic ultrasound is designed to intentionally produce certain biological effects.

The prime difference is in the frequency ranges. Ultrasound is measured in megahertz. One megahertz is one million cycles per second. This is a frequency, says Porter, which is ideal for sound absorption in body tissues.

The frequency for diagnostic units might range up to 10 megahertz.

Porter takes us a step deeper into understanding the ultrasound wave itself:

“Ultrasound waves are pressure waves of mechanical energy moving in a straight line from the source. There is very little divergence of the ultrasound beam, which gives it more power to penetrate into the tissues. This differs from audible sound in that the sounds we hear are vibrations which are propagated in all directions from the source.

“A sound wave is one cycle of compression and rarefication. Particle density is greatest during the compression phase and is less dense than when at rest during the rarefication phase. Rarefication is a term meaning to make something less dense. The compression phases move along in a row as the wave moves deeper into the tissue. Thinking of ocean waves coming into shore or how a ‘slinky’ toy moves will give you the idea of the wave motion of the compression wave as it travels through tissues.

“The rate of repetition of these waves is called the frequency and is measured in hertz, or cycles per second. The human ear can typically hear in the range of 16,000 to 18,000 hertz. The bat navigates, using echo location, at around 200,000 hertz. Ultrasound is ultra high frequency sound, well above detection by the human ear–in the range of 1 million to 10 million hertz. (One million hertz becomes one megahertz.)

“Thermodynamics is the science of the transformation of energy into heat and heat into various forms of energy. This field of study has determined that the body readily converts and stores various energy forms as heat. The energy in the sound wave enters the tissues as kinetic energy, or energy of motion, and is converted to heat, resulting in a rise in the tissue temperature.

“The sound head is often called a transducer. Any device which converts energy from one form to another is known as a transducer. An ultrasound head converts electrical energy to kinetic energy in the form of sound waves.

“Ultrasound is produced by applying electrical current to a specialized material which is housed in the sound head of the unit. Components within the typical unit transform 110-volt, 60-cycle electricity from a wall outlet to 500 volts or more and to one million cycles per second.”

The two prime problems that exist with therapeutic ultrasound, Porter indicates, are improperly trained applicators and out-moded equipment.

“Out-dated units in need of calibration are often used,” she says, “and tales of damage from deep tissue burns are often heard. An understanding of what is going on in that mysterious machine and of what it is doing to the horse are essential before consistently good results will be obtained.”

Use Of Ultrasound

Appropriate equipment in the hands of a skilled individual, however, can effect beneficial changes for a variety of problems. Some of them, as outlined by Porter, include:

Joint MobilityUltrasound is often used to increase joint range of motion, which might have been compromised by scar tissue, muscle spasm, tendonitis, and bursitis. Elevating tissue temperatures provides a level of comfort for the horse when stretching exercises are used to increase the joint’s range of motion.

Tendon ExtensionA tendon, when injured, will tend to shorten or contract. To achieve normal elongation once again, the therapist will normally make use of both heating and stretching. Heating a tendon to 37-40° Centigrade and applying static or repeated steady stretches manually can increase tendon length.

Scar TissuesThere is evidence that ultrasound can soften scar tissue, which is capable of inhibiting normal range of motion. Scar tissue will absorb more of the sound wave energy and can thus be selectively heated by ultrasound because it is more dense than the surrounding tissue. Scar tissue can be compromising to the horse’s ability to perform and can be cosmetically unattractive.

Bony GrowthsUltrasound is sometimes used to treat pain and inflammation associated with splints, or exostosis (a spur or bony outgrowth of a bone) of the interosseous ligament between the second and third or third and fourth metacarpal bones. This condition is caused by new bone formation in response to trauma from a blow or from weight bearing on imperfectly positioned bones. However, it appears unlikely that ultrasound can be used to stimulate the reabsorption of calcium deposits. Reduction of splint size is probably due to a reduction in the swelling in the soft tissue and a resolution of scar tissue around the injury, not a reduction in the size of the actual bone formation. The most satisfying results normally are obtained from treatment that is begun at the first sign of inflammation.

Pain ReliefWhile ultrasound can alleviate pain, there is some uncertainty as to specifically how this is accomplished. It is known that ultrasound can stimulate acupuncture points and trigger points for pain relief.

Muscle Spasm RelaxationUltrasound produces relaxation of muscle spasms through its direct effect on the gamma fiber activity. The gamma efferent fibers originate in the spinal cord and terminate in specialized muscle fibers called the intrafusal fibers. Vigorous muscle contractions heighten the electrical activity between the muscle and the spinal cord. Impulses traveling from the spinal cord through the gamma fibers to the intrafusal fibers cause the muscle to contract. Contraction stretches an organ called the muscle spindle. Stretching of the muscle spindle results in reflex contractions of the muscle fibers. Constant muscle tension creates obstruction of the circulation resulting in ischemia (localized tissue anemia due to the obstruction of the inflow of arterial blood). The ischemia causes pain. Pain brings about reflex muscle activity and a vicious cycle called muscle spasm is born. Heating of the gamma fibers slows their transmission rate, which reduces the stimulation of the muscle spindle, allowing the muscle to relax. Heating the muscle tissue also causes an increase in circulation, thus breaking the cycle.

Reduction Of EdemaThe physiological effects of ultrasound include an increase in the quantity of blood flowing through the capillaries and an increase in cell membrane and vascular wall permeability, all of which help in the reduction of edema, which is essential for rapid recovery from soft tissue injuries. An example of edema reduction, as described by Porter, involved a serious hematoma (a local swelling filled with blood) on a yearling’s chest that was resolved in nine days with ultrasound therapy. In another case, edema involved with tendonitis was resolved by 70% with 10 days of ultrasound therapy.

Wound HealingIt has been observed that wound closure is significantly more rapid with ultrasound. Experimental evidence indicates that ultrasound can increase the rate of protein synthesis in fibroblasts, which are responsible for the repair of injured tissue. Studies of tendon repair, says Porter, indicate that the stage of the healing process in which ultrasound is applied is very important. Ultrasound applications begun too early in the repair process could actually retard healing. Ultrasound applied within the first two weeks of injury or surgical repair could hinder initiation of the healing process. On the other hand, ultrasound applied at low intensities two weeks after injury, when collagen formation and fibroblastic infiltrations has begun, can be beneficial.

“This is an area of ultrasound use which requires further study,” says Porter. “Certainly, veterinary guidance would be in order when using ultrasound following surgery or an injury resulting in a wound. Special consideration must be given to avoiding contamination of the wound. The wound area must be thoroughly cleaned before and after ultrasound applications.”

Porter also points out that there are treatment areas where ultrasound should not be used:

  • Ultrasound should not be applied to the eyes, since the lens has poor vascularization and cannot conduct heat away. Also, cavitation (the formation of partial vacuums in a liquid) may occur in the fluid media and lead to irreversible damage.
  • The uterus should not be exposed to therapeutic ultrasound during pregnancy because of the possibility of cavitation in the amniotic fluid.
  • The heart should not be directly sonated at therapeutic levels because the possibility exists that it may change the action potentials and contractile properties of the heart tissue.
  • Ultrasound should not be used on a regular basis over or near growth centers of the bone until bone growth is essentially complete. Changes in the epiphysal area would disrupt normal growth of the bone.
  • Healing fractures should not be treated with ultrasound. This could cause delay in the calcification process and retard healing.
  • Ultrasound should be avoided in areas which have been anesthetized or have diminished sensation.
  • Ultrasound should not be applied over areas of vascular insufficiency because the blood supply would not be able to adjust to the increased metabolic demand.
  • Inflammed blood vessels or vessels with known clots should not be treated with ultrasound due to the dangers of dislodging the clot and creating an embolus.
  • Tumors should not be treated with ultrasound because the vibrations might stimulate the tissues to grow and encourage metastis (the migration of cells to other organs).
  • Ultrasound should not be used over areas of cellutis or infections as it will result in dissemination of the infection process.
  • One final, but rather important, word of caution. Because of its heating effects, ultrasound should not be used on an injured area immediately after exercise. Irritation from the exercise would be increased by ultrasound.

Thus, we can conclude, the key to beneficial therapeutic ultrasound lies in proper utilization for maladies that it can benefit. If the correct level of ultrasound is administered, it can result in positive effects for a number of equine problems. Porter summarizes, in part, her feelings on the use of ultrasound, as follows:

“The opportunity for misuse of ultrasound will increase as the popularity of ultrasound use grows. It has been stated repeatedly that overdosing with ultrasound either in terms of too high an intensity level or with an application schedule that is too frequent or prolonged can result in deep tissue

“It is important to know how to apply ultrasound to cause a rise in the target tissues without overheating tissue interfaces. The Arndt-Shultz principle certainly applies to ultrasound; that is, too small an amount of energy produces no useful reaction, too much energy destroys tissue, but the appropriate amount of energy provides the desired response.”