Physical Modalities in Rehabilitation: Laser, E-Stim, and Ultrasound, Oh, My!
Janet B. Van Dyke, DVM, DAVSMR
Most of us are familiar with diagnostic ultrasound and surgical lasers. When we hear physiotherapists discuss the use of ultrasound and laser, the temptation is to believe that we already have these tools at our disposal. In reality, the ultrasound used by physios is quite different from the diagnostic ultrasound with which we have experience, and therapeutic laser cannot be interchanged with the higher-powered surgical laser. Physical modalities commonly used in human physiotherapy, and now in veterinary rehabilitation, include electrotherapy, electromagnetic fields, light, heat, sound, and water.
Electrotherapy can affect the sensory and the motor nerves. Indications for electrotherapy include wound healing, pain control/relief, reduction of inflammation, muscle re-education, reversal of atrophy, and strengthening.
Electrotherapy works at many levels. At the cellular level, it causes excitation of nerve cells, changes cell membrane permeability, and stimulates protein synthesis, osteosynthesis, and fibroblast formation. On the tissue level, electrotherapy causes skeletal muscle and smooth muscle contraction. On the segmental level, electrotherapy facilitates muscle-pumping action that improves joint mobility and circulatory and lymphatic drainage.
TENS works by stimulating faster sensory nerves with an electrical impulse that causes overloading of interneurons. This limits the ability of sensory nerves to transmit their pain signals to the brain, creating analgesia for the patient. This effect is short lived, generally not lasting more than 1 hour past the treatment period. In veterinary rehabilitation, this modality is used immediately postoperatively, and during therapy, allowing the patient to work through a potentially painful treatment.
Motor nerve stimulation is called neuromuscular electrical stimulation (NMES), and is achieved with a different piece of equipment from the TENS. NMES works on motor nerves and impacts muscle fibers. It is particularly useful in the patient who cannot perform voluntary movement, who have edema, and who have delayed wound healing. In the patient who is able to stand, NMES can cause increased muscle contraction during weight bearing, and in the ambulatory patient, timed NMES can assist in muscle re-education during gaiting.
The NMES has several variables, including intensity, pulse duration, current, frequency, on-off times, ramp duration, and treatment duration. Intensity is measured in milliamps (mA), and pulse duration in microseconds (ms). Frequency is measured in hertz (Hz) or pulses per second (pps). Increasing the frequency causes faster firing, with increased strength of muscle contraction and a more rapid fatigue. On-off times are measured in seconds, with ratios ranging from 1:1 (10 seconds on and 10 seconds off), for prevention of atrophy, to 1:4 (10 seconds on and 40 seconds off) used for weaker muscles. Ramp duration is particularly important in veterinary rehabilitation, as it is the period of time from the onset of the current until the full strength current is delivered. Ramp duration is generally set for 2 to 8 seconds. Treatment duration is generally 15 to 20 minutes, and this therapy has its best results when used two to three times per week.
NMES is delivered to the patient via leads and electrodes. Most NMES machines come with disposable electrodes which have a sticky side, designed for multiple uses on human patients. These electrodes can be used on our veterinary patients, but quickly lose their adhesive due to hair and dander accumulation. We recommend the use of non-sticky carbon electrodes, which use gel as a medium to transmit current. These electrodes can be taped on for the 15 to 20 minutes of treatment time. There is no need to clip the patient, as long as sufficient gel is used.
The physiological effects of laser stimulation include accelerated cell division via mitochondrial stimulation, increased leukocyte phagocytosis, stimulation of fibroblast production, enhanced synthesis of ATP, and angiogenesis. Treatment with laser is indicated for pain management, control of inflammation, and tissue healing. Penetration of laser energy is determined by the wavelength, and many wavelengths are patented.
The power of a laser is measured in milliwatts to watts. There are four classes of laser: Class 1 has less than 0.5 mW of power. A common Class 1 laser is a garage door opener. Class 2 lasers have up to 1.0 mW of power. Most laser pointers are in this class. Class 3 is divided into 3a and 3b, with Class 3a having 1.0 to 100 mW, and 3bs having up to 500 mW of power. These represent the majority of therapeutic lasers available today. Class 4 lasers have over 500 mW of power. These are 'hot' lasers commonly used in surgery.
Laser therapy is measured in joules of energy. A joule is the energy delivered by one watt of laser energy in one second. To get 1 joule of energy delivered from a 500 mW laser, one would need to treat for 2 seconds. (500 mW x 2 seconds = 1 watt second). Most therapeutic regimens from current research call for 1 to 8 joules of energy. As the common therapeutic laser delivers 500 mW of power, treatment times are 2 to 16 seconds per site. As stated above, the depth of penetration is determined by the wavelength of the laser. Higher power simply delivers this energy to the same depth at a faster rate.
Heat can be applied by hot packs, infrared light, hydrotherapy, and by therapeutic ultrasound. The effects of thermotherapy are vasodilation with secondary increased local circulation, decreased pain, relaxed muscle tone, reduced muscle spasm, increased tissue extensibility, increased cellular metabolism, and increased local tissue oxygenation. Heat is generally used to reduce pain from arthritis, trigger points and muscle spasms, and to prepare tissues for exercise or stretching.
Cryotherapy can be applied via ice bath, ice massage, ice pack, vapocoolant gel, or circulating ice compression units. The beneficial effects of cryotherapy include vasoconstriction; reduced cellular metabolism; decreased nerve conduction velocity, leading to analgesia; reduction of edema and decreased muscle spasm. Cryotherapy is generally used in the acute period after injury or surgery.
Recent research has shown that intermittent compression with cryotherapy is superior to simple application of cold packs in reducing swelling and edema as well as in creating analgesia.
Like a diagnostic ultrasound, the therapeutic ultrasound converts electricity to sound waves by means of a piezoelectric effect upon the crystal housed in the transducer head. Unlike the diagnostic equipment, therapeutic ultrasound works at 1 MHz to 3 MHz, with the desired results being thermal effects and tissue modulation. Common uses for therapeutic ultrasound in veterinary rehabilitation include heating areas of muscle spasm to create relaxation, treating bicipital tenosynovitis to break up the debris within the tendon sheath, and stimulating the insertion of the iliopsoas muscle to encourage healing of a strain lesion.
There are many variables that can be applied to ultrasound, including duty cycle, frequency, intensity, and treatment duration. Duty cycle can be set at continuous or pulsed. The continuous setting is chosen when tissue heating is desired. Pulsed duty cycle can be set from 10% to 90%, with the ratio referring to the amount of time that the sound waves are being applied during any given treatment period. Frequency determines the depth of penetration of the sound waves, with 1 MHz penetrating deeper (up to 5 cm) than 3 MHz (1–2 cm). Intensity is measured in watts per cm2 and can be set from 0.1 to 2.0 W/cm2. Treatment duration is determined by the size of the transducer head, which can range from 1 cm2 to 10 cm2.
Generally, treatment time will equal approximately 5 minutes for a space that is twice the size of the transducer head. It is important for the person using ultrasound to be aware of how fragile the transducer crystal is. It must be held in contact with fluid or transducer gel at all times to prevent the crystal from damaging itself by generating sound waves in dry air. It is also important to keep the transducer head in motion during treatment, to prevent burning the tissue below the crystal.
Ultrasound waves will be attenuated by the tissues through which they pass. Bone attenuates the waves the most, followed by cartilage, tendon, skin, blood vessel, muscle, fat, and finally blood. This must be taken into account when determining the treatment intensity for a tissue or lesion that sits deep to any of these tissues. Hair is another possible attenuator of sound waves.
Some therapists advise shaving the fur over the area to be treated. Others say that this is not necessary if the fur is wet, and sufficient gel is applied to the area.
There are many physical modalities available to the veterinary rehabilitation therapist, each of which can add tremendous benefit to the patient. This is not to say that by simply purchasing these modalities and applying them to your patients you can expect great results. Excellent manual skills, meticulous diagnostic techniques, and creative problem solving are all essential before adding the potential benefit of a modality.