Janet B. Van Dyke, DVM, DACVSMR
Therapeutic plans in veterinary rehabilitation generally involve a combination of manual therapies (joint mobilizations and soft tissue mobilizations), physical modalities (laser, therapeutic ultrasound, e-stim, shockwave), and therapeutic exercises. The modalities are generally used to prepare the tissues for the manual therapies and therapeutic exercises. Physical modalities should never be the sole therapeutic method applied to any patient. Therapeutic parameters for each modality are chosen based upon the acuity of the injury, so the therapist must be well versed on the definitions of the acute, subacute, and chronic phases of healing. Therapeutic ultrasound works on a frequency of 1 MHz (megahertz) or 3 MHz via a reverse piezoelectric effect. This is the conversion of electricity to sound waves and takes place within the piezoelectric crystal housed in the transducer (sound head). Therapeutic ultrasound can create thermal and non-thermal effects, with the latter causing tissue modulation instead of heat. Tissue heating caused by therapeutic ultrasound causes increased collagen extensibility and blood flow, decreased muscle spasm, and creates a mild inflammatory effect. Non-thermal effects of therapeutic ultrasound cause enhanced tissue repair via stimulation of fibroblasts, increased protein synthesis, blood flow and glycosaminoglycans synthesis, facilitated inflammation, and improved cartilage healing. Indications for therapeutic ultrasound include muscle spasm, muscle pain secondary to IVDD, tendinopathy, delayed wound healing, and muscle strain injuries. Setting variables for therapeutic ultrasound include duty cycle, frequency, intensity, and treatment duration. Duty cycle refers to the ratio of on time to off time and can be either continuous or pulsed. Continuous treatment causes tissue heating. Pulsed duty cycle can range from 10% to 90%, with increased potential for tissue heating at higher levels.
Frequency, measured in megahertz (MHz), determines the depth of penetration of the sound waves. One MHz penetrates to 5 centimeters and 3 MHz penetrates to 1 to 2 centimeters. Intensity is measured in watts per square centimeter and is generally set between 0.1 and 2.0 W/cm2. Treatment duration is determined by the size of the transducer head and the size of the area to be treated. Transducer heads come in sizes from 1 cm2 to 10 cm2, and the general rule is to treat for 2.5 minutes per treatment area equal to the head diameter. This means that if the patient has a treatment area of 20 cm2 and the therapist has a 5 cm2 treatment head, the treatment will take 10 minutes.
There are precautions that the therapist must take when using a therapeutic ultrasound device. The crystal in the transducer head is very fragile. The head must maintain contact with fluid at all times when the unit is turned on. If the head is allowed to remain in dry air while turned on, the crystal will be damaged. As the potential for tissue heating exists, the transducer head must be kept in motion at all times during the therapy session. Fur can attenuate the sound waves preventing transmission to the deeper tissues and creating heat at the surface. To prevent this, the fur must be clipped or at least soaked in water or gel. Contraindications for use of therapeutic ultrasound include therapy over open physes, over fractures, or over a pregnant uterus. There are few clinical studies looking at the use of therapeutic ultrasound in veterinary practice. One study demonstrated healing of partial gastrocnemius muscle avulsions in dogs using therapeutic ultrasound, with follow up of 6 to 12 months.