Ultrasound is high frequency sound waves that transmit energy by compressing and 
rarefying material in its path. Ultrasound waves have a frequency of greater than 20,000 Hz, or cycles per second. This is based on the limits of normal human hearing of 16 to 20,000 Hz. Therapeutic ultrasound units have frequencies of between .7 and 3.3 MHz, frequencies that are absorbed by soft tissue to a depth of 2-5 cm. Recently, a 45 kHz ultrasound unit has been introduced. This lower frequency unit, called long wave ultrasound, dissipates sonic energy over a wider area.
A study by Morcos and Aswad, Histological Studies of the Effects of Ultrasonic Therapy on Surgically Split Flexor Tendons (1), was published in the Equine Veterinary Journal in 1978. The researchers describe a favorable clinical response in horses with injured superficial flexor tendons, to ultrasound therapy. The histological picture of the repair process in surgically split tendons is described following 10 minutes of treatment 6 times/week, for 2 weeks. One month after surgery the treated tendons showed an absence of the inflammatory reaction and the degenerative areas that were observed in the control tendons. A network of newly branching blood vessels was seen crossing the incision line and passing into the tissue. The authors reported that this vascular network was associated with massive invasion of young fibroblasts migrating to the incision gap from the surrounding tissue. Six weeks after surgery, collagen fibrils were seen to join with the cut tendon surfaces. The ultrasound treated tendons healed with little scar.