Ultrasonically induced motion in tissue during fracture treatment?

Abstract

Objective: Ultrasound has been shown to increase mechanical strength of the callus of healing fractures in animal models. In human trials, the time to healing has been decreased using low-power pulsed ultrasound. The mechanism both physical and biological for this phenomenon is not known. We have reported accelerated endochondral ossification in the callus after ultrasound exposure, whereas other authors have reported that ultrasound exposure changes the calcium content of cultured cells. We report here the first measurement of movement of bone and tissue caused by the low power ultrasound signal (30 mW/cm∧2 average at 1.5 MHz, tone pulsed 200ux every 800 us). Methods: Fresh cadaveric human forearms were cut down over the distal radius to expose the bone. The radius was cut through to simulate a fracture. The underside of the arm was exposed to the healing signal mentioned above. A laser interferometer was used to measure the 1 kHz motion of the distal and proximal edges of the bone cut. The interferometer was also used to measure the motion of the tissue and fluid surface between the cut ends of the bone. Results: The ends of the bone moved at a velocity of 1 to 2 microns per second at 1 kHz. The tissue was found to move 3 to 5 microns per second. Movies of the motion of the bones were made with a scanning laser interferometer and showed modes of vibration. Conclusions: Small cyclic motion of the fractured bone is induced by ultrasound treatment. Because it has been shown that CW treatment does not accelerate healing but that pulsed ultrasound does, this ultrasound radiation pressure induced tissue motion may contribute to the healing effect of ultrasound.