Low-intensity pulsed ultrasound initiates bone healing in rat nonunion fracture model

Low-intensity pulsed ultrasound exposure has been shown clinically to short en the fracture repair process and to induce healing of nonunions in humans , but its mechanism of action remains unclear. In this study we investigate d the effect and mechanism of low-intensity pulsed ultrasound on nonunion f racture healing in rat tibias. A consistently reproducible nonunion was pro duced in rat tibias by muscle interposition without osteotomy This model wa s produced by creating a closed tibial fracture with only the distal end of the tibialis anterior muscle interposed into the fracture site. One limb w as noninvasively exposed to low-intensity pulsed ultrasound (a 200-millisec ond burst of sine waves of 1.5 MHz, repeating at 1.0 kHz) for 20 minutes da ily. The incident intensity was approximately 30 mW/cm(2). Rats were killed at intervals between 2 and 6 weeks. The events were assessed by radiograph s, microfocus X-ray computed tomograms, and histologic examination. After 6 weeks of exposure, 7 of 14 nonunion fractures showed healing on radiologic assessment. The results of three-dimensional microfocus X-ray computed tom ographic reconstruction and histologic examination also supported this find ing. On the other hand, all control tibias remained in a state of nonunion during the same period. These results indicate that low-intensity pulsed ul trasound promotes healing in the rat nonunion fracture model.