The effect of low intensity pulsed ultrasound on regenerate bone in a less-than-rigid biomechanical environment

The purpose of this study was to investigate effects of low intensity pulsed ultrasound (LIPU) on distraction osteogenesis in a less-than-rigid biomechanical environment in a rabbit model. A less-rigid mini-lengthener was applied and a mid-tibial osteotomy performed in 20 New Zealand White rabbits. After a 7 day latency period, the tibiae were distracted 0.5 mm every 12 hours for 10 days. Ten of the rabbits received LIPU for 20 min/day (ultrasound group) and 10 received sham LIPU (control group) from day 17 until sacrifice on day 37. Radiographs were taken weekly after distraction and the callus area was measured. After sacrifice, dual-energy X-ray absorptiometry (DEXA), torsional testing to failure, and histomorphometry were performed. On radiographs, all the treatment tibiae displayed persistent radiolucencies; however, only one of the control tibiae displayed a radiolucent interzone. Torsional strength of the treatment group was 54% of the contralateral tibia compared to 139% in the control group (p<0.008). Bone density and callus size were not significantly different between the 2 groups; however, the ultrasound group displayed a tendency towards more cartilage and fibrous tissue formation (p<0.16) and less bone (p<0.16) than controls. In a biomechanically unstable environment, LIPU appears to stimulate more cartilage formation in regenerated callus than in controls. This callus is biomechanically inferior to unstimulated callus at the early stage of healing tested. During distraction osteogenesis a sound biomechanical environment is important to achieving anticipated results.