Low intensity pulsed ultrasound (SAFHS, Exogen Inc.) was used to treat 15 immature New Zealand white rabbits following a mid diaphyseal tibial osteotomy and 1cm bone lengthening using an Orthofix M-100 device. Fifteen matched controls underwent an identical procedure but the ultrasound transducer was not switched on. At 4 and 6 weeks postoperatively the tibiae were analysed using DXA, QCT and 4 point bend mechanical testing. There were no differences identified between the active and control groups at 4 or 6 weeks with respect to bone mineral content or cross-sectional area of the regenerate, nor the bone proximal and distal to it. No improvement in strength of the regenerate was identified in either group. We cannot, therefore, support the use of the SAFHS to accelerate bone healing in patients undergoing limb lengthening.

Low intensity pulsed ultrasound has been shown to accelerate fracture healing in animals and humans. The mechanisms of action are discussed and we propose that the intensity of the ultrasound may need to be increased mechanically to stimulate a bone that is rigidly fixed using the M-100 fixator.

Aim: To examine the effect of the low intensity ultrasound stimulation (SAFHS, Exogen) on new bone formation and stress shielding in a distraction osteogenesis model in New Zealand white rabbits.

Methods: Thirty male rabbits underwent a right tibial osteotomy at eight weeks of age. Distraction of the osteotomy by 0.75mm/day was performed for two weeks. Ultrasound stimulation commenced on the seventh day after wound healing. The ‘active’ group was stimulated for 20 minutes daily. The controls had identical dummy stimulators applied. Half of the animals were culled at four weeks and half at six weeks. Dual-energy x-ray absorptiometry scans evaluated BMC and BMD. Quantitative computerised tomography measured the cross-sectional areas. Four-point bend testing of distracted and non-operated tibiae was performed in a standardised fashion.

Results: No differences were identified between the active and control groups at four or six weeks with respect to BMD, BMC or cross-sectional area of the regenerated bone. Stress shielding osteopaenia was unaffected by ultrasound stimulation. No significant improvement in strength of the regenerate was identified in either group – there was a trend towards improved strength at four weeks.

Discussion: Low intensity pulsed ultrasound accelerates fracture healing in humans when immobilised by plaster of Paris. One published study purporting to show improvement in distraction osteogenesis is fatally flawed. We believe the intensity of the ultrasound may need to be increased to stimulate mechanically a bone rigidly fixed by an external fixator. Other interventions such as the use of growth factors or bisphosphonates provide much greater improvements in experimental animals and are much more convenient to apply.