Combined use of low-intensity pulsed ultrasound and rhBMP-2 to enhance bone formation in a rat model of critical size defect.

Background: Bone repair is regulated by biological factors and the local mechanical environment. We hypothesize that the combined use of low-intensity pulsed ultrasound (LIPUS) and recombinant human bone morphogenetic protein-2 (rhBMP-2) will synergistically or additively enhance bone regeneration in a model simulating the more difficult scenarios in orthopaedic traumatology. Methods: Femoral defects in rats were replaced with absorbable collagen sponges carrying rhBMP-2 (0, 1.2, 6, or 12 µg; n = 30). Each group was divided equally to receive daily treatment of either LIPUS or sham stimulation. At 4 weeks, new bone formation was assessed using quantitative (radiography and microcomputed tomography), qualitative (histology), and functional (biomechanical) end points. Results: LIPUS with 1.2 µg of rhBMP-2 significantly improved the radiographic healing as compared with its sham control starting as early as 2 weeks. Quantitatively, the use of LIPUS with 6 µg of rhBMP-2 significantly increased the bone volume. However, using LIPUS with 12 µg of rhBMP-2 indicated a reduction in callus size, without compromising the bone volume, which was also observable histologically, showing organized lamellar bone and repopulated marrow in the original defect region. Histologically, 1.2 µg of rhBMP-2 alone showed the presence of uncalcified cartilage in the defect, which was reduced with LIPUS treatment. Biomechanically, LIPUS treatment significantly increased the peak torsion and stiffness in the 6- and 12 µg rhBMP-2 groups. Conclusions: LIPUS enhances rhBMP-2–induced bone formation at lower doses (1.2 and 6 µg) and callus maturation at 12-µg dose delivered on absorbable collagen sponge for bone repair in a rat critical-sized femoral segmental defect.