Bone Morphogenetic Protein-2 Retained in Synthetic Polymer/β-Tricalcium Phosphate Composite Promotes Hypertrophy of a Vascularized Long Bone Graft in Rabbits

Abstract

Vascularized bone grafting is a useful method for repairing critical bone defects. The repaired bone often presents compromised mechanical strength because of insufficient thickness of the graft. To promote the hypertrophic response, bone morphogenetic protein (BMP) with osteoinductive capacity may be effective. This study was designed to evaluate the efficacy of BMP in rabbits. The first metatarsal bones with or without feeding vessels were transferred to critical-sized bone defects in the femurs of rabbits. Recombinant human (rh) BMP-2 (0, 30, or 60 μg) with a carrier material composed of 30 mg of β-tricalcium phosphate powder, 30 mg of a polymer gel (p-dioxanone/polyethylene glycol block copolymer) was placed in contact with the cortical surface and the elevated periosteum of the vascularized bone graft. After surgery, hypertrophic changes were evaluated radiographically. Twelve weeks after surgery, the reconstructed femurs were harvested for investigation by biomechanical and histologic methods. Rapid hypertrophy was observed on radiographs in the vascularized bone grafts with rhBMP-2-retaining implants. The increase of bone volume and the biomechanical strength were dependent on the dose of rhBMP-2 (p < 0.01). Histologic sections in the vascularized bone grafts with rhBMP-2-retaining implants revealed a large amount of newly formed bone. The potential use of BMP may improve clinical outcome by promoting hypertrophy of the vascularized bone graft and shortening the treatment time.