Prefabrication of an Axially Vascularized Bone Graft: Proof of Concept for a BMP-2-Based Cell-Free Approach

Reconstruction of large craniofacial bone defects typically relies on vascularized autografts, but involves donor-site morbidity and limited availability. Tissue-engineered vascularized grafts could address this gap; however, current strategies remain limited by in vitro cell manipulation, regulatory complexity, high costs, and difficult clinical scaling up. We previously prefabricated a maxillary graft combining a devitalized bone matrix (DBM), freshly isolated human adipose-derived cells, known as the stromal vascular fraction (SVF), and Bone Morphogenetic protein-2 (BMP-2). Here, we tested if cells were essential by developing a cell-free approach with only DBM (SmartBone) and BMP-2. SmartBone blocks were loaded with BMP-2 (60 µg/mL in a fibrin hydrogel) with or without SVF cells and implanted subcutaneously in nude mice for 12 weeks (ectopic model) or combined to an axial arteriovenous (AV) bundle in nude rats (785 mm³) for 2 or 12 weeks (vascularized model). Bone formation was assessed by X-ray tomography and histology. Across both mice and rats, BMP-2 alone was sufficient to generate robust and organized bone, and adding SVF cells provided no additional benefit at this BMP-2 dosage. In mice, BMP-2 grafts formed lamellar bone with marrow, while grafts without BMP-2 produced only calcified fibrous tissue. Bone and marrow volumes did not differ between BMP-2 and BMP-2 + SVF groups (p > 0.05). In rats, both conditions showed significant increases in mineralized volume from 2 to 12 weeks (p < 0.05). In conclusion, the dose of BMP-2 tested here, combined with DBM, is enough to robustly generate pedicled bone grafts while using off-the-shelf, approved materials and microsurgical prefabrication, providing a simplified, translationally feasible approach for the reconstruction of large bone defects.