Regenerative Repair of a Critical-Size Rat Femoral Segmental Defect Guided by a Chemically Crosslinked Degradable Shape Memory Polymer-Hydroxyapatite Composite Scaffold

Facile surgical delivery and stable fixation of synthetic bone graft substitutes play roles just as critical as osteoconductivity and osteoinductivity in ensuring successful scaffold-guided bone regeneration. Shape memory polymers (SMPs), when engineered with a multitude of appropriate characteristics, may serve as smart synthetic bone graft substitutes to meet these challenges. We previously showed that rigid polyhedral oligomeric silsesquioxanes (POSS) nanoparticles could be covalently integrated with urethane-crosslinked polylactide (PLA) to prepare high-strength, degradable POSS-SMP nanocomposites that exhibit stable temporary shape fixation at room and body temperatures, and efficient shape recovery with a safe thermal trigger. By crosslinking star-branched POSS-PLA building blocks with hydrophilic polyethylene glycol diisocyanates (PEG-DI), we recently prepared amphiphilic POSS-SMPs in the presence of 20 wt% osteoconductive mineral hydroxyapatite (HA). The resulting amphiphilic mineral composite exhibited desirable surgical handling characteristics (compliance, toughness) and excellent shape recovery (&gt;95%) at a safe triggering temperature. The incorporation of HA mitigated the negative impact of the composite’s acidic hydrolytic degradation products on the proliferation and osteogenesis of bone marrow derived stromal cells.<br/>Here we further examine the <i>in vivo</i> efficacy and safety of macroporous 20 wt% HA/POSS-SMP composite scaffolds, tailored for a 5-mm rat femoral segmental defect and fabricated using 3D printed sacrificial molds, in guiding long bone regeneration with or without 400-ng recombinant human osteogenic bone morphogenetic protein-2 (rhBMP-2). These compliant macroporous scaffolds may be conveniently press-fit within the defect and allowed to recover to their pre-programmed shape at body temperature and/or upon hydration<i> in vivo,</i> resulting in stable graft fixation within the femoral segmental defects in skeletally mature SASCO SD rats. Using a combination of longitudinal microCT analyses over 16 weeks and terminal femoral histology analyses, we demonstrated robust new bone formation throughout the defect, guided by the macroporous degrading amphiphilic scaffolds in the absence of exogenous rhBMP-2. With the addition of 400-ng rhBMP-2, new bone formation, new bone remodeling, and scaffold resorption were significantly accelerated, resulting in complete bridging of the defects as early as 5 weeks after implantation. Histological analyses confirmed scaffold-guided osteointegration and coordinated osteoblastic/osteoclastic remodeling by 8 weeks in both treatment groups. Systemic organ pathology did not reveal any detrimental effects of the degrading nanocomposites. To further accelerate the full resorption of the scaffold and functional restoration of the mechanical integrity of regenerated long bone, an amphiphilic scaffold with larger macroporosity and/or more expedited degradation rates may be considered. These findings combined support this amphiphilic osteoconductive degradable SMP composite as a promising class of next-generation synthetic tissue grafts.