Repair of bone defects is a major challenge in orthopaedic surgery. Current bone graft treatments, including autografts, allografts and xenografts, have many limitations making it necessary to develop a biomaterial to be a bone graft substitute. One such biomaterial is bioactive resorbable silica-calcium phosphate nanocomposite (SCPC). SCPC was processed using a 3D rapid prototyping technique and sintered at different temperatures to create porous scaffolds. SEM analyses and mercury intrusion porosimetry showed SCPC to be highly porous with micro- and nanopores. BET analysis indicated that SCPC had high surface area. Mechanical testing demonstrated that SCPC had a compressive strength similar to trabecular bone. Analysis of different thermal treatment temperatures indicated as the temperature was increased, the porosity decreased and the mechanical strength increased. When loaded with rhBMP-2 (SCPC-rhBMP-2), SCPC provided a sustained release profile of rhBMP-2 for 14 days. This was shown to be a greater release than hydroxyapatite (HA)-rhBMP-2. After immersion in SBF, ICP analyses showed the calcium concentration of SBF dropped drastically after one day of immersion. In conjunction, FTIR showed the formation of a hydroxyapatite layer on the SCPC surface and was confirmed by SEM. SCPC thermally treated at 850 ??C demonstrated the greatest dissolution/precipitation reactions when immersed in SBF. Processing the SCPC-rhBMP-2 hybrid using a rapid prototyping technique allowed for an exact replica of the rabbit ulna to be fabricated. This was implanted into a 10 mm segmental defect in the rabbit ulna. CT scans during the healing of the defect showed intimate union between SCPC-rhBMP-2 and the bone and about 65% healing of the defect after 4 weeks. Rabbits were euthanized after 12 and 16 weeks. Digital images show almost complete healing of the defect after 16 weeks. Torsional testing of the ulna after 12 weeks demonstrated restoration of maximum torque and angle at failure. Histological evaluation after 12 weeks showed the regenerated bone has all the morphological characteristics of mature bone. Through in-vitro and in-vivo testing, it can be recommended that the porous bioactive SCPC can serve as a successful delivery system for biological growth factors and serve as an alternative to autologous bone grafting.

Non-healing bone defects have a significant socioeconomic impact in the U.S. with approximately 600,000 bone grafting procedures performed annually. Autografts and allografts are clinically the most common treatments; however, autologous donor bone is in limited supply, and allografts often have poor mechanical properties. Therefore, tissue engineering and regenerative medicine strategies are being developed to address issues with clinical bone grafting. The overall objective of this work was to develop bone tissue engineering strategies that enhance healing of orthotopic defects by targeting specific osteogenic cell signaling pathways. The general approach included the investigation of two different tissue engineering strategies, which both focused on directed osteoblastic differentiation to promote bone formation. In the first cell-based strategy, we hypothesized that constitutive overexpression of the osteoblast-specific transcription factor, Runx2, in bone marrow stromal cells (BMSCs) would promote orthotopic bone formation in vivo. We tested this hypothesis by delivering Runx2-modified BMSCs on synthetic scaffolds to critically-sized defects in rats. We found that Runx2-modified BMSCs significantly increased orthotopic bone formation compared to empty defects, cell-free scaffolds and unmodified BMSCs. This gene therapy approach to bone regeneration provides a mineralizing cell source which has clinical relevance. In the second biomaterial-based strategy, we hypothesized that incorporation of the collagen-mimetic peptide, GFOGER, into synthetic bone scaffolds would promote orthotopic bone formation in vivo without the use of cells or growth factors. We tested this hypothesis by passively adsorbing GFOGER onto poly-caprolactone (PCL) scaffolds and implanting them into critically-sized orthotopic defects in rats. We found that GFOGER-coated scaffolds significantly increased bone formation compared to uncoated scaffolds in a dose dependent manner. Development of this cell-free strategy for bone tissue engineering provides an inexpensive therapeutic alternative to clinical bone defect healing, which could be implemented as a point of care application. Both strategies developed in this work take advantage of specific osteoblastic signaling pathways involved in bone healing. Further development of these tissue engineering strategies for bone regeneration will provide clinically-relevant treatment options for healing large bone defects in humans by employing well-controlled signals to promote bone formation and eliminating the need for donor bone.