A tissue engineering approach to trabecular bone replacement

Due to the limitations of current bone graft materials, tissue engineers have looked to develop a synthetic alternative to trabecular bone. This thesis has examined the development of a process for the fabrication of bone regeneration implants, the mechanical and physical characterization of the resulting structures, the in vitro evaluation of the cellular response to the implants, and the in vivo evaluation of the implant as bone replacement scaffold. Matrix fabrication was based upon the use of poly(lactide-co-glycolide) microspheres arranged in a 3-dimensional structure. Mechanical optimization of the gel microsphere matrix resulted in a structure with a compressive modulus of 1651 MPa. However, this structure lacked the necessary porosity for bone regeneration. Optimization of the sintered matrix showed relationships between microsphere diameter and matrix properties. Although the sintered matrix had a modulus only in the mid range of trabecular bone (250-350 MPa), this structure possessed a pore system optimal for bone ingrowth (pore diameter 100-260). In vitro cellular evaluation of the sintered matrix using primary culture rat osteoblasts and fibroblasts showed that the matrix structure was conducive to cellular attachment and proliferation. In vitro release studies indicated that the matrix could be used as a delivery vehicle for the controlled release of drugs. It was also shown that the matrix could be used as a flexible onlay for spinal grafting procedures. A degradation study indicated that the 85:15 PLAGA copolymer (Mw = 420,000) was an optimal copolymer for bone regeneration. Optimization of the 85:15 sintered matrix showed a significant effect of microsphere diameter on the porosity and compressive modulus of the matrix. In vitro studies were also repeated using human osteoblasts. Cells attached and proliferated throughout the sintered matrix. An immunofluorescence stain for osteocalcin, a bone specific protein, showed that the cells were functioning normally within the matrix pore system. The matrix was also evaluated in an in vivo ulnar defect model in rabbits. X-ray and histological analysis indicated that by 6 weeks the sintered matrix coated with osteogenic protein-1 (OP-1) and seeded with autogenous marrow cells showed the greatest bone formation. This work represents a significant step towards the development of a clinically available trabecular bone replacement.