The Ingrowth of New Bone Tissue and Initial Mechanical Properties of a Degrading Polymeric Composite Scaffold

Abstract

Trabecular bone deficiency causes a dilemma at surgery in a variety of clinical situations, including trauma, tumor resection, and reconstruction. A synthetic material to replace trabecular bone would be biocompatible, provide temporary mechanical strength to the reconstructed region, and serve as a scaffold upon which new bone could grow (i.e., osteoconduction). In addition, it should serve as a carrier for osteoinductive biomolecules, degrade into nontoxic materials that the body can excrete via normal metabolic pathways, and allow the new bone to remodel along lines of local stress. A particulate filled composite based on an unsaturated linear polyester was designed as a candidate material for this application. The components are mixed with a monomer that cross links the double bonds of the unsaturated polyester. Degradation occurs via hydrolytic degradation of the backbone polymer's ester linkages. This strategy of prepolymer synthesis via condensation polymerization in the laboratory followed by cross linking the unsaturated prepolymer via radical polymerization at surgery offers design flexibility. The radical polymerization allows curing during surgery to facilitate reconstruction of various shaped defects. The laboratory synthesis of the prepolymer allows alterations of its composition and physical properties to effect desired properties in the resulting composite. This study investigates the effect of several composite material formulations on the in vitro mechanical properties and the associated in vivo histologic characteristics of the resulting material. The prepolymer molecular weight, presence of a leachable salt, and amount of cross linking monomer had strong effects on the resulting strength and modulus of the composite. These strengths were on the order of 5 MPa, a magnitude appropriate for consideration of the material as a temporary trabecular bone substitute. The in vivo studies in a rat proximal tibia model demonstrated progressive growth of new bone against the receding surface of the degrading material, and ingrowth of new bone trabeculae into the interior of the degrading specimen. The specimen was also well integrated with the surrounding bone, with no internal fibrosis. There was an absence of a foreign body inflammatory response to the presence of this material over a 5-week time span. This material may thus be an attractive candidate for temporary replacement of trabecular bone, facilitating both osteoconduction and osteoinduction.