(450b) Effects of Bone Surface Composition On the Mechanical Properties and Biocompatibility of Polyurethane/Allograft Bone Composite Cements

Injectable allograft bone/polyurethane composites offer a minimally invasive procedure for treating bone defects. The biodegradable polyurethane phase synthesized from lysine-derived and aliphatic polyisocyanates has tunable mechanical properties, degrades to non-toxic compounds, and supports cell attachment and proliferation in vitro. The allograft bone reinforces the polymer and promotes cellular infiltration in vivo through the process of creeping substitution. The mechanical properties of the composites must match those of the host bone to guarantee load support and avoid stress shielding near the surface of the material. In order to achieve polymer reinforcement a strong interaction must be present between the polymer and bone filler. This work focused on investigating the extent of interfacial binding between the allograft bone particles and polyurethane phase. Composite cements with an allograft content of 56 vol% exhibited compressive elastic modulus and strength of the order of 1500 MPa and 50 MPa, respectively. The effects of: (a) surface demineralization using HCl to activate the allograft, and (b) blockage of the OH and amine groups at the bone surface using an isothiocyanate to inhibit the reactivity of the bone, on the mechanical and biological properties of the composite cements were evaluated. Although the removal of minerals from the surface was expected to increase the reactivity of the bone with isocyanates, there were no significant differences in mechanical properties between the composites with defatted allograft particles and those with surface demineralized allograft. However, the composites with the blocked OH and amine groups exhibited an order of magnitude decrease in their compressive properties compared to those with unmodified filler. All of the composites supported cell attachment and proliferation in vitro. These observations suggest that defatted mineralized allograft bone provides a reactive surface that participates in the liquid molding reaction of injectable allograft bone/polyurethane composites. It is proposed that the extent of available reactive surface can be controlled in order to tune the mechanical properties of the composites and reach target values for treatment of bone defects.