(655c) Development of a New Generation of Bone Cement Using Nanotechnology

A new generation of bone cement with mechanical properties significantly higher than that of commercially available acrylic bone cements are strongly desired in order to ensure the long term clinical performance of the cemented arthroplasty. Currently commercial acrylic bone cement formulations contain micron-sized particles of either BaSO4 or ZrO2 to provide radiopacity, however these particles lead to a deterioration in mechanical properties. In this work, we have introduced nanosized fibrous and tubular titania particles with high aspect ratio, in the acrylic polymer matrix which not only provides radiopacity, but also acts as a reinforcing agent for the cement. The challenge for preparing nanocomposite is the agglomeration of the nano fillers in the polymer matrix which leads to poor performance of the composite. We have presented a novel method of functionalizing titania nanostructure in order to avoid the nanophase agglomeration when it is blended with the polymer matrix.

Methacrylic acid (MA), a functionalization agent that can chemically link TiO2 nanomaterials and polymer matrix, was used to modify the surface of titania using a Ti-carboxylic coordination bond. Then, the double bond in MA was copolymerized with methyl methacrylate (MMA) to form a TiO2-PMMA nanocomposite. In order to enhance the adsorption of carboxylic group of methacrylic acid on titania surface, the pH of the reaction medium has also been adjusted taking into account the amphoteric nature of the naturally occurred hydrated titania surface. In a typical experiment for functionalization, 0.1 g of calcined titania powder was dispersed in 35 mL of 2-propanol with the aid of ultrasonic agitation, followed by reacting with 3 mL of methacrylic acid at 80-85 °C for 24 hours with constant stirring. Two different extents of functionalization have been considered with two different pH of the reaction medium. In the case of nanofibers pH 3.85 and 5.5 yield 3 and 10wt% functionalization respectively, while for the tubes, pH 4.5 and 5 yield 4 and 10wt% functionalization respectively. The nanocomposites were synthesized at room temperature using PMMA powder and MMA liquid. The functionalized TiO2 was dispersed in liquid monomer portion by ultrasonic agitation for 15 minutes. The resulting materials were then hand-mixed with the powder portion into a dough state and then injected into a polysiloxane mold. After complete curing at room temperature, specimens were taken out of the mold and aged for 24 h at room temperature. For wet tests, the samples were immersed in distilled water at 37 ºC for one week. Distribution of the nanofillers in the cement matrix was examined by electron microscopy. Fracture toughness and dynamic and static elastic moduli of the nanocomposites were evaluated both in the dry and wet conditions. Moreover, radiopacity of the nanocomposites have been evaluated in order to explore its efficacy as bone cement. Commercial bone cements have also been subjected to all these characterization for a comparative study with the experimental composites.

Composites with functionalized titania nanotubes and nanofibers were shown to possess sufficient radiopacity and significantly higher fracture toughness, and dynamic and static elastic moduli, compared to commercial bone cements containing BaSO4 at p<0.05. High interfacial area of nanotubes and fibers promote the adhesion between the functionalized nanofillers and surrounding polymer matrix in allowing an external load to be effectively transferred to the nanofillers from the polymer matrix resulting in producing tougher and stiffer cements. More specifically, composites with nanotube exhibits stronger mechanical properties than that of composites with nanofibers. The improved mechanical properties are due to the higher aspect ratio and ability of the hollow nanotubes to mechanically interlock with the polymer matrix.