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Hydroxyapatite Composites Developed for High Strength Bone Cement

4/23/2008

Ideally, a bone cement material must functionally match the mechanical behavior of the tissue to be replaced, must form a stable interface with the surrounding natural tissue, and be effective in guided tissue regenerative procedures. It should also be easy to handle, biologically compatible, non-supporting of microbial growth, and non-allergenic.

Polymethylmethacrylate (PMMA), is a plastic material commonly used as bone cement in orthopedic surgery where it has a central role in the success of total joint replacement. PMMA is used for the fixation of biomaterials such as artificial joints to bone, the filling of bone defects and, also, as a drug-delivery system. However, PMMA has low mechanical strength, requires a high curing temperature, and has toxicity problems due to leakage of the monomer of methylmethacrylate.

To overcome these problems, an international research team from the Nanotechnology Research Division in the Department of Mechanical Engineering at the University of Aveiro in Portugal, Georgia Institute of Technology in Atlanta, Georgia, and the University Louis Pasteur in Strasbourg, France has developed hydroxyapatite/PMMA composites reinforced with multi-walled carbon nanotubes (MWCNTs). These nanotubes are an outstanding reinforcement material for composites, which led to a novel, high-strength material suitable for bone cement.

When preparing such nanocomposites, a homogenous dispersion of the MWCNTs is required so that they are individually coated with the PMMA-HA material in order to produce an efficient load transfer to the MWCNT network and a uniform stress distribution. This is achieved using a freeze-granulation technique. The use of freeze-granulation, an already mature and well-understood technique based on instant freezing of sprayed drops and subsequent freeze-drying, increases material homogeneity and also enhances the dispersion of the MWCNTs in the composite matrix.

 Mechanical tests indicated that 0.1% concentration in weight of MWCNTs (purity >95%, with lengths of 1-5 µm and diameters of 5-10 nm) in the PMMA-modified HA nanocomposite material yields the best mechanical properties. In fatigue tests, after one million cycles, these nanocomposites did not present any sign of crack propagation and delamination.  Above this concentration, a significant decrease in hardness and elastic modulus occurs. In addition, in vitro tests to assess the eventual toxicity of MWCNTs were conducted with excellent results, according to principal researcher Dr. M. K. Singh. Several batteries of animal in vivo tests were also concluded and preliminary results are highly promising.

Other potential applications include orthopedic tools, such as bone fixation screws, which can be fully absorbed by the living organism after their mission is accomplished. The research was supported with funding from the Foundation of Science and Technology (FCT, Portugal). For further information contact Dr. M. K. Singh at mksingh@mec.ua.pt.