The effect of water in inorganic microsponges of calcium phosphates on the porosity and permeability of composites made with polymethylmethacrylate

Bone-substitute compounds were obtained by mixing pre-polymerised powders of polymethylmethacrylate and liquid methylmethacrylate monomer with two aqueous dispersions of α-tricalcium phosphate (α-TCP) grains and calcium-deficient hydroxyapatite (CDHA) powders. The final composites appeared to be porous. The total open porosity was a function of the amount of water added. The water, which was the pore-forming agent, vapourised after the polymerisation process, leaving behind empty spaces in the polymeric matrix. The inorganic powders placed inside the polymeric matrix were shown to act as local microsponges. The water capacity of these microsponges can be determined by a centrifugation step carried out on aqueous dispersion of α-TCP and/or CDHA powders that occur before any reaction with the organic compound. The relationship between the total open porosity of the composites and the amount of water trapped inside the inorganic agglomerates proved to be almost linear. The effect of the chemical composition of the powder on the total open porosity is not too great, provided that the two kinds of pellets are prepared with the same amount of water. Both the permeability and shape of the pores proved to be a function of the total open porosity. An increase of the latter parameter produces an increase in permeability and a decrease in tortuosity. Osteoconductivity and osteoinductivity of the final composites were tested in vivo by implantation in rabbits. Formation of new trabecular bone was observed inside the pores where the inorganic powders had been placed.