Degradation and swelling issues of poly-(d,l-lactide)/β-tricalcium phosphate/calcium carbonate composites for bone replacement.

Recently a tri-phase material consisting of poly-(d,l-lactide) (PDLLA), β-tricalcium phosphate (β-TCP), and calcium carbonate (CC) was proposed as a novel bone substitute candidate. β-TCP is suitable because of its bone-like mineral phase, PDLLA is introduced as a biodegradable adhesive phase, and CC is essential for buffering the acidic degradation of the lactate component. We hypothesize that the amounts of the three different components in the composite material must be carefully balanced in order to avoid issues such as accelerated degradation or pronounced volumetric swelling. To prove this, granulates made of different mixing ratios of the tri-phase compound were prepared by grinding. Specimens of the different compounds were manufactured by a hot pressing process. The bending strength of the specimens was determined before and after storing in demineralized water and phosphate buffered saline (PBS). The particle size of the compound granulates was smaller than 100µm. A ratio of 60wt% of the PDLLA component indicated the best compromise between stability of test specimens based on a strong melting network and bone-like properties. The specimens exhibited a bending strength up to 90MPa. The strength increased with an increasing ratio of β-TCP to calcium carbonate (based on 60wt% PDLLA). A vast volumetric swelling up to 40%, and thus a huge reduction of the bending strength, was observed during the storage of specimens in PBS. A swelling and thus a volume increase could be critical, especially for using the tri-phase bone substitute compound as 3D scaffold with defined dimensions. This must be considered with regard to the composition of the compound and the scaffold design.