Biomimetic hydroxyapatite/collagen composites: injectable cements versus pre-set microspheres

Method:  Seventeen female New Zealand white rabbits were used. Critical size defects (diameter: 5mm) were created in the lateral aspect of the  femoral condyle and the MS were mixed with 1 ml of the animal’s blood to make a clot, implanting it afterwards in the defect. As a control, dense formulations of the same CPC compositions were prepared by mixing the liquid and the powder phase in situ to form a paste, injecting it afterwards in the defect. The animals were sacrificed one month post-operative to analyse material resorption and cortical and trabecular bone formation by scanning electron microscopy.

Result: A significant increase of material resorption was found for the MS as compared to the dense CPC after 1 month. Furthermore, woven bone and chondroid tissue were able to grow within the implanted biomaterials, with higher values for the MS, both with and without collagen, compared to the dense CPC. MS samples showed a longer and thicker cortical repair than CPC, less so for the collagen-covered samples. CPC allowed bone formation in form of chondroid tissue trabeculae surrounding the biomaterial, whereas the MS allowed higher trabecular infiltration throughout the whole defect site, showing perfect integration of the new woven bone formed without disrupting the pre-existing trabecular structure.

Conclusion: The results show an enhancement of the MS resorption that was even higher in the presence of collagen, as well as an increased bone formation. A second series of experiments, with 3 months post-operative period, is being processed. The use of MS as carriers for other biologically active molecules, enabled by their high microporosity and low-temperature processing is under investigation.