Worldwide 500,000 cases of maxillofacial cancer are diagnosed each year. After surgery, the reconstruction of large bone defect is often required. The induced membrane approach (Masquelet, 2000) is one of the strategies, but exhibits limitations in an oncological context (use of autografts with or without autologous cells and Bone Morphogenetic Proteins). The objectives of this work are to develop an injectable osteoinductive and osteoconductive composite matrix composed of doped strontium (Sr) hydroxyapatite (HA) particles dispersed within a polysaccharide scaffold, to evaluate in vitro their ability to stimulate osteoblastic differentiation of human mesenchymal stem cells (hMSC) and to stimulate in vivo bone tissue regeneration.

HA particles were synthesized with different ratios of Sr. X-ray diffraction (XRD), Inductively Coupled Plasma (ICP), and particle size analysis (Nanosizer™) were used to characterize these particles. HA and Sr-doped HA were dispersed at different ratios within a pullulan-dextran based matrices (Autissier, 2010), Electronic scanning microscopy Back Scattering Electron microscopy (ESEM-BSE) and ICP were used to characterize the composite scaffolds. In vitro assays were performed using hMSC (cell viability using Live/Dead assay, expression of osteoblastic markers by quantitative Polymerase Chain Reaction). Matrices containing these different particles were implanted subcutaneously in mice and analyzed by Micro-Computed Tomography (micro-CT) and histologically (Masson's trichrome staining) after 2 and 4 weeks of implantation.

XRD analysis was compatible with a carbonated hydroxyapatite and patterns of Sr-doped HA are consistent of Sr substitution on HA particles. Morphological evaluation (TEM and Nanosizer™) showed that HA and Sr-doped HA particles form agglomerates (150 nm to 4 µm). Matrices composed with different ratios of HA or Sr-doped-HA, exhibit a homogenous distribution of the particles (ESEM-BSE), whatever the conditions of substitution. In vitro studies revealed that Sr-doped HA particles within the matrix stimulates the expression of osteoblastic markers, compared to non-doped HA matrices. Subcutaneous implantation of the matrices demonstrated the formation of a mineralized tissue. Quantitative analyses show that the mineralization of the implants is dependent of the amount of HA particles dispersed, with an optimal ratio of 5% of particles. Histological analysis revealed osteoid tissue in contact to the matrix.

In conclusion, the ability of this injectable composite scaffold to promote ectopically tissue mineralization is promising for bone tissue engineering. Osseous implantation in a femoral bone defect in rats is now in progress. 5% of doped HA particles were implanted within the induced membranes in a context of radiotherapy procedure. Micro-CT analyses are ongoing. This new matrix could represent an alternative to the autografts for the regeneration of large bone defects in an oncological context.