Efficacy of silicate-substituted calcium phosphate with enhanced strut porosity as a standalone bone graft substitute and autograft extender in an ovine distal femoral critical defect model.

A synthetic bone graft substitute consisting of silicate-substituted calcium phosphate with increased strut porosity (SiCaP EP) was evaluated in an ovine distal femoral critical sized metaphyseal defect as a standalone bone graft, as an autologous iliac crest bone graft (ICBG) extender (SiCaP EP/ICBG), and when mixed with bone marrow aspirate (SiCaP EP/BMA). Defects were evaluated after 4, 8, and 12 weeks with radiography, decalcified paraffin-embedded histopathology, non-decalcified resin-embedded histomorphometry, and mechanical indentation testing. All test groups exhibited excellent biocompatibility and osseous healing as evidenced by an initial mild inflammatory response followed by neovascularization, bone growth, and marrow infiltration throughout all SiCaP EP-treated defects. SiCaP EP/ICBG produced more bone at early time points, while all groups produced similar amounts of bone at later time points. SiCaP EP/ICBG likewise showed more favorable mechanical properties at early time points, but was equivalent to SiCaP EP and SiCaP EP/BMA at later time points. This study demonstrates that SiCaP EP is efficacious as a standalone bone graft substitute, mixed with BMA, and as an autograft extender.

Silicate-substituted calcium phosphate with enhanced strut porosity stimulates osteogenic differentiation of human mesenchymal stem cells.

While many synthetic ceramic bone graft substitutes (BGSs) have osteoconductive properties (e.g. provide a physical scaffold for osteointegration of surrounding bone tissue), certain BGSs are osteostimulative in that they actively upregulate mesenchymal stem cell proliferation and stimulate differentiation into osteoblast-like cells. The osteostimulative properties of silicate-substituted calcium phosphate with enhanced porosity (SiCaP EP) were evaluated in vitro with STRO-1+ immunoselected human bone marrow derived mesenchymal stem cells (HBMSCs). Osteostimulative materials (SiCaP) and Bioglass 45S5 (Bioglass) were also assessed as positive controls along with non-silicate substituted hydroxyapatite as a negative control. HBMSCs were also assessed on Thermanox discs cultured in basal and osteogenic media to determine when osteogenic differentiation could be significantly detected with this in vitro cell system. HBMSC viability and necrosis, total DNA content, alkaline phosphatase (ALP) expression, and osteocalcin expression were evaluated after 7, 14, 21, and 28 days. It was demonstrated that SiCaP EP is osteostimulative based on its propensity to support STRO-1+ HBMSC proliferation and ability to promote the differentiation of HBMSCs down the osteoblastic lineage from ALP-expressing, matrix-producing osteoblasts to Osteocalcin-producing pre-osteocytes without the presence of external osteogenic factors. SiCaP EP permitted greater HBMSC attachment as well as ALP and Osteocalcin expression than Bioglass which may be attributed to its microstructure and chemistry.

Biomimetic synthesis of calcium-deficient hydroxyapatite in a natural hydrogel.

A novel composite material consisting of calcium-deficient hydroxyapatite (CdHAP) biomimetically deposited in a bacterial cellulose hydrogel was synthesized and characterized. Cellulose produced by Gluconacetobacter hansenii was purified and sequentially incubated in solutions of calcium chloride followed by sodium phosphate dibasic. A substantial amount of apatite (50-90% of total dry weight) was homogeneously incorporated throughout the hydrogel after this treatment. X-ray diffractometry (XRD) showed that CdHAP crystallites had formed in the cellulose. XRD further demonstrated that the CdHAP was comprised of 10-50 nm anisotropic crystallites elongated in the c-axis, similar to natural bone apatite. Fourier transform infrared (FTIR) spectroscopy demonstrated that hydroxyl IR bands of the cellulose shifted to lower wave numbers indicating that a coordinate bond had possibly formed between the CdHAP and the cellulose hydroxyl groups. FTIR also suggested that the CdHAP had formed from an octacalcium phosphate precursor similar to physiological bone. Scanning electron microscopy (SEM) images confirmed that uniform approximately 1 microm spherical CdHAP particles comprised of nanosized crystallites with a lamellar morphology had formed in the cellulose. The synthesis of the composite mimics the natural biomineralization of bone indicating that bacterial cellulose can be used as a template for biomimetic apatite formation. This composite may have potential use as an orthopedic biomaterial.