Early new bone formation in ovine intramuscular defects: comparison between different silicate-containing calcium phosphate synthetic bone grafts

Abstract Background The property of in vivo osteoinductivity (OI) has been reported in synthetic calcium phosphates bone grafts, including silicate-containing calcium phosphates in different animal intramuscular defect models. However, most studies that have evaluated osteoinductivity in these models only focus on the formation of new bone at only one endpoint, typically 12 weeks, and without reporting evidence of graft resorption. Methods Three clinically available silicate-containing calcium phosphate bone graft substitutes were characterised and then implanted into an ovine intramuscular defect model for 6 or 12 weeks to assess their bone forming potential. Bone formation was evaluated with radiographs, micro-CT (µCT) and qualitative histology. Result The main physical/chemical differences between the three materials were the morphology and surface areas of the graft materials, but also the form that the silicate was incorporated. One of the bone grafts with a nano-scale microstructure (nano-Si-Ap) resulted in significant new bone formation that was sufficient to bridge between granules after 6 weeks which progressed further after 12 weeks implantation, and evidence of graft resorption/remodelling was observed at both time points. The other nano-scale bone graft (nano-SiO2-HA) showed a more limited quantity of new bone formation at 6 and 12 weeks and did not show evidence of resorption/remodelling. The bone graft with a micron-scale microstructure and low surface area (micro-Si-CaP) exhibited very limited evidence of new bone formation or resorption at either time points. Conclusions Implantation in ovine intramuscular defects was found to be an effective model to differentiate the relative bone forming potential of three silicate calcium phosphate bone grafts, particularly using a short implantation time of only 6 weeks. Positive outcomes in such a pre-clinical model when evaluating synthetic bone graft substitutes may be clinically relevant to their potential use in challenging bone defects.