Injectable Calcium Phosphate Nanocomposite: Balancing Mechanical Support and Osseointegration for Enhanced Spinal Screw Fixation in an Osteoporosis Sheep Model

This study assessed the use of injectable calcium phosphate nanocomposite (CPN) for enhancing spinal screw fixation in a sheep model for osteoporosis, with the aim of addressing the limitations of polymethyl methacrylate(PMMA) and calcium phosphate cement(CPC). In vitro, CPN showed superior compressive strength (58.2 MPa vs. 15.0 MPa for CPC) and modulus (2.6 GPa vs. 1.51 GPa for PMMA), with favorable injectability and degradation. In the osteoporosis sheep model (induced via ovariectomy and methylprednisolone), CPN-augmented screws exhibited enhanced static mechanical performance (pull-out force, torque) at 12 weeks, surpassing that of PMMA and CPC, with comparable short-term fatigue durability to PMMA and better long-term residual stability. Micro-CT and histology confirmed that CPN promoted more new bone formation (BV/TV: 56.1 % at 12 weeks) and bone-implant contact (78.1 % at 12 weeks) via synergistic degradation and osteogenesis. Biosafety was validated by the absence of organ damage and abnormal blood parameters. These results indicate that CPN balances immediate mechanical support and long-term biological adaptation, yielding promising findings for application in spinal fixation for patients with osteoporosis.