Material-driven fibronectin assembly enables low-dose BMP-2 to regenerate structurally mature bone in critical-sized load-bearing defects

Abstract

Bone grafts for critical-sized load-bearing defects are usually judged by how much bone forms, and rarely by whether that bone is structurally and mechanically competent. Here, degradable ceramic monetites coated with poly(ethyl acrylate) (PEA) and functionalised with fibronectin (FN) and rhBMP-2 at 10-25 g/mL, 60-150-fold below the approved clinical concentration, are assessed across length scales. MicroCT showed that only monetites functionalised with rhBMP-2 bridged the defect, with a significant increase in new bone volume in both the monetite vicinity and the whole defect volume. In contrast, platelet-rich plasma (PRP) functionalisation significantly reduced bone formation relative to FN, showing that the identity of the adhesion cue, and not merely its presence, determines the outcome. Monetite degradation and increased interconnected porosity favoured bone ingrowth and produced a continuous interface between monetite and new bone. The multiscale analyses of quantitative polarised light microscopy, SAXS and nanoindentation show that PEA-coated monetites functionalised with FN and 25 g/mL rhBMP-2 promoted structurally and mechanically more mature bone, with a hierarchical organisation and local mechanical properties approaching those of native cortical tissue. In the bridged region this group recapitulated the preferential collagen fibre orientation of native cortical bone and reached an elastic modulus and hardness that were not statistically distinguishable from it, although mineral particles throughout the regenerated tissue remained thicker and less aligned than in cortical bone. Synchrotron microCT in an ovine model extended these findings to a clinically relevant scale, supporting the translational potential of the system. Taken together, these findings position PEA-coated monetites functionalised with FN and low-dose rhBMP-2 as a strategy for the structural and functional regeneration of large bone defects.