Experimental study on the repair of rabbit femoral condyle critical-sized bone defect using polyphosphoester-amino acid copolymer-composite nanohydroxyapatite filling material

To engineer a porous nanohydroxyapatite/polyphosphoester-amino acid composite bone graft substitute (nHA/PPE-AA) in granular form and evaluate its reparative efficacy and in vivo safety in a rabbit femoral condyle critical-sized defect (CSD) model. Porous nHA/PPE-AA granules were fabricated using polyphosphoester-amino acid copolymer (PPE-AA) as the organic matrix and nanohydroxyapatite (nHA) as the inorganic phase. Material characteristics were examined by scanning electron microscopy, X-ray diffraction, and Fourier-transform infrared spectroscopy. A rabbit femoral condyle CSD model (10 × 10 × 10 mm ³ ) was randomly assigned to nHA/PPE-AA, positive control (calcium sulfate), and blank groups. Bone repair was assessed by gross inspection, serum bone turnover markers, digital radiography (G/N ratio), micro-computed tomography, sequential fluorochrome labeling, histology, interfacial SEM, and BMP-2 mRNA expression. Safety was evaluated by ESR, WBC, CRP, and histology of major organs and peri-implant tissues. The nHA/PPE-AA composite exhibited an interconnected porous granular structure (35%–55% porosity, 3–7 MPa compressive strength). XRD and FTIR confirmed stable coexistence of nHA and PPE-AA. Gross and imaging analyses demonstrated progressive defect repair, with higher G/N ratios and more trabecular regeneration in the nHA/PPE-AA group. Fluorochrome labeling and histomorphometry indicated accelerated mineralization and shorter lag time. Histology and SEM confirmed mature neobone formation and superior bone-material integration. RT-qPCR showed sustained BMP-2 upregulation from 1 to 3 months, peaking at 3 months. Safety assessments revealed only transient changes in ESR, WBC, and CRP, with no significant organ pathology. Porous nHA/PPE-AA granules provide interconnected porous structure, moderate mechanical support, osteogenic activity, and good in vivo biocompatibility. In a rabbit femoral condyle CSD model, they promoted new bone formation, accelerated mineralization, and improved bone-material interface, highlighting their potential as a translationally relevant bone void filler.