Multimodal Microstructural and Cellular Assessment of Magnesium Phosphate Cement in Rat Unicortical Diaphyseal Defect Healing

Background: Magnesium phosphate cement (MPC) has emerged as a promising bone substitute due to its rapid setting, biocompatibility, controlled degradation, and favourable osteoconductive characteristics. However, comprehensive in vivo evaluations that integrate radiographic, histological, histomorphometric, and microstructural analyses in diaphyseal cortical defects remain scarce. Objective: This study aimed to investigate the healing dynamics of a standardised unicortical femoral diaphyseal defect in rats treated with MPC, using a multimodal evaluation strategy to characterise inflammatory response, osteoid formation, vascularisation, defect bridging, and biomaterial resorption. Materials and Methods: A total of 21 adult male Wistar rats underwent surgically created 2 × 6 mm unicortical femoral defects, filled with MPC. Animals were monitored for 2, 6, and 12 weeks. Healing progression was evaluated by histopathology, histomorphometry, and scanning electron microscopy (SEM). Results: Histologically, early phases were characterised by inflammatory infiltration and limited woven bone; by week 6, trabecular networks had expanded with coexisting woven and lamellar bone. By week 12, mature lamellar bone predominated, with reduced fibrosis and improved cortical continuity. Histomorphometry confirmed progressive decreases in inflammation and vascularity and increased osteoid deposition. SEM revealed early interfacial gaps that gradually consolidated into organised lamellar-like structures with visible mineralisation fronts. Conclusion: MPC demonstrated favourable biocompatibility, osteoconductive behaviour, and controlled biodegradation over the 12-week healing period. While remodelling remained incomplete in some regions, the cement supported progressive bone infill, stable integration, and gradual replacement with organised bone matrix. The findings highlight MPC as a bone graft substitute for small to moderate cortical defects and support its further optimisation for higher load-bearing applications.