Effect of axial stress-loaded magnesium alloy intramedullary nails on bone fracture healing.

Biodegradable magnesium (Mg) alloys are promising orthopedic implants. However, the synergistic effects of their degradation and physiological loading on fracture healing require further investigation. This study evaluated Mg-Zn-Y-Nd alloy intramedullary (IM) nails in a rat tibiofibular fracture model under 10 N axial loading. A combined approach of finite element analysis (FEA) and in vivo experiments was utilized to assess stress distribution, degradation kinetics and biological response. FEA validated a dynamic load-sharing mechanism, where maximum callus strain decreased from 1530 to 233 με as mineralization progressed, effectively shifting mechanical load from the IM nail to the regenerated bone and mitigating stress-shielding. In vivo, the Mg-Zn-Y-Nd nail exhibited controlled degradation (peaking at 0.74 mm per year) synchronized with healing phases, without premature stress-corrosion failure. Radiographic and micro-CT analyses revealed that Mg-based implants significantly accelerated healing compared to stainless steel, with the Mg-load group exhibiting superior bone volume fraction and trabecular architecture. Mechanistically, FITC staining showed that early-stage high strain (1530 με) modulated osteocyte morphology into a spindle-shaped alignment (aspect ratio reached 2.5), which is essential for establishing structural anisotropy and bone quality.The Mg-Zn-Y-Nd IM nail acts as a bioactive interface where Mg release and mechanical loading synergistically promote rapid, high-quality bone regeneration. This study provides a robust biomechanical foundation for the clinical translation of load-bearing biodegradable fixations.